Prosthodontics PDF

3 Hiba al Shami Walaa Shaban Wijdan Al Manaseer  Primary Impression for Edentulous Arches The Pathway of Complete Denture Construction The goal of the prosthodontics, is to restore and maintain teeth and adjacent structures. The Pathway of Complete Denture Cons...

3 Hiba al Shami Walaa Shaban Wijdan Al Manaseer  Primary Impression for Edentulous Arches The Pathway of Complete Denture Construction The goal of the prosthodontics, is to restore and maintain teeth and adjacent structures. The Pathway of Complete Denture Construction. it is divided into two steps: Secondary impression via the special tray. Mounting via the articulator. Note that we will learn about the primary impression as we now know the landmarks, which are observed via the examination. History and Clinical Examination Why? To make the right decision for the patient’s case. How? By collecting all the information from the patient (medical and dental history) and the area we’re building a prosthesis in. Anatomy of the maxillary & mandibular arches Areas of either Maxillary or Mandibular arches are classified mainly into: A stress-bearing (or supporting) area. A peripheral (or limiting) area. Relief area. Histology The foundation for dentures is made up of bone and covering soft tissues. The denture base rests on the mucous membrane, which serves as a cushion between the denture base and the supporting bone. Anatomy of the maxillary and mandibular arches  The primary stress-bearing areas generally have thicker mucosa and/or underlying cortical bone. Relief areas usually have thin mucosa leading to near attachment of the denture and underlying bone. The total area of support in the mandible is significantly less than the maxillae. The edentulous maxillae it is 24 cm2 The edentulous mandible is 14 cm2 Therefore, the mandible is less capable of resisting occlusal forces in comparison to the maxilla which has more area to handle the stress. Buccal shelf, is a primary stress bearing area due to: 1- its made of cortical bone. 2- its horizontal, therefore, it withstands the perpendicular occlusal forces. MUCOUS MEMBRANE Composed of mucosa and sub-mucosa. The submucosa is formed by connective tissue (Varies from dense to loose areolar tissue and in thickness).depending on the type of oral mucosa and function it performs  The submucosa may contain glands, fat, or muscle fibers and transmits the blood and nerve supply to the mucosa. Where the mucous membrane is attached to bone, the attachment occurs between the submucosa and the periosteal covering of the bone. The mucosa covering the hard palate and the crest of the residual ridge is classified as masticatory mucosa. Characterized by: Its well-defined keratinized epithelium. Lack of tissue movement → firm when pressed, feels solid and resistant when touched. The maxilla A: 1- Labial frenum (not visible). 2- Labial sulcus 3-Buccal frenum. 4- buccal sulcus 12- incisive papilla. B: 1- Labial notch. 2- labial flange 3- Buccal notch. 4- buccal flange The mandible 5- Residual alveolar ridges 7- retromolar pad.  Primary Impression Impressions are negative replica capturing the same dimensions of the oral cavity and duplicating Anatomical areas and landmarks into a workable cast (replica) So we need it to be: Accurate Precise Careful Why don’t we work directly inside the patient’s mouth? Time consuming Material could harm the patient’s mouth Preliminary impressions: Trays, Materials, Techniques The clinician’s goal when making definitive impressions is to record as accurately as possible the shape of the mucosa overlying the alveolar ridges and hard palate together with the functional depth and width of the sulci. Anatomical depth of the sulcus when the muscles are at rest where as functional depth when they are contracted we care to functional depth to accommodate the movements during oral function. Preparation of the mouth: It is essential that the oral tissues be healthy before impressions are made. There should be no distortion or inflammation of the denture foundation tissues. dentures stomatitis: common fungal infection affecting oral mucosa common among denture wearers, causes redness and swelling  The most effective way of resolving the inflammation is: Leave dentures out for at least 24 hours If patient complains; use tissue conditioner (very soft material that we can put inside the denture to relief pain). Oral hygiene instructions is almost always indicated. Stock trays can be found in stock markets pre made and ready to use only in primary impression as they are not customized for each patient. Stock trays are constructed in either metal or plastic, perforated or unperforated each one used in specific materials. Stock trays: rarely fit the jaws without tissue distortion. An edentulous stock tray that is approximately 3 - 5 mm (based on the material used) larger than the outside surface of the residual ridge is selected. How is a stock tray selected? In the maxilla, the distal extension covers to the vibrating line, allowing space and coverage of the tuberosities. Anteriorly covers residual ridges, sulci labial frenum with 3-5 mm space The mandibular tray should extend distally to the ascending ramus, and should cover the labial frenum anteriorly. The tray is the most important part of the impression-making procedure. If too large: it will distort the tissues around the borders of the impression. If too small: the border tissue will collapse inward onto the residual ridge. A properly formed tray enables the dentist to carry the impression material to the mouth and control it without distorting the soft tissues that surround it or with minimal distortion. The preliminary impression should be as accurate as possible. Overextension of peripheral borders is preferred to underextension. ** denture is held in place by physical retention not mechanical so we need to capture the whole area to fabricate the denture with correct dimensions to achieve more support. Therefore, it is advisable to select an impression material that has a relatively high viscosity, thereby allowing the material to compensate more easily for the deficiencies of the tray as it gives us a better record of the area. The most suitable materials are: Alginate (irreversible hydrocolloid) Silicone putty Impression compound Alginate: Good details Must be poured immediately: **it’s dimensionally not stable because of moisture absorption and evaporation, when exposed to oral environment it absorbs water leading to expansion. Conversely, during drying it undergoes shrinkage and reduction in size They can exhibit defects in the area of the palate (big voids). Wax or compound may be added to the stock tray border before impression making to extend the borders, if the tray had deficiencies in its hight alginate will not reach the full hight of the sulcus as alginate can’t support itself. If it reached it’ll tear easily due to its low tear resistance The weight of the stone, if they are not supported by the borders of the tray or supported with a thin alginate borders, may be sufficient to distort the borders of the impression.  