Dentistry Series Part 3: Malocclusions

In the last two installments of this equine dental series, you've digested an overview of the basics of equine dentistry, and I hope you've gained an understanding of just why dental care for horses is rising to the forefront of equine veterinary medicine. You've also been indoctrinated into the intricacies of a thorough equine dental exam. It's a far cry from the days of wrestling with the horse, jumping up and down, and hoping to actually see something in the horse's deep, dark mouth!

In this installment you will be introduced to some of the finer points of equine dental malocclusion corrections. Too many horse owners have never seen the inside of a horse's mouth. Even some of my own clients really don't have a good idea of what exactly it is that I do when I "float" their horse's teeth. They just trust me to do the right thing. That's fine, but my goal is to have the best-educated clientele on the planet.

I entertain this lofty goal because once an owner really and truly understands how the horse's mouth works, what throws it out of balance, and how equilibrium is restored, they will be forever diligent in obtaining quality dental care for their horses.

Back to Basics

First, a few definitions:

Occlusion is used to designate the interface where the grinding surfaces of the teeth meet. When your dentist puts that tape between your teeth and asks you to bite down and tap, tap, tap, he or she is checking to see how your teeth meet to make sure that you can effectively and comfortably chew your food. Herbivores--in this case horses--have teeth that are somewhat different than ours, but they must still effectively occlude to allow chewing that is both efficient and comfortable.

Malocclusion is an abnormal occlusion or contact of two or more teeth. For our purposes here, the chief difference between horses' teeth and our teeth is that horses' teeth are designed to wear away incrementally throughout their lives. Under conditions of normal wear, a horse's teeth should last into his 20s. When malocclusions develop, that wearing process can accelerate dramatically, effectively shortening the life of the teeth.

One goal of comprehensive dental care is to normalize the wear patterns of teeth to prolong their useful life. This corrective process is known as dental "equilibration." The term "floating" is an old one that's deeply ingrained in the horseman's vocabulary, but it really only refers to removal of sharp enamel points. You know from earlier articles that these points are only one relatively minor dental abnormality. Strictly speaking, they are not considered malocclusions except in extreme cases. They can cause extreme soft tissue pain and, as such, should be removed routinely as part of any dental equilibration procedure.

The bulk of equilibration work is focused on making changes to the occlusal surfaces that restore and maintain normally wearing equine teeth.

What Is Normal?

The first thing one appreciates upon examining the horse's oral cavity is the impressive appearance of the horse's cheek teeth. They are designed uniquely to crush, mash, grind, and generally masticate coarse forage. The chewing surfaces are composed of undulating ridges of very hard enamel and slightly less tough dentine and cementum. This difference in density of tissue allows certain areas of the tooth to wear a bit faster than others, so ridges develop on the chewing surfaces. These transverse (lying in a crosswise direction) ridges are normal and aid in the efficient grinding of plant matter.

I like to describe the visual appearance of the occlusal surfaces as "regularly irregular."

Each tooth should have one or two transverse ridges with a shallow trough in between. These ridges should be uniform in size from the front of the mouth to the back.

When opposing teeth do not meet in the normal manner, one or more teeth can wear at an accelerated rate. If unopposed, one tooth or part of one tooth can overgrow due to lack of an opposing surface to grind against. If even one tooth is too short or too tall, the rest of the system compensates to accommodate the primary abnormality.

In one sense, this is the beauty of the equine dental system: It is very adaptable and can tolerate a great deal of deviation from normal. On the other hand, if the dental system compensates for too long, it will approach failure on one or more levels. The more dramatic the abnormalities become, the more difficult they are to correct.

The older horse has less remaining tooth to erupt, so there is less time to return the system to normal function.

There are some cases so severe that they cannot be corrected, or the horse might be so old that complete correction is impossible. In either case, the practitioner is often limited to palliative therapy. Even treatment aimed at "damage control" and pain alleviation is very important to the patient's current and future health.

Just to reiterate, the more normal the wear patterns, the longer the teeth will last to do their job. Following this line of reason, the sooner abnormalities are recognized, the easier they are to correct.

When It's Really Bad

What happens when malocclusions are so severe that they cannot be corrected at the time of diagnosis?

Let's examine a 13-year-old with a significant wave mouth, which is characterized by a wavelike or stair-step configuration of the premolars and molars from front to back. Theoretically, in order to correct this malocclusion completely, one would have to shorten (reduce) each tooth in each arcade to the level of the shortest tooth in that arcade.

This plan is dangerous on several levels. After the first few millimeters of tooth surface are removed, the dentinal tubules (minute canals that comprise the dentin) are likely to become irritated and cause the horse some discomfort that could result in reluctance to eat. It is also possible to expose the sensitive pulp tissue that could result in infection or death of the pulp and loss of the tooth.

