Temporomandibular joint (TMJ) abnormalities are a common problem in humans and domestic animals. In humans, it is an umbrella term encompassing both intra-and extra-articular conditions and accounts for significantly higher health care costs and sick leave in affected compared to non-affected individuals (1). There have been numerous human studies on the neuromuscular effects of intra-oral splints, and reports detailing the correlation between different TMJ disorders and vertigo in elderly patients (2–4). Dental occlusion and TMJ abnormalities have also been reported to affect the center of gravity, gaze stability, physical performance, and fitness (5).

The inter-relationship between the TMJ and the rest of the body is of particular interest in equine performance, primarily because pain effects equine performance, whether or not it results in overt lameness (6). In addition, the reins and bit, by which the rider mainly communicates with the horse, can be a source of oral discomfort that changes the way a horse moves (7–9). Horses which are not necessarily classically lame but move differently when under saddle or tacked are known colloquially as “rein-” or “bridle-lame”; in German, the term “Zügel-lahmheit” is used. At times, this change can only be felt by the rider and is not appreciated by external observers. The effect makes accurate diagnosis extremely difficult. There have been several recent reports of TMJ osteoarthritis causing lameness and poor performance in sport horses (10, 11). There has been no research into the effect of TMJ inflammation on equine performance and motion, despite the fact that the TMJ is the conduit through which controlling rein pressure is exerted via the bit in the mouth to the rest of the horse under saddle.

Kinematic analysis and the use of instrumented reins represent objective methods of quantifying sub-clinical poor performance when used in combination with other diagnostic modalities such as perineural nerve or joint analgesia (12, 13). Using these methods, a significant amount of research has been published on rider impacts on equitation (14–18), the roles of tack (saddles, bits, and bridles) (19–22), and disease [specifically osteoarthritis (OA)] on movement of the horse (23–25).

The objectives of this study were 2-fold. Firstly, to evaluate the effect of acute inflammation of the TMJ using a lipo-polysaccharide (LPS) model on rein tension when horses were cued using long-reins at walk and trot. Secondly, to determine the effect of this inflammatory process on the 3D kinematics of horse movement at the same gaits.

The hypotheses were that; TMJ inflammation would affect the rein inputs necessary to control horse movement; and that this inflammation would change the 3D-kinematics of movement.

There were no effects of injection order on peak rein forces at the walk (session 1: 5.6 N, session 2: 4.6 N, p = 0.335, η_p² = 0.23), while overall trot peak forces were slightly lower at the second injection time (session 1: 11.0 N, session 2: 8.5 N, p = 0.046, η_p² = 0.67). TMJ inflammation had no effect on average peak forces at the walk but there was a significant injection side × time interaction at the trot (p = 0.002, η_p² = 0.92) such that there was a decrease in peak force post-injection on the injected side (Figure 3). Asymmetry was significantly different after injection for both the walk (p = 0.043, η_p² = 0.68), and the trot (p = 0.001, η_p² = 0.96), with all horses showing a shift to reduce force (i.e., rein tension) on the inflamed side (Figure 4). There was no effect of injection order on asymmetry.

The only kinematic variable to show any significant change due to rein tension or TMJ inflammation during the walk or trot (Tables 2, 3) was forward head tilt which increased in the presence of rein tension in the trot (p = 0.042, η_p² = 0.68) after injection.

The results of our study confirmed the first hypothesis, namely that rein inputs necessary to control horse movement would be affected by TMJ inflammation. The second hypothesis, that TMJ inflammation would change the horses’ locomotion, was not supported by the kinematic data.

The inter-relationship between mandibular position, TMJ inflammation, and the rest of the body is a complete unknown in the horse and a controversial topic, at best, in human orthodontics. Early work (31) showed that 70% of patients with TMJ osteoarthritis had improvements in balance, coordination and, or ataxia, after mandibular manipulation and this has been supported by a number of other researchers (2–4, 32, 33). This is contradicted by other reports in which patients with temporomandibular joint disorders, or abnormal disk position, did not have abnormal posture or muscular pain (34, 35).

