Equine Temporomandibular Disorders: Review of the Current Literature and Experiences of an Integrative Veterinary Dental Practitioner

Janelle S. Renschler, DVM, PhD, Dipl ACVP, cVMA, CVMMP, IAED/C

Equine temporomandibular joint (TMJ) disorders have historically been classified as “rare” by the academic community. Integrative/holistic veterinarians have had a much different view, although they were lacking scientific evidence to characterize temporomandibular disorders (TMD). This discrepancy has frustrated me, as a high percentage of my new equine dental patients have evidence of TMJ pain on myofascial and acupuncture exams. Furthermore, equine dental practitioners have long recognized performance benefits in horses following occlusal equilibration, and this could most readily be explained by improving TMJ function (and hence relieving whole body fascial tension patterns). Recently, I came across several new enlightening articles on equine TMD, and it seemed fitting to do a literature review of the topic.  

Where were we before?  

TMD has been called rare (Balducci 2021), and the impact on performance in sport horses was unknown or at least not part of mainstream discussion. Articles such as “TMJ Pathology: Is it Real?” were published only 11 years ago (Witte 2015). Literature searches would have led one to believe that osteoarthritis of the TMJ was a rare clinical phenomenon, and there was no evidence for TMJ myofascial pain or any impact on performance.  

n humans, however, TMD has been well documented with a prevalence of up to 30-50% worldwide (Alrizki 2023) with associated issues including arthralgia, myalgia, myofascial pain (+/- referral), degenerative joint disease, disk displacement, subluxation, and headache (Schiffman 2014). Integrative veterinarians have long recognized that humans, horses, and small animals have similar myofascial connections and transpositional acupuncture meridians; therefore, it is not a huge leap to project that animals might experience similar TMJ pain. Acupoints Stomach (ST)-7, Triple Heater (TH)-16 and –17, and Dental Point have been associated with dental and/or TMJ disorders in horses (May 2008, Xie and Preast 2007), and Coneglian et al. (2023) showed a decrease in pain reaction at these points following dental treatment. These points have been used by holistic veterinarians as diagnostic indicators of TMJ myofascial pain. 

The difficulty in characterizing TMD in horses has likely been related to challenges in imaging the joint, the lack of training to recognize myofascial pain, and the complicated interrelationship of the TMJ with the rest of the stomatognathic system and distant myofascia.  

Latest systematic review article on equine TMD 

Jasinski et al. (2024) published a systematic review article to compile clinical data and research on equine TMD. Only 20 case reports were identified since 1999, including 9 cases of septic arthritis, 5 cases of naturally occurring osteoarthritis (OA; not trauma-related), and 6 other non-OA causes including dentigerous cyst, fractures, luxation, and neoplasia. The OA cases included 1 horse with a cyst and 1 horse with an osteochondrosis-like lesion (both resulting in euthanasia), and 3 other cases (1 euthanized after intra-articular medication, 1 treated with intra-articular medication, and 1 treated with a mandibular condylectomy). All cases that had a dental exam (3/5) had cheek teeth overgrowth. Clinical signs for these OA cases included noises during mastication, tongue protrusion, performance issues, TMJ swelling, head shaking, yawning when bridled, quidding, and inability to open the mouth.  

This systematic review also identified research studies involving TMJ OA. Guerrero Cota et al. (2018) evaluated cadaver heads from horses euthanized for unrelated reasons, and 21/32 TMJs had OA changes in the intra-articular disc. Carmalt et al. (2018) described CT findings from 1018 horses with no clinical signs of TMD, with OA changes in 594/2162 TMJs.  

These findings indicate that TMJ OA is common and often associated with no recognizable clinical signs; however, clinically significant OA does occur (but is uncommonly reported). None of these reports have evaluated TMJ myofascial pain, and this type of disorder would likely be much more responsive to interventions such as occlusal equilibration, acupuncture, chiropractic, and myofascial therapies. Anecdotally, I have observed that horses are commonly reactive at the TMJ/dental acupoint indicators, and the reactivity is typically eliminated following dentistry and myofascial treatment. Occasionally horses do not improve, and these may represent other underlying pathologies such as OA, justifying further diagnostics and treatment.  

Emerging role of TMD in performance horse medicine 

Intra-articular TMJ injection to improve performance is a current trend in equine sports medicine. As with anything, practices vary widely in reasons for injecting and diagnostics to justify the procedure. Anecdotally, many horses have shown improved performance after these injections. So, is there any evidence for TMD affecting performance? 

Several case reports have shown decreased performance attributed to TMD. Jorgensen et al. (2015) described a high-level dressage horse with TMJ changes on CT, responsive to intra-articular injection. Carmalt and Reisbig (2022) showed improvement of a hindlimb lameness following diagnostic analgesia of bilateral TMJs in a horse with CT findings of bone cysts and OA. 

One recent study was specifically undertaken to answer this question. The authors induced unilateral TMJ inflammation by injection of LPS, and 5/5 horses had reduced rein tension on the injected side (Reisbig 2023). One limitation of this study is that this only reflects the response to acute inflammation, rather than chronic pain. Another problem with this literature is that we currently lack a way to repeatably characterize TMJ myofascial pain; therefore, its impact on performance, as its general existence, remains anecdotal.  

Carmalt (2023) did describe peri-articular TMJ pain in his recent article suggesting diagnostic steps to rule in TMD in horses with poor performance. He maintained that TMD is an uncommon cause, with a low prevalence, and that TMJ peri-articular pain is often associated with other distant causes such as neck or back pain, hock OA, impinging dorsal spinous processes, etc. His guidelines propose that all other causes of lameness/poor performance should be ruled out before TMD is considered as a primary cause, and that indiscriminate TMJ intra-articular treatment may overly inflate the clinical significance of TMD. 