Obviously, these are situations to avoid. So how do we correct these if one can't simply "level" the arcades?

The most sensible approach is to stage the correction over a period of 12 to 24 months. Remember, the teeth did not wear into this abnormal pattern in just a few months. A 13-year-old with a severe wave has been working on that wave since his permanent teeth were in place at four or five years of age. In most cases, there is no need to make huge, aggressive corrections in one day. A more reasonable approach would be to reduce the severity of the wave; that is, to reduce the height of all the components of the wave to allow better mobility of the jaw during mastication.

Horses chew in a modified circular motion. Most of the movement is side-to-side, but there is significant front-to-back movement, too. Severe malocclusions will limit or prevent this front-to-back movement, which only perpetuates the malocclusion.

If the practitioner can restore some of that normal range of motion, it's likely the horse will maintain the corrections.

Wave mouth is one of the most common malocclusions in the horse and can be one of the most challenging to correct properly because it involves the entire arcade.

Although the teeth meet in an abnormal pattern, each of them does contact an opposing tooth. This is why the horse might not show any outward signs of dental problems. At this point, even with a severe malocclusion, he is still able to masticate his food and maintain or even gain body weight (due to the compensatory ability of the system I spoke of).

Remember, we want to correct this malocclusion as early as possible so that the masticatory system does not fail prematurely.

Even with a severe wave mouth, this horse's teeth still grind food because of their contact along the grinding surfaces. When I plan correction of this particular wave, even in a stepwise manner, I know the horse is going to lose some of that contact for a period of time after the correction. There is no way to restore 100% contact from front to back and still make a significant correction of the wave.

I explain this to clients as a "short-term loss for a long-term gain." The art of successful equine dentistry lies in knowing exactly how to make those corrections and how much and where the horse can tolerate some loss of occlusion while still moving toward the future goal of complete correction.

Sometimes a lot of fine tuning is necessary, and the veterinarian repeatedly must check the occlusion with the mouth closed, watching as he or she moves the teeth across each other to simulate mastication.

There is no "instruction manual" for these types of corrections. A practitioner gains expertise in this area only after a significant amount of extra education and a hefty caseload over several years. In other words, practice makes perfect (almost).

No two cases are exactly alike in presentation or underlying cause. But if a skilled practitioner is armed with a very clear understanding of the anatomy and biomechanics of the equine masticatory apparatus, he or she can apply basic concepts to individual circumstances, then formulate and execute a treatment plan for each patient. Successful treatment results in a horse that wears his teeth in a normal or as near a normal manner as possible, maintaining functional teeth well into old age.

All Teeth Need Attention

This installment has focused on cheek teeth corrections, but you should understand that the incisor teeth are an integral part of the horse's masticatory system. An imbalance in the incisor arcades usually results in improperly functioning cheek teeth. Every exam should include an evaluation of incisor health and function.

The same principles of correction apply. Caution must be exercised so as not to damage individual teeth. Severe incisor imbalances should be corrected in stages over time. Proper incisor corrections depend upon an intimate familiarity with the interplay between incisors and cheek teeth function. Small changes in the incisor arcades can result in dramatic changes farther back in the mouth. Again, the expertise of the practitioner is of paramount importance.

Tools of the Trade

A final note on instrumentation: I intentionally have avoided a discussion of dental instruments. There are many different types of dental instruments available to equine practitioners. The particular type of tool a given practitioner uses is not of much consequence; what matters is how well the practitioner knows his or her instrument and its limitations and strengths, and how talented he or she is in manipulating that instrument.

The old adage, "there is more than one way to skin a cat" applies here.

Most veterinarians choose motorized instruments of some kind because they afford the experienced practitioner greater precision, greater speed, less soft tissue trauma, and less strain to both the operator and the patient. Precise corrections in a fraction of the time mean the horse is under sedation and his mouth has to be open for less time.

Motorized instruments are powerful and have the ability to remove large amounts of tooth very quickly. This is a blessing and a curse. For this reason, motorized tools should only be used in qualified hands.

An equine dental practitioner should have thorough knowledge of equine medicine, dental anatomy, and biomechanics. The type of tool is not nearly as important as the expertise and knowledge of the practitioner holding it.

About the Author

Mary DeLorey, DVM

Mary S. DeLorey, DVM earned her veterinary degree from University of Missouri in 1992. Since 2000, she has devoted her entire professional energies to equine dentistry. Her practice, Northwest Equine Dentistry, Inc. serves the states of Washington and Idaho and is based near Seattle. Dr. DeLorey has traveled internationally to instruct veterinarians in equine dentistry techniques and speaks to horse owners nationwide. She trail rides and raises sport ponies from her ranch in Eastern Washington when she's not on the road.

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