minimizing the risk of alginate separating or dislodging from the tray : 1- The wax and tray are covered with adhesive. 2- We use perforated trays to add mechanical retention and ensure mechanical inter locking after it set. Preloading with injection or wiping of material into the floor of mouth or posterior buccal vestibules of the maxilla especially the hard palate to avoid voids. Mixing powder to liquid ratio Extra step: marking the sulcus on the impression so it replicate accurately in the resulting cast. Silicon putty: Has a high viscosity. It will flow beyond the tray to compensate for underextension of the stock tray (if so). Once set, it will support itself in this position.  It exhibits some degree of elasticity and so will record undercuts with reasonable accuracy. Its high viscosity often records surface detail poorly. Impression compound: A thermoplastic material with a high viscosity. Like silicone putty, the material will flow beyond the tray and will maintain position after it is chilled. Not necessary to correct any underextension of the stock tray. Additions can be made to it if part of the impression is deficient (adjustable). Its high viscosity means that it records surface detail poorly. It is non-elastic and will cause trauma in severe boney undercuts **silicon has some degree of elasticity while compound is a non- elastic material. Generally we can’t use it for severe undercuts (sharp and bilateral), but sometimes we can handle it by rotating it in these anatomic areas High value of coefficient of thermal expansion Poor thermal conductivity **the impression compound is at 37 degree inside the patient’s mouth, when removed it cools from the outside while it still hot inside which causes internal stresses which if released, distortion happens so IMMEDIATE POURING IS RECOMMENDED.  If impression compound or silicone putty is used: Tray borders do not require modification. No need for tray adhesive for compound, although necessary for silicone. Preloading in the mouth not required (not like alginate as alginate is less viscous). The tray is seated in the mouth in exactly as for alginate material. Positioning: Mandibular impression: 8 o’clock at the level of the elbow. Maxillary impression: from behind; 10 o’clock at the level of the elbow. REMEMBER we aim for a well fitted denture starting with a well recorded impression. Retention, Support, Stability Retention: The resistance of the denture to removal in a direction opposite that of insertion (away from the tissues). ** resistance to vertical forces; the denture won’t fall down from the patient’s mouth. Support: The resistance of the denture to displacement towards the tissues. ** role of bone and underlying tissue resisting occlusal force.  Stability: The resistance of the denture to displacement in any direction other than the path of insertion. ** resistance of horizontal forces; the denture shouldn’t move while eating, talking or any movement. Different concepts in tissue displacement Soft tissues are normally displaceable (1-3 mm) Three techniques can be applied: 1. Mucostatic technique 2. Mucodisplacive technique 3. Selective pressure technique Mucostatic technique If minimal pressure is applied to the tissues, and therefore records their resting shape. Distribution of occlusal loads will be uneven. However, physical retention will be optimal. ** denture base is in direct contact with the underlying soft tissue. Mucodisplasive technique If more pressure is applied to the tissues, and therefore records their displaced shape Distribution of occlusal loads will be even. However, physical retention will be reduced. no intimate contact between the denture base and resting shape of mucosa A selective tissue displacement technique: Generally: Primary support area are compressed Secondary support areas are not compressed Periphery under compression to achieve seal (discussed later). Can also be done when a patient has an area of highly displaceable tissue due to hyperplasia and fibrous mucosa resulting a mobile mucosa. Typically the upper anterior region. Taking impression of displaced tissue: 1. Cut a window on the special tray above the defect. 2. Take a conventional impression of the rest of the denture bearing area (with mucocompressive technique). Trim to the defect. 3. Take another impression over the defect in a low viscosity material Should the mucosa be recorded in its resting state or in its displaced state? There is no evidence to indicate that one technique produces better long-term results than another. The choice is made by the dentist on the basis of the oral conditions based on the clinical examination. **It is variable among individuals In most maxillary cases: usually a mucostatic technique. **because residual ridges in maxilla is a secondary stress bearing area In most mandibular cases: usually a mucodisplasive technique How to control the displacement of the tissues? Viscosity of the impression material. **high viscosity →mucocompressive. Spacer in the tray. **e.g. zinc oxide eugenol is mucocompressive if there is no space. Size and distribution of perforations in the tray. **perforations → mucostatic. fabricating primary cast from our primary impression using plaster of Paris or dental stone We use POP as we don’t need high rigidity for the primary cast because we only use it to make the special tray we end up with all the anatomical landmarks replicated on the cast Perfect cast results from perfect impression. Wish you all the best