Introduction

The anatomy of the temporomandibular joint (TMJ) is similar between humans and horses in that it is comprised of 2 independent joint compartments separated by a biconcave intra-articular disk.^1,2 The latter, along with the articular surfaces of the joint, is fibrocartilaginous and adapts to masticatory loading over time.^3–5

Pain or discomfort associated with the TMJ is extremely common in humans.⁶ It is associated with a wide array of clinical signs and symptoms. Interestingly, there are several reports detailing a correlation between dental and TMJ abnormalities and balance.^7,8 TMJ abnormalities in the horse have been reported to occur across a wide range of ages. Osteochondrosis-like abnormalities have been described in a young filly,⁹ but the majority of published cases are those associated with wounds, sepsis, or end-stage disease in older animals.^10–13 In contrast, some TMJ abnormalities can be nebulous and only be appreciated in certain situations, including a change in behavior or under saddle, or the development of stereotypies.^14,15 In addition, it has been found that TMJ inflammation may only be appreciated by the astute rider as a change in bit contact when riding.^16–18 Despite these recent publications, the prevalence of equine TMJ problems is considered to be low. Attributing the TMJ to having any role in poor performance should, therefore, be a process of exclusion, not inclusion. Other more common problems such as lameness and cardiovascular, respiratory, and dental disease should be considered, and eliminated, before the TMJ is cited as the sole, or major, cause of the problem.^19,20

History

A detailed history, including the result of any previous veterinary examination or treatment, is often critical to understanding the nature of the problem. TMJ disorders in the horse (other than obvious swelling or wounds) are not usually associated with any specific clinical finding or history. Some horses are reported to resent opening their mouth wide (noted specifically when a speculum is used); they might have a clicking sound when chewing or have changed the way that they chew. Others may develop stereotypies such as frequent yawning, lip smacking, or resting with the tongue protruded from the mouth. However, the majority have nothing to suggest TMJ involvement. As such, open-ended questions designed to understand exactly how and when the abnormal behavior (or performance) manifests are of paramount importance. The conditions of its occurrence and the client’s varied attempts to ameliorate it may guide the veterinarian toward or away from the TMJ as the source of the problem. Questions around feed type and whether there has been any change in the way the horse prehends and chews its feed are important. Information on how the horse responds to bit placement, how it feels in hand, and its response to collection, gait, and lead changes are invaluable. Similarly, the results of changing bits and the use of bitless bridles—if safe and appropriate—are useful. Carefully phrased, empathetic, personal questions regarding the rider’s own physical health and dominant hand are also included. There have been multiple cases in which the rider, through myriad physical ailments (such as osteoarthritis or anterior cruciate ligament problems), have changed the way in which they ride their horses. This results in problems that may be incorrectly attributed to the horse. Ultimately, when the rider is appropriately treated the horses no longer manifest any of the performance issues for which veterinary attention was sought.

Clinically important TMJ pain in horses tends to occur in combination with other performance-limiting problems, many of which are substantially more common (eg, osteoarthritis of the tarsometatarsal and distal intertarsal joints). In the end, all of these may require identification and sequential treatment to return the patient to a useful performance level.

Distant Examination, Palpation, and Lameness Examination

TMJ disease may present as local swelling or pain. Similarly, the response of TMJ pain in this region may be muscle development or pain associated with other regions of the body such as the neck or back. Primary neck or back pain can manifest in similar presenting signs and so needs to be excluded.

The head is examined from both sides and from directly in front of the animal, paying attention to symmetry and the relative protuberance of boney landmarks. It is important to note that a particularly prominent TMJ may be due to joint effusion, periarticular soft tissue swelling, bone formation (osteophytosis), or simply from a loss of masseter muscle mass. Palpation of the TMJ and surrounding area may help determine which of these is the cause, but further information may be gained from diagnostic imaging. It is advisable to be cautious interpreting an adverse response (avoidance behavior) when the joints are palpated. An adverse response is simply that—an adverse response. It may mean everything or nothing. In humans, intra-articular problems of the TMJ are only 1 facet of the disease, and one can have peri-articular pain in the absence of intra-articular disease.²¹ The author has found periarticular TMJ pain associated with dental pathology, but also in horses with forelimb lameness. At times, avoidance behavior in the face of TMJ palpation can be found in combination with tight or hypertrophied “strap muscles” or poll and nuchal ligament pain in the region of the insertion on the occiput and associated bursa. In the former case, there may be a history of fighting the bit. This chronic, head and neck carriage can promote abnormal neck muscle development with or without associated back pain.

