Prosthodontics PDF

16 Prosthodontics  Key learning points A factual knowledge of the various prosthodontic techniques available to restore a damaged tooth the various prosthodontic techniques available to replace a missing tooth A working knowledge of the various impression material...

16 Prosthodontics  Key learning points A factual knowledge of the various prosthodontic techniques available to restore a damaged tooth the various prosthodontic techniques available to replace a missing tooth A working knowledge of the various impression materials and techniques used in prosthodontics fixed prosthodontic techniques, including instruments and materials used removable prosthodontic techniques, including instruments and materials used A factual awareness of other removable prosthetic procedures fixed and removable orthodontic appliances, including instruments and equipment used the use of dental implants in tooth replacement Prosthodontics is the branch of dentistry that involves the restoration or replacement of damaged or missing teeth by the use of artificially constructed devices. In this specialty, teeth that have been damaged (whether by dental caries, trauma or some other means) are restored by dental techniques other than fillings, namely inlays, crowns and veneers, or they are extracted and replaced. Missing teeth are replaced by dentures, bridges or implants. Tooth restorations or replacements that are permanently cemented to existing teeth are also referred to as fixed prostheses, while those that can be removed from the mouth by the patient are referred to as removable prostheses. Implants are a stand‐alone category of tooth replacement that are provided by dentists who have undergone specialist additional training in this field. For those dental nurses with an interest in this area of specialist surgical dentistry, the NEBDN now have a post‐registration qualification available, the Certificate in Dental Implant Nursing. Further details are available at www.nebdn.org. All the artificial devices used to restore or replace the teeth are constructed outside the oral cavity by a technician, rather than within it by the dentist or therapist, as for fillings. For this reason, accurate copies of the prepared teeth and/or the dental arches must be taken and provided to the technician for them to create the artificial restoration or replacement. This is then returned to the dentist for placement or fitting in the patient’s mouth, at a later date. These accurate copies are made by taking impressions of the teeth, after the necessary tooth preparation has been carried out by the dentist beforehand. In addition, the occlusion of the individual patient’s dental arches must also be recorded accurately, as any disruption to the normal occlusion will be uncomfortable for the patient, sometimes to the point of being painful. This is because the musculature surrounding the temporomandibular joint, especially the lateral pterygoid muscles, will become strained as the teeth attempt to bite in their correct positions, and the patient will experience facial pain as the muscles are stretched, as well as dental pain due to premature contacts on the teeth. The skill of the dental technician involved in fixed prosthetic dentistry is to construct the restorations with the same tooth morphology as the original tooth, and to fit the restoration into the occlusion of that individual patient. So, each restoration is consequently constructed by hand as a unique artificial device. An inlay or crown made for one specific tooth in one dental arch would therefore fit no other tooth accurately in any other patient. Although the teeth used in denture construction are preformed, the technician involved in removable prosthetic dentistry is equally skilled in constructing prostheses that accurately fit the individual oral anatomy of the patient, as well as sitting comfortably in the correct occlusion. Again, each removable artificial device is handmade and unique to that patient. The techniques used to cement fixed prostheses (crowns, inlays, veneers or bridges) to teeth are similar to those used with fixed orthodontic appliances, while the construction of removable prostheses (dentures) is the same for removable orthodontic appliances. Consequently, orthodontic appliances are overviewed at the end of this chapter. Occlusion and malocclusion are discussed in detail in Chapter 12. Impression materials used in prosthodontics As mentioned above, all prosthodontic devices are constructed outside the patient’s mouth, and impression materials are used to record an accurate copy for that construction to take place. An impression is also taken of the opposing arch of the patient (the dental arch that does not contain the tooth to be restored or replaced), and this may involve a different impression material. The variety of impression materials available for use in dentistry is vast, but they must all have the following properties: To be easily mixed: if their correct mixing is too difficult to achieve by the average member of staff, their use will be limited. To be cost‐effective: certainly within the NHS where treatment costs are fixed, materials that are overly expensive to use routinely will not be cost‐effective and are likely to be avoided by the profession. To have an adequate working time before setting: the working time is that available to correctly mix the material before it begins to set; if this is too short then the impression will not be in place before it begins to set, and the mix will be unusable. To have a relatively short setting time: the setting time is that taken for the material to fully set so that it can be removed from the mouth without any tearing or distortion and needs to be as short as possible for the patient’s comfort. To record the tooth details accurately: a high level of accuracy must be achieved with every impression, so that tooth morphology, tooth preparation and occlusion can be reproduced correctly. To be stable when set: models cast from the impression must be accurate and not distorted, so the material must not deteriorate at normal room temperature and conditions before it is received by the technician and the models are cast up. To be elastic: this property ensures that tearing of the impression on removal from the mouth does not occur, while any distortion that does occur as the impression is pulled out of any undercuts  is not permanent, and the impression ‘bounces’ back into its original shape and maintains the recorded details accurately. To be able to be disinfected without affecting the accuracy of the details recorded: this is to avoid cross‐infection from the patient to the dental staff and the technician; the impression must be able to withstand the use and concentrations of any recommended disinfectants. Where no undercuts are present in the mouth, such as in some edentulous patients (those with no remaining teeth), non‐elastic impression materials may be used, but they have been largely superseded by the more modern elastic materials. The more commonly used elastic types of impression material fall into one of the following categories: Irreversible hydrocolloids (alginate). Addition silicones and vinyl polysiloxanes (from heavy‐ bodied putty to light‐bodied paste). Polyethers. A far less commonly used impression material is agar, which is a reversible hydrocolloid. This material is used very little now, having been superseded by the more modern alternatives. Details of the more common materials available are shown in Table 16.1, but some of the more modern ones can be mixed automatically in special machines, rather than by hand. However, impression material mixing is a daily task of the dental nurse in the vast majority of dental workplaces, and all should be proficient in the hand mixing of all commonly used materials. The techniques and skills required should be covered in all good training courses. Table 16.1 Common impression materials used in prosthodontics. Name Type of Mixing components and technique material Alginate Irreversible Powder and room‐temperature water in hydrocolloid equal portions, mixed by spatulating in a bowl Addition Elastomer Base and catalyst, as putty and liquid or silicone and two pastes, mixed in equal portions by vinyl spatulation, or in preloaded tubes, or in polysiloxanes a mixing machine Polyether Elastomer Base and catalyst pastes, mixed in equal portions by spatulation, then loaded into a syringe for direct application (Agar) Reversible Gel in a sealed tube, becomes fluid by hydrocolloid heating the tube and is mixed by manipulation within the tube before use Used in the laboratory to produce duplicate models but obsolete otherwise Alginate impression material This is the impression material most commonly used in the dental workplace, as it is easy to mix and relatively cheap. It is suitable for producing impressions for models for the following: Opposing arch models for crown, bridge, inlay and veneer construction. Models for the construction of full and partial acrylic dentures. Models for the construction of removable orthodontic appliances. Study models, for any purpose. Models for the construction of special trays, bleaching trays, orthodontic retainers. Reproduction of models, as more than one cast can be made from a single impression. However, the set material is not accurate enough to be used to take the working model for crown, bridge, veneer or inlay construction. It is presented as a coloured dry powder of calcium salt, alginate salt and filler, with a measuring scoop, which is mixed with water at room temperature using a similar