Like most other disciplines within veterinary medicine, recognition of disease begins with familiarization of what normal looks like. Proper occlusion is important for several reasons in veterinary medicine for not only cosmetic reasons, but also for function and comfort. Probably the most important aspect of veterinary orthodontics and occlusion is functionality of a bite. If the bite is not functional, or is lacking, prehension of food and normal mastication may be altered. Comfort is also important in that our role, as veterinarians, is to help treat or prevent disease and ease suffering whenever possible. Some malocclusions may potentiate disease conditions such as periodontal disease, lead to chronic infections such as oro-nasal fistulas, or result in uncomfortable situations when chewing food or simply at rest. “Proper” occlusion depends on breed standards and what may be normal for one breed could be considered abnormal for another. For example the occlusion of a boxer can be considerably different than that allowed for a Doberman pincer.
WHAT IS NORMAL?
There are basically 6 points to evaluating the occlusion in most dogs:
- The “scissor” incisor relationship
- The canine interlock
- The premolar interdigitation
- Alignment of developmental grooves of the carnassial teeth
- A consistent freeway space between arcades
- Head symmetry
If you ask most people familiar with dog dentition what the proper occlusion for most breeds is, they typically will say a “scissor” occlusion. And, for the most part they are correct. What they mean is the incisor relationship should be in a scissor fashion. This means the six maxillary incisors should be slightly more forward and overlap the six mandibular incisors. In more specific terms, the cusp tips of the mandibular incisors should occlude or strike the cingulum or flat area on the backside (palatal side) of the maxillary incisors. Jaw length problems are usually most obvious here, and since jaw length is known to be a hereditary trait, this relationship is very important in those animals intended for breeding.
Proper canine interlock is very important in maintaining jaw length relationships. This is important from early in life with the deciduous (baby)teeth as the jaws are growing. The mandibular canine tooth should rest or occlude between the maxillary 3 rd incisor and the maxillary canine tooth. This creates an interlock situation that prevents one or the other jaw from overgrowing the other. Observant owners may recognize that around 4-5 months of age, a jaw length discrepancy develops shortly after the deciduous canines are shed, and before the permanent canines have time to erupt into full occlusion. Also, malocclusions such as base narrow mandibular canine teeth do not allow for a proper interlock, therefore releasing the jaws to grow at will. The ideal occlusion would be if there were equidistance between each of the teeth involved when held in occlusion.
One of the best indicators of jaw length relationship is the premolars. Both the interdigitation of the cusp tips of the premolars and the alignment of the developmental groove of the carnassial teeth show evidence of proper jaw length relationship. The cusp tips of the premolars should point to the interdental (diastema) space of the opposing premolars creating a “pinking shear” effect. If there is a shift in one or the other jaws, this effect will be altered. An even more precise measurement is the alignment of the developmental groove of the maxillary 4 th premolar with the same groove on the mandibular 1 st molar. Although not as sensitive an indicator to subtle jaw length discrepancies, this is a good visual tool to show differences in jaw lengths.
The last two points of evaluation are the space between the arches, and the symmetry of right and left sides. A consistent freeway space implies that the space between the upper and lower arch has a relatively even angle from caudal to rostral (back to front). This means there is no bowing of one or the other jaws. The head symmetry can best be evaluated from a rostral (front) perspective. The midline between the central incisors of the maxilla and mandible should line up and both left and right sides ideally should be mirror images of each other. Any alteration of these may indicate a wry occlusion.
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WHAT IS ABNORMAL?
Malocclusions are any deviation from the previously mentioned normal parameters. A classification has been adopted for animal use based on a similar system developed by Angle for human orthodontic patients. The classification groups malocclusions together based on relative jaw lengths:
- Class I Malocclusions: those abnormal occlusions were the jaw lengths are relatively normal, but one tooth or a group of teeth are in an abnormal position.
- Class II Malocclusions: abnormal occlusions due to a long maxilla or a short mandible
- Class III Malocclusions: abnormal occlusions due to a short maxilla or a long mandible
- Class IV Malocclusions: abnormal occlusions due to a right or left deviation of one of, or both, of the jaws. A wry occlusion.
COMMON CLASS I MALOCCLUSIONS AND TREATMENT OPTIONS:
Class I malocclusions occur when one or more teeth are out of normal position but the jaw length is relatively normal. Examples of this include: base narrow mandibular canine teeth, “lance” canine teeth, and anterior crossbite. These may be influenced by retained deciduous teeth, or may be genetically pre-programmed to occur. Each of these conditions can be treated orthodontically, but there are other treatment options that can work just as well to relieve an uncomfortable or abnormal occlusion such as extractions or alteration of tooth structure.
