Tooth Preparation Concepts and Final Impression PDF

Tooth Preparation Concepts and Final Impressions: Do’s and Don’ts Alper Çomut, DMD, DMSc Teeth do not possess the regenerative ability found in most other tissues Therefore, careful attention to every detail is imperative during tooth preparation, because what ever you do is irreversible princip...

Tooth Preparation Concepts and Final Impressions: Do’s and Don’ts Alper Çomut, DMD, DMSc Teeth do not possess the regenerative ability found in most other tissues Therefore, careful attention to every detail is imperative during tooth preparation, because what ever you do is irreversible principles of tooth preparation may be divided into 3 broad categories: Biologic considerations, which affect then health of the oral tissues Mechanical considerations, which affect the integrity and durability of the restoration Esthetic considerations, which affect the appearance of the patient. Often, improvement in one area will adversely affect another and striving for perfection in one may lead to failure in another. For example: PFM preparation: Sufficient thickness of porcelain needed for lifelike appearance. However, if too much tooth structure is removed to accommodate a greater thickness of porcelain for esthetic reasons, the pulpal tissue may be damaged (biologic consideration) and the tooth unduly weakened (mechanical consideration). Optimum tooth preparation entails the best combination of compromises among the biologic, mechanical and esthetic considerations. Biologic considerations Prevention of damage during tooth preparation Adjacent teeth . Metal matrix band . Proximal enamel of the tooth being prepared Teeth are 1.5 to 2 mm wider at the contact area than at the CEJ. Thin tapered diamond bur to leave a slight lip of enamel without causing excessive tooth reduction or angulation Soft tissues . Tongue and cheek Pulp . Extreme temperature : Irrigation, fresh burs . Morphology of the pulp, the size decrease with age, radiographic evaluation Biologic considerations Conversation of tooth structure The thickness of remaining dentin is inversely proportional to the pulpal response Ways to conserve tooth structure: 1. Use partial-coverage rather than complete coverage restorations 2. Preparation of teeth with minimum taper between axial walls 3. Anatomic occlusal reduction 4. Orthodontically repositioning teeth to avoid uneven axial wall reduction 5. Avoidance of unnecessary apical extension of the preparation. Mechanical considerations 1. Convergence angle (Taper) 2. Retention 3. Resistance Retention Taper The recommended taper is 6 degrees The angle between the hands of a clock showing 12:01 is 5.5 degrees Retention Resistance Tooth Preparation 1. Occlusal surface / Incisal edge reduction 2. Axial wall reduction - Buccal surface reduction - Palatal / Lingual surface reduction - Proximal surface reduction 3. Margin preparation Occlusal reduction Burs: occlusal reduction bur, football bur, chamfer bur Amount: material dependent :1.5 to 2.5 mm Check occlusal reduction in maximum intercuspation. Explorer should easily pass by between the prepared tooth and opposing tooth when the patient is in occlusion Functional cusp reduction Occlusal anatomy should be followed. Not a flat surface but anatomical, preserving cusp forms Central fossa is critical: - Not enough reduction : thin restorative material - Too much reduction : short clinical crown Anatomical occlusal reduction will increase surface area of the preparation, hence increase retention and resistance of the crown Buccal surface reduction Burs: chamfer or shoulder bur Amount: material dependent :1.5 to 2.5 mm 5 6 8 12 13 14 Hold bur parallel to the long axis of the root Place the bur to the height of contour, progressively prepare the buccal surface. The margin preparation will form automatically Follow the buccal wall of adjacent teeth, similar to denture tooth set up Palatal / lingual surface reduction for anterior teeth Burs: football bur (incisal edge to cingulum), chamfer or shoulder bur (cingulum to CEJ) Amount: material dependent :1.5 to 2.5 mm Anatomical tooth preparation requires concave surface at the palatal / lingual surface. This can only be achieved by football bur. Do not use flat surfaced bur. Evaluate the amount of reduction when the patient is in occlusion Proximal surface reduction Burs: chamfer or shoulder bur, very thin chamfer or flame bur Amount: material dependent :1.5 to 2.5 mm Attention not to damage the proximal surface of the adjacent tooth. Break the contact with very thin chamfer of flame bur to leave a slight lip of enamel without causing excessive tooth reduction or angulation. Then continue with the chamfer or shoulder bur. Hold the bur parallel to the long axis of the root Margin preparation Burs: chamfer or shoulder bur Preferred finish lines are chamfer and shoulder finish lines. Attention not to create J-lip with chamfer preparation. J-lip is unsupported enamel and will break overtime, leaving open margin. J General rules for tooth preparation 1. At the proximal, if there is root proximity and margin touches next tooth,, trim the outermost part of finish line to break the contact with a thin flame bur. If not, during die ditching the finish line will be cut away and the margin of the crown will not cover tooth margin. 