measuring cup (Figure 16.1). Once the lidded container holding the powder constituents has been shaken to ensure their even distribution, and then the powder measured out into the flexible mixing bowl using the scoop provided, a 1:1 proportion of water is added and the constituents are mixed together with a large spatula. Correct mixing is achieved by folding the powder into the water initially, then vigorously spreading it against the bowl side – this is called spatulating (Figure 16.2). The mix needs to be spatulated thoroughly to be free of air bubbles, and to create a stiff and creamy consistency. Figure 16.1 Alginate measuring scoop and water measurer. Figure 16.2 Alginate mixing stages and tray loading. The mix is then loaded into an impression tray before insertion into the patient’s mouth (see later). A set impression is shown in Figure 16.3. Figure 16.3 Alginate impression set in an upper tray. The working time of alginate is affected by the temperature of the mixing water used, and the setting time is affected by the room temperature. In both cases, the higher the temperature, the less time is required. Room temperature water and surroundings provide the optimum conditions of use but are not always possible, such as on cold winter days and hot summer days. Some alginates are presented as ‘chromogenic’ materials which change colour during the mixing and setting stages, so that the tray can be loaded and the impression taken at the optimal points of the procedure. So, an initial white powder changes to pink during the working time, and the tray is loaded and inserted into the patient’s mouth. Once the material has changed to a purple colour it is set, and the impression can be removed from the patient’s mouth. The uses and advantages of alginate are listed above. Its disadvantages are as follows: Can undergo dimensional changes in the presence or absence of water. If left immersed in water, the impression expands. If allowed to dry out, the impression shrinks. Ideally, then, the model should be cast immediately after disinfection. When this is not possible, the impression should be wrapped in a damp gauze and sealed in an airtight plastic bag before sending to the laboratory. Addition silicone impression material This is one of the elastomer impression materials and is highly accurate when set. It is used specifically for all fixed prosthetic work and some removable prosthetic work. It has a variety of presentations. Tubs of heavy‐bodied putty with liquid or paste activator, a chemical which starts the reaction to produce the impression material (e.g. Express; Figure 16.4). Tubes of light‐bodied paste with liquid or paste activator (e.g. Xantopren; Figure 16.5). More recent preloaded gun syringes which mix the constituents automatically (e.g. Express; Figure 16.6). Figure 16.4 Express heavy‐bodied putty material. Figure 16.5 Xantopren paste and liquid wash material. Figure 16.6 Express light‐bodied material in delivery gun. As with alginate, measuring scoops are provided for accurate mixing, but it should be noted that it is possible for the mixing and setting times to be affected by some types of rubber PPE gloves. If mixing is to occur by hand, it is advisable that vinyl gloves are worn. When putty materials are being mixed, equal numbers of measured scoops are laid out ready for hand mixing; when lighter‐bodied materials are being mixed, either similar measured lengths of paste or the correct number of liquid drops are laid out ready for spatulation on the mixing pad. Those products which are available in side‐by‐side tubes are dispensed via a gun syringe with a specific mixing nozzle to combine the base and activator together as they are squeezed out (Figure 16.7). Many of the materials can also be measured out and mixed in an automatic mixing machine. As each component is usually highly coloured, adequate mixing can be seen to have occurred when a non‐streaky mix is produced. Unlike alginates, silicones are not affected by temperature. Figure 16.7 Detail of self‐mixing of the activator and base while passing through the mixing nozzle. The silicones can be used either in a one‐stage technique (the most widely available, and using addition cured silicones) or a two‐stage technique (using condensation cured silicones). With the former, both the heavy‐bodied putty and the light‐bodied paste are mixed at the same time. The putty is mixed and loaded into the impression tray by the dental nurse while the paste is either syringed onto the prepared tooth or placed onto it using a flat plastic instrument by the dentist. The loaded tray is inserted into the patient’s mouth where both materials then set and are removed together in the tray. With the latter, the putty is mixed, loaded into the tray, inserted into the mouth and allowed to set first. It is then carefully removed and spaced in the area of the preparation, while the mixed paste is syringed or wiped onto the tooth. The set putty and tray are reinserted and the whole is removed when the paste has set. While the one‐stage technique is obviously quicker, the two‐stage method ensures that adequate paste remains around the prepared tooth during tray insertion and gives a very accurate impression, whereas it can be displaced by the putty during tray insertion in the one‐stage method. Adhesive is usually supplied by the manufacturer to ensure the putty material remains in the tray while being removed from the mouth, rather than pulling out of the tray, but perforated trays can also be used which allow some material to squeeze through the perforations and then lock the set impression in place. Setting time for the silicones is usually 4 minutes or more, so adequate moisture control to maintain patient comfort is of great importance during this period. The advantages of silicones are as follows: Are dimensionally stable in the presence of moisture. Mixing techniques prevent the formation of air bubbles so that the impression is highly accurate. Have excellent elasticity, strength and accuracy that allow for: use in deep undercuts, without tearing of the impression undistorted final impression for model casting, as their elasticity allows the material to ‘bounce’ back to its original shape once it has been removed from the mouth several tooth preparations to be recorded accurately in one impression, without tearing. Suitable for use for all types of denture construction, as well as for fixed prostheses. The disadvantages of silicones are as follows: More complicated and time‐consuming technique of impression taking than for alginate. More expensive materials. Longer setting time may be too uncomfortable for some patients to tolerate. Paste materials are particularly sticky before setting and need to be carefully handled to avoid causing an unnecessary mess. Polyethers These are also highly accurate impression materials, used specifically for fixed prosthetic work and certain removable prosthetic work. An example of this type of impression material is Impregum (Figure 16.8). Figure 16.8 Impregum base and catalyst paste material. They are presented as two pastes which are usually different colours to ensure that uniform mixing occurs. They are mixed in equal proportions by spatulation on a waxed paper pad, and then collected into special syringes for administration to the prepared tooth (Figure 16.9). Figure 16.9 Polyether collection technique. The remaining material is loaded into the impression tray. Again, adhesive is supplied by the manufacturer to ensure the set impression remains in the tray while being removed from the mouth. Polyethers have a similar setting time to silicones but set more stiffly than other elastomers, and therefore need to be removed with a sharp displacing action from the mouth, otherwise they can be difficult to remove. Their advantages and disadvantages are as for silicones, except that they are slightly less dimensionally stable when moist. Impression handling As all the impressions taken have been inside the patient’s mouth, they will obviously be contaminated by their saliva and perhaps even their blood. To avoid cross‐infection from the patient to either staff or the technician, the impressions (and bite records) must be disinfected immediately after their removal from the mouth. This is done as follows: Rinsed under cold running water to remove any visible debris. Fully immersed in a disinfectant bath of a recommended impression disinfectant, such as a solution of up to 10% sodium hypochlorite (bleach) or products such as Proforma (Figure 16.10). Immersed for up to 10 minutes, depending on the manufacturer’s instructions. Rinsed under cold running water again, to remove the disinfectant solution. Alginate impressions: covered with wet gauze and sealed in an airtight bag. Elastomer impressions: blown dry using the triple syringe and then sealed in an airtight bag. All stored at room temperature or below before transportation to the laboratory. Work ticket enclosed, detailing dentist, patient name and age, prosthesis to be constructed, material to be used, shade, additional features, date of delivery for fitting, disinfection details. The work ticket details should also be recorded on the patient’s record card or computer notes. Figure 16.10 Impression immersed in disinfectant bath. As indicated, the majority of impressions are sent away to a laboratory and this can take some considerable time, especially if they are posted. During this period, they must remain stable so that the cast models eventually produced are accurate, otherwise the fixed prostheses will not fit onto the patient’s tooth or into their mouth