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Prosthodontics

Fraser Hale Canada

Prosthodontics is the branch of dentistry that involves replacement of lost teeth or tooth structure with a prosthetic device. This includes crowns, bridges and implants. Space permits discussion of crowns only. The presentation will touch on bridges and implants.

PROSTHETIC CROWNS

There are many factors when considering the treatment plan for a tooth that might be a candidate for a prosthetic crown. Your job is to look at each candidate tooth, the animal it is attached to and the owner that brought the problem to you. Then decide if a crown is indicated or not.

A tooth that has a crown fracture with pulp exposure usually requires total pulpectomy and root canal obturation. Drilling of the access holes and filing of the canals removes natural tooth tissue and so weakens the remaining crown. With the pulp gone, there is no longer a source of moisture being supplied to the dentin from within and this lack of moisture also weakens the crown (makes it more brittle). Therefore, it is quite safe to say that an endodontically treated tooth is weaker than an intact, vital tooth. If the behaviour that led to the fracture/wear is going to continue unabated after endodontic therapy, then a protective metal prosthetic crown may well be indicated.

The most commonly fractured teeth in dogs are the canines. These long, conical crowns taper to a fine point. Depending on the nature of the trauma, it is usually these thin tips that break off. After endodontic treatment, you are left with a much shorter, stockier crown. This new shape is mechanically much less prone to fracture and this may counter-balance the weakening effects mentioned in the last section.

The next most commonly fractured tooth is the fourth upper premolar. Typically, it is the small, prominent mesial cusp that bears the biting force and fails. Once this tip is gone, the new crown shape allows the biting forces to be spread over a larger surface area. Again, the change in mechanics resulting from the alteration in crown shape may protect it from further damage.

Historically, teeth were restored with amalgam that was held in place by mechanical retention only. This meant that the cavity preparation had to have undercuts, which further weakened the crown and led to stress risers in the bottom of the cavity preps. The amalgam itself did nothing to add to the strength of the tooth. Now, we have bonded composite resins that require no undercutting and so allow a more conservative cavity preparation. In addition, the bonded restoration, though not nearly as strong as natural tooth structure, does add some strength to the tooth compared to amalgam, which just sits passively in the hole.

In order to place a metal crown on a tooth, some of the natural tooth structure usually must be sacrificed. A metal crown is usually at least 1 millimeter thick. Therefore, a millimeter of tissue (enamel and dentin) must be removed from all surfaces of the natural crown. This further weakens the tooth and that is contrary to the purpose of the crown. In a young dog that has a large pulp chamber and a thin crown wall, the loss of tissue around the outside of the crown may be unacceptable.

The portion of the tooth that is covered by the metal prosthesis is certainly well protected from further damage and this is the reason for placing a metal crown. However, a metal crown does not prevent a fracture of the portions of the tooth it does not cover.

In humans, metal crowns are often used to protect molars that endure occlusal forces directed along their long axis. In dog canine teeth, the forces are usually at 90 degrees to the long axis and the margin of the metal crown can act as a fulcrum, leading to a fracture of the tooth at the apical margin of the crown.

Crowns can fall off and when they do, they are rarely found. To replace a lost crown requires a new set of impressions and so there is at least one anesthetic to clean up the tooth and take the new impressions and another to place the crown. Are the owners prepared to accept this risk and cost?

For many clients the decision to treat the tooth rather than extract it is a big jump. In my practice, root canal and conservative restoration costs about twice as much as extraction. If we add a metal crown (crown prep, impressions, lab fees, second anesthetic to place crown, cost of luting agentÖ), the total bill doubles yet again. For many pet owners, it is just not practical to put so much of their veterinary budget into the treatment of a single tooth.

The main purpose of the prosthetic crown is the prevention of further damage to the treated tooth. This can also often be achieved by avoiding the behaviour that led to the fracture in the first place. Many dogs break their posterior teeth chewing on bones and hard toys that the owners unwittingly provided to them. Once informed that this causes dental fractures, they are usually only too happy to get rid of the offending toys. With the risk thus reduced, the need for a metal crown is less pressing.

