Incisor Diagonal ("Slant mouth"): What does it mean?

Let's talk about incisor diagonal bite, which is a slant in the incisor alignment. I find this fairly often in a new patient's mouth, and I know that I can correct it and the horse's body will improve.

Unlike the cheek teeth, owners can easily see the incisors by lifting their horse's lips. However, the majority of my clients seem to be unaware of the situation until I point it out. So LIFT YOUR HORSE'S LIPS and take a look.

Here is a series of diagonals that I saw just in one month. Note that most of them (66% upon my review of records over several years) have a slant to the left side of the horse. These are associated with a shift in the lower jaw to the side of the slant, so that the central incisors are misaligned. Consequently, the upper incisors overgrow on the opposite side, blocking the forward jaw motion. The situation will continue to worsen over time.

A shift in the jaw will put tension on the hyoid apparatus (connected to the tongue), causing downstream effects in the poll, sternum, lower neck, diaphragm, lumbar spine, pelvis/sacrum, stifle, and basically everything in the body. Horses with a DGL-3 are typically restricted in a left sidebend at the poll, and they weight the left front foot more heavily. This creates a larger, lower foot on the left front and a more contracted foot with a higher heel on the right front.

Occasionally, a diagonal bite can be caused by a primary dental issue (e.g., fractured tooth) leading to the horse chewing only on one side. However, more of the milder diagonals are likely caused by body asymmetry leading to tension patterns on the TMJ. This explains why there is a higher prevalence of left slants, as most horses bend easier to the left for a variety of reasons.

Despite the inciting cause, leaving a diagonal bite will make it challenging, or impossible, to straighten the horse through bodywork or classical training. Many horses receive monthly chiropractic, acupuncture, massage therapy, etc., but it never truly resolves their issues. Few people seem to realize that correcting the structural issues in the mouth (alone) will get the horse further than adjusting them over and over again.

Even more challenging is that most of these horses (all of the horses with incisors pictured here) have had dental care regularly by otherwise competent veterinarians. Therefore, dental issues have been "ruled out" as a cause of their body or behavioral issues. There is an irrational fear among some veterinarians about working on the incisors; therefore, most just do not touch the incisors unless some pathology is present. I learned almost nothing about balancing incisors from conventional veterinary continuing education.

Reductions in diagonal bites can be done safely by monitoring the color of the secondary dentin overlying the incisor pulps. Incisors should never be cut down, but rather a disk can be used to make micro-adjustments. Millimeters matter here, and the correct incisor angle must be maintained. Incisor work is an art, as well as a science, and is an important part of the standards for International Association of Equine Dentistry (IAED)-certified practitioners.