Developmental Abnormalities of the Mouth and Dentition in Small Animals
Proper growth and development of the oral cavity depends on a series of events that must occur normally and in the proper sequence. Genetic abnormalities or trauma that affects either the developing tissues or the timing of their development can cause abnormalities. Defects that decrease comfort, health, or function require treatment; those that result in only an esthetic problem do not. Common developmental problems include persistent deciduous teeth, unerupted teeth, malformed teeth, malocclusion, and malformed jaws.
Deciduous teeth of kittens and puppies are designed to function in a small mouth (fewer in number and smaller in size) and for a temporary period. Dental trauma during this time of energetic oral exploration is often compensated for by the exfoliation of the damaged teeth as the permanent teeth erupt. The permanent teeth are larger and more numerous, erupting as the jaws lengthen to accommodate them.
Exfoliation of deciduous teeth is a complex process, part of which involves pressure exerted by the crown of the subjacent permanent tooth against the root of the deciduous tooth. If the permanent tooth does not erupt in the correct position, the deciduous tooth may remain firmly in position. This may be due to hypodontia with no succedaneous permanent tooth, a genetically malpositioned permanent tooth bud, or traumatic displacement of the tooth bud. Persistence of a deciduous tooth in areas of wide tooth spacing may not cause a problem. However, if the deciduous tooth causes crowding with the permanent tooth (often the case with canine teeth in dogs), then the area is predisposed to periodontitis. Additionally, the displaced permanent tooth can itself result in traumatic occlusion that requires treatment. Timing of deciduous tooth exfoliation and permanent tooth replacement are genetically determined. In rare cases, trauma during tooth development can cause displacement of a tooth bud that affects exfoliation.
Most commonly, two canine teeth (one deciduous and one permanent) are present at the same time. The permanent maxillary canine tooth erupts mesial (“rostral”) to the deciduous one, giving the appearance of a wider and more blunt canine tooth rostral to a narrower one with a sharper cusp. The permanent mandibular canine tooth erupts lingual (“medial”) to the deciduous one, giving the appearance of a wider and more blunt canine tooth toward the tongue next to a narrower one with a sharper cusp positioned toward the lip. In the premolar area, it is common to see a deciduous tooth in an area with no simultaneous permanent tooth. A smaller than normal premolar should be radiographed for evaluation of its anatomy and root structure to determine whether it is a deciduous tooth.
A deciduous tooth should be extracted when it remains firmly attached (no mobility) after its successor permanent tooth has erupted. Persistent deciduous teeth that do not have a permanent tooth replacement may be left in place if the roots are strong. However, radiographs should be taken to verify there are no embedded or impacted permanent teeth at the site and that the roots are not being resorbed.
Because most persistent deciduous teeth are genetic, pets with this problem should not be bred unless the condition is known to have been caused by trauma.
Tooth eruption is genetically programmed. Some breeds, particularly small breeds (eg, Maltese), are predisposed to delayed or incomplete eruption. Some brachycephalic breeds are predisposed to malpositioned first premolar teeth that remain unerupted because of their abnormal position. Trauma can also move a tooth bud into a position in which it is unable to erupt because of impact against another structure.
In some breeds (particularly the terrier breeds), missing premolars are considered a variation of normal. But in most animals, an edentulous area where there should be a tooth is an indication for radiography. An unerupted tooth is easily identified. Embedded refers to an unerupted tooth covered in bone, the eruption of which is compromised by lack of eruptive force. Impacted refers to an unerupted or partially erupted tooth, the eruption of which is prevented by contact with a physical barrier.
Teeth that are incompletely erupted with a persistent gingival covering can be treated with operculectomy (a form of gingivectomy) to sculpt the tissue to a normal architecture. Individual teeth that are completely unerupted after maturity may remain quiet and require only monitoring. However, they can also form dentigerous cysts that can destroy large areas of the jaws. Mandibular first premolars are significantly predisposed to cyst formation, particularly in brachycephalic breeds. For that reason, any missing mandibular first premolar should always be radiographed; any that are discovered should be removed or closely monitored with periodic radiographs. Other unerupted teeth should be removed if they cause a problem. Surgical removal of deeply unerupted mandibular canine teeth can be challenging.
Animals with unerupted teeth should not be bred unless it is known the condition was caused by trauma.
Any interruption during tooth formation can result in a deformed tooth. The insult can be traumatic, metabolic, infectious, or rarely genetic. Insults to epitheliogenesis (eg, parvovirus, distemper virus, high fever) that occur during amelogenesis causes enamel hypoplasia or hypomineralization. Insults to dentin formation can cause deformed or missing roots.