Placing a crown on a treated tooth can have a negative impact on the remaining teeth. If the owners are convinced to have the crown work done to prevent further damage to the treated tooth, they may feel that it is all right to allow the animal to continue the destructive behaviour that led to the fracture in the first place. Therefore, the remaining teeth continue to be placed at risk. If no crown is placed, the owners are more likely to be cautious about their petís chewing habits, to the benefit of the treated tooth and all the others as well.

From all this, you may be getting the impression that I never place crowns, but that is not the case. I am just very selective.

Some dogs damage their teeth in the performance of their duties. Police and security dogs, ringsport dogs, shutzhund dogs, and flyball dogs are all involved in activities that place their teeth at risk. In cases such as this, the risky behaviour cannot be altered. Some dogs are chronic fence chewers and the situation cannot be changed. In cases like this, where the preservation of the teeth is a high priority and the dental abuse is going to continue, I recommend cast metal crowns.

I do not do cosmetic crowns (crowns made of tooth coloured material). The purpose of the crown is to prevent further damage. None of the esthetic crown materials comes close in durability to the metal alloys. In fact, there are quite prone to chipping and cracking, and in human patients, they commonly require repair. In a dogís mouth, they may look pretty at discharge, but they are not likely to hold up in the long term.

Breeders and show types sometimes request cosmetic crowns for their show dogs. However, the AKC and CKC would frown on this. My understanding of the rules is that it is acceptable to perform the root canal treatment and fill the access holes, but that the natural crown cannot be augmented in any way. The reasoning is that the judges would have no way of knowing what is under the prosthesis and what congenital defect it might be covering up. Therefore, placing a beautiful ceramic crown on the canine of a show dog might lead to disqualification.

GUIDELINES FOR DESIGN OF CAST METAL CROWNS

Much of the following can be found in greater detail in the veterinary dental literature, but I will give a few lines to some aspects of crown design now.

One over-riding principle to keep in mind is preservation of natural tooth structure to maximize the strength of the final result. Whenever tooth structure must be removed to satisfy other criteria of design, it should be removed only to the extent absolutely necessary.

The tooth to be crowned is often in close contact with the proximal and/or occluding members. Therefore, in order for there to be room for the thickness of material used to make the prosthesis, the tooth must have its outside dimensions reduced. For example, there may be less than one millimeter between tooth 404 and teeth 103 and 104. If tooth 404 receives a 1.5-millimeter thick jacket of restorative, it would then be too wide to fit in the space available. Therefore, it must be reduced to allow the addition of the restoration. For the maxillary fourth premolar tooth, the palatal face must not be over built, as it is in close proximity to the labial face of the mandibular first molar when the mouth is closed. In order to plan an appropriate tooth reduction, the operator must carefully assess all occlusal relationships involved.

The material to be used in the fabrication of the prosthesis will also impact on the requirements of tooth reduction. For example, a gold cast crown should be at least 1 millimeter thick on the occlusal, incisal or cuspal surface but can be tapered to a very fine gingival margin. On the other hand, a porcelain-fused-to-metal jacket needs to be at least 2 millimeters thick incisally or cuspally, 1.5 millimeters occlusally and at least 1.5 millimeters thick at the gingival cavosurface. Therefore, it is important to decide what restorative will be used before starting the crown preparation. It is advisable to discuss reduction requirements with your dental laboratory prior to treatment to ensure that you provide them with the amount of space they need.

In order for the emergence profile (shape of the tooth as it emerges from the gingiva) of the restored crown to be similar to the natural tooth and acceptable for maintenance of periodontal health, care must be taken in designing and preparing the gingival cavosurface. The factors to be considered include the anatomy of the natural tooth and it relationship to other oral structures and the material to be used for the restoration. There are several marginal finish lines to choose from as outlined in standard texts.

A chamfer finish has some advantages. It allows for some micro-movement of the crown without damage to the underlying tooth margin. It also allows the lab to fabricate a crown with a substantial gingival margin, which is less technically exacting than margins that taper to a fine point (feather, occult, chisel, bevel).

