Occlusion describes the way in which teeth contact each other, particularly the relationship between the maxillary and mandibular teeth as they approach each other during chewing or at rest. It is difficult to define normal occlusion as the arrangement of teeth and their supporting structures commonly found in health and that which is the common or standard arrangement varies both between and within breeds. The state in which teeth are in a normal relationship with each other is dependent on number, size, shape and position of teeth and the length, width and relative relationship of the jaws and therefore there is in both cats and dogs (the species which will be considered in this article) considerable variation around what is considered normal.
The development of occlusion is controlled primarily by genetic factors. It has been shown that tooth bud position, tooth size and jaw length are inherited (Stockard, 1941; Thomas et al, 1998; Hlusko et al, 2006; Suto, 2008) and that the control of tooth eruption and the growth of the maxilla and mandibles are regulated independently. Development and growth of the facial muscles and other soft tissue is also important in the growth and development of the jaws. Disruption of the normal development of any of these tissues, either through genetic mutation or environmental factors such as trauma or nutritional changes, can result in malocclusion.
The intention of this article is to describe malocclusion in order that it may be recognised and possibly more importantly that the consequences of the malocclusion and potential corrective measures may be understood.
Normal occlusion
Malocclusion can be defined as the abnormal positioning of teeth. It is important to understand, therefore, what is considered normal occlusion in cats and dogs (AVDC, 2017).
The jaws are the part of the skull supporting the teeth. The upper jaw is called the maxilla and lower jaws the mandibles. There are considered to be three classic skull shapes; mesocephalic, brachycephalic and dolicocephallic.
In dogs in the mesocephalic skull shape, which is seen in breeds like labradors and collies, there is even spacing of the dentition without overcrowding or wide spacing between them (Figure 1). The maxilla is slightly longer and wider than the mandibles. The maxillary incisors occlude just rostral to the corresponding mandibular incisors (Figure 2) and the mandibular canines are inclined labially and occlude centrally between the maxillary canine teeth and the maxillary third incisors. The incisor scissor bite and the canine occlusion form the dental interlock (Figure 3) which helps maintain the normal relationship of the maxilla and mandibles during growth. The maxillary and mandibular premolars interdigitate up to the the maxillary third and mandibular fourth premolars (Figure 1) with the mandibular first premolar being the most rostral of these. The mesial cusp of the maxillary fourth premolar sits outside the space between the mandibular fourth premolar and mandibular first molar and the occlusal surfaces of the mandibular first and second molars occlude with the occlusal surface of the maxillary molars.
In cats the mesocephalic skull shape leads to occlusion between the canine and incisor teeth which is the same as in the dog. Though the cat has fewer premolars and molars than the dog the principles of the occlusion are similar with the exception of the maxillary molar which does not occlude with another tooth (Figure 4).
In the brachycephalic skull shape, seen in breeds like Bulldogs and Pugs and Persian cats, the maxilla is shorter. This is often associated with rotation and overcrowding of teeth (Figure 5).
The dolicocephallic skull shape is associated with a longer maxilla and abnormally wide tooth spacing which is seen in breeds such as Rough Collies, Greyhounds and some Oriental cats.
Malocclusion
Malocclusion is associated with the abnormal positioning of one or more teeth, with the retention of deciduous teeth, with an abnormal number of teeth and an abnormal relationship between the maxilla and mandibles. It can be associated with the deciduous or permanent dentition or when both permanent and deciduous teeth are present (mixed dentition).
Malocclusion can cause abnormal tooth to tooth or tooth to soft tissue contact. Abnormal tooth to tooth contact will be associated with repeated concussive trauma to a tooth surface resulting in inflammation of the pulp (pulpitis), consequent pain, abnormal wear, pulp exposure and can prevent normal jaw closure (Figure 6). Abnormal tooth to soft tissue contact can result in trauma to the oral soft tissues, inflammation, ulceration and infection (Figure 7).
It is in these cases that treatment should be considered. There are many cases where malocclusion does not cause abnormal tooth to tooth or tooth to soft tissue contact and where there is clearly no pain or abnormal function associated with the malocclusion. In these inconsequential cases treatment is usually neither indicated or ethically acceptable. Some cases fall between the two clear examples above and here treatment or no treatment is at the discretion of the clinician.
