Loïc F. Legendre, DVM, DAVDC, DEVDC
Read the French translation: Occlusion Normale et Inocclusions Communes
There exist three types of canine skulls:
1. Mesaticephalic--mid size skull with a snout not too long and not too large. This type is seen in the Beagle, Labrador retriever, German shepherd, etc.
2. Brachycephalic--Short jaws and short wide skull as seen in the English Bulldog, Shih Tzu, Pekinese, etc.
3. Dolichocephalic--Long and narrow skull and jaws as seen in the Collie, Doberman, Borzoi, etc.
The normal occlusion, based on the mesaticephalic skull, is as follows: the lower incisors rest on the cingulum of the upper incisors. The mandibular canine teeth fit, without touching, between the maxillary lateral incisors and the maxillary canines. Maxillary and mandibular premolars interdigitate. The tip of the central cusp of the mandibular fourth premolar points to the gingiva between the maxillary third and the fourth premolar. The tips of the crowns of maxillary and mandibular second premolars are at the same level. Finally, the angle of the mandible lies directly below the posterior border of the coronoid process of within 3 mm of that point. Any deviation from the above is an abnormality.
|Figure 1. Diagrammatic description of normal or class 0 occlusion.|
Class I Malocclusion (Neutrocclusion)
It is characterized by dental irregularities, but with a normal rostrocaudal (mesiodistal) relationship of the mandible and the maxilla. This category comprises:
Individual teeth malpositions--examine to find out whether or not the malposition results in trauma. If yes, treatment has to be instituted. If not, just enter the malposition in the dental records.
Rotations--the most common rotation involves the maxillary third premolar, in brachycephalic patients. The affected tooth ends up crowded between second and fourth premolar. The crowding exacerbates periodontal disease and often leads to early tooth loss. Moreover, in some cases the distal root of the third premolar is displaced buccally and is actually found outside the maxilla. In other words, the bone support is missing which, by definition is a form of periodontal disease. Treatment consists in extracting the rotated tooth to make space for second and fourth premolars and to minimize the periodontitis.
Variations in tooth sizes--fairly rare. Here too one wants to ascertain that the condition does not results in dental trauma, in which case treatment is indicated.
Missing teeth--an abnormality that rarely requires any correction. It suffices to note it in the file.
Supernumerary teeth--these on the other hand, cause trauma secondary to crowding. In the majority of cases, the affected tooth needs to be removed to make space for the other.
Gemini teeth--one adult bud tries to divide to create two teeth. The division is partial and as a rule, one ends up with two crowns fused and one root system. One needs to verify that the crowns are completely covered with enamel, that there is space for the extra crown, and that the endodontic system is normal (need x-ray). If the answer is yes to all three, no further treatment is required. If not, restoration, endodontic treatment, or extraction is necessary.
Fused teeth--two adult buds fuse and create a doubled crown tooth but with two radicular systems. Examination and treatment are similar to those done on Gemini teeth.
Rostral open bite--this condition is often the result of bone deformation. The incisors do not overlap when the mouth is closed.
Caudal open bite--there is space between the premolars when the mouth is closed.
Rostral cross bite--upon closure of the mouth, the mandibular incisors land labial to their maxillary counterparts. If no other abnormality exists, this condition can be corrected using orthodontic appliances.
Level bite--in this situation, both maxillary and mandibular incisors occlude on top of each other. This is definitely traumatic and results in fractures if untreated. Correction consists of extractions or orthodontic movement.
Caudal cross bite--upon closure of the mouth, the mandibular premolars end up buccal to their maxillary counterparts. In case of traumatic occlusion, extraction of one tooth is necessary for correction of the problem.
Base narrow mandibular canine teeth--common orthodontic condition in dogs. The mandibular canine teeth are too close to each other or are not tipped buccally sufficiently and end up contacting the palate upon closure of the mouth. Two treatments exist; 1) crown amputation of the canine teeth and direct pulp capping (vital pulpotomy) to eliminate the trauma to the palate. 2) Orthodontic movement of the canine teeth to their intended position. Either treatment is adequate; the choice will depend on the exact presentation and on the desiderata of the owners.
Mesially displaced maxillary canine tooth--the adult canine is tipped mesially into the space where the mandibular canine normally occludes. Correction takes the form of extraction, crown amputation and direct pulp capping, or orthodontic movement.
Lance canine--an extreme form of mesially displaced maxillary canine tooth: the canine points straight rostrally out of the maxilla. Its tip is usually dorsal to the neck of the lateral incisor. Correction consists of extraction (careful, this is a nasal surgery) or orthodontic movement. The orthodontic treatment is by the best for the patient.
Class II Malocclusion (Distocclusion, Mandibular Brachygnathism, Mandibular Retrusion--Overshot)
The mandible is too short relative to the maxilla; therefore the mandibular incisors do not touch the cingulum of the maxillary incisors. The amount of space between the maxillary and the mandibular incisors varies with the severity of the condition. The position of the mandibular canines varies from being rostral to but touching the maxillary cuspids to being caudal to the maxillary cuspids. Finally, the mandibular premolars are shifted caudal in relation to the maxillary premolars, to varying degrees, disrupting the "pinking shear" effect normally seen. The central cusp of the lower fourth premolar is now touching or hidden behind the upper carnassial.
|Figure 2. Diagrammatic representation of a class II malocclusion.|
|Figure 3. Diagram of a class III malocclusion.|
Class III Malocclusion (Mesiocclusion, Prognathism, Mandibular Protrusion, Undershot)
It is the opposite condition to class II malocclusion. The mandible is too long in relation to the maxilla. In fact, most of the time, it is the maxilla that is too short. The mandibular incisors contact the incisal edge of the maxillary incisors or even are rostral to them. The position of the mandibular cuspids varies from being caudal to the lateral maxillary incisors but touching them, to being rostral to the lateral incisors. The mandibular premolars are shifted rostrally, to varying degrees, in relation to the maxillary premolars.
Always make sure that you look at all the teeth when doing a bite evaluation, not just at the incisors and the cuspids.