People have been warning others not to look a gift horse in the mouth since the 1500s, and maybe even earlier. The adage was borne out of the fact that horses’ teeth change continuously over time and are often used to approximate their age. So, looking at a gift horse’s teeth is, essentially, indicative of distrust toward the giver.
But at the 2016 American Association of Equine Practitioners’ Convention, held Dec. 3-7 in Orlando, Florida, Bruce Whittle, DVM, encouraged attendees to look inside horses’ mouths, as there’s substantial information hiding there. Not only can a veterinarian estimate the animal’s age, as we’ve already ascertained, but these important structures can also help guide treatment or even become injured or diseased themselves, he said.
Whittle, a practitioner at Honey Creek Veterinary Hospital, in Trenton, Missouri, reviewed basic dental anatomy and function with attendees during the convention.
The equine tooth—like that of other animal and human species—is comprised of four layers: pulp, dentin, enamel, and cementum.
The innermost layer, the pulp is “basically the heart and brain of the tooth,” Whittle said. It contains vital structures including nerves, the blood supply, lymphatics (which play a part in immune responses), and odontoblasts (cells that produce dentin).
This important, but soft, structure is protected by the outer layers. Pulp damage can result in tooth disease or death, but it can also take years to become clinically apparent, he said.
The average horse has 140 or more pulp horns, ranging from one to six or seven in each tooth, depending on the type of tooth. The challenge, Whittle said, is that it’s almost impossible to tell where in a tooth the pulp is located; it can be as close as 1 millimeter under the chewing surface, or up to 36 millimeters below it. As such, it’s crucial for veterinarians to use extreme caution when floating teeth. Not only can they inadvertently directly damage the pulp with the floating device but they can also damage it indirectly.
“Thermal damage is a concern,” he said. “Only 20 seconds of grinding can increase the tooth temperature by 4.6 degrees Celsius (40 degrees Fahrenheit), and it takes 5 to 10 minutes to cool down” to normal.
Veterinarians can image pulp using radiographs (this is most useful for incisors, he said, which are located at the front of the mouth) and CT or MRI, although it’s uncommon to place a horse under general anesthesia to image their heads for dental issues, he added.
The next layer is dentin, which makes up the bulk of the tooth, Whittle said.
“Dentin is more mineralized (about 70%) than cementum (65%) but less than enamel (96-98%),” he said. “Dentin, therefore, wears more quickly than enamel, contributing to the ‘rough’ occlusal surface important for grinding fibrous feed material.”
There are four subtypes of dentin—primary, regular secondary, irregular secondary, and tertiary—all of which aid in protecting the pulp.
Enamel is the tooth’s hardest substance and forms the ridges on the horse’s rough occlusal (or chewing) surface. However, enamel doesn’t have the capacity to heal itself if it’s damaged like the other tissues in the tooth can—once it’s gone, it’s gone. As such, it’s located between the dentin and cementum to stay protected from damage as much as possible.
Finally, cementum is the outermost layer of the tooth. Whittle said it’s a tissue similar to bone and serves as an attachment between the tooth and the periodontal ligament, which secures the tooth in the socket (or alveolar bone) and provides support as the horse chews.
Cementum fills the infundibulum (a crescent-shaped depression in a tooth’s crown) formed by the enamel on the occlusal surface, but Whittle said it might not be filled completely in the incisors and upper cheek teeth. In fact, one study revealed that only 11% of infundibula were completely cementum-filled, he said.
Whittle said horses have a hypsodont tooth structure, meaning they continue to erupt throughout the horse’s lifetime; the only teeth that do not continue erupting are the first premolars, or the wolf teeth, he said. Horses wear their teeth down about 2.5 millimeters per year, on average, he said.
Young horses develop and erupt 24 deciduous (or baby) teeth, including the incisors and premolars. Starting at about 1 year of age and continuing approximately through age 5, horses sequentially erupt 36 to 44 permanent teeth.
Whittle cautioned that if horses’ baby incisors don’t fall out at the right time, their permanent teeth can grow in behind them and cause dental issues. Additionally, he said, the order in which horses’ cheek teeth erupt can lead to malocclusion (abnormal contact between opposing teeth). For example, the last cheek tooth to erupt—the fourth premolar—must grow in straight between two other permanent cheek teeth. Any deviation from this could lead to misaligned teeth and associated problems that require frequent care.
Finally, Whittle reviewed how horses use their teeth during chewing and how their jaw anatomy impacts that movement.
The horse’s lower jaw (the mandible) is narrower than his upper jaw (the maxilla), and the lower teeth are positioned slightly to the inside of the upper teeth. As the horse chews in a triangular pattern, the outside edges of the upper teeth and the insides edges of the lower teeth can develop sharp points. And, because there’s little extra room in a horse’s closed mouth once the tongue and the palate (the top of the mouth) are considered, these sharp points have the potential to damage soft structures they contact. As such, routine dental exams and treatment are crucial to keep horses’ mouths healthy.
Whittle wrapped up by highlighting a few important points for attendees to remember. Teeth are living structures, he said, and the pulpar depth beneath the chewing surface varies from horse to horse and tooth to tooth. Because the pulp is vulnerable to damage, it’s important to use caution when floating teeth. On that note, he said, when floating, be sure to leave enough rough surface on the cheek teeth so horses can chew properly. Finally, evaluate young horses’ teeth regularly, catch malocclusions early, and administer treatment to reduce the likelihood of ongoing problems.