Oral Anatomy and Charting
Norman Johnston United Kingdom
An understanding of dentistry requires an awareness of the structure and function of oral tissues, both the teeth and supporting structures. This is fundamental to the appreciation of dental pathology and treatment regimens. Dental patients in practice vary from “traditional” pets, such as dogs and cats, to small herbivores.
The main muscles of mastication that close the jaws are: temporalis muscle, masseter, and pterygoid (lateral and medial). Innervation of all four is the mandibular nerve, which is the only motor branch of the trigeminal nerve.
The only muscle to open the jaw is the digastricus. Only the rostral belly innervates via the mandibular branch of the trigeminal while the caudal belly innervates via the facial (C7) nerve.
The oral cavity itself is split into the vestibule and the oral cavity proper. Within the oral cavity proper are the hard palate, soft palate, tongue, and the floor of the mouth. The bones of importance to the oral cavity are the incisive bone, mandibles, and maxillae. The sockets in all these bones that contain the teeth are termed alveolar processes. The mandibles comprise of two bones joined rostrally by a strong fibrous joint termed the mandibular symphysis. The mandible contains a horizontal and vertical ramus. The horizontal ramus comprises the symphyseal area and the body while the vertical ramus comprises the coronoid process, condylar process, and the angular process. The tongue is a structure comprised of both intrinsic and extrinsic muscles and functions as the structure that takes fluids and solids into the mouth and has important functions in grooming and vocalisation.
DENTAL ANATOMY: THE TOOTH AND ATTACHMENTS
The anatomical components of teeth will be discussed in terms of their structure and function.
Enamel. Enamel is 96% inorganic, mainly hydroxyapatite crystals, with 4% water and fibrous organic material. It is the hardest substance in the body and covers the exterior surface of the crowns only. The enamel consists of hexagonal prisms or rods of hydroxyapatite crystals held together by a cementing organic matrix. Enamel is formed by ameloblasts within the tooth bud before eruption. It is capable of only very limited repair when damaged once the tooth has erupted.
Dentine. Dentine is the main supporting structure of the tooth and is the second hardest tissue in the body after enamel. It is 70% mineral and acellular as hydroxyapatite crystals and 30% organic as water, collagen and mucopolysaccharide. The main structure is the dentinal tubule, which extends from the external surface to the pulp. There are some 30,000–40,000 tubules per mm2, which can transmit pain to the pulp if the dentine is exposed.
Alveolar bone. The roots are encased in the alveolar processes of the jaws. The densest bone lines the alveolus and is called the cribriform plate. It may be seen radiographically as a white line called the lamina dura.
Cementum covers the enamel free roots and provides a point of attachment for the periodontal ligament. Similar in composition to woven bone it is 45-50% inorganic, primarily as hydroxyapatite crystals, and 50-55% organic material. Cementum is capable of formation, destruction, and repair. It is nourished from vessels within the periodontal ligament.
Periodontal Ligament. Comprised of taut collagen fibre bundles (called Sharpey’s fibres where they are trapped in cementum and alveolar bone) that are anchored to the cementum of the tooth and the alveolar bone. Blood vessels are widely dilated and evenly distributed in the periodontal ligament along with nerves that are capable of transmitting heat, cold, pain, and pressure in addition to proprioception in some species.
Pulp. This living tissue within the tooth is located in the pulp chambers and root canals. It is well innervated and vascularised and comprises connective tissue, nerves, lymph and blood vessels, collagen, and undifferentiated reserve mesenchymal cells (e.g., odontoblasts). Odontoblasts line the pulp cavity and branch into the dentine tubules. These branches, together with the fine nerve endings, cause the dentine to be sensitive to temperature and pain. The odontoblasts lay down secondary dentine and reduce the pulp cavity in size as the animal ages. The pulp is nourished via vessels entering and leaving the root canal at the apical delta and, occasionally, via accessory canals.
GINGIVA AND SURROUNDS
Attached gingiva. The attached gingiva is tightly adherent to the subgingival connective tissue and bone via deep rete pegs. It is keratinised to withstand the stress of ripping and tearing food.
Mucogingival junction (MGJ). This is the junction between the soft, fleshy mucus membrane of the oral cavity and the tough, collagen rich gingiva. The MGJ remains stationary through life although the gingiva around it may change in height due to hyperplasia, recession or attachment loss.
Free gingival. Free gingiva forms the gingival margin visible during exam. It surrounds the crown of the tooth.