The poll and cervical vertebrae are palpated by standing either to the side of the horse or under the neck immediately in front of the chest. The author prefers the latter position and reaches both hands around the neck, pulling down gently on the midline of the neck, as well as compressing the articular facets toward the midline while closely watching the horse for evidence of discomfort.²² This may manifest as overt withdrawal from the pressure, but at times will only be noticed as a slight change in facial expression or flicking of the ear. Should any of these be noted, radiographic examination of the cervical spine may be prudent, especially if there is any evidence of a neurological deficit apparent during the rest of the evaluation. A number of horses have been presented for problems related to collection, some of which have continued even when the tack is changed to a bitless-type bridle. The horses are presented for suspected TMJ disease, but in fact have advanced cervical arthritis that is painful when the rider collects the horse. Radiographic examination of the neck in these cases may lead to a diagnosis and exonerate the TMJ from having any role in the problem.

The epaxial muscles of the thoracic and lumbar regions are palpated, noting any hypertrophy, localized swelling, or spasms associated with the pressure. Particular attention is paid to whether the pain is uni- or bilateral, where (anatomically) the pain is located, and whether the muscle spasms continue after pressure is removed. Finally, the sacral region, sternebrae, and limbs are palpated. A complete lameness examination with flexion tests (divided into upper and lower limbs), local analgesia, and subsequent radiographic assessment usually follows the palpation phase of the examination. It has been the author’s observation that in his clinical population of horses, bilateral back pain is more commonly secondary to an underlying hind limb lameness. In these horses, the worst back pain is typically contralateral to the lamest limb. One caveat to the previous statement is the presence of impinging dorsal spinous processes (IDSPs; ie, “kissing spines”). These present with bilateral generalized epaxial muscle pain with or without point pain located directly over the dorsal spinous processes. This specific pathology may be present in addition to a limb-associated lameness or identified in isolation. A clinical suspicion of IDSP-related pain needs to be confirmed radiographically and investigated further with local interspinous ligament treatment to determine the clinical significance of the radiographic finding. Seemingly unrelated, IDSP and TMJ disease are mentioned together because a high head carriage associated with some forms of bit avoidance (such as may be seen with TMJ pain) may result in back extension and exacerbation of the impingement of the dorsal spinous processes of the thoracic and lumbar vertebrae and its associated pain.

If necessary, inertial sensors, diagnostic analgesia, and nuclear scintigraphy can be used to determine the seat of the lameness, but this avenue typically means that the TMJ is not the primary cause of the problem. It is also important to note that strange lameness problems may benefit from a complete neurological examination.

Physical Examination

If, after obtaining the history, the author feels that the performance problem is more likely not associated with an underlying lameness issue, a complete physical examination is undertaken. Particular attention is paid to the cardiovascular and respiratory systems. Atrial fibrillation, exercise-induced pulmonary hemorrhage (EIPH), and asthma are relatively common diagnoses in some sport horse disciplines.²³ These problems may occur in combination with lameness and result in poor performance or behavioral issues that can mimic those seen in horses with TMJ issues. As such, it is important not to stop the examination protocols once a singular problem has been identified. Ultimately, 1 issue will likely be the rate-limiting one, but a complete understanding of the entire horse needs to be obtained before this can be determined.

Endoscopic Examination of the Airways and Temporohyoid Articulation

TMJ problems can result in nonspecific behavior changes. Some of these may manifest as subtle alterations in the normal mentation. Relaxed horses may become nervous and in some cases be reluctant to enter arenas. Clients whose horses have developed an upper or lower airway abnormality (such as dorsal displacement of the soft palate, left laryngeal hemiplegia, or EIPH) have reported similar changes. To rule these other possibilities out, depending on the previous findings, an endoscopic evaluation of the upper and lower respiratory tracts should be performed. If possible, the upper airway examination should be performed without sedation, as it negatively affects the neuromuscular control of the soft palate and movement of the arytenoid cartilages of the larynx. A flexible endoscope is used to evaluate all 3 meatuses, the ethmoids, and the nasomaxillary opening of each nostril. Each guttural pouch is entered, the temporohyoid articulation is visually inspected (as described later in this narrative review), and the presence or absence of fluid accumulation is noted. The nasopharynx, larynx, and trachea are similarly evaluated with or without performing a tracheal wash or bronchoalveolar lavage (if EIPH is suspected).

