Fillings, crowns, and implants

Introduction

Damaged teeth are common but often go unnoticed, or the consequences are ignored or at best underestimated. Damage is usually traumatic and is most commonly seen following excessive tug-of-war games with abrasive or breakable toys. The importance of a thorough medical examination that includes assessment of the oral cavity, combined with a good understanding of the potential consequences of dental trauma, cannot be over-emphasized. Advances in dental treatments have enabled easier access to modern surgical techniques to preserve, strengthen, or replace damaged or missing teeth. The practitioner should be familiar with these options, be capable of making an accurate diagnosis, and be able to formulate a therapeutic plan. 

Clinical indications for endodontic treatment

The most obvious clinical sign, and the most important for both practitioner and patient, is pain. This is always present with dental trauma, but often goes unnoticed by the owner and requires good observation of the animal or careful history taking (Table 1). The dental pulp, composed of connective tissue, blood vessels, lymphatics and nerves, extends continuously from the tooth into the periapical periodontal space via the root apex. Pain is therefore experienced when mechanical or thermal stimuli cause inflammation of the dental pulp, and the sensation is increased when the periapical periodontium is compressed by biting and as inflammation develops through the acute and chronic phases. The owner becomes accustomed to the animal’s condition and lacks knowledge about the expression of pain, thus delaying detection. Broken teeth also provide an ideal site for bacterial colonization; infection develops instantaneously in the pulp canal, but it takes several days for the local signs (periapical periodontitis) to become apparent 1. Since the tooth is a closed system, despite the persistence of periapical inflammation, disease development depends on host factors (periapical environment, age, general immune status) and may present as an acute disease (abscess, fistula, suppurative inflammation) or chronic inflammation (granuloma, cyst) (Figure 1). Irreversible periapical inflammation can also occur after trauma that causes contusion rather than fracture. These lesions are common in small breeds and can cause severe damage if they are left untreated.

Clinical indications for a dental prosthesis

Functional deficits, such as malocclusion resulting from a broken crown, are often regarded as the deciding factor for treatment. The owner, convinced that restoring tooth height will re-establish a correct bite, generally underestimates the pain resulting from periapical inflammation. In other situations it is the clinician who advises a prosthesis, with the aim of strengthening the devitalized tooth. The prime objectives of any prosthesis are to provide better protection against mechanical wear, future damage, and potential bacterial contamination of the pulp canal.

Recent advances in dental restoration resins have led to varying opinions as to the best approach to broken teeth. Some clinicians argue that periodontal management, root canal treatment and appropriate restoration of the dental access is sufficient to re-establish good function, irrespective of the height of the remaining crown. As a restored tooth is never as strong as the original, the owner/handler must be made aware that a deficient crown has limitations as to its function, and that it is necessary to minimize shocks and avoid excessive biting forces by appropriate training. For other clinicians, human studies point to the advantages of dental prostheses, notably by improving mechanical strength and protecting the devitalized tooth from fluids gaining access by microleakage (Figure 2) 2. However no veterinary studies have yet been undertaken to support or refute either approach, leaving the practitioner to make the decision, taking into account the animal’s well-being and the efficacy of the prosthesis.

Functional defects are always accompanied by an esthetic defect, but the latter is rarely taken into consideration, in that direct restoration (i.e. restoring the defect in a one-step procedure) is performed without attempting to re-establish the original height, or indirect restoration is achieved via a metal (rather than ceramic) dental crown, as appearance is rarely an issue. Occasionally (e.g. with show dogs) owners request a ceramic crown to restore the original appearance of the tooth, but given the relative fragility of such prostheses, such requests should be very carefully assessed by the practitioner (Figure 3).

Human advances in implantology to resolve periodontal problems have naturally led some to attempt these techniques on animals. The advantages and disadvantages should be carefully discussed with the owner and should include advice on the limitations and prognosis associated with such treatments. There are of course fundamental distinctions between veterinary and human indications (e.g. dental agenesis, dental avulsion, and dental extraction) to be considered 3.

Patient and tooth assessment 

The dental examination must always be preceded by a full clinical examination and a neurological examination where necessary. Particular attention should be paid to the temporomandibular joints, the bony maxillofacial structures, and the oral mucosa. The examination of a damaged tooth starts with the animal conscious but is completed under general anesthetic. During the conscious examination note the following;

• Any difference in color of all or part of the dental crown 4 (Figure 4).

• The structural integrity of the crown

• Any reaction when the tooth is percussed

• The dental occlusion

The simplest and most useful technique is scanning the surface of the tooth using a probe. This will immediately identify any opening to the pulp cavity; if present, an opening has a considerable bearing on possible complications and therapeutic options.

Under anesthesia the following are essential;

• Assessment of the periodontal integrity (using a probe to estimate any subgingival extension of the fracture).

• Intra-oral radiography to assess the dental canal: root wall ratio, and to check for any periapical inflammation. 

• The medical status of the patient (i.e. age, history, cardiovascular and metabolic status).

• The dental occlusion and what the mechanical forces are likely to be.

