Orthodontics for Horses
- Jun 1, 2007
The equine jaws and teeth are designed to interact in an efficient manner that maintains normal forces during the entire life of the teeth. If forces and alignment are normal, the horse is comfortable and teeth have the most longevity.
Consequently, the horse has a significantly reduced chance of developing other complications, such as abnormal dental wear and periodontal disease.
The term occlusion describes how teeth contact their opposite partners. Normal equine occlusion occurs when the incisors (front teeth) are in contact while the cheek teeth are not touching, as in Figure 1.
The upper jaws are slightly farther apart than the lower jaws, thereby accounting for the overlapping occlusion seen in the normal horse. Contact in cheek teeth and chewing occurs as the lower jaw moves from side to side and grinds feed. There also is a slight "back-to-front" motion of the mandible.
A malocclusion is characterized by abnormal tooth contact.
Types of Malocclusions
Malocclusions can be classified in a similar fashion to those in man and small animals. Class I malocclusions relate to abnormalities on single teeth. An example of this is shown in Figure 2.
In the example shown, the tooth cannot be rotated into normal position by current orthodontic methods. This patient should be monitored for accumulation of feed material between teeth that do not fit together normally. As feed accumulates, these teeth can develop periodontal disease. Regular attention and removal of debris between teeth by your equine veterinarian will result in maintenance of normal occlusal forces and extended tooth life.
Class II malocclusions are found in the so-called "parrot mouth" horses, in which the upper jaw is longer than the lower jaw. This is a good example of a condition that can be best addressed in the young, growing individual.
Many treatments exist for this condition. It is important to understand that the goal of cosmetic perfection might be unreasonable in many cases. Complications of this condition include direct contact of long, unopposed teeth with gums and limited range of motion during chewing. The primary goal should be achieving incisor arcade contact with the purpose of maximizing normal occlusion. In so doing, all complications are minimized and the patient's teeth can generally be addressed routinely on an annual basis.
Class II malocclusion treatment is demonstrated in Figures 3 and 4 (page 72). Many Class II malocclusions are effectively treated with routine dental care. An important part of treatment is reduction of long teeth, such as "hooks." These overlong teeth act to limit forward movement of the jaw in chewing. Reduction, or shortening, results in more freedom of movement and allows proper growth of the mandible. Other cases require further intervention.
An additional method of treatment is a bite plane (shown in Figure 5). The bite plane serves to force the upper jaw upward into normal position and provides a surface of contact for the lower incisors.
In more severe cases, the veterinary dentist might need to apply wires that wrap around the front teeth like braces. It is imperative that this treatment be performed in concert with a bite plane apparatus.
Figure 6 (page 76) shows what happens when wire is used alone--the upper teeth are simply pulled downward with no improvement in the condition.
A Class II malocclusion is the result of the interaction of multiple genes. Generally this type of malocclusion results from mating of an individual with a short head to one with a longer head. While heritability is an important aspect of managing the condition, it is by no means a given that a single individual will produce offspring with this malocclusion. The general recommendation is not to use affected individuals for breeding and to refrain from repeated matings that have produced the problem. It is a complex problem with no easy answer. Affected individuals should be given the best chance for a normal life and, therefore, they should be treated with the best methods available.
Editor's Note: There are ongoing studies to look at the genetic aspects of Class II malocclusions. Owners who have horses with this condition should refer to their breed registry requirements on whether they need to disclose the condition, and whether these malocclusions can be altered, so that they don't misrepresent these horses.
Class III malocclusions are those horses affected with "sow mouth," in which the lower jaw is longer and teeth are not in a position of normal occlusion. These malocclusions can result from delayed development of the jaws, or they can be inherited, as in the dwarf condition in some Miniature Horses. The Miniature Horse breed associations do an excellent job of selecting against individuals that demonstrate these conditions, and while they continue to exist, the incidence in the author's practice area appears to be declining.
Class III malocclusions can be improved in most patients, regardless of the cause. In those that have the inherited conditions, regular dental care serves the same purpose as in Class II malocclusions, where long teeth are shortened to allow more freedom of movement of the lower jaw. Their reduction also removes obstructions to normal growth of the upper jaw, and it allows the patient to reach its maximal genetic potential. An example of a Class III malocclusion is shown in Figure 7 (page 77). After routine dental care the patient had a significantly improved range of jaw motion. The treatment consisted simply of reducing the overlong teeth. The result is shown in Figure 8.
Shedding of Baby Teeth
Shedding of baby (deciduous) teeth is a complex process. It is not an inflammatory course of action. It is a result of a genetically predetermined breakdown of roots and supporting structures that can be hastened by normal chewing forces and delayed or accelerated by abnormal chewing forces.