Feather, occult, chisel, and bevel margins allow some micro-movement of the crown under stress, which is fine for some materials and can protect the underlying tooth from damage by absorbing the stress. However, these finishes all taper to a fine point which can be more difficult to wax up accurately and would be subject to distortion during handling.

For porcelain and ceramic restorations, micro-movement of the crown can lead to marginal cracking and chipping of these hard, but brittle, materials. Therefore, butt joints are generally required. Again, consult with your laboratory technicians to ensure that your marginal prep is appropriate for the material to be used.

For the periodontal health of the tooth, this gingival cavomargin should be place 1 to 2 millimeters coronal to the free gingival margin. This keeps the tooth-restoration interface away from the gingiva and out in the open where it is accessible for home-care. In human prosthodontics, the margin is often placed subgingivally for cosmetic reasons; with judicious flossing and brushing, this can work well. With dogs, in view of typically less than effective home-care, function and durability should take priority over cosmetics; and keeping the margin out in the open improves the periodontal prognosis.

While the luting agents available today claim to be incredibly strong, it is still important to design a crown preparation that will maximize the retention of the crown. One of the keys is to maximize the surface area contact between the tooth and the crown. There should be at least 4 millimeters of sound tooth covered by the crown and 6 millimeters would be preferred.

Occasionally, fractured teeth have less than 4 millimeters of natural crown left coronal to the free gingival margin. In order to achieve sufficient retention, something must be done. Crown lengthening procedures (see next presentation) are surgical procedures that expose more of the natural tooth structure by repositioning the free gingival margin of the tooth.

Another approach is to add material to the remaining crown. Placing a pre-fabricated or custom-made post into the endodontic canal and/or pins into the dentin can do this. These pins and posts are then used as scaffolding to retain some form of core build-up material. Though these strategies can improve the retention of the crown, they also actually weaken the natural tooth structure. An endodontic post may help hold the crown in place but the forces on the crown may be transmitted through the post to the canal within the root and can lead to root fractures. Therefore, the goal of achieving retention may be in conflict with the principles of maintaining the integrity of the tooth.

Rotational stability is often a concern on prepared canine teeth as the prepared surface may approximate a cylindrical cone. To prevent rotation of the crown, retention grooves may be cut in the tooth. These grooves must be parallel to each other and the long axis of the natural crown to allow the prosthetic crown to slide on properly. They should be deep enough to offer resistance to rotation while avoiding excessive removal of natural tooth tissue.

Once the lab has created a wax pattern (for a full metal crown) or the uncured porcelain or ceramic crown on the stone model, they must be able to remove it and you must be able to place the finished product on the patientís tooth. Therefore, the tooth must be tapered from the gingival cavosurface coronally so the crown can slide on and off. The desired degree of taper is five to six degrees from parallel sides. With parallel sides, friction between tooth and crown would make seating the crown difficult. Without some vent slots to allow air and glue to escape, air and hydraulic pressure would prevent proper seating of crown. Excessive taper reduces the surface area available for retention and is mechanically less resistant to dislodgment of the crown.

Any under cuts in the prepared tooth will make it difficult for the lab to remove the pattern from the model and for you to place the crown on the tooth. When undercuts are unavoidable, they can be filled with composite or glass ionomer prior to crown preparation.

When placing a metal crown on a fractured canine tooth, the owners may envision that the finished product will be the same size and shape as the undamaged tooth, but this is not recommended. In order to reduce the risk of tooth fracture apical to the crown (between the crown margin and the alveolar crest) and to improve the retention of the crown, it is recommended that the restored tooth be no more than 2/3 of the natural crown height. Building the crown higher than that makes it too much like an efficient lever and the amount of force (perpendicular to the long axis of the tooth) needed at the tip to dislodge the crown is greatly reduced.

Canine teeth have a distal curvature to the crown, which helps keep prey from slipping out of the grasp of the teeth. When fabricating a cast crown for a canine tooth, it is recommended that the lab remove this distal curvature by creating a smooth slope from the distal cavosurface to the crown tip. In this way, if the dog grasps something firmly and pulls hard, it is more likely to slide off the tooth rather than being caught behind it and break the tooth or pull the crown off.


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