Prosthodontics: The Metal - Ceramic Crown Preparation PART 2

Hello!!

My studies have started and I am having anatomy this month, so you probably know how much to learn I have right now because as far as I saw on Instagram most of you are dentistry students or doctors... Anyway, I think quality beats quantity haha 

This is the second part of "The Metal-Ceramix Crown Preparation". I am also planning to do the third one with short summary of part 1 and part 2 + videos. 

Axial reduction of the proximal and lingual surface

Sufficient tooth structure must be removed to provide a distinct, smooth chamfer of about 0.5 mm width.

M and N: Proximal reduction. O:Placing a 0.5-mm lingual chamfer. 

A football-shaped diamond is recommended for lingual reduction of anterior teeth.

6. Reduce the proximoaxial and linguoaxial surfaces with the diamond held parallel to the intended path of placment of the restoration. These walls should converge slightly from cervical to incisal or occlusal. A taper of approximately 6 degrees, as measured as the angle between opposing axial walls, is recommended. On anterior teeth, a lingual concavity is prepared for adequate clearance for the restorative materials. Typically, 1 mm is required if the centric contacts in the completed restoration are to be located on metal. When contact is on porcelain, additional reduction is necessary. For anterior teeth, usually only one groove is placed, in the center of the lingual surface. For molars, three grooves can be placed in a manner similar to that described for the all-metal complete cast crown.

7. Make a lingual alignment groove by positioning the diamond parallel to the cervical plane of the facial reduction. When the round-tipped diamond of appropriate size and shape is aligned properly, it is submerged almost halfway into the tooth structure. Verify the alignment of the groove, and carry the axial reduction from the groove along the lingual surface into the proximal; maintain the originally selected alignment of the diamond at all times.

8. As the lingual chamfer is developed, extend it buccally into the proximal to blend with the interproximal shoulder placed earlier.

A lingual chamfer is prepared to allow adequate space for metal. A smooth transition from interproximal shoulder to chamfer is essential.

Alternatively, a facial approach may be used. Although this is slightly more difficult initially, after some practice it should be easy to eliminate the lingual guiding groove and to perform the proximal and lingual axial reduction in one step; however, this requires that the diamond be held freehand parallel to the path of placement. The proximal flange that resulted from the shoulder preparation can be used as a reference for judging alignment of the rotary instrument. 

A: Proximal reduction of the flange with a facial approach. B: One sufficient tooth structure has been removed, the cervical chamfer is prepared simultaneously with the lingual axial surface. After distolingual preparation had been completed, the mesial chamfer is blended into a smooth transition with the shoulder.

The interproximal margin should not be inadvertently placed too far gingivally and thereby infringe on the attachment apparatus. It must follow the soft tissue contour. On posterior teeth, the lingual wall reduction blends into the functional cusp bevel placed during the occlusal reduction. Anterior teeth require an additional step: After preparation of the cingulum wall, one or more depth grooves are placed in the lingual surface. These are approximately 1 mm deep.

9. Use a football-shaped diamond to reduce the lingual surface of anterior teeth. It is helpful to stop when half this reduction has been completed, to evaluate clearance in maximum intercuspation and all excursions. The remaining intact tooth structure can serve as a reference.

Finishing

The margin must provide distinct resistance to vertical displacement of an explorer tip, and it must be

smooth and continuous circumferentially. (A properly finished margin should feel like smooth glass slab.) All other line angles should be rounded, and the completed preparation should have a satin finish free from obvious diamond scratch marks. Tissue displacement is particularly helpful when subgingival margins are being finished. Sometimes this step is postponed until just before impression making after tissue displacement.

Controlled tissue displacement can be helpful when finishing the margin with a fine-grit diamond or another rotary instrument.

10. Finish the margins with diamonds, hand instruments, such as the off-angle hatchets or carbides. 

All internal line angles should be rounded to facilitate the impression-making and die –pouring steps. 

