Equine malocclusions: Are you overfiling?

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Horses are nonruminant herbivores, so their skull anatomy and tooth structure allows them to grasp and chew fresh grass and dry roughage, and to grind grain and pelleted roughage.

Horses' teeth are hypsodont, meaning they erupt continuously. They're numbered by the Triadan system (100-400) and are sectioned into four quadrants. Each quadrant has three incisors, typically four canines, three premolars (premolars 2-4), three molars and occasionally a first premolar (wolf tooth).

The three premolars (2-4) are “molarized” in form and-combined with the three molars-are referred to as the cheek teeth. The cheek teeth of each mandible and maxilla are the primary teeth of mastication, while the incisors are used mostly for the prehension of food. The canine and wolf teeth have less significant function to the modern-day horse.

Horses chew in an oval or figure-eight motion, with the lower jaw grinding against the upper jaw. Proper occlusion of the teeth, especially the cheek teeth, is essential for adequate mastication and digestion of feed.

Which malocclusions are which?

Equine malocclusions are classified into four categories-class 1 to 4-plus class 0, which is normal occlusion.

• Class 1 is neutrocclusion, with normal jaw length of both the maxilla and the mandible but with the teeth crowded and displaced in a mesial, distal, buccal, lingual or palatal orientation.

• Class 2 is distoclusion, with either a short mandible or long maxilla.

• Class 3 is mesioclusion, with either a long mandible or short maxilla.

• Class 4 is mesiodistoclusion, with one jaw in mesioclusion and the other in distoclusion.1

This article will discuss treatment of class 1 malocclusions.

Before you start, know your bone

Odontoplasty, also known as floating or filing of the teeth, is the removal of tooth structure or adjustment of the tooth contours. But before treating any teeth, equine veterinarians need to carefully consider the surrounding bone structure, says Edward Earley, DVM, DACVD (equine), a lecturer at Cornell University School of Veterinary Medicine and practitioner at Laurel Highland Veterinary Clinic in Williamsport, Pennsylvania.

“There's a reason why the cheek teeth may be at a different angle,” Dr. Earley suggests. “I try to take that into account as opposed to coming up with a preconceived idea of what the angle should be, and then just needlessly filing the teeth to achieve that angle.”

Asymmetry can trip you up, he says, because most veterinarians want dental symmetry when they see skulls, mandibles and maxillae that are asymmetrical.

“Basically, the teeth are moving within the bone to position themselves for the best angle for the anatomy of the asymmetric skull,” he says. “When imaging asymmetric skulls with computed tomography, the asymmetry can be noted to extend into the hard palate and associated bone surrounding the cheek teeth.”

For example, one side of the palate may be shorter and more angled than the opposite side, so the teeth are just following that asymmetry and maxillae.

It's more important than ever before, Dr. Earley says, to preserve teeth in these situations. In the mid-1980s the average horse lived into its mid-20s, while now it lives into the low 30s. And it's important to consider the amount of reserve crown that remains in a tooth. Initially, as the tooth grows, it elongates, then in time erupts out. By design, with normal wear of 2 to 3 mm per year, a cheek tooth will be functional for about 25 years.

“We should be trying to extend the life of the tooth,” Dr. Earley says. “If we file down more tooth than we need to every time, we're taking away time for that tooth to be a functional tooth.”

Your takeaway? Never treat without a specific diagnosis of periodontal, endodontic, restorative or orthodontic disease for a particular tooth, Dr. Earley says.

Sources of class 1 malocclusions

Class 1 malocclusions can result from a number of factors.

Skull asymmetry predisposes equine patients to occlusal abnormalities, with the teeth orienting themselves in the skull relative to the anatomy. Typically, these class 1 malocclusions involve the incisors, premolars or molars, and they may develop into class 4 malocclusions.1

In some of these cases the incisors become diagonal, and many practitioners think these teeth should be leveled, Dr. Earley says. But radiographs of these patients show that the asymmetry originates in the maxilla or the mandible. The teeth don't wear evenly because the incisor bone has shifted and the incisor arcade is longer on one side.

“Many practitioners are looking at these asymmetric incisors and leveling them but not looking at the reason why they're angled,” Dr. Earley says.

Horses may be trying to align their chewing teeth (premolars and molars, or cheek teeth) for function, and if the front of the skull is a little crooked, the incisors won't wear evenly.

Corrective odontoplasty should be mild and focus on the premolars and molars, Dr. Earley says, so the occlusion can allow for functional mastication.

Developmental maleruptions may occur when there's a change in chewing forces due to missing or overlong teeth.1 For example, Dr. Earley notes, miniature horses tend to have rotated incisors. As the teeth erupt they can be crooked or rotating, but if they aren't causing a problem, they don't need to be addressed.

If the incisors begin to cause malocclusion or block the eruption pathway of another tooth, then periodontal, endodontic or extraction treatment may be needed.

Tooth elongation refers to a tooth that is taller than the neighboring teeth in the same or contralateral arcade.1 The bone and jaw shift due to external forces that affect the form and function of bone, tooth and surrounding soft tissue, Dr. Earley explains.

In a horse's mouth, the bone and tooth try to respond to uneven forces on their own, he says. Try to correct it and you'll ruin the “wave pattern” or equilibration.