Oral Examination

Equine TMJ disease can present as problems prehending or masticating feed, quidding, clicking while chewing, or changes in the way the animal uses its tongue. In these cases, horses are usually fed before sedation such that the abnormalities might be re-created. Myriad oral and dental problems can present with the same clinical signs and are more common, so they need to be excluded before the TMJ is considered.

The incisors, the interdental space (“bars of the mouth”), and the symmetry of the head are examined before placement of the speculum. Attention is paid to any evidence of equine odontoclastic tooth resorption or facial nerve paresis (muzzle deviation, differences in palpebral fissure size, drooping ear, etc). The presence of the former condition may explain why a horse might resist a full-mouth speculum being opened, while the latter can be suggestive of temporohyoid osteoarthritis, which may occur alone or in combination with TMJ osteoarthritis.¹³ At absolute minimum, a good light source, dental mirror, and full-mouth speculum (eg, Haussman, modified-Guenther, or Conrad) should be used to examine the oral cavity. Use of an intraoral endoscope with image storage capabilities is becoming more widespread and is preferable to a dental mirror when used correctly.²⁴ Fractures of the clinical crown of cheek teeth, open pulp chambers, diastemata, and intraoral foreign bodies can all present with problems masticating. The author has also had 1 case of a jumping horse that did not have any (apparent) chewing problems but, when a bit was placed into the mouth, became dangerous (rearing and bucking). TMJ disease was suspected, but an oral examination performed under heavy sedation revealed the presence of a small stick that had become lodged across the hard palate between the 110 and 210 teeth. This had cut into the gingiva on the palatal aspects of the teeth and had to be cut in midline before extraction.

A study²⁵ examining the oral examination findings and CT appearance of the equine TMJ found that animals with a TMJ abnormality were more likely to have infundibular disease and less likely to have periodontal disease than those with no TMJ abnormality. However, the authors noted that these oral examination findings may have preceded the TMJ changes and questioned the biological relevance of the statistical link. A disparity between the occlusal angles of the cheek teeth on opposite sides of the mouth that may, at times, tend toward a shear mouth has been reported to be associated with abnormal mastication due to pain. However, this finding is not pathognomonic of oral, dental, or TMJ pain.

Compilation of Clinical Findings

At this point in the evaluation, it is likely that multiple anomalies have been identified. The preference is to address each of them individually in a sequential fashion, focusing on those that are the most performance limiting. Only then would the author consider the TMJ. There are times when problems that may be interrelated are treated at the same time. An example would be osteoarthritis of the tarsometatarsal and distal intertarsal joints, sacro-iliac pain and muscular discomfort of the back. Treating all these problems at the same time makes it difficult to determine the primary problem and may result in potentially unnecessary treatments in the future (eg, hock osteoarthritis causing secondary sacro-iliac and back pain).

Having excluded common causes of poor performance, or treating them, an attempt is frequently made to determine whether the issue is associated with pain or behavior. A 2- to 3-week trial using NSAIDs may help to answer this question. The author suggests that owners make a video of the horse walking, trotting, and doing its normal job at the start of the program and then repeat this video assessment in the middle and end of the treatment period. A nonbiased analysis of the videos is then made by the author in discussion with the owner.