I have several thoughts about Carmalt’s article. He is confirming that peri-articular TMJ pain is common, likely by palpation around the ST-7 acupoint in the masseter muscle. I also find that ST-7 is reactive commonly with distant lameness (hock, stifle), lumbosacral pain, and stomach pain. Any tension along the ST acupuncture channel may cause this point to be reactive. Also, many horses with underlying pain issues (foot pain, TMJ pain, etc.) are predisposed to stomach ulcers. I would agree that finding only reactivity at ST-7 cannot be classified as a TMJ disorder. May (n.d. webinar) originally described reactivity at all of 3 acupoints (ST-7, TH-17, and Bao sai) as indicating TMJ myofascial pain, and I have worked based on that schematic.  

To complicate the picture, distant myofascial issues might induce TMJ pain and tension, thus changing mastication patterns, leading to dental malocclusions such as overjet, hooks, ramps, etc. These would limit the rostral-caudal movement of the mandible and further impact the TMJ, requiring occlusal equilibration to restore function (even if the original problem was a distant body issue). Interestingly, in my clinical experience, many of the horses with TMJ myofascial pain improve in both acupoint sensitivity (peri-articular TMJ pain), chewing ability, and performance following good dentistry alone. I would not expect this to happen if the primary cause (e.g., lumbosacral or other joint pain) was not also addressed. However, relieving tension patterns in the TMJ might also improve other conditions along the affected acupuncture channels (e.g., stifle pain along the ST channel). Determining the primary cause may not be possible since everything in the body is connected. 

The anecdotal performance enhancement of “indiscriminate” TMJ intra-articular injection may indicate that clinically significant OA and/or TMJ inflammation is more common than experts indicate. Alternatively, some of these horses may have TMJ myofascial pain (but no underlying pathology) that is somehow benefited by injection. Equine acupuncturists frequently inject substances such as vitamin B12, Sarracinea extract, homeopathic remedies, etc. at acupoints to have an impact on the fascia. Therefore, other injections along the acupuncture channel might also have this effect. Finally, the performance enhancement in these cases may not be real, as ridden performance can be highly subjective, and horse owners/trainers may be biased in their assessment. At any rate, I would not expect any effect to be lasting (in the absence of true TMJ pathology as the primary cause) if dental occlusion and other body issues were not also addressed. 

Clearly from this body of literature, some horses with performance issues do have significant TMD that requires further investigation if occlusal equilibration and myofascial therapies do not correct the problem. Carmalt (2023) describes the first step as diagnostic analgesia with a ridden assessment before and after. Arthrocentesis of the TMJ was described in an older article (Rosenstein 2001), targeting the discotemporal joint compartment. The discomandibular joint compartment is more difficult to access, but an approach was described by Carmalt and Tucker (2020). As these joint compartments likely do not communicate in the absence of disc damage, both should ideally be injected for diagnostic analgesia or treatment (Pimental and Carmalt 2021).Imaging of the TMJ could include radiography, computed tomography, arthroscopy, and (less beneficial) ultrasound. Skyline radiography (rostral 45° ventral 30° lateral-caudodorsolateral oblique) view of the TMJ allows for superior visualization of the joint space and mandibular condyle (Ebling et al 2009). 

Currently there are no studies demonstrating the most effective intra-articular treatments for TMD. Since this is a fibrocartilagenous joint, it may respond differently to corticosteroids than the typical hyaline cartilage joints. Carmalt (2023) described that, anecdotally, corticosteroid-based injections for performance-limiting TMJ OA were possibly not as effective or long-lasting as in other arthritic joints. No reports in the literature describe the use of platelet rich plasma (PRP) or autologous conditioned serum (IRAP) in the equine TMJ. However, Carmalt and Reisbig (2022) did publish the use of autologous protein solution in one case of TMD.  

In conclusion, recent literature does begin to help characterize equine TMD including pathologies, diagnostic techniques, imaging practices, treatment, and role in poor performance. TMJ myofascial pain and the impact of occlusal equilibration on TMD have not yet been recognized by the academic community, so more research on this topic is needed by integrative veterinarians. 

References:

Alrizqi AH, Aleissa BM. 2023. Prevalence of temporomandibular disorders between 2015-2021: A literature review. Cureus 15(4):e37028.  

Balducci J, Ruby J, Hall C, Williams J. 2021. Arthrotomy, curettage and medical management of septic arthritis and osteomyelitis of the temporomandibular joint in a horse. Eq Vet Educ. 33:e5–e11.  

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Carmalt JL, Kneissl S, Rawlinson JE, Zwick T, Zekas L, Ohlerth S, Bienert-Zeit A. 2016. Computed tomographic appearance of the temporomandibular joint in 1018 asymptomatic horses: a multi-institution study. Vet Radiol Ultrasound. 57(3):237-45. 

Carmalt JL, Tucker ML. 2020. Arthroscopic approach and intra-articular anatomy of the equine discomandibular joint compartment of the temporomandibular joint. Vet Surg. 49:1326–33. 

Carmalt JL and Reisbig NA. 2022. Arthroscopic treatment of bilateral mandibular condylar cysts and associated osteoarthritis of the temporomandibular joints in a horse. Eq Vet Educ. 34(9), e352-e358. 

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Pimentel KL, Carmalt JL. 2021. The frequency of communication between the synovial compartments of the equine temporomandibular joint: A contrast-enhanced computed tomographic assessment. Front Vet Sci. 8:753983.  

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