Anatomy and Physiology

Periodontal disease is the most common disease of the oral cavity in dogs. Every dog is affected during its life to some degree. Curiously, compared with other diseases it's often neglected, although it can be treated or even prevented with care. In oral hygiene, the benefit of any therapeutic intervention is of short duration if it isn't prolonged with daily care by the owner.

The aim of this care is to fight dental plaque. While brushing is accepted as the most effective means of protection, there are alternatives - both physical and chemical - to help control plaque.

Philippe HENNET
DVM, Dipl AVDC, Dipl EVCD

A graduate of the National Veterinary School of Toulouse in 1988, Philippe Hennet conducted a research project in canine endodontics with the dental faculty at Toulouse in 1987 - 1989, which led to a veterinary doctoral thesis in 1990. He continued his research as part of a clinical residency at the Dentistry Section of the University of Pennsylvania Veterinary School at Philadelphia in 1990 - 1992. Philippe now specializes in oral and ENT diseases at his practice in Paris. He has been a dentistry assistant at the National Veterinary School of Alfort since 1993. He has also been the scientific co-organizer of the dentistry course at the European School for Advanced Veterinary Studies (ESAVS) in Luxembourg since 1995. A diplomate of the American Veterinary Dentistry College (AVDC) since 1993, and of the European Veterinary Dentistry College (EVDC) since 1998, he is the author of many publications in English and in French.

1. Anatomy and Physiology

Teeth

(Figure 1 and Figure 2)

Figure 1. Lateral view of the denture of the dog.

Figure 2. Frontal view of the denture of the dog.

The first primitive mammals that appeared 250 million years ago during the Mesozoic era already possessed two hemi-mandibles connected ventrally by a symphysis and joined to the squamosal's glenoid cavity by a condyle (temporomandibular articulation). The enamel-covered teeth were divided into cutting incisors, rounded canines and grinding or hacking molars, implanted in the alveolar borders of the maxilla and the mandible. Present-day mammals retain these essential anatomical characteristics, with modifications depending on diet (Lavergne et al., 1996).

The carnivores are diphyodonts (having two successive sets of teeth, deciduous and permanent) and heterodonts (having different types of teeth with different functions). The incisors - prehensile cutters that number three per hemi-jaw - only have one root. The conical canines are tearing teeth adapted to the diet of a carnivore.

The premolars have two roots, with the exception of the first premolar, which is regressive and has a crown formed by three cusps in a line. There is alternate occlusion of the crowns of the maxillary and mandibular premolars with the necessity of diastema between the teeth.

Occlusion of permanent teeth in dogs. Note the regular alternation of maxillary and mandibular premolars: the main cuspid of maxillary premolars is lodged in the middle of the interdental space of the two mandibular premolars. The mandibular canine is lodged in the corner maxillary canine interdental space, thus forming a powerful triad. In mesocephalic and dolichocephalic dogs, and in certain brachycephalic dogs, the incisors have a scissor bite (the cuspid of the mandibular incisors in contact with the cingulum of the maxillary incisors). (© Ph. Hennet).

Jaws

In the carnivore specialization, temporomandibular articulation is located in the extension of the occlusal plane. It consists of a deep transversally-oriented, hemi-cylindrical mandibular fossa, bordered ventrally by a powerful retro-articular process into which an elongated mandibular condyle fits transversally. This mechanism principally permits the raising and lowering of the mandible as well as the lateral movements in dogs that are essential for ripping through prey (Lafond, 1929; Gaspard, 1967).

Physiology of Manducation

The term manducation designates all the actions involved in eating, including prehension, mastication, insalivation and deglutition (Verchère et al., 1992). Contrary to humans, carnivores do not chew their food. They divide them into scraps that are not completely crushed and scarcely insalivated, but that are quickly swallowed. Manducation principally consists in breaking up large pieces of food. In the wild, the canids capture their prey with their powerful canines. The incisors serve to cut and tear large pieces, which are then introduced deeper into the oral cavity. This action may be supplemented with jerks of the head driven by the muscles in the nape. The piece of food - a muscle mass for instance - is cut by the scissors formed by the cuspids of the mandibular and maxillary carnassials (Figure 3). To enable this, the vestibular surface of the mandibular carnassials must come into contact with the lingual surface of the upper carnassials through an opening of the symphysis and an external torsion of the mandibular body (Gaspard, 1967).

Figure 3. Cutting effort (From Gaspard, 1967).

Only one mandibular branch works at any one time (Lafond, 1929). When the piece is soft and less voluminous the canid can lock its jaws without bringing its carnassials together. This is also what happens when it works simultaneously with the two hemi-mandibles. Here the piece breaks due to an extension associated with shearing. The deformation is accompanied by lateral movements. Carnivores rip the elastic body and tear the fibrous tissues by laceration, which consists of violently moving the mandibular teeth across the maxillary teeth. The food is violently stretched, which results in it breaking at the point of least resistance (Figure 4).

Figure 4. Break by extension (From Gaspard, 1967).

When faced with a long, rigid body like a bone, the canid immobilizes it between its forepaws by pressing one extremity against the ground and energetically seizing the other extremity in its mouth. It then revolves its head from one side to the other to subject the body to flexion and torsion. The body ultimately breaks at the point of the carnassials. Thus, the body is squeezed then crushed between the first upper molar and the crushing talon cusp of the lower carnassial. These food fragmentation techniques and the major forces developed explain the powerful chewing muscles that allow carnivores to lock their jaws.