It is not ethically acceptable to treat a patient with an inconsequential malocclusion with the sole purpose of altering its appearance. Changing a patient's appearance as a result of treating a clinically significant malocclusion is acceptable, but alleviation of pain and restoration of normal function should be the primary concern and not aesthetic considerations.
Patients with skeletal (associated with discrepancies of jaw length) malocclusions, because of the likely hereditary nature of this problem, should not be bred from and ideally should be neutered. Any treatment that involves a change in a dog's conformation should be reported to the Kennel Club.
Terminology
In describing a patient's occlusion it is helpful to have an understanding of some specific dental terminology.
The occlusal surface of a tooth is the surface closest to the corresponding teeth on the opposite dental arch. The surface of a tooth closest to the tongue is called the lingual surface, this surface can be called the palatal surface for the maxillary teeth. The tooth surface closest to the cheeks is called the buccal surface and that closest to the lips is called the labial surface. Both of these terms can be replaced with facial. The terms rostral and caudal are not useful terms when describing tooth surfaces as they would for example describe different tooth surfaces for incisors and premolars. They are therefore replaced with the terms mesial, which is the surface of a tooth which intersects with a continuous line connecting all teeth and is closest to the midline drawn between the two central incisors, and distal which is the opposite tooth surface.
Classification of malocclusion
Class 1 malocclusion
Malocclusion associated with the abnormal positioning of one or more teeth with normal jaw relationship is called a Class 1 malocclusion (Figure 8). This can often be the result of trauma affecting the eruption of deciduous or permanent teeth or tooth overcrowding. A tooth may be completely malpositioned as for instance in the case of a supernumerary tooth and where teeth are overcrowded or may be tilted in an abnormal direction (terms to describe this would include mesioversion, distoversion, linguoversion and labioversion).
Examples of common Class 1 malocclusions include linguoversion of a mandibular canine tooth in a patient with a normal jaw relationship and otherwise normal occlusion. Mesioversion of the maxillary canine teeth in Shetland Sheepdogs (Figure 9a), often called lance canines, and occasionally in cats is another example (Figure 9b).
Class 2 malocclusions
Class 2 malocclusions (or mandibular brachygnathia) are skeletal malocclusions which present with the lower jaw relatively shorter and often narrower than the maxilla such that the teeth do not occlude normally. This is a relatively common malocclusion and can be seen in a wide range of breeds though it is more common in mesocephalic or doliocephallic rather than brachycephalic skull types.
The commonest Class 2 malocclusion involves malocclusion of the mandibular canine teeth with the palatal mucosa (Figure 10) or gingiva and often abnormal tooth to tooth contact between the mandibular and maxillary canine teeth. The trauma can be significant and painful producing obvious signs of oral pain and dysfunction and associated inappetence and morbidity. Left untreated this malocclusion can lead to oronasal fistula formation in severe cases.
Class 2 malocclusions can occur with the primary dentition (Figure 11a) and permanent teeth (Figures 11b and c). Puppies with a Class 2 malocclusion will commonly have a similar malocclusion affecting their permanent dentition. However, as Class 2 malocclusion involving the primary dentition is often associated with the mandibular canines penetrating into the palatal mucosa, it produces an interlock between the maxilla and mandibles and therefore fixes them in their abnormal relationship during growth. Extraction of the primary mandibular canine teeth at this stage can allow increased growth of the mandibles and resolution of the malocclusion in some cases as well as resolving pain and discomfort. Treatment of a malocclusion that involves the primary dentition with the aim of preventing malocclusion developing with the permanent teeth is called interceptive orthodontics.
Class 3 malocclusions
Class 3 malocclusions (or mandibular prognathia) are also skeletal malocclusions and are associated with a relatively short maxilla compared with the mandibles (Figure 12). This is most commonly seen in some brachycephallic breeds such as Boxers and Bulldogs. In fact it is considered normal by certain breed associations.
This sort of skeletal malocclusion is often associated with a reverse scissor occlusion (anterior cross bite — the mandibular incisors occlude rostral to the maxillary incisors) and the mandibular canine teeth may occlude directly with or rostral to the maxillary third incisors (Figure 13 showing reverse scissors occlusion affecting the incisors).