accurately. For this reason, impressions should not be exposed to any heat sources or chemicals, and alginate impressions must be kept moist and not allowed to dry out, otherwise they will distort and any models cast from them will be useless. Where simple study models are required, they can be cast up at the dental workplace rather than being sent to a laboratory and the casting technique is one of several skills that can be acquired by the dental nurse as an extended duty. Further details are given in Chapter 18. Impression trays Impression trays are devices used to hold the semi‐solid impression material in the shape of a dental arch, so that it can be inserted into the patient’s mouth and held in place without dripping, while it sets over the teeth and other oral structures. The tray then holds the set impression in a horseshoe shape while it is removed from the mouth, inspected and disinfected, then sent to the laboratory for model casting. The trays are available for use with edentulous patients (Figure 16.11) and dentate patients (Figure 16.12), the latter examples being referred to as ‘boxed trays’. They can be plastic and single use, or metal and autoclavable for reuse. As many impression materials are not adhesive to plastic or metal, the trays are either perforated so the set material locks itself into the tray or are unperforated and require the use of an adhesive so that the impression sticks to the tray. Obviously, the shape of upper and lower trays differs by the inclusion of the palatal coverage required in the upper trays. Figure 16.11 Edentulous impression trays. Figure 16.12 Boxed impression trays. Those impression trays available in a variety of child and adult sizes and preformed by the manufacturer are called ‘stock trays’, while those handmade in acrylic by the technician from an initial study model are called ‘special trays’ (Figure 16.13). These are custom‐made and individual to the patient and are used when a high level of accuracy is required, such as when chrome‐cobalt dentures are being constructed. Figure 16.13 Special trays with models. A final type of tray is that used for fixed prosthodontic work, which records a partial section of both dental arches and the occlusion of the area in one impression. Examples are the triple trays shown in Figure 16.14, which are discussed later. Figure 16.14 Examples of triple trays. Fixed prosthodontics These tooth restorations or replacements are cemented within, or onto, a tooth and include the following prostheses (Table 16.2): Temporary or permanent crown: a cap or shell‐like device made to cover three‐quarters to the whole surface of a single tooth. Temporary or permanent bridge: two or more crown‐like units joined together as a single device, at least one of which is to replace a missing tooth. Veneer: a facing made to fully cover the labial surface of a tooth. Inlay: an insert into a tooth cavity that has been constructed in a laboratory. Table 16.2 Types of fixed prostheses available. Fixed Purpose of prosthesis Construction prosthesis materials Temporary To cover the prepared tooth while Preformed crown awaiting a permanent crown acrylic or (Figure As an emergency restoration polycarbonate 16.15) Cold‐cure acrylic Permanent To protect a heavily filled or root‐filled Porcelain crown tooth from fracture during chewing ceramic (Figure Aesthetics All ceramic 16.16) Tooth shape change Bonded porcelain to metal Precious metal alloy Non‐precious metal alloy Temporary To cover prepared teeth and replace Acrylic bridge missing teeth while awaiting the Resin‐based permanent bridge materials To temporarily replace missing teeth after extraction while resorption occurs Permanent To replace missing teeth All ceramic bridge Aesthetics Bonded (Figure porcelain to 16.17) metal Precious metal alloy Non‐precious metal alloy Veneer Aesthetics, to cover the labial surface Porcelain (Figure of an anterior tooth when it is 16.18) discoloured or misshapen Fixed Purpose of prosthesis Construction prosthesis materials Inlay (Figure To restore a cavity in a tooth with a All ceramic 16.19) material stronger than conventional Precious metal filling materials alloy Non‐precious metal alloy Figure 16.15 Temporary polycarbonate crown forms. Figure 16.16 Full gold crown on model. Figure 16.17 Permanent two‐unit bridge on model. Figure 16.18 Porcelain veneers on six upper anterior teeth. Figure 16.19 Cemented gold inlay. All are provided for varying reasons but involve the use of similar impression and cementation materials, and similar instruments. The material used depends on the following considerations: Tooth involved: are high chewing forces likely to occur? Aesthetics: is an anterior tooth involved? Longevity: is the prosthesis temporary or permanent? Occlusion: is the patient’s bite unusual in any way? Although some temporary crowns and bridges can be constructed at the chairside, using either stock crown forms (see Figure 16.15) or preoperative impressions to construct them, all other fixed prostheses are sent to a laboratory for construction by the technician. Crowns A crown is a laboratory‐constructed artificial restoration which replaces at least three‐quarters of the natural crown surface of the tooth. There are several different types, made of various materials, and they usually require at least two visits for the tooth preparation, crown construction and fitting to be completed. However, the use of modern three‐dimensional imaging techniques in the workplace which can scan the tooth preparation and construct the crown that day, or the use of newer composite materials (such as Luxacrown) which can be used to construct crowns at the chairside, has enabled patients to receive treatment much more quickly. The surgery procedure for the conventional tooth preparation is summarised below. All staff and the patient are provided with suitable PPE. Unless the tooth is non‐vital, local anaesthetic is administered to anaesthetise the tooth to be prepared. An alginate impression of the opposing arch is taken, using the appropriate impression tray. An occlusal registration is taken, using softened wax which the patient bites into, or with the use of a specific occlusal registration material such as Blu‐Mousse or Express Bite registration putty and catalyst (Figure 16.20). In more complicated cases where several crowns are being constructed in the same arch, a face bow technique is carried out to enable the technician to articulate the models at the laboratory. The tooth is prepared by reducing its overall dimensions by 1 mm for metallic or ceramic crowns or 1.5 mm for bonded crowns, using tapered diamond burs which produce near‐parallel sides to provide optimum retention, but without producing undercuts (Figure 16.21). The prepared tooth shape to be achieved is illustrated in Figure 16.22. To ensure accurate recording of the crown preparation margins, gingival retraction cord can be pushed into the gingival crevice and removed immediately before the impression is inserted. This is cord soaked in either adrenaline or alum, both of which cause the gingivae to retract and pull away from the tooth, thus allowing impression material to flow into the crevice created and accurately record the prepared tooth margins. An elastomer impression is then taken of the working arch, using a silicone or polyether material. When satisfactory impressions have been produced, a temporary crown is made at the chairside and cemented temporarily to the prepared tooth (see later). A shade of the tooth is taken by comparing the adjacent teeth to a suitable shade guide (Figure 16.23) and ensuring that any surface characteristics such as root darkening or hypomineralised spots are also mimicked. This stage may be carried out at any point in the procedure. All relevant details are accurately recorded on the laboratory slip, which is sent to the laboratory with the disinfected impressions and occlusal registration for construction of the permanent crown. A correct return date should be given, to coincide with the patient’s next appointment for fitting of the crown. Figure 16.20 Bite registration record using Express bite registration putty and catalyst material. Figure 16.21 Examples of tapered diamond crown preparation burs. Figure 16.22 Crown preparations of three upper right teeth. Figure 16.23 Shade guide. Laboratories vary in the time required for the conventional crown to be custom made, and the period may range from a few days to 2 weeks. Accurate and detailed information provided on the laboratory slip will ensure that unnecessary delays are avoided, and a professional and trusting relationship between the practice and the laboratory technician often allows for a speedier completion on occasion. The surgery procedure for the fitting of the crown is summarised below. Provide suitable PPE for the patient and all staff. Local anaesthesia is administered, unless the tooth is non‐vital. At this point, some dentists may choose to apply a rubber dam to the prepared tooth, so that it is isolated from possible oral contamination. Removal of the temporary prosthesis, using specific crown removal instruments or a bur in the high‐speed turbine to cut the temporary prosthesis off the tooth. Try‐in of the permanent prosthesis onto the tooth (or teeth). The marginal fit of the crown will be checked for accuracy, along with the occlusion and the shade of the prosthesis. Occlusion will be checked using articulating paper: high spots will leave a coloured mark to indicate the point that needs reducing. Reduction is carried out using burs in the high‐speed handpiece, and polishing burs or stones to