A tooth with reduced attachment apparatus is less capable of withstanding chewing forces as the tooth's supporting structure begins to break down, so the breakdown process is sped up. If chewing forces are reduced, breakdown might be slowed. If the adult tooth is not present, breakdown occurs anyway, but at a reduced rate. In some teeth without adult counterparts, the deciduous tooth might remain functional for years beyond its normal life. Premature loss of a deciduous tooth results in abnormal development of the adult tooth.
How does one know when to remove baby teeth? There is a normal schedule by which the majority of horses shed their baby teeth and erupt their adult teeth. The problem is horses don't read well! Many patients shed teeth at variance with these normal times, so it's critically important that one understand when to remove baby teeth and when not to do so. They should not be extracted just because they fit into the schedule. There are two indications for deciduous tooth removal:
- The presence of the adult tooth.
- A loose deciduous tooth that can easily be "wiggled."
Bone growth is another aspect of orthodontics. The flat bones of the head grow by addition of new bone on the end and side surfaces. Their method of growth differs from that of the long bones of the leg that undergo epiphyseal growth (the ends of the bone grow). Most growth in the flat bones of the jaws occurs at the back of the bone, not at the front.
This backward-directed bone growth is important in maintaining proper alignment of teeth, especially in Class II malocclusion (parrot mouth) patients. If the upper incisors are allowed to drop down, giving the patient the classic parrot-mouth look, growth of the mandible is restricted by the resulting dental interlock. The same occurs to a lesser degree with overlong cheek teeth such as hooks. As the lower jaw is forced back toward the neck, its growth is limited. Treating these malocclusions removes these restrictive conditions, thus allowing maximal growth potential to be reached.
Normal tooth eruption results as normal tooth attrition (wearing down) is compensated. The equine tooth is composed of a crown that can be about four inches in length (in cheek teeth), most of it submerged in the jaw. As the chewing surface undergoes attrition, the remainder of the crown erupts at a slow rate to replace the lost tooth material. If all forces of mastication are normal, this results in even occlusal surfaces.
If chewing forces are abnormal, tooth attrition is abnormal. In such a situation, eruption is uneven, with some teeth erupting faster than others. The faster erupting teeth become longer than others, resulting in hooks, ramps, steps, and waves. Without outside intervention, these conditions become progressively worse. They can result in many other complications such as cavities, periodontal disease, and pulpitis (inflammation of the dental pulp, which is located in the central cavity of a tooth). This can lead to premature loss of teeth. This is one reason it is very important to have yearly dental care for your horse. Routine dentistry results in even tooth occlusion, which spreads the forces of mastication evenly throughout the arcade. The net result is even attrition, even eruption, minimal chance of complications, and increased tooth longevity.
Normal Tooth Movement
There is normal tooth movement over time in equine patients. The back teeth move forward as the patient ages. This process is called mesial (forward) drift. It also occurs in man. The teeth move through bone toward the midline in the center of the front teeth. This process, together with the process of tooth eruption, assists in keeping the teeth crowded together with no spaces between them. If a malocclusion occurs where one or more teeth are elongated for an extended period of time, the drifting process can be interrupted, creating spaces between teeth. Periodontal disease results, and premature tooth loss is inevitable if the process is not treated. Early recognition is critical to successful treatment. Prevention is best, as with most conditions. Therefore regular annual dental care is important.
In aged patients, such as those older than 20, the eruptive process and the drifting process are slow and limited. In these patients, if spaces develop between teeth, they might not be resolved. Thus the importance of prevention is highlighted.
In some patients, elongated teeth become forced together by chewing forces, resulting in increased wear on the sides of the teeth. This also results in the production of spaces between teeth that cannot be reversed or otherwise treated. This occurs in geriatric patients that have not had dental care for many years. Unfortunately, it leads to premature tooth loss.
As you can see, equine dentition is very dynamic. Teeth undergo attrition and erupt to replace lost crown. Attrition is not even over the entire arcade, thus, malocclusions, such as hooks, ramps, steps, and waves develop. As a consequence, eruption is not even throughout the arcade. Uneven attrition and eruption can be minimized with regular dental care.
Teeth move and bones grow in equine patients, just as in kids. The direction of bone growth is away from the nose--a somewhat counterintuitive notion! The importance of this concept comes in management of conditions such as parrot mouth foals. Tooth movement continues throughout life, as adult teeth move forward in the arcades. This helps prevent problems, such as periodontal disease and other causes of premature tooth loss.
Regular dental care is necessary in all horses for the purpose of treating and preventing dental disease.
POLL: Rehabbing the Injured Horse