• The state of the periodontal tissue (checking for periodontal disease and assessing if restorative surgery is feasible).

• If orodental hygiene is satisfactory.

• Appropriate endodontic treatment (Table 2).

• Dental radiography (to look for periapical lesions, ankylosis or root resorption).

• Use of dental models that accurately reproduce the mandibular and maxillary dental arcades.

Success rates for endodontic treatment have rarely been studied in veterinary dentistry. However it is important to stress two major differences compared to human dentistry: 

• Treatment is often instigated long after the initial insult and the periapical periodontitis is well advanced.

• The canal anatomy is complex and a canal may be as long as 40-42 mm, making effective mechanical and chemical debridement difficult.

One retrospective study noted that treatment for irreversible pulpitis offers a clinical and radiographic success rate of ~ 85%, but if pulp necrosis has developed, the success rate drops to ~45% 9. However, if one only considers animals with no clinical signs (pain, inflammation) and no radiographic signs of aggravation, the overall success rates of root canal treatment are close to those reported for human dentistry at around 96% 9. These results may explain why opinions vary between practitioners, but should also alert clinicians to a probable underestimation of the chronic pain experienced by treated animals. It is therefore important to recommend an additional disinfection step if there is established periapical periodontitis, even though this means a second general anesthetic (Figure 6).

To achieve this, calcium hydroxide is applied to the dental canal at the end of the canal preparation phase. It is protected from external contamination by a temporary hard filling material which is left in situ for 15 days; definitive filling of the canal is then performed as a second procedure, thus allowing greater healing of the periapical periodontitis. The advantage of this two-session treatment was highlighted by a prospective study using 2D and 3D dental imagery and histology 10. The disadvantages of an additional general anesthetic should be assessed against parameters such as pain, status of the pulp inflammation (pulpitis vs. pulp necrosis) and the degree of periapical inflammation 11.

Indirect dental restoration

Indirect restorations involve several surgical procedures, and the choice between the different possible restorations should consider the extent of the tooth damage, the mechanical stress that will be supported by the restored tooth, and the need to control dental plaque, as well as financial and esthetic aspects.

Ideally a crown will have both excellent retention and optimal mechanical resistance. The quality of the retention of the restoration is directly related to the percentage of a tooth’s surface area covered by the prosthesis, and a full crown, i.e. one that completely covers a damaged tooth, is thus largely preferred in veterinary dentistry. A molded metal prosthesis strengthens the damaged tooth by spreading occlusal forces over a large surface area, and by eliminating forces directed onto the actual site of the fracture. It is important to ensure that the crown itself does not weaken the tooth 12. To achieve this one must consider the five main principles of dental preparation prior to crown placement;

• Preservation of the tooth structure

• Retention and resistance of the crown

• Durability of the crown

• Integrity of the crown margins

• Respect for the underlying periodontal tissues 

The tooth is prepared during an initial general anesthetic. The axial surfaces must be reduced using a conical diamond burr to allow retention of the molded crown; the amount of enamel removed should be minimal (0.5 mm depth) and it is desirable to attain an optimal angle of reduction of 6% 12. This is not easy; a study of preparation angles performed by human dental students showed that the ability to achieve this theoretically ideal angle varied markedly 13. The retention of the dental prosthesis is a result of micromechanical and chemical bonding, and it is recognized that whilst a significant part of crown retention is related to the quality of the adhesion of the dental resin, a minimal reduction angle is also fundamental for effective retention 13. Poor preparation is the primary cause of crown dehiscence. The quality of the shape of the margin at the base of the crown, the optional use of an intra-canal retention post, and the final supragingival prosthetic coverage (which should remain above the gum line) are all essential parameters that the clinician must master.

A silicon impression made during surgery is sent to a specialist laboratory which prepares the dental crown using a metal alloy (nickel-cobalt or cobalt-chromium); this offers good resistance to mechanical forces. During a second anesthetic the crown is positioned, adjusted if necessary, and finally sealed with a suitable liquid resin. Treatment is deemed a failure if the tooth fractures under the crown or the crown becomes detached; midterm therapeutic success (3 years post-procedure) is assessed to be ~80% 13, and it would appear that the failure rate is directly related to residual tooth height, i.e. the lower the crown, the poorer the retention (Figure 7). 

 

Conclusion

The use of dental prostheses in veterinary medicine need to be carefully considered. The esthetic appearance is rarely the primary consideration; protecting the pulp and ensuring a pain-free tooth are paramount, and any prosthesis must always be used appropriately and only after effective endodontic and periodontal treatment. Finally it may be worthwhile noting that the ethics and practicalities of replacing a missing tooth should always be fully discussed with the owner. The four possible restorative techniques (a removable prosthesis, a removable or fixed partial denture or bridge, or a prosthesis supported by a periodontal implant (Figure 8)) all carry risks; the mechanical forces exerted by a dog, as well as the difficulties associated with ensuring satisfactory behavioral control, make such treatments problematic and must be clearly explained to the owner before proceeding.