smooth the area afterwards. When the dentist and patient are happy with the aesthetics and the fit, the prosthesis can be cemented into place using one of a variety of luting cements. These materials are summarised below and discussed in detail in Chapter 15. If the fit is poor or the occlusion is completely incorrect, the dentist will take new impressions and bite registration and request a remake of the prosthesis. Instruments and materials required The majority of dentists have a normal ‘conservation tray’ set up as the basic instruments required for crown preparation and fitting, and some may work under a rubber dam too. The additional equipment and materials specifically required are shown in Table 16.3. Figure 16.24 Beebee crown shears. Table 16.3 Crowns: additional equipment and materials. Item Function Diamond Tapered so that no undercuts are produced on the burs (see prepared tooth or teeth, otherwise the fixed prosthesis Figure will not seat fully onto the tooth 16.21) Retraction Cord soaked in an astringent solution (adrenaline or cord alum) that is then packed into the gingival crevice to cause shrinkage of the gingiva away from the prepared tooth. This provides a definitive tooth margin which is reproduced in the impression and also the cast model Impression Variety of plastic or metal boxed trays, sized to fit fully trays (see over the dental arch: upper and lower styles Figures Also triple tray system which takes both part‐arch 16.12 and impressions and the bite registration in one procedure 16.14) Crown Preformed plastic or polycarbonate tooth‐shaped former formers, in a variety of sizes and available for each tooth (see Figure shape 16.15) Beebee Short beaked shears for cutting and shaping the margins crown of temporary crowns shears Alternatively, acrylic burs can be used to adjust the (Figure temporary crown margins 16.24) Shade Shaded teeth in holder, to determine the required shade guide (see of the prosthesis by comparing each example to the Figure adjacent teeth and determining the best match available 16.23) When all‐ceramic crowns are to be constructed, both the core and tooth shade must be recorded using special shade guides (Figure 16.25) Figure 16.25 All‐ceramic core and tooth shade guides. The fixed prosthesis is permanently cemented to the prepared tooth using a luting cement. These are adhesive to the dentine of the tooth and are mixed to a creamy consistency so that the prosthesis can be seated fully onto the tooth before the cement sets. Types available are discussed fully in Chapter 15 and summarised in Table 16.4. Table 16.4 Types of luting cement. Type Action Mixing Self‐cure resin Chemical bonding between tooth and Double prosthesis syringe mix Light‐cure Light‐cure bonding between tooth and Double resin prosthesis syringe mix Dual‐cure resin Combination of self‐cure and light‐cure Double bonding between tooth and prosthesis syringe mix Polyester resin Chemically adhesive, and inert in saliva Waxed pad and spatula Glass ionomer Chemically adhesive to tooth and inner Waxed pad surface of prosthesis and spatula Zinc Chemically adhesive to tooth and inner Glass slab polycarboxylate surface of prosthesis and spatula Zinc phosphate Mechanically adhesive to rough inner Glass slab surface of prosthesis, and surface of and tooth spatula Modern types of cement tend to be provided in double syringe form with little or no mixing necessary, but older types (such as phosphate, polycarboxylate and some glass ionomer cements) require correct proportioning and thorough mixing before use. All can be mixed on a cool glass slab with a small spatula, by incorporating increments of powder into the relevant liquid and spatulating thoroughly until a smooth, creamy mix is produced. The types of permanent crown available can be summarised as follows: Porcelain jacket crown (PJC): an early type of all‐porcelain crown used for anterior teeth only, to provide good aesthetics when the only other alternatives were metal crowns. All‐ceramic crown: the modern successor to PJCs, constructed of stronger ceramic materials than porcelain alone (such as zirconia), and therefore able to be used both anteriorly and posteriorly to give a more ‘tooth‐like’ appearance than other crowns. They also require less tooth removal during preparation than a conventional bonded crown. An example of this type of crown is the Emax (Figure 16.26) which mimics the translucency and shine of enamel far better than porcelain or bonded porcelain and appears much more tooth‐like. Porcelain bonded crown (PBC): these consist of a substructure of metal for strength with a buccal or labial face of porcelain for better aesthetics than an all‐metal crown (Figure 16.27); these crowns were popular for years before the advent of the stronger and more aesthetic all‐ceramic crowns. They appear less opalescent than the all‐ceramic crowns and their porcelain can be cracked off the underlying metal in patients with a heavy bite. Full gold crown (FGC): these can be made of yellow gold (see Figure 16.16) or a mixture of precious or non‐precious metals to give a silvery appearance, and are the strongest of all crowns available, making them ideal for posterior teeth, especially in patients with a heavy bite. However, their aesthetics are poor compared to bonded crowns and all‐ceramic crowns. These can be made as full coverage crowns or three‐quarter crowns which leave the buccal or labial surface of the tooth intact but cover the rest of the tooth. This gives better aesthetics while still providing adequate coverage of the tooth cusps, so providing strength to the device. Three‐quarter crowns have tended to be superseded by bonded crowns, which provide both good aesthetics and strength in the same situations. Figure 16.26 Example of an all‐ceramic molar crown with tooth‐like aesthetics. Figure 16.27 Porcelain bonded crowns cemented to three upper right teeth. Post crowns As discussed in Chapter 15, when teeth die and are preserved by root filling and restoration, the remaining tooth structure often becomes brittle with time and fractures. Sometimes the fracture is so extensive that there is not enough tooth structure left to restore it without the use of additional support. This support is often achieved by the placement of a metallic post and core structure which is then shaped to hold a conventional crown – these restorations are called post crowns (Figure 16.28). Figure 16.28 Illustration of a conventional metal core and post crown. The metallic post and core system can be constructed from preformed posts, such as Paraposts or Dentatus posts with a core constructed at the chairside, or the prepared root forms part of the crown preparation impression, and the post and core are handmade by the technician, along with the crown. The chairside procedure differs only in the preparation of the post hole in the root, and the chairside post and core placement or impression technique, as follows: The root face margins of the fractured tooth are shaped as for a conventional crown preparation. The root‐filling material in the root canal is carefully removed to a suitable depth using Gates Glidden drills (see Figure 15.56). The post needs to be as long as possible to provide adequate support for the new crown, but drilling should not be so deep that there is a risk of root fracture. The canal is then prepared widthways, using drills specific to the type of post to be placed, so that a parallel‐sided hole is produced. This will give the maximum retention for the post, once cemented. A prefabricated post is then either screwed into the canal (Dentatus system) or cemented into the canal (Parapost system, Composipost system) using one of the usual luting cements. Examples of the post systems are shown in Figure 16.29. Alternatively, a wax post is placed in the hole and forms part of the impression to be sent to the technician for post crown construction. Using this technique, the post hole must then be retained as an unblocked channel while the post crown is under construction, often by the insertion of a temporary post. If a prefabricated post has been placed at the chairside, its top end is then used as the retainer for the core to be suitably shaped to hold the eventual crown itself. Suitable materials for core construction are hardened glass ionomer cements such as Vitremer. Once the impression has been taken, the core then holds the temporary crown in place while the final crown is under construction. Otherwise the technician will construct the post and core as a single structure, and then the crown as a separate structure to be cemented onto it at the fitting appointment. More recent developments in this area of prosthodontics include the development of titanium posts, which can be bent and customised to fit into individual root canals and require no drilling of the post hole before fitting, thus eliminating the potential for root fracture during preparation. Other developments include glass fibre posts which are cemented into the post hole using self‐adhesive resin cements before a core is constructed using a bulk‐fill composite material. Examples of each include Filpost and RelyX Fibre Post, respectively. Figure 16.29 Parapost, Composipost and Dentatus post systems. Temporary crowns Temporary crowns are placed for a limited time only while the permanent crown is being constructed and are used for the following reasons: To maintain the appearance. To prevent sensitivity of the prepared teeth between the preparation and fitting visits. To maintain the correct space between adjacent teeth so that the permanent crown fits. Sometimes the adjacent teeth