As in other herbivores, the premolar and molar teeth of horses are important for grinding herbaceous food (primarily grass) in order to maximize the extent to which the intestinal tract can obtain nutrients from it. The premolar and molar teeth are collectively referred to as the "cheek teeth" and, unlike the situation for humans, dogs and cats, these teeth are continually growing throughout the life of the horse.
In the young adult, the root of each of the cheek teeth is several centimeters in length, but they are progressively worn down by their grinding function, known as dental attrition. The age of horses can be defined by the length of the roots of their cheek teeth. By the age of 30 years, most horses have expended their cheek teeth by normal chewing activity throughout life; in the aged horse, the length of the roots of the cheek teeth might be less than 1.0 cm. They are not generally able to live beyond this point because, in the absence of effective cheek teeth, it is not possible to acquire sufficient nutritional value from food. The rate at which dental attrition occurs differs among horses based on genetic differences, such as the shape of the mouth, the type of food that the horse has been fed and the extent to which the horse's teeth have been cared for throughout life.
Grass-based diets increase the wear on teeth because of the abrasive content of silica in the dirt and grass, compared to the low silica content of grain. Therefore, the extent to which grain is used in a ration is an important predictor of early dental attrition. Veterinary dental health care for horse's teeth, when applied conservatively, can promote oral hygiene and possibly increase the useful life of horses.
However, excessive dental "treatments" may actually quicken the rate of dental attrition to the eventual demise of the patient. Currently, it is fashionable for horse "dental prophylaxis" programs to be undertaken regularly with little heed paid to the fact that these treatments may often be unnecessary and may undermine the true function of the cheek teeth.
Although some unusual horses are able to attain ages beyond 35 years, it's often the result of special diets. Many of these very old horses tend to have difficulty maintaining their body weight and they are at risk for other dental problems. As the horse ages and the cheek teeth become progressively worn down, the risk of secondary dental disease increases.
A very important contributor to normal oral hygiene is the movement of the tongue within the oral cavity. If the horse is unable to "clean" the interior surfaces of the oral with the tongue, food material will lodge and accumulate. If food material accumulates in the recesses between adjacent teeth or between the teeth and the lining of the cheek, bacterial infection occurs and leads to gingivitis. As with people, gingivitis progressively leads to other dental problems.
Rasping the Cheek Teeth
The on-going grinding process of the cheek teeth causes the cheek teeth to develop sharp edges. If these sharp edges are left unattended, the horse may experience pain during normal cleaning movements with the tongue that will lead to poor oral hygiene, food retention, and gingivitis. To prevent this sequence of events, the most common dental prophylaxis that is routinely afforded adult horses is "rasping" or "floating" the cheek teeth.
Dental rasping entails the use of "dental floats" – hand-held instruments that are comprised of a handle and a head. At the head is a grinding surface, often composed of carbide-tungsten. The grinding surface of the dental float is moved backwards and forwards along the sharp edges of the cheek teeth with the intention to smoothing off the sharp corners and improving the ease with which the tongue can be moved inside the oral cavity.
Remarkably, dental rasping is often accomplished in the standing horse with minimal restraint. For some horses, a light sedative may be useful to facilitate the procedure. Some dental rasping gear is powered so that the veterinarian's work in completing the procedure may be lessened. The use of powered gear is especially helpful when a large number of horses are being treated. Unfortunately, some veterinarians use the powered gear to excess and cause marked smoothing of the dental occlusal surfaces. Excessive rasping clearly quickens dental attrition and removes the important grinding surface of the cheek teeth to a detrimental extent (these surfaces are supposed to have a degree of irregularity to them).
The most common sign of "normal" dental attrition is the tendency of old horses to become thin. In younger horses, the presence of sharp enamel edges may interfere with the normal maintenance of oral hygiene and cause food retention and gingivitis.
Symptoms of excessive sharp enamel edges include:
Certainly, there are numerous other and unrelated causes for signs associated with weight loss and difficulties when being ridden. Of note, some individual horses develop the habit of "grinding" their cheek teeth ("bruxism"); although this behavior may be a sign of internal disease (it is often just a habit), it is not a sign of dental problems.
In advanced cases of dental disease, teeth may be broken (fractured) and/or infected (peri-apical abscess). In these cases, signs of dental pain may be more prominent. Infection of a tooth may lead to abscess formation, swelling, and purulent drainage from the tissue between the tooth root and the overlying skin. The jaw bone may be enlarged at the point of the affected tooth root. For some of the "upper" cheek teeth, infection and/or fracture leads to infection of the maxillary sinus and a foul-smelling unilateral nasal discharge.
If a substantial tooth problem has been present for a sufficient length of time, it will be apparent that the rate of attrition of the cheek teeth (based on inspection of the oral cavity) may be uneven. Irregular attrition causes marked deviation in the grinding plane, often known as a "step mouth" or a "wave mouth."
The most important component of dental disease diagnosis is a thorough examination of the horse's mouth, including careful palpation of each of the teeth. Prior to a thorough oral/dental examination, it is often necessary to provide tranquilization.
In the last few years, there has been a widespread increase in the level of interest in equine dentistry. Extensive equipment is now available to assist the veterinarian during an oral examination. Based on current standards, and using one of several methods, it is important that the whole oral cavity and each of the cheek teeth be scrutinized very carefully. Good record keeping is also very important. In some cases, radiography of the teeth (and tooth roots) may be useful.
It is widely recommended that each horse be subjected to a routine health maintenance examination once yearly. During such examinations, both the oral cavity and the teeth should be examined (with sedation if needed). For those horses with recognized problems (narrow jaws or previous dental problems), it may be appropriate that oral/dental examinations be undertaken with greater frequency as dictated by the needs of the individual. A complete oral/dental examination should be an essential part of any pre-purchase examination.
The frequency with which the cheek teeth are rasped (floated) must be determined for each individual horse. It is currently fashionable to undertake dental rasping on a very frequent basis, regardless of individual need. As a rule, most horses probably do not require extensive rasping more frequently than once yearly – many horses maintain excellent oral hygiene by having their cheek teeth rasped every two to three years. It should be noted that it is regular examinations and not regular rasping that precludes the development of severe dental disorders. It is easier to correct a problem in the early stage before it has had time to cause unbalanced dental attrition.