Focusing on the TMJ

If, at this point in the management of the horse and the client, TMJ pain is still a possibility, then further investigation is necessary. If the performance issue is not repeatable or the horse is resistant to the use of local analgesia, a nuclear scintigraphy examination may be necessary. Using this methodology in the face of recalcitrant back pain, the author has found thoracic facet osteoarthritis in a horse, which the client was adamant had TMJ problems based on its behavior under saddle (unpublished data, 2018). In some horses the performance issue is repeatable, and in these cases the first step towards localizing the problem to the TMJ is usually a baseline assessment (which may include being ridden by a non-biased individual such as a test rider), before injection of local analgesia (mepivicaine or bupivacaine) into the most accessible joint compartment (the discotemporal (DTJ) joint).^17,26 Ultrasound guidance is not usually necessary.²⁷ After an appropriate period (10 to 15 minutes), the assessment is repeated to determine whether there has been a change. The interpretation of the assessment after analgesia is not simple. If a change is noticed, then the joint block has been determined as positive. If no change is seen, then TMJ disease is not necessarily excluded. It may be that some pain, specifically that generated by increased intraosseous pressure, may not be ameliorated by injecting local analgesia into the joint space, a finding reported in other joints.²⁸

Specific radiographic projections have been reported for imaging the equine TMJ.^29–31 These specifically target the condylar process of the mandible and the joint space. It is important to remember that the targeted portion of the mandible is rounded, and thus some important pathology may be overlooked as a function of projection angle. That said, periarticular osteophytosis and some subchondral bone cysts can be identified and confirmed arthroscopically (Figures 1 and 2). Pathologies of the intra-articular disk may not be recognized radiographically in the absence of contrast enhancement.³² Ultrasound examination of the joint has been extensively documented.^33,34 Clinically, given the concave shape of the joint in the rostro-caudal and abaxial-axial directions, the use of ultrasound is limited to the external (lateral aspect) third of the joint.

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A rostral 45° ventral 30° lateral-caudodorsal lateral oblique (R45°V30°L-CdDLO) radiographic projection of the right temporomandibular joint of a horse. Note the presence of the osseous densities in the joint space (arrows) representing marginal osteophytosis.

Citation: Journal of the American Veterinary Medical Association 262, 3; 10.2460/javma.23.09.0513

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An arthroscopic view of the caudal pouch (CP) of the discotemporal joint showing the mature osteophytes (OP) and associated damage to the intra-articular disk (IAD).

Citation: Journal of the American Veterinary Medical Association 262, 3; 10.2460/javma.23.09.0513

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The most common advanced imaging technique used to investigate the equine TMJ is CT with or without contrast enhancement.^2,35,36 A whole-head CT with slices ≤ 1 mm is typically requested.³⁶ This approach allows the tongue, teeth, sinuses, hyoid apparatus (including the temporohyoid articulation), and TMJ to be assessed. Findings suggestive of clinically apparent TMJ disease have been the presence of marginal osteophytes, flattening of the condylar process of the mandible, and subchondral cysts with or without pericystic sclerosis. The ease of acquiring a standing CT, where available, is such that some veterinarians may elect to forgo an initial radiographic assessment in favor of a better modality. As MRI of the head becomes more available, it is likely that this modality will be preferred, as it gives exquisite soft tissue resolution.³⁷ It is important to note that these advanced imaging techniques should not be utilized without a reasonable index of suspicion that the problem lies within the head. Similar to what is reported in humans, many horses have CT abnormalities of their TMJs but have no reported clinical signs of disease attributable to that joint.³⁶ If advanced imaging is used as the sole diagnostic tool, then horse TMJs will be overtreated.

Management, Further Investigation, or Treatment

Once there is a high likelihood that the TMJ is the source of the problem, the next steps are dependent on severity, availability of treatment, owner goals, expectations and finances. Some veterinary-owner combinations want to pursue a treatment above chasing a diagnosis, while others want a final diagnosis and prognosis. There are also clients that are adamant that the TMJ is the seat of the problem regardless of the other problems. In these cases, the clients are counseled that their treatment choice may not result in resolution of their concerns, but the author will treat the TMJ. No other concurrent problem is addressed at this time, and the horse is re-checked in 3 to 4 weeks to determine the response to treatment.