tend to tip into the space once the crown preparation has been carried out,  as the contact points between the teeth are removed during the procedure. To maintain the correct occlusion between opposing teeth. The opposing tooth to the prepared tooth will have no occlusal contact after the crown preparation procedure and may therefore tend to overerupt. Temporary crowns can be handmade at the chairside on the day of crown preparation, or prefabricated types can be adjusted to fit the individual tooth. Those handmade on the day are created as follows: An impression of the tooth is taken before crown preparation begins, using an elastomeric material which sets firmly, such as a silicone putty. A cold cure acrylic material (such as ProTemp) is then mixed and placed in the impression after crown preparation and reinserted into the mouth over the prepared tooth. This material contracts during setting and will not easily be dislodged, even without the use of a cement. Indeed, this ‘shrink‐ fit’ technique often requires a smear of lubricating jelly such as Vaseline to be coated onto the tooth preparation first, otherwise the temporary crown can be very difficult to remove. The material takes just minutes to set and produces a temporary crown of exactly the shape of the original tooth. Shades are rather restricted, so colour matching is as accurate as can be expected. Temporary crowns placed using a prefabricated product are made by fitting a crown form over the prepared tooth. For anterior teeth a clear plastic crown form such as an Odus pella (see Figure 15.37) may be used. It is trimmed with crown scissors (see Figure 16.24) and filled with a material which matches the teeth, such as composite. The filled crown form is placed over the tooth preparation and the filler is set, usually by light‐curing. The plastic crown form is then peeled off, leaving the underlying temporary crown in place over the tooth preparation. Alternatively, tough tooth‐coloured polycarbonate crown forms are used, such as Directa (see Figure 16.15) and these only need trimming with acrylic trimming burs in the slow handpiece before being filled with either a temporary cement or a ‘shrink‐fit’ material such as ProTemp and fitted onto the prepared tooth to set (Figure 16.30). Both techniques (handmade and prefabricated) can be successfully used for posterior teeth. Figure 16.30 Prefabricated temporary crowns in place on the upper left incisors. Trimmed prefabricated temporary crowns can be cemented with a material which is adhesive to the tooth but easily and cleanly removed for fitting the permanent crown, for example Temp Bond. Metal crown forms made of aluminium, nickel‐chromium or stainless steel can be used on posterior teeth (Figure 16.31), but they have largely been replaced by more aesthetic temporary crowns, either prefabricated or handmade. Figure 16.31 Examples of prefabricated metal posterior temporary crowns. Stainless steel crown forms, often referred to now as preformed metal crowns (PMCs), are still often used as the best restoration for caries management in deciduous molars instead of a conventional filling, via the Hall technique, as well as in cases of non‐carious tooth surface loss (such as erosion) and trauma. The Hall technique involves the cementation of a PMC onto a carious deciduous molar tooth, without removing any caries or preparing the tooth under local anaesthetic beforehand. By isolating the caries bacteria under the crown and away from any further food debris nutrients, the bacteria are unable to continue attacking the tooth and the cavity is unable to progress further. Research shows that the technique has equal success rates to those where caries removal and tooth preparation are carried out under local anaesthetic and is therefore a useful treatment alternative for young and anxious patients. Providing successful restorative treatment to anxious young patients, without the need for local anaesthetics and tooth preparation, should result in compliant, healthy adult patients in the future. Bridges A bridge is a laboratory‐constructed artificial device which is composed of two or more units, one of which will replace a missing tooth. Essentially, a conventional bridge is composed of one or more units which are each exactly the same as a single crown, but as a bridge they are all joined together to make one structure. Within that structure will be one or more units that lie over the dental ridge where a tooth is missing, while the other units sit over the prepared teeth that will hold the bridge in place. The unit replacing the missing tooth is called a pontic, the units holding the bridge in place are called retainers, and the teeth that they are cemented onto are called abutments. A conventional bridge is illustrated in Figure 16.32. Figure 16.32 Conventional bridge components. Bridges have several advantages over removable prostheses (dentures), which may also be used to replace missing teeth. There is no embarrassment of a loose prosthesis falling out, as bridges are fixed to the teeth permanently. Overall, their aesthetics are superior to dentures. They are more hygienic than dentures, because there is no involvement of any teeth except the retainers and therefore fewer stagnation areas. Usually only two appointments are required for their provision, while denture construction may require up to five visits. The materials used in their construction are better able to resist occlusal forces than the acrylic used to construct many dentures. The shades available can be customised in any way by the laboratory technician to mimic the patient’s other teeth, whereas  those available for dentures are mass produced in a finite shade range and are unalterable. They solve the problem of patients with a strong gag reflex who require tooth replacement, and who usually cannot cope with a denture. They are also better tolerated because of the minimal amount of soft tissue coverage involved. However, good oral hygiene control postoperatively is of paramount importance with bridges, as they produce stagnation areas unlike any others in the mouth (that is, under the pontics), and therefore require special techniques for effective cleaning to be carried out. Due to the complexity of their design and construction, as well as the cost of the materials used in their manufacture, bridges also tend to be far more expensive than dentures. Several different types of bridges have been developed, but all designs rely on retaining teeth (abutments) to hold the bridge permanently in place, and they are joined to the missing teeth (pontics) in one structure as follows: Fixed–fixed bridge where retaining teeth are involved to either side of the missing teeth, as one solid design (Figure 16.33). Fixed–moveable bridge where a joint is incorporated in the design to allow some degree of flexibility to the bridge (Figure 16.34). Cantilever bridge where the retaining tooth or teeth are to one side of the pontic only. Simple cantilever design where retaining teeth are those immediately to one side of the pontic only (Figure 16.35). Spring cantilever design where the retaining teeth are to one side but several teeth away from the pontic (Figure 16.36). Adhesive bridge where the retaining teeth undergo minimal tooth preparation and retention is provided by lingual or palatal metal wings only (Figure 16.37). Figure 16.33 Fixed–fixed bridge replacing upper right central incisor. Figure 16.34 Fixed–moveable bridge. Figure 16.35 Simple cantilever bridge replacing the lateral incisor off the canine tooth. Figure 16.36 Spring cantilever bridge replacing the upper left central incisor off upper left first molar tooth. Figure 16.37 Adhesive bridge replacing the left central incisor. The choice of which type of bridge is used depends on several factors. Whether an anterior or a posterior tooth is being replaced, as the latter usually experience heavier occlusal forces, so full crown retainers are generally required. Like crowns, bridges can be constructed of all‐metal or all‐ ceramic materials and obviously the former would not be provided anteriorly. Fixed–fixed bridges tend not to be used so frequently nowadays, as their inflexibility during use can cause damage to retaining teeth: their solid structure, especially with long bridge spans, allowed occlusal forces on one end of the bridge to gradually loosen the other end from the abutment tooth. While undetected, this would allow caries to seep under the retainer and eventually destroy the abutment tooth. Wherever possible, adhesive bridges are used, as they involve minimal tooth preparation. If a patient has natural spaces between the teeth, only a spring cantilever design can be used so as to maintain the spaces and give good aesthetics. The health of the abutment teeth is of paramount importance to the success of the bridge; if there is any cause for concern, an adhesive type of bridge is advisable so that any problems would result in its dislodgement rather than causing damage to the abutments. All types of bridge except adhesive ones rely on the retaining teeth being of full crown coverage. Indeed, the tooth preparation is exactly the same as for a single crown, as are the instruments and impression materials used. Some additional procedures and techniques are used when constructing and fitting a bridge. While radiographs are always taken to determine the health of any tooth involved in fixed prosthodontics, study models are  often also taken before bridge construction, so that: the occlusion can be checked from all angles the bridge design can be visualised and decided upon any potential