2. Place margins on the tooth structure, never on fillings, core materials or cast post and core. If the margins ends up very subgingival, crown lengthening will be needed. The fact that the finish line is sub gingival is not an excuse to place the crown margin on fillings or post and cores. 3. Do not tilt the bur during margin refinement. It will cause undercuts. General rules for tooth preparation 4. Remove all pre-existing fillings that you have not placed in to make sure there tooth structure is caries-free. Radiographs may be misleading and not show recurrent caries due to overlapping filling material and carious lesion. General rules for tooth preparation 5. Follow the geometric shape of the root throughout the axial wall - Developmental grooves on the roots of canines and premolars - Furcations of premolars and molars If not ➞ Overcontour at the gingiva level ➞ Periodontal problems, caries at the margins General rules for tooth preparation 6. Do not remove the existing fillings before finishing the tooth preparation. Full contour of the tooth will give you the guidance for preparation Most of the time, the filling will pop out during preparation on its on, so do not waste your time removing it as the first step. Once the tooth preparation is completed, if there is still any filling material left, remove it at that time, remove any recurrent carious lesion and place your core material and adapt it to the newly prepared tooth surface. General rules for tooth preparation 7. When multiple abutments will support the fixed partial denture, the path of insertion is important. To evaluate the common path of insertion of multiple teeth, a firm finger rest is established and the mirror is maneuvered until one preparation is centered. Axial walls should be in view 360 degrees. Then, pivoting on the finger rest, the mirror is moved, without changing its angulation, until is is centered over the second preparation. If there is a common path of insertion, the axial walls of the second abutment should also be in view 360 degrees. 8. If all abutments of the fixed partial denture will not be prepared the same visit, under-prepare the teeth at the first visit and refine the preparations at the appointment when all abutments are prepared. This will allow you to create path of insertion without over preparation. final impressions prerequisites 1. tissue health 2. saliva control 3. tray selection 4. displacement of gingival tissues tissue health 1. if periodontal disease is present, it must be treated and resolved before fixed prosthesis is fabricated 2. the surrounding soft tissues must be reevaluated after tooth preparation and use of provisional restoration 3. soft tissue trauma due to tooth preparation is transient as long as patient receives properly made provisional restoration and maintains adequate oral hygiene 4. if the provisional restoration is poorly contoured or not polished or has defective margins, plaque accumulation will lead to a localized inflammatory response. if this is the case, a properly adapted and well-contoured polished provisional must be fabricated and cemented and attention shifted from the teeth to the soft tissues, these must be returned to a state of optimum health before impression making is even considered saliva control 1. cotton rolls should be placed to the buccal and lingual site of the teeth to be impressed and suction should be in place the whole time 2. impression tray, impression material, impression syringe, retraction cords should be prepared ahead of time. once the cotton rolls and suction are placed and a dry field is obtained, the retraction cords should be placed and impression should be taken without interruption. during this time patient will not be allowed to close his/her mouth to prevent saliva presence 3. Saliva control is essential for easy cord packing, to obtain chemical retraction of gingival tissues and for elimination of any air bubbles in the impression tray selection 1. tray should cover all teeth in the arch 2. Diagnostic models can be used to select the proper size tray 3. materials: Plastic, metal, custom trays 4. types: Full tray, quadrant tray, triple tray displacement of gingival tissues impression materials steps for impression making 1. anesthetize the teeth even for non-vital tooth impression 2. prepare impression tray, impression material, retraction cord etc. 3. remove provisional restorations, clean temporary cement on the teeth 4. final check your preparations 5. if there is bleeding, use viscostat to stop 6. obtain complete saliva control with cotton rolls and suction 7. place retraction cord steps for impression making 8. no closing for the patient from the moment of cord packing until the end of impression ! 9. remove the retraction cord, make sure there is no bleeding 10. Inject the light body 360 degrees, while keeping the tip of the syringe in contact with the tooth on the margin steps for impression making 11. if the contact of the tip is lost, possibility of air bubble 12. blow air to make sure all margin is washed with light body, if not inject more 13. Place the regular body loaded tray immediately 14. do not remove before the timer is off 15. inspect the impression when it is dry