Interestingly, there appears to be a role of dental occlusion and TMJ inflammation in physical fitness and human performance, which remains valid even at the elite level of human athletics (5, 36, 37). This inter-relationship is of particular interest to investigators involved in assessing equine performance because pain effects equine performance regardless of whether it results in overt lameness (6). This pain may manifest only when horses are under saddle, or tacked (rein-or bridle-lameness) as described above.

The findings of the current randomized, controlled, blinded study support previously published clinical papers linking TMJ abnormalities to poor performance in sport horses (10, 11). It shows that, at a trot, when rein tension was applied, the tension required to maintain the horse in the optimal treadmill position actually increased on the non-injected side. This makes sense from a control standpoint. The horse was worked into the bit (as evidenced by the significant increase in head tilt angle with rein tension) but was avoiding pressure on the injected side. To keep the horse straight on the treadmill, increased tension on the non-injected side was required.

The lack of statistical difference seen kinematically could be a function of only using five horses. Depending on the variable examined, a post hoc power calculation revealed that up to 63 horses would have been necessary to have been confident in a lack of statistical difference between pre-and post-injection movement. Subjectively, horses which had trotted in a stable, controlled, manner before TMJ injection became “unstable” when being “worked into the bit” by the hander after injection. In two (2/5) horses, seconds were being counted down toward the end of the data collection minute, because there was a concern that the horse would uncontrollably exit the treadmill by stepping on one of the side-rails. Interestingly, this observation was not corroborated by the objective kinematic data. It is possible that the handler controlled the instability sufficiently through the added rein-tension on the non-injected side to negate any objective differences. Had the investigators used a surcingle and side-reins at a set length (resulting in the same pressure being applied to each side of the bit), the kinematic results may have been different because there would have been no corrective input to any avoidance behavior displayed toward the injected side while the horse was at trot.

Finally, this study only investigated the role of acute inflammation. As osteoarthritis develops, the inflammatory process becomes chronic. When combined with the other classical manifestations of this chronic disease process (osteophytosis, synovitis, fibrosis, reduced range of motion, etc.), it is possible that the response to rein-input would be different and a secondary classic lameness may become apparent, similar to that noted in human patients (31). While this may seem implausible, chronic TMJ-OA in a sport horse was recently reported to effect an underlying baseline lameness (11); so a link between the two has already been established.

Determining that TMJ inflammation is causing poor performance, or rider-appreciated asymmetric rein-contact, is not easy. When presented with a suspected case it is important to have a logical, step-wise, approach to the evaluation. A detailed history, including a good understanding of the specific equestrian discipline, is critical before assessing the horse. Distant, complete physical, and lameness examinations should be performed; and only after excluding common problems (such as cardiac or musculoskeletal disease), should an investigation into the role of the TMJ be instigated. Options for assessment include injection of local analgesia, or diagnostic treatment of the joint(s); upper airway endoscopy; specialized tangential radiographic projections; computed tomography and, or, arthroscopic assessment of the offending joint (38). Overall, our experience has been that clinically significant TMJ pain in horses tends to occur in combination with other inter-related problems. All of these may require identification in order to successfully treat the patient. Despite this, the TMJ should not be excluded when considering the myriad causes of poor performance in the horse.

This study shows, under rigorous scientific conditions, that TMJ inflammation in the horse can be a source of rein contact avoidance. It elevates the importance of this enigmatic joint from relative obscurity—like neck and back pain 30 years ago—into one of serious contention when investigating nebulous equine performance concerns. Ultimately, more work is necessary, with a greater number of horses, to gain a better understanding of the relationship between inflammation of the equine TMJ and performance.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The animal study was reviewed and approved by the Institutional Animal Research Ethics Board and adhered to the Canadian Council on Animal Care guidelines for humane animal use (20200038). Written informed consent was obtained from the owners for the participation of their animals in this study.

NR, JP, JL, MW, and JC contributed equally to study concept, design and execution of the trial, data interpretation, and manuscript preparation. All authors contributed to the article and approved the submitted version.

This research was supported by the Mark and Pat Dumont Equine Research Fund at the University of Saskatchewan. The funding source had no role in study design, data collection, analysis or interpretation, in the writing of the manuscript, or in the decision to submit the manuscript for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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