The finishing steps for the facial margin depend on the design of margin chosen 

A: completed preparation. Note that the transition from incisal to axial walls is rounded, and a distinct 90-degree or slightly sloping shoulder had been established. B: An even chamfer width and a smooth transition between lingual and axial surfaces. The chamfer is distinct and blends smoothly into the facial shoulder.

 A porcelain labial margin requires proper support for the porcelain. A shoulder with a 90-degree cavosurface angle is recommended. This type of shoulder can also be used for a crown with a conventional metal collar and offers the advantage of allowing the collar to be kept narrow. However, there is then the risk of leaving unsupported enamel. For this reason, the margin is often beveled or sloped to create a more obtuse cavosurface angle. A flat-ended diamond in a low-speed handpiece creates the 90-degree shoulder. Any unsupported enamel must be removed subsequently by careful planing with a sharp chisel. Care must also be taken to orient the rotary instrument as it moves around the tooth if inadvertent undercuts are to be avoided. When a metal-collar design of ceramic restoration is planned, the need for a 90-degree shoulder is less crucial. A sloping shoulder has been advocated to ensure the elimination of unsupported enamel and to minimize marginal gap width. Such a shoulder (cavosurface angle of about 120 degrees) can be accomplished with a flat-ended

diamond by changing its alignment, with particular attention to the configuration of the tooth structure cervical to the margin. Alternatively, a hatchet can be used to plane the margin to the correct angulation. Again, be careful to avoid undercutting the axial wall of the preparation where it meets the shoulder during finishing. A shoulder-bevel margin is most effectively achieved with a flame-shaped carbide bur or hand instrument, depending on the length of bevel required. 

The shoulder bevel.

In general, a short bevel with a cavosurface angle of 135 degrees is advocated, although longer bevels have been recommended for improved marginal fit. Special care must be exerted where the bevel meets the interproximal chamfer. The chamfer and bevel should be continuous with each other. Care must be taken not to damage the epithelial attachment during beveling; tissue displace ment before preparation of subgingival bevels is recommended.

11. After a satisfactory facial margin has been obtained, round all sharp line angles within the preparation.

S: Finishing the preparation with a fine-grit diamond. T: The completed preparation.

 This facilitates surface wetting and expedites subsequent procedures (impression making, pouring of casts, waxing, and investing). A fine-grit diamond operating at low speed is particularly useful. However, where access allows, a slightly larger tapered diamond may be preferred because the greater diameter of its tip prevents "ditching" of the chamfer. Blend all surfaces together, and remove any sharp transitions.

Facial (A) and lingual (B) views of metal-ceramic preparations.

Evaluation

Areas often missed during finishing are the incisal edges of anterior preparations and the transition from occlusal to axial wall of posterior preparations. The completed chamfer should provide 0.5 mm of space for the restoration at the margin. The chamfer must be smooth and continuous, and when it is evaluated, a distinct resistance to vertical displacement of the tip of an explorer or periodontal probe should be felt. The chamfer should be continuous with the interproximal shoulder or beveled shoulder. The cavosurface angle of the chamfer should be slightly obtuse or 90 degrees. Under no circumstances should any unsupported tooth structure remain, especially at the facial margin. Care is also needed to avoid creating an undercut between the facial and lingual walls. This aspect of the preparation should be thoroughly evaluated. Excessive convergence should also be avoided, because this may lead to pulpal exposure. All residual debris is removed with thorough irrigation. 

Metal-ceramic crowns used to restore maxillary incisor teeth.

A: Metal-ceramic preparations on the maxillary premolars in conjunction with more conservative preparations on the molars. B: Buccal view of the preparations. Note that, by comparison, considerable tooth reduction was needed on the premolars to accomodate metal-ceramic restorations. C: Except for the molars, all remaining teeth in this patient have been prepared for metal-ceramic restorations. Note that subtle variations and modifications of the same underlying theme: wing - type preparations on the anterior teeth, wingless on the premolars. D: Mandibular arch of the same patient. Many of the smaller mandibular teeth were prepared with wingless restorations. Because of previously existing restorations, excessively heavy shoulder - like chamfers resulted on some of the posterior teeth.