L7-Comut-Principles of tooth preparation and concept of margin design.pdf

Lecture objectives Alper Çomut, DMD, DMSc Clinical Associate Professor Department of Prosthodontics PRINCIPLES OF TOOTH PREPARATION AND CONCEPT OF MARGIN DESIGN What is tooth preparation? To remove tooth structure with certain principles to replace it with dental materials in order to restore form...

Lecture objectives Alper Çomut, DMD, DMSc Clinical Associate Professor Department of Prosthodontics PRINCIPLES OF TOOTH PREPARATION AND CONCEPT OF MARGIN DESIGN What is tooth preparation? To remove tooth structure with certain principles to replace it with dental materials in order to restore form, function and esthetics. • Principles of tooth preparation • Retention and resistance form • Angle of convergence • Height/width ratio • Arc of displacement • Auxillary grooves • Margin preparation • Types of margin design • Selection of margin design • Location of the margin Aims of tooth preparation • Preserve as much tooth structure as possible to provide retention and resistance to the restoration • Provide enough space for restorative materials to withstand occlusal forces • Create harmonious axial contours and margin location to provide proper emergence profile Retention path of insertion Resistance apical or oblique direction Optimum angles of convergence for various teeth groups 6° taper angle of convergence 3° 3° angle of inclination Correlation between the height and angle of convergence 15° Anterior teeth 10° Premolars 15° Molars 20° 7° Correlation between the height and the arc of displacement Minimum height requirement To obtain resistance, the length must be great enough to interfere with the arc of the casting pivoting about a point on the margin on the opposite side of the restoration. ✓ ✓ ✗ ✗ molars ≥4 mm molars: . greater taper . greater diameter . greater occlusal forces Height / width ratio d o m c oc: height md: width height/width ratio≥0.4 other teeth ≥3 mm Correlation between the width and To obtain resistance, the length great enough to interfere with the arc of the arcmust ofbedisplacement the casting pivoting about a point on the margin on the opposite side of the Correlation between the height and the arc of displacement restoration. ☺ ☹ ☺ Alternatives: 1. Prophylactic endodontic treatment + Post and core 2. Crown lengthening Axillary grooves Auxillary grooves Proximal grooves ✓ Faciolingual grooves facial groove complete resistance to complete resistance to faciolingual dislodgment mesiodistal dislodgment distal groove mesial groove ✓ ✓ lingual groove mesiodistal preparation faciolingual preparation more parallel, less taper less parallel, more taper Axial wall preparation ✓ ✗ ✗ Occlusal surface preparation ✓ ✗ ✗ Consequences of poor tooth preparation dentist insufficient reduction excessive reduction problem no space for restorative material retention resistance technician patient 1. overcontoured 1.esthetic and restoration periodontal problem 2. properly contoured 2. porcelain restoration fracture 1. properly contoured restoration 1. decementation 2. porcelain fracture 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 No undercuts Mostly happens when preparing the margin, due to overtipping the bur. Remember: On a non-prepared tooth, the height of contour is at the mid portion of the clinical crown. On a prepared tooth, the height of contour should be at the closest portion to the margin. if undercuts ➞ seating problems ➞ cement wash-out Rule of thumb Remove all pre-existing fillings that you have not placed in to make sure the tooth structure is caries-free. Margin Preparation Types of margin design Selection of margin design Location of the margin Radiographs may be misleading and not show recurrent caries due to overlapping of filling material and caries lesion. Chamfer Rounded shoulder Chamfer ✓ ✓ c - Full ceramic restorations - Ceramo-metal restorations c - Full metal restorations Shoulder with bevel Knife-edge - Easy to form - Readily visible on prepared tooth, impression and die. ✓ ✗ c - Strength and esthetics for ceramic c Rounded shoulder Labial butt margin / Ceramic butt margin - Full ceramic restorations - Ceramo-metal restorations - Full metal restorations - Strength and esthetics for ceramic Shoulder with bevel Shoulder with bevel ✓ ✗ ✓ ✗ - Ceramo-metal restorations - No esthetic concern - Cement may prevent full seating ✗ Knife-edge Location of the margin - No definite finish line Supragingival - Causes overcontour Gingiva level - Difficulty in waxing and casting Subgingival Advantages of supragingival margins Dentist Technician Patient easy tooth preparation readable margin easily cleansible easy impression no tearing of impression no biological width injury Subgingival margin indication previously existing restoration caries retention and resistance esthetics easy fit evaluation hypersensitivity Summary 1. Ideal 6° of angle of convergence Rule of thumb Place margins on the tooth structure, never on fillings, core materials or cast post and cores. 2. Minimal height: 4 mm for molars 3. Height/width ratio of 0.4 or greater very subgingival ➞ crown lengthening 4. Proximal grooves to resist faciolingual dislodgment The fact that the finish line is subgingival is not an excuse to place your crown margin on fillings or post and cores. 5. Follow the geometric shape of the root throughout the axial wall 6. Anatomical occlusal surface preparation with functional cusp reduction. Summary Summary 7. Remove all pre-existing fillings 8. Prefer supragingival or gingiva level margin location Dentist Technician Patient 9. Prefer chamfer or shoulder preparations. Avoid knife-edge preparation. ☺ ☺ ☺ 10. Bevel should be more than 45° angled 11. Place margins on the tooth structure, never on fillings, core materials or cast post and cores