Enamel abnormalities can be regional, with circumferential lines of missing enamel (rough surface with staining), or generalized with complete loss of enamel. Radicular dysgenesis can present with relatively normal appearing crowns that are mobile. The lack of roots is readily identified on radiographs. An interesting individual tooth abnormality that appears to be genetic is convergent roots of the mandibular first molar. This anomaly less commonly affects other teeth. The crown may appear normal, or it may have a small developmental groove on the buccal surface extending from the gingival margin. On a radiograph, the roots converge apically instead of having their normal divergent position. The crown sometimes appears too large in relation to the size of the roots. The convergence causes the floor of the pulp chamber to arch dorsally into the main pulp chamber, giving it the radiographic appearance of a “dens-in-dente” or dens invaginatus. These teeth commonly have a communication from the periodontal ligament to the pulp chamber in the furcation area, resulting in an extremely high rate of endodontic disease. Many other individual tooth anomalies are seen occasionally, such as supernumerary teeth, twinning and fusion of teeth, supernumerary roots, and “peg” teeth (short cylindrical teeth).
Enamel hypoplasia or hypomineralization is treated with early dentin sealant to prevent bacterial ingress to the pulp. Composite resin veneers can also protect the softer dentin from abrasion and provide a smooth surface on which plaque is less able to form, but they will eventually wear or chip. Root dysgenesis carries a poor longterm prognosis. The teeth can be maintained for years with strict oral care and avoidance of any dental trauma or overuse. Individual anomalous teeth should be evaluated for associated pathology; many cause no problem and do not require treatment.
Malformed teeth are the result of trauma, infections, or genetics. Routine caution and care during tooth development prevents most of them.
Malocclusion is nearly always genetic; however, trauma during development can interfere with normal growth. Maxillary length is easier to manipulate than mandibular length through selective breeding. As a result, a preference for longer faces and noses inadvertently selects for mandibular distocclusion (ie, overbite, or lower jaw appears shorter than upper jaw), whereas selecting for a “blockier” head or shorter nose results in mandibular mesioclusion (ie, underbite, or lower jaw appears longer than upper jaw). The upper and lower jaws develop at different rates, making the timing of tooth eruption critical. If the jaws have an abnormal relationship to each other at the time the permanent teeth gain enough height to occlude, then the dentition is locked into the abnormal position. If this occurs unilaterally, it can allow continued jaw lengthening on one side while arresting it on the other side, resulting in a mismatch of the central incisor midlines (ie, asymmetric skeletal malocclusion such as “wry” bite).
The most common maxillary-mandibular discrepancy is a horizontal symmetric skeletal malocclusion, resulting in mandibular mesioclusion (class 3 malocclusion) or mandibular distoclusion (class 2 malocclusion). The latter problem often causes traumatic occlusion when the mandibular canine teeth impact against the most rostral hard palate. Linguoversion of the mandibular canines often accompanies this problem, because they can be directed palatally as they erupt along the palatal surface of the maxillary canines. Individual tooth malposition (dental malocclusion or class 1 malocclusion) can also be genetic, such as mesioversion of the canine teeth (ie, “lance projection”) in Dachshunds and Shetland Sheepdogs.
During the deciduous dentition period, interceptive orthodontics can be performed by selectively extracting deciduous teeth. If there is dental interlock, then extracting locked teeth can allow the jaws to grow to their genetic potential. Deciduous rostral crossbite can be treated by extraction of the deciduous maxillary incisors. This not only relieves the interlock but also encourages the permanent incisors to erupt in a more labial angle (they normally erupt on the palatal side of the deciduous incisors) to help correct the malocclusion. Likewise, deciduous mandibular distoclusion can be treated by extraction of the deciduous mandibular canine teeth. Again, this not only relieves the dental interlock but also encourages the permanent mandibular canine teeth to erupt in a more labial angle (they normally erupt on the lingual side of the deciduous canines) to help correct the malocclusion. Whenever deciduous teeth are extracted, touching the tooth bud of the developing permanent teeth must be avoided so as not to damage the enamel organs or developing enamel. This damage can cause brown spots on the crowns of permanent teeth due to focal enamel defects. Instruments should not be inserted on the palatal side of deciduous maxillary incisors or on the lingual side of deciduous mandibular canines. Even with proper technique, enamel damage can occur, because the enamel epithelium can be tugged as the deciduous tooth is extracted from the alveolus.
Mandibular mesioclusion in the permanent dentition is considered normal for many brachycephalic breeds and does not require treatment unless it results in traumatic occlusion. If the mandibular canines impact against the palatal aspect of the third or second maxillary incisors, then extraction of the maxillary incisors in contact will create a wide diastema into which the canine tooth can fit, resolving the problem. The rostral crossbite (ie, maxillary incisors positioned lingual to the mandibular incisors) rarely causes discomfort or health problems. In contrast, mandibular distoclusion often requires orthodontic or surgical intervention. Canine teeth can be moved into a nontraumatic (not always normal) position that is comfortable and functional. Alternatively, the tooth can be shortened and the pulp treated with vital pulp therapy. This approach requires sterile technique to avoid introduction of infection into the pulp and followup radiographs throughout life to monitor the need for definitive endodontic treatment.
Only animals with a normal, healthy occlusion should be bred.