Gingival sulcus. Between the tooth and the free gingival margin is the gingival sulcus. It is the crevice that surrounds the tooth. The normal depth of the sulcus is 0.5 mm to 1 mm in cats and 1–3 mm in dogs. As a rule 1mm for cats and 1–2 mm in dogs can be considered normal. The sulcus lining epithelium renews itself rapidly every 4–6 days compared to 6–12 days for oral epithelium. The sulcus is bathed in crevicular fluid, which contains many of the elements of immunity.
Junctional epithelium. At the bottom of the sulcus is the junctional epithelium (or epithelial attachment), which is so important in the control of periodontal disease. This attaches the gingival tissues to the tooth using hemidesmosomes.
Teeth Types and Function
The masticatory forces in the dog have been estimated to be 300 to 800 psi as passive bite force with a sudden localised bite force when snapping the jaws shut of as much as 30,000 to 80,000 psi.
Incisors. Used for cutting, scooping, picking up objects and grooming.
Canines. Used for holding prey, slashing and tearing when fighting and act as a cradle for the tongue.
Premolars. Used for holding, carrying and breaking food into small pieces.
Molars. Used for grinding food into small pieces with flat occlusal tables.
Tooth Numbering: Triadan
The modified Triadan system provides a consistent method of numbering teeth across different animal species. The system is based on the permanent dentition of the pig, which has 11 teeth in each quadrant: three incisors, one canine, four premolars and three molars. The grand total is 44 teeth. The same framework exists for all species BUT other species may have missing teeth from evolution, and in these cases, the numbers take this into account as long as certain rules are followed.
First digit of the modified Triadan system denotes the quadrant:
Second and third digits denote the tooth position within the quadrant, with the sequence always starting at the midline.
The system relies on definite landmarks, the so-called “rule of fours and nines.” The canine teeth are always number four when present. The first molars are always number nine when present. In addition, the carnassials are always the upper fourth premolars and lower first molars. In cats, where tooth numbers are reduced, the use of the carnassials as landmarks will help considerably.
The central incisor is always 01 with the following incisors 02 and 03.
The canines (when present) are always 04.
The premolars (when present) are 05 to 08. The upper carnassials in carnivores are always the last upper premolar, therefore must be either 108 or 208.
The molars (when present) are 09 to 011. The lower carnassials in carnivores are always the first molar, therefore must be either 309 or 409
Why do You Need to Chart Your Dental Cases?
1. Charting is essential to record the presence of health and/or disease in a form that can be used now and later. At its very simplest form, it is necessary for medico-legal protection reasons to know what teeth and pathology was present before treatment was started.
2. The success or otherwise of treatments is impossible to gauge over time without the proper information gathered at initial treatment.
3. It is clinically a good habit to develop. The client is often impressed by the time taken to gather information that they can easily understand and use to play their part in the maintenance of the oral cavity.
Many different charts exist in the vet dental literature. Use one that suits your purposes. The main chart discussed during this paper is used with grateful permission from Pharmacia Animal Health.
How to Chart
Chart the mouth starting at the side uppermost. It is best to use a microcassette recorder and transcribe later for speed and cleanliness:
1. Count the teeth and note missing or extra teeth.
2. Determine level of calculus as per the legend. (0=none to 3 = 100% cover)
3. Determine the level of gingival inflammation as per the legend. Using the blunt perio probe, gently run probe round the buccal sulcus to determine the degree of gingival inflammation. Take care not to use too much apical pressure or pass the same way twice—you do not wish to create damage where none exists.
4. Note any major abnormalities visible: fractured teeth, enamel defects, neoplasms, and attrition with tertiary dentine exposure.
5. Optional: use disclosing solution to determine location and level of plaque. This step is of most use in consulting room examinations post-op. Apply to buccal surfaces with a soaked cotton bud. Duo-128™ (Virbac) contains two dyes; a pink deposit indicates immature plaque and purple is mature, mineralised plaque.
After initial scaling:
1. Examine the sulcus of each tooth in a minimum of six locations: three buccal and three palatal/lingual. Use the graduated probe in the long axis of the tooth in an apical direction. Apply no more than one ounce of pressure. Use the probe as your eyes under the gum line and feel for missed subgingival calculus, abnormal pits and depressions in the root surfaces and periodontal pockets; describe depth and location.
2. Note location of teeth extracted at this procedure by crossing out tooth on graph and crossing out the boxes.
3. Note other important features such as: a) gingival recession and root furcation exposure—draw new line on buccal graph; b) caries—draw location; c) enamel defects—draw location; d) mobile teeth—use index as per code key; e) other significant lesions (e.g., fractured crowns or cusps, abrasion sites, FORL’s in cats etc.)
4. Note treatments performed—fillings, root planing, extractions, pocket management and gingival flaps, and root canals.
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