Class 3 malocclusions can be subtle in their presentation with evidence of the mandibular canine teeth pressed against the maxillary third incisors and the incisors either with their cusps level or in reverse scissor occlusion. The canine interlock prevents further forward growth of the mandibles and can result in ventral bowing of the mandibles and an open bite (increased distance between the occlusal surfaces of the premolars).
In extreme Class 3 malocclusions the mandibular canines and incisors may be so far rostral to the maxillary teeth that they are not in contact with any other teeth or soft tissue and are therefore comfortable and functional.
Posterior crossbite
Posterior crossbite describes the abnormal relationship of the maxillary fourth premolar and mandibular first molar (Figure 14) occasionally seen in doliocephalic breeds where the cusp of the mandibular first molar sits outside the maxillary fourth premolar.
Posterior malocclusion in cats
A specific malocclusion is seen in cats where the cusp of the maxillary fourth premolar occludes directly with the gingiva and oral mucosa on the buccal aspect of the mandibular first molar. This may cause ulceration and inflammation of the mucosa in this area which may lead to gingival and mucosal hyperplasia which in turn will cause a more significant traumatic malocclusion and a progressive deterioration in the condition (Figure 15).
Wry occlusion
Wry occlusion develops if there is differential growth between either side of the maxilla or mandible. This may result from trauma to one side of the jaw or an abnormal dental interlock developing on one side of the jaws restricting normal growth. A wry occlusion is evident when the midline between the central maxillary and mandibular incisors are not in line.
Diagnosis of malocclusion
Malocclusion can develop in young and adult animals as a result of periodontal disease which allows increased tooth mobility, neoplasia where teeth are moved from their normal position and trauma. Therefore it is important when carrying out routine examinations of cats and dogs in the consulting room, or when triaging patients in emergency situations, to include an assessment of the patient's occlusion.
Examination of patients to evaluate occlusion should initially involve assessment of skull morphology, symmetry and jaw length. Following this tooth position and occlusion should be assessed by lifting the lips and assessing the patient with its mouth closed looking for a normal scissor bite of the incisors, canine interlock, interdigitation of the premolars and a normal relationship of the most caudal premolars and molars. Individual teeth should then be examined for signs of wear and trauma and oral soft tissue checked for evidence of abnormal tooth to soft tissue contact.
Many of the occlusal problems that are commonly seen are associated with abnormal or disturbed growth and development. Veterinary nurses are in an excellent position to identify these abnormalities as they should see young and growing puppies and kittens on a regular basis. An occlusal evaluation should be part of every contact that veterinary nurses and veterinary surgeons have with developing patients until they are fully grown. Therefore a knowledge of normal anatomy and occlusion and an awareness of how malocclusion presents and its significance is essential.
Treatment of malocclusion
Malocclusion that does not involve abnormal tooth to tooth or tooth to soft tissue contact may not require treatment. However malocclusion that involves even minor abnormal contact should be evaluated by a veterinary surgeon who has an interest and understanding of veterinary dentistry. Oral pain can be difficult to identify in veterinary patients and pulpitis resulting from abnormal tooth contact is easily overlooked. Abnormal tooth to tooth contact can also place abnormal stresses on the temporomandibular joints (TMJ) and results in pain, arthritis and joint dysfunction (Jung et al, 2013).
Malocclusion that results in abnormal tooth to soft tissue contact is often both easier to see and to appreciate its consequences because of the resultant soft tissue injury, ulceration and infection.
A detailed account of the treatment options for malocclusion are outside of the scope of this article. However they can be summarised as one or a combination of orthodontic treatment which involves moving teeth (Figures 16a and b) into a more favourable position and extraction or partial amputation and endodontic treatment of a tooth or teeth (Figure 16c) removing problematic contact between teeth or a tooth and soft tissue in the oral cavity.
Conclusion
The development of normal occlusion is a complex process which, if all goes well, results in a precise relationship between the maxillary and mandibular teeth. Relatively minor disturbances in the relative growth of the maxilla or mandibles or in the timing of the exfoliation of deciduous teeth and eruption of permanent dentition can lead to significant and traumatic malocclusion. Veterinary nurses, because of their existing role in supervising growing puppies and kittens, are ideally placed to recognise and provide appropriate advice for these patients.