undercuts of adjacent teeth can be identified. With bonded bridges, the metal substructure is often tried on the abutment teeth before proceeding with the porcelain work so if the fit is then found to be incorrect, a full remake will not be required. Ensuring the correct occlusion is present with a multiunit bridge is a complicated process and needs to be checked and finalised before the bridge is cemented onto the abutments. It is best carried out as follows: High spots are identified by the patient closing onto a fine film of articulating foil or Mylar (shimstock). Alternatively, fine coloured articulating paper may be used (that used for removable prosthodontics is too thick). The foil or paper can be held in place using Miller forceps (Figure 16.38) which can be slid gently into the buccal or labial sulcus without compromising the occlusion. Figure 16.38 Miller forceps with articulating paper in place. Adhesive bridges These bridges are primarily used to replace just one or two front teeth, although modern luting cements and advances in preparation techniques are now enabling their use posteriorly in carefully chosen cases. The pontic has a porcelain‐bonded facing while the metal backing has wing‐like flanges which rest against the palatal or lingual surface of the abutments and are bonded directly to their acid‐etched enamel. These Maryland‐type bridges (see Figure 16.37) conserve tooth tissue, as the only preparation required is to roughen the palatal/lingual enamel where the flanges will adhere, and possibly prepare a defining ridge in the enamel to help the technician to determine the margins of the flanges. Adhesive bridges are accordingly ideal for younger patients, who are more likely to have few if any restorations present. They are far quicker to make and can be replaced much more easily than conventional bridgework, as they do not have to be cut off the abutment teeth. However, they will not withstand heavy occlusal forces without becoming dislodged, so suitable cases have to be chosen carefully. The ideal cases are younger patients with a minimal overbite, or even an open bite, where the pontic is likely to experience little if any occlusal loading. When used posteriorly, the palatal or lingual preparation of the abutments is more defined and may extend over the full surface or even up over the palatal or lingual cusps. However, this is still far less tooth preparation than for a conventional bridge and worth considering for suitable cases requiring only one tooth replacement. Ideal cases for posterior adhesive bridges include patients with no opposing teeth to the bridge, or a denture is present as the opposing teeth so that occlusal forces are lower than if natural teeth were present. The adhesive bridge requires special dual curing resin cements with primers, to provide a strong chemical bond between the retaining teeth and the metal wings of the bridge. The fitting surface of the flanges is made retentive by acid etching and sand blasting, and a chemical‐cure adhesive resin, such as Panavia Ex, which bonds to both metal and enamel, is used as a luting cement. Temporary bridges A temporary bridge is necessary between the bridge preparation and fitting visits to prevent tooth sensitivity, space closure and tipping or overeruption of the abutment teeth. It may be made directly in a similar fashion to that of a chairside constructed temporary crown. Before the abutment teeth are prepared, the gap of the missing tooth is filled with a piece of cotton wool roll, to mimic the presence of the missing tooth. A putty or heavy‐bodied elastomer impression of the bridge area is then taken, the cotton wool discarded, and the impression put aside. The abutments are then prepared and an impression for the permanent bridge is taken. The first impression is now used to make a temporary bridge. A composite‐type resin (such as Temphase or ProTemp) is placed in the part of the impression containing the abutment teeth and pontic area, and the impression is then reinserted over the prepared teeth until the resin sets. On withdrawal of the impression, the temporary bridge is removed, trimmed and cemented back into place with a temporary cement until the permanent bridge is cemented at another visit. Alternatively, temporary bridges may be used as tooth replacements for up to 6 months after the extraction of a tooth, to allow bone resorption to occur before a permanent bridge is constructed. The abutment teeth are prepared in the same way and the impression is taken and sent to the technician. The abutment teeth have a temporary crown‐like covering placed. The technician then removes the tooth to be extracted from the working model and constructs the temporary bridge to replace it, using composite‐type resin materials or acrylics. Once the temporary bridge is returned, the abutment covers are removed, the tooth is extracted and the prosthesis cemented to the abutment teeth. Bone resorption can then progress without leaving unsightly gaps beneath the pontic of a permanent bridge. Any gaps that do become apparent under the temporary bridge can be closed using composite materials, until the risk of further resorption is over, usually around 6 months post extraction. The permanent bridge can then be constructed to replace the temporary bridge. Oral hygiene instruction for crowns and bridges No matter how well fitting the crown or bridge is to the tooth, microscopically the junction between the two is a potential stagnation area for plaque to gather. Thorough brushing at the margins of the crown will ensure that plaque does not accumulate and cause recurrent caries or periodontal problems. The general oral health messages to be relayed to the patient following crown or bridge cementation are: regular and thorough toothbrushing daily use of fluoride toothpaste and medium‐textured toothbrush, ideally a good‐quality electric toothbrush regular flossing or interdental brushing to clean crown margins interproximally careful use of floss so as not to dislodge crown attend for dental examinations so that margins can be checked professionally sensible diet, low in free sugars regular use of good‐quality mouthwash, to reinforce plaque control. In addition, bridges provide a challenge to the patient with regard to adequate oral hygiene, as they are fixed prostheses producing stagnation areas actually beneath the pontics, where conventional brushes and floss cannot be used. As well as the oral hygiene instructions for crowns, patients with bridges need to be instructed in the use of Superfloss (Figure 16.39). This is a type of dental floss with a stiff end, which can be threaded under the pontic and then drawn through to a sponge part which is used to clean beneath the pontic. When used regularly, it keeps this region of the bridge plaque‐free and prevents caries undermining the retainers, with catastrophic consequences. Alternatively, a correctly sized interdental brush can be used to clean under the pontic by being gently inserted into the area from the labial or buccal side. Figure 16.39 Illustration of Superfloss. More recently, the use of sonic toothbrushes has been shown to provide excellent cleaning in these areas, without dislodging the bridge, and these are being recommended more frequently in these cases. Veneers Conventional crown preparation requires the removal of a significant amount of dentine from the tooth, involving all of the tooth surfaces. While this may be harmless in fully developed adult teeth, it could result in pulpal damage in younger patients as the pulp chambers are larger in recently erupted teeth. In other cases, it may be felt that labial enamel defects in incisors that require a restoration to improve appearance do not justify a full jacket crown preparation, and the teeth are more suitable for restoration by veneers. Veneers are either a composite or porcelain facing made to cover the labial surface of anterior teeth. Where composite is used, the dentist carries out the restorative procedure at the chairside, as for a routine filling with this material. Porcelain veneers require the input of a technician to construct each one by hand, in the laboratory. They are used in the following situations: To mask a discoloured tooth (such as with tetracycline staining). To mask a root‐filled tooth that has become darkened with time. To close diastemas between teeth and improve the appearance. To change the shape of rotated teeth so that they appear aligned. To change the shape of malaligned teeth so that they appear aligned. To correct poorly shaped teeth, such as peg laterals. As a cosmetic procedure, to lighten the whole labial segment, although this has been largely superseded by the use of tooth‐ whitening techniques. Porcelain veneers are fragile once constructed and can break if the patient is careless with them. Ideally, they are only fitted to patients with low incisal edge forces and they are sometimes constructed so as not to cover the incisal edge of the tooth at all but finish just in line with it. The instruments and impression materials used for porcelain veneer construction are the same as for crowns and bridges, but often no opposing arch impression is required as veneers rarely encroach on the occlusion. The surgery procedure for veneer preparation is as follows: Unless the tooth is non‐vital, local anaesthetic will be required. On the rare occasion that an opposing arch impression is required, this is taken in a stock tray using alginate. The labial surface of the tooth is prepared by removing enough enamel to allow the technician to construct the veneer; this is especially