The most common medications used in the TMJ for clinical disease in humans are corticosteroids, hyaluronic acid, and platelet-rich plasma (PRP).^38–42 Despite being used clinically, there is little experimental evidence upon which to base the practices. The use of biological therapies seems to be limited to PRP, as the author can find only 1 paper⁴³ on the use of autologous conditioned serum, which is a review paper postulating treatment. In horse TMJ cases that are treated by intra-articular injection, the medication chosen is usually a corticosteroid. The author prefers to use triamcinolone rather than methylprednisolone acetate given that the former has been shown, in some experimental conditions, to have fewer negative effects on the chondrocytes than the latter.⁴⁴ Additionally, methylprednisolone acetate has been shown to result in periarticular dystrophic mineralization of the soft tissues. There are no reports in the equine literature of using autologous conditioned plasma (eg, PRP) or autologous conditioned serum (IRAP) in definitively diagnosed TMJ arthritis. Autologous protein solution (Pro-Stride; Zoetis; nSTRIDE; Zimmer Biomet) has been used by the author in 2 cases, one of which was published.¹⁷ In both cases, this treatment was instigated after extensive investigation, which included local analgesia, radiography, CT, and arthroscopic examination.

It is important to understand that there are no data on the use of the common intra-articular medications in the TMJ using fibrocartilage models. It may be that the TMJ, being fundamentally different from other commonly injected joints (having hyaline cartilage), responds differently to treatment. Indeed, the author has anecdotally found that steroid-based intra-articular treatments of performance-limiting TMJ osteoarthritis may not be as effective and may have a shorter refractory period than typically seen in other osteoarthritic joints. The other important consideration is that steroids injected into the TMJs have the potential to reduce inflammation extra-articularly in a region that is heavily innervated and has other high- (atlanto-axial) and low-motion (temporohyoid) joints. An example of this effect was an erroneous interpretation of treatment success made by the author. A 21-year-old pony was presented for acute-onset dysmastication, which progressed through esophageal choke to anorexia over the course of 48 to 72 hours. A diagnostic workup failed to reveal the cause of the problem other than the fact that palpation in the region of and caudal to the left TMJ was strongly resented by the pony. No radiographs were taken (which was an oversight). The joint was injected, after appropriate aseptic skin preparation, with 10 mg of triamcinolone and 100 mg of gentamicin. Twenty-four hours later, the pony began to eat and thereafter returned to normal. Approximately a month after the injection, the pony suffered a violent bout of colic, for reasons other than TMJ disease, and was humanely euthanized. A postmortem examination of the head and neck region was approved by the client and revealed a pristine TMJ, with normal joint fluid, and no gross periarticular osteophytosis, synovitis, or cartilage defects. However, there was evidence of a fractured left paracondylar process with early callus formation. The supposition was that steroid injection into the left TMJ had diffused out of the joint and affected the tissues in the region, which included the paracondylar process in this pony.⁴⁵

In some cases, before or after the effect of treatment is assessed, the client wishes to pursue an absolute diagnosis to better understand the problem and hopefully improve the long-term prognosis. In these cases, diagnostic arthroscopy can be performed.^{17,32,46–49} This has corroborated computed tomographic findings showing significant cartilage degeneration and the presence of condylar cysts, but has also identified cartilage loss and eburnation of subchondral bone in horses where CT examination found no significant abnormalities - despite the horse having a positive, repeatable, response to the installation of local analgesia into the affected joint (Figure 3).

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A—An arthroscopic view of the right discotemporal joint in a horse with dysmastication and problems under saddle that improved dramatically with intra-articular analgesia but had unremarkable radiographic and CT examinations. Note the lack of appreciable articular cartilage on the retroarticular process (RAP), the mandibular fossa (MF), and the articular tubercle (AT). B—A closer view of the MF and AT in the same horse showing the lack of articular cartilage and exposed subchondral bone under the curette (C).

Citation: Journal of the American Veterinary Medical Association 262, 3; 10.2460/javma.23.09.0513

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In cases where osteoarthritis of the TMJ has advanced to the point that intra-articular medication will not, or will no longer, work, a mandibular condylectomy can be performed. Initially reported as a salvage procedure,⁵⁰ a recent case series described the return to work of most of their horses.¹³ Notably, these horses were involved in Western-type disciplines, which require little collection; however, the author is aware of several horses that have returned successfully to low-level jumping and dressage following this procedure.

In summary, equine TMJ abnormalities are uncommon causes of poor performance. A definitive diagnosis of TMJ disease should not be made without rigorous investigation and the exclusion of other potential causes. Indiscriminate treatment associated with diagnostic failure will inflate the significance of pathology in this region and artificially elevate the prevalence of disease.

Disclosures

The author has nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.

Funding

The author has nothing to disclose.