“We've looked at the occurrence of wave patterns within our own practice and have found that approximately 60 to 70 percent of these horses have a wave pattern on their arcades,” Dr. Earley says. “We suspect a wave could be viewed as a normal variant related to the staging of eruption as the cheek teeth develop and mature.”

If a tooth is long and causing a diastema to the opposite arcade, then you can make a diagnosis, Dr. Earley says. However, complete leveling of the tooth isn't indicated. If it's reduced by just 1 to 3 mm, the tooth will be taken out of occlusion and the pathology addressed.

“Typically, the tall or overlong tooth is the healthy tooth, with the pathology on the opposing arcade,” Dr. Earley says. Possible causes include excessive attrition, a missing tooth, a wedging or diastema effect or periodontal disease. One study found that if a practitioner files a tall tooth to the level of the other teeth, there could be a 58 percent chance of exposing the pulp on that tooth.2

Go easy, Dr. Earley says: “With just 1 to 3 mm of tooth reduction, the tooth will be taken out of occlusion, eliminating the direct effect of the overlong tooth.”

Missing dentition-resulting from extractions, injuries, broken teeth or dental agenesis-may occur in the incisor arch or the cheek teeth arcade.1

When floating to correct for missing teeth, veterinarians should evaluate for pathology on a tooth-by-tooth basis and specifically correct any individual tooth's problem related to that diagnosis.

“With missing cheek teeth, the remaining teeth will attempt to move together and function as a single arcade unit,” Dr. Earley says.

For that movement to occur, the caudal teeth tip forward initially, with radicular and translation tooth movement following and the rest moving forward within the arcade.

“Tooth tipping is the easiest type of orthodontic movement,” Dr. Earley says, “and the staged movement will affect how the overlong teeth are reduced.” Initially, the overlong tooth will be directly opposed to the missing tooth, and over time as the arcade shortens, the opposing overlong teeth will be at the rostral and caudal aspect of the arcade.

Dr. Earley's example is the maxillary first molar (09). Remove the tooth, and during the next one or two years, the opposing mandibular first molar (09) probably won't wear. That could cause problems if it continues to erupt in that space, so Dr. Earley suggests floating the mandibular first molar a few millimeters down-not level with the other mandibular cheek teeth, but enough to keep it from causing problems.

“Due to that procedure, the remaining maxillary second and third molars (10 and 11) will shift or tip forward and try to close that gap,” Dr. Earley says. “When that happens, the entire maxillary arcade is shorter, and then you'll start looking for a little elongation in the opposing mandibular third molar (11) and possibly the opposing mandibular second premolar (06). The reason is that the maxillary arcade is now shorter.”

After that, Dr. Earley recommends bringing the affected tooth down just 1 mm and being careful not to overfile.

Enamel points, transverse ridges and excessive cingula all have anatomical function for mastication. The enamel points should be sharp to help the horse masticate properly. The maxillary cheek teeth have prominent vertical ridges (cingula or styles) along the buccal aspect.1

“The cingula bulge along the teeth before the enamel point develops,” Dr. Earley says. “The cingula give the tooth structure, and the enamel point is that part of the cingulum that didn't wear. That cross-section of the cingula gives more strength and support to the cheek teeth and more cross-sectional area. They are then able to withstand the forces of mastication.”

The other important concept for veterinarians to understand is that sharp enamel points on the teeth aren't necessarily a bad thing. In fact, they serve an important purpose, according to Dr. Earley.

“One's first instinct is to want to file those points down,” he says, “but if one were to do that, it would be more difficult for those animals to graze and chew natural roughages, making eating more difficult and decreasing food intake.”

If the points cause pain for the animal, dig into a cheek or the tongue, or cause a sore or callus, they might need to be slightly reduced.

“But in general, I like transverse ridges and enamel points,” says Dr. Earley.

Transverse ridges are linear cusps that run along the occlusal surface in a buccal-to-palatal/lingual direction. The transverse ridges allow for a greater occlusal surface area for mastication. The combination of the enamel point, cingulum and transverse ridge enables an enhanced tearing and shearing function with roughage and forage and an improved mastication process.

“The only time a transverse ridge would be too tall is when it's wedging two teeth below or above and trying to split them apart,” Dr. Earley says. “Most of the time, a transverse ridge is a good thing. Unless it's causing pathology of the above or below tooth, it's normal and should be there.”

Bottom line? Dr. Earley cautions against filing cingula down along the sides of teeth, because aggressive odontoplasty of cingula, enamel points and transverse ridges weakens dentition and reduces functional mastication.

“An orthodontic examination should entail an accurate evaluation of how masticatory forces are affecting class 1 malocclusion,” Dr. Earley says. “Occlusal forces can be reduced or eliminated with minimal focal odontoplasty of the involved teeth. A diagnosis should be made on a tooth-by-tooth basis so that a treatment of precise focal odontoplasty can be developed and implemented.”

References:

1. Earley, ET. 2016. How to diagnose class I malocclusions in the horse, in Proceedings. AAEP Annual Convention & Trade Show 2016;55.

2. Marshall R, Shaw DJ, Dixon PM. A study of sub-occlusal secondary dentine thickness in overgrown cheek teeth. Vet J 2012;193(1):53-57.

Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.