- Base Narrow Mandibular Canine Teeth (BNMC): This occlusion occurs when the canine teeth of the mandible erupt more medial (toward the midline or tongue) than they should, causing the teeth to point more straight up and occlude into the mucosa of the hard palate. If the deciduous teeth are seen in this position, many times the permanent teeth will follow that same eruption pattern and erupt into the base narrow condition. In other cases, retained deciduous teeth, which are normally shed and lost lateral to the erupting permanent canines, can influence the permanent teeth to erupt in a more upright position sending them into the mucosa of the hard palate resulting in the base narrow condition. BNMC can result in an uncomfortable occlusion where the cusp tips of the mandibular canine teeth embed into the mucosa of the hard palate. In severe instances, one or both of the involved mandibular canine teeth can penetrate the mucosa, through the bone of the palate, and enter the nasal cavity causing an oro-nasal fistula. This opening into the nasal cavity will allow food, saliva and bacteria to enter the nasal cavity resulting in chronic nasal infections. BNMC also disrupts the normal “canine interlock” and can contribute to jaw length discrepancies. Correction of BNMC can be either with orthodontic movement, mandibular canine tooth height alteration, or extraction of the tooth or teeth causing the problem. There are several ways to orthodontically correct BNMC: acrylic incline planes, active force springs to push the teeth apart, removable orthodontic appliances, surgical repositioning, and in mild cases altering the gingiva or tooth structure to encourage movement. Orthodontic movement does take time (usually 4-16 weeks) with rechecks and the possibility of multiple sedations or anesthesia. The reward is maintenance of all the dentition and creation of a comfortable occlusion. If a solution is desired at one visit with minimal follow-up, then crown reduction with endodontic therapy or extraction can be performed. Remember, the mandibular canine teeth help hold the tongue in the mouth, so if these teeth are removed, expect the tongue to periodically hang out one side of the mouth or the other. More importantly, the roots of the mandibular canine teeth constitute a significant portion of the mass of the rostral mandible. If these teeth are removed, packing an osseopromotive (synthetic bone replacement) material into the empty alveolus and suturing the gingival closed would be advisable.
- “Lance” canines: This condition, commonly seen in the Shetland sheepdog, is a result of one or both of the maxillary canine teeth deviating or pointing rostral. It has been named the “lance” canine tooth due to the way the tooth points forward like a lance or spear. The result is a closed diastema space between the maxillary 3rd incisor and maxillary canine tooth, creating a crowding of these two teeth. This can cause occlusal problems with the mandibular canine teeth causing them to deflect out labially. The close, and abnormal position of the maxillary incisor and the maxillary canine tooth to each other creates an area for plaque retention and in many cases predisposes these teeth to periodontal disease. Correction may involve orthodontic movement by means of an active force elastic chain to pull the maxillary canine tooth caudally into position or extraction of the malpositioned canine tooth. In some instances, removal of the crown of the affected canine tooth with endodontic therapy may remove the occlusal interference, but does little to prevent potential periodontal problems.
- Anterior crossbite: A crossbite type malocclusion occurs when one or more teeth is out of normal arch alignment. The anterior crossbite malocclusion occurs when the normal scissor relationship of the maxillary incisors is lost due to one or more of the maxillary incisors occluding behind, or lingual to the opposing mandibular canine teeth. Retained deciduous incisors that deflect the normal eruption pattern of the permanent maxillary incisors may influence this. Other causes include genetic predispositions and developmental accidents such as trauma. This malocclusion has minor consequences, but can contribute to abnormal wear of the incisors and may increase periodontal disease. Correction of this condition involves movement of the maxillary incisor teeth forward, or the mandibular teeth caudal, or a combination. Active force appliances such as maxillary arch bars, palatal expansion devices, or elastic power chains can orthodontically move these teeth.
Classes II, III, and IV are primarily jaw length problems and are very difficult to manage orthodontically in dogs and cats. In some cases, there may combinations of malocclusions that result in deviated teeth that cause uncomfortable occlusions such as a Class II with BNMC. In these situations, creating a comfortable and functional occlusion is the goal of treatment. Not every dog is entitled to a perfect occlusion, but as veterinarians, we should provide the options for therapy that will help create a comfortable and functional occlusion.