important if the veneer is to give the appearance of an improved alignment to the tooth (Figure 16.40). An impression is taken of the labial segment using one of the highly accurate elastomer materials, as for crowns and bridges. The prepared tooth is covered temporarily for appearance and sensitivity reduction, using composite material etched just to the centre of the tooth, so that it can be removed easily at the veneer fit appointment. This stage may not always be necessary when minimal tooth preparation has been carried out. An accurate shade is taken, recording all tooth characteristics for the technician, as for crowns and bridges. Figure 16.40 Veneer tooth preparations of all six upper anterior teeth. As with all fixed prostheses, veneers are custom made in the laboratory by a highly skilled technician. The shades taken in the surgery will be accurately replicated as the veneer is constructed by hand from porcelain, before the final firing in an oven to produce the surface glaze. The fitting surface of the veneer will be abraded and chemically roughened using hydrofluoric acid in the laboratory, to produce a rough surface for cement adhesion. The finished product is then returned to the surgery for fitting. The veneer fitting procedure is as follows: Again, local anaesthetic may be required. The temporary veneer is removed, by flicking it off carefully with a hand instrument, such as a flat plastic or an excavator. The veneer is carefully tried onto the tooth and the fit and shade are checked.  Special light‐cure or dual‐cure luting cements are used for veneer cementation, such as RelyX. They contain little filler content and are often available in different shades so that the final veneer appearance can be further matched to the adjacent teeth. If the fit and shade are satisfactory, the fitting surface of the veneer is coated with a silane agent, which allows the luting cement to chemically bond to it for good adhesion. The tooth is isolated with either a rubber dam, celluloid matrix strips, or with PTFE tape (Figure 16.41) stretched over the adjacent teeth and then etched, washed and dried. The dual‐cure resin bond and cement are applied to the tooth and the veneer is carefully pushed onto it with a paddling action, in the correct position. Excess cement is carefully removed before full light curing occurs, without disturbing the position of the veneer. Flecks of cement trapped interproximally can be removed using abrasive diamond strips, otherwise they will act as stagnation areas and hold plaque. Figure 16.41 PTFE tape used to isolate a tooth under treatment. The final appearance possible is shown in Figure 16.18. Inlays These are fixed prostheses used to restore a cavity in a tooth, rather than to cover the whole or part of the surface of a tooth, as the other fixed prostheses do. Unlike fillings, though, which are also used to restore cavities, inlays are constructed indirectly in a laboratory by a technician rather than placed directly into the tooth by the dentist or hygienist. They are constructed of gold alloy, porcelain or a special type of composite which contains more filler than usual and is therefore stronger than conventional composite filling materials that are placed at the chairside. However, the advent of modern composite materials, especially the bulk‐fill varieties, has reduced the general usage of indirect composite inlay techniques as the results now possible at the chairside with newer materials is as good as the inlays, but less time‐ consuming. The purpose of using an inlay rather than a filling is to produce a restoration of higher strength than that possible with conventional plastic materials, and of a more permanent nature, although with the continual improvement of filling materials, gold alloy inlays are being provided less frequently nowadays. They are generally confined to teeth which have lost cusps, undergo heavy occlusal forces or are otherwise too weak to be satisfactorily restored with amalgam. Small uncomplicated cavities do not usually warrant the extra time and expense of restoring them with inlays. Their use in anterior cavities has also declined with the development of better aesthetic anterior filling materials. As the inlay is inserted into the tooth rather than cemented onto it, less tooth preparation is also necessary than if the tooth were restored using a conventional crown. The equipment, materials and impression techniques are the same as for other fixed prostheses. Inlay preparation is as for a conventional filling, with the full removal of all carious tooth tissue to sound dentine, but then the resultant cavity preparation is adjusted to ensure that the sides are not undercut but parallel (Figure 16.42). This may involve any undercut walls being filled in with plastic materials, such as glass ionomer cements. This allows the inlay to be inserted fully, without becoming stuck on an undercut. The maximum retention possible is produced, by ensuring that the inlay fits snugly against all the cavity walls. Only a fine cement layer will then be required, which reduces the risk of cement dissolution in saliva with time, and the gradual loosening of the inlay. Figure 16.42 Illustration of an inlay preparation. Once the cavity has been suitably prepared, the necessary impressions and occlusal registrations are taken. Gingival retraction cord may be used to ensure that deep cavity margins are sufficiently exposed for an accurate impression to be taken. The tooth is restored with a temporary filling while the inlay is being constructed. At the fit of the inlay, the occlusion is checked as for crowns and when correct, the inlay is cemented into place using any of the luting cements available. Gold alloy inlays have their margins well adapted to the tooth by burnishing at the fit stage, so that the wafer‐thin edge of the gold is pressed firmly against the cavity wall. This prevents ingress of saliva and reduces the possibility of the cement being dissolved out (dissolution), with subsequent loss of the inlay. Removable prosthodontics Removable prostheses are all types of dentures – appliances that are made in the laboratory in various stages to replace missing teeth. They can be removed from the mouth by the patient, for example for cleaning, and reinserted again easily, without the use of cements. Generally, removable prostheses are made to replace several missing teeth rather than just one or two, as bridges do, or even to replace all the teeth in some patients. When there are no teeth left in a jaw, it is said to be edentulous (edentate) and the artificial replacement is called a full or complete denture; if some teeth are still present, the replacement is called a partial denture. The majority of dentures are made completely of acrylic, although many may also be constructed with a base of chrome‐cobalt metal. Teeth may need to be replaced by a removable prosthesis (or indeed by a bridge or an implant) for the following reasons: Prevent excessive masticatory forces on the remaining teeth, which may cause their eventual fracture. Prevent overeruption of the opposing teeth, which may cause occlusal problems. Prevent tilting of the adjacent teeth into the edentulous spaces, causing stagnation areas. Prevent soft tissue trauma of the alveolar ridges during mastication. Allow adequate mastication and avoid digestive problems and malnutrition, especially in the elderly. Provide good aesthetics, especially if anterior teeth are missing. Not all patients are suitable for tooth replacement by the use of dentures, and the following points are considered for every case before treatment commences. Is there any previous denture experience, and was it successful or not?  If not, is there a cause which can be remedied? Is the shape of the patient’s mouth naturally retentive for full dentures, with good ridges and a high palate, or might preprosthetic surgery be necessary? Are there any potential retention problems for partial dentures and, if so, can they be remedied by tooth shape adjustment? Might the patient’s occlusion cause problems with the provision of a denture? Is there enough clearance without premature contact onto the denture? Are there any medical contraindications to dentures, such as epilepsy or an adverse reaction to the acrylic material? Are there other dental problems which need addressing first, such as caries or periodontal disease? If the teeth have been lost within the previous 6 months, bone resorption is likely to occur and this will affect the fit of a denture adversely. Good co‐operation and perseverance by the patient are paramount to the success of dentures. If there is any doubt about these then the treatment is likely to fail. Can the patient afford the treatment? Full and partial acrylic dentures These are the most common types of denture: full ones (Figure 16.43) for edentulous patients and partial ones (Figure 16.44) for patients with any number of missing teeth up to one tooth short of being edentulous. The material used for their construction, and that of removable orthodontic appliances too, is either pink or transparent acrylic. Figure 16.43 Full upper denture. Figure 16.44 Partial upper denture on model. Acrylic consists of a powder called a polymer and a liquid called a monomer. When mixed together, they form a plastic mass which has the consistency of dough. This sets into a hard acrylic by a process called curing. Curing is effected by heating the dough slowly in a special flask in an oven, or by adding a catalyst which allows it to cure at room temperature. These two methods of curing are known respectively as heat curing and cold curing. Heat‐cured acrylic is used for dentures and orthodontic appliances, and the curing process is carried out by a technician in the laboratory. Cold‐cured acrylic (also called self‐cured or autopolymerised acrylic) can be used by the dentist at the chairside to make temporary crowns, and to carry out denture repairs. It is also used by the technician for the construction of special trays to take accurate second impressions. As dentures are removable prostheses, their retention must be adequate to keep them in position in the mouth during speech and chewing, but weak enough so that the patient can easily remove the device from their mouth as they wish, say for cleaning purposes. The level of retention achieved relies on the following factors: A suction film of saliva developing between the denture and the patient’s soft tissues, especially the palate. A post‐dam along the back border of the upper denture, to help the suction film to develop. An accurate design and fit of denture, to allow the film to develop adequately. Use of any natural undercuts in the patient’s mouth, such as the alveolar ridges or any suitably shaped natural teeth. Use of stainless steel clasps around standing teeth with partial dentures, to increase the retention of the denture by the clasps gripping the teeth and preventing it from being dislodged by normal soft tissue movements (Figure 16.45). Figure 16.45 Denture clasp example. Sometimes no natural undercuts are present in the patient’s mouth so their own teeth are adjusted to provide them, in the following ways: Use of a crown to change the overall shape of the tooth. Use of composite build‐ups to provide a retentive area for clasps to engage. Shape change of an existing restoration for similar reasons. With edentulous patients, the alveolar ridges can be changed surgically, to improve retention and comfort. Alveoplasty: changing the shape of the existing ridge, such as by the removal of gross undercuts which would prevent the denture being seated. Flat ridges can be built up by the addition of artificial bone substitutes under the mucoperiosteum, to increase natural retention by creating a ridge that the denture can sit on. Alveolectomy: the surgical removal and smoothing of sharp ridges to allow comfortable wearing of the denture. More recently, a special type of denture acrylic material has been developed which becomes flexible when warmed by insertion in warm water, so that it can be flexed while being inserted into the mouth. This enables any undercut ridges to be utilised in the retention of the flexible appliance, whereas a denture constructed of conventional acrylic (which is rigid, not flexible) would not be able to be inserted into the undercuts. The warmth of the oral cavity enables the flexible denture to be removed again, but otherwise it stays in place during chewing and speech. An example of this material is Valplast resin. Denture construction Usually, acrylic dentures are made in four or five stages, with each stage being returned to the technician at the laboratory between patient appointments. The dentist prepares and records the details of the patient’s oral cavity, and the technician uses these records to construct the dentures to fit that patient’s mouth. Each laboratory stage is returned to the dentist for the next clinical stage to be recorded in the patient’s mouth, until the end result – the acrylic dentures, with or without clasps, are produced for fitting. The stages are as follows, although not every stage is required in every case. First impressions: using stock trays and alginate impression material (see Figure 16.3); the tooth shade and shape (mould) are often decided at this stage too. The impressions are correctly disinfected, as described previously, and suitably wrapped for dispatch to the laboratory. Laboratory: study models are cast in plaster of Paris from the impressions, and special acrylic impression trays are custom made from them if required; in simple cases, the first impression may be accurate enough for denture construction to proceed. Second impression: using special trays and either alginate or elastomer impression material to produce a very accurate impression, and the tooth shade and mould may be chosen at this stage if not already recorded. Laboratory: working models are cast in dental stone and wax occlusal rims are constructed on them. Alternatively, the special tray and wax rims are both provided at a second‐stage appointment (Figure 16.46). Bite registration: the existing, or required, occlusal face height of the patient with an edentulous arch is measured using a Willis bite gauge (Figure 16.47) and recorded on the occlusal rims by warming them or using bite registration paste to stick them together. Patients with some teeth present in both arches are guided to their usual occlusal position with the rims in place to record their normal occlusal face height and tooth contacts. The rims then hold the models in the correct position and angulation for the dentures to be constructed (Figure 16.48). Laboratory: models in their recorded face height positions are mounted onto an articulator, so the technician can construct the wax try‐in dentures in these correct horizontal and vertical positions.  Try‐in: wax try‐ins with the actual acrylic teeth mounted in them (Figure 16.49) are inserted and checked for accuracy of fit and occlusion, as well as shade; any major inaccuracies will result in new records being taken and a retry being requested. Laboratory: stainless steel clasps are added as necessary, then the try‐ins on their models are sealed into flasks and the wax is replaced by heat‐cured acrylic to form the final dentures, which are then cleaned and polished to provide a shiny outer surface to the denture. Fit: acrylic dentures are inserted in the patient’s mouth and checked for comfort, fit and aesthetics, then specific denture care information is given. Figure 16.46 Second stage of denture construction: (a) wax bite rims on models; (b) special tray. Figure 16.47 Willis bite gauge in position to record occlusal face height. Figure 16.48 Wax rims on working models. Figure 16.49 Try‐in stage of lower full denture. Each stage of denture construction in the surgery involves the use of specific instruments, materials and equipment which the dental nurse must be able to recognise and lay out at each appointment. These are summarised in Tables 16.5–16.9. Table 16.5 First impressions: instruments, materials and equipment. Item Function Stock To be sized and used to take the initial impression trays impressions, so that special trays can be (see Figures 16.9 constructed; they may be upper and/or lower, and and 16.10) edentulous or dentate Alginate To be mixed, loaded into the trays and inserted to impression produce the initial impressions material and room‐ temperature water Shade and To determine the colour and shape of the denture mould guides teeth, to be as close in appearance to any remaining teeth as possible Work ticket or To record the patient and dentist details, the docket (Figure denture design and base material to be used, the 16.50) tooth shade and mould, the type and position of any clasps, and the return date Table 16.6 Second impressions: instruments, materials and equipment. Item Function Study models and To take the more accurate second impressions special trays where required, to produce the working models Alginate or To take the more accurate second impressions elastomer impression material Work ticket To record the next stage request and the return date Table 16.7 Bite registration: instruments, materials and equipment. Item Function Wax bite rims Adjusted in height so that correct face height of the patient can be recorded Wax knife To remove or add additional wax to the rims, as (Figure 16.51a) necessary Bite To be mixed and applied to the rims, so that they registration are held in the correct position once set paste (optional) Pink sheet wax For addition to the rims, as necessary (Figure 16.51a) Willis bite To record the desired occlusal face height in gauge (Figure edentulous patients, where no natural teeth remain 16.51b) as a guide Heat source To warm the hand instruments and rims for (Figure 16.51c) adjustment Work ticket To record the next stage request and return date Table 16.8 Try‐in. Item Function Try‐in To determine if fit, occlusion and aesthetics are correct prostheses before finishing the dentures Heat source To warm the wax and make adjustments, as necessary Le Cron To make fine adjustments to the try‐in, as necessary carver (Figure 16.51a) Wax knife To warm and smooth the wax after adjustments, as necessary Shade and To check or alter the shade or mould, as necessary mould guides Pink sheet For addition to the try‐in, as necessary wax Patient To allow the patient to view the try‐ins and decide if mirror they are happy with the appearance, before completion of the dentures Work ticket To record any changes required for a retry, or to record the fit return date Table 16.9 Fitting. Item Function Completed removable To fit, to the patient and dentist’s prostheses satisfaction Straight handpiece and To remove any acrylic pearls or occlusal selection of high spots before polishing and trimming/polishing burs smoothing the adjusted area for comfort (Figure 16.52a) Patient mirror To allow the patient to view the completed prostheses Articulating paper (Figure To identify occlusal high spots, for 16.52b) adjustment as necessary Pliers (Figure 16.52c) To adjust clasps as necessary Pressure relief paste To identify high spots on the denture fitting surface, for removal as necessary