AAEP Health Coverage: Rein Lameness Associated with the Temporo-Mandibular Joint

Portrait sports red stallion in the bridle. Rein lameness in horses can be associated with the TMJ.
During performance, the TMJ is under a load through bit control, which could tie in the presence of pain with movement with bit and bridle. iStock

It is not uncommon for riders to complain that their horse’s performance is suffering despite no obvious cause. At the American Association of Equine Practitioners Convention in San Antonio November 18-22, 2022, James Carmalt, MA, VetMB, MVetSc, PhD, FRCVS, DABVP, DAVDC, DACVSMR, DACVS of the Western College of Veterinary Medicine in Saskatchewan, Canada, presented information on the potential role of temporo-mandibular joint (TMJ) disease as a contributing cause of rein lameness.

Carmalt notes that these horses do not present with a classic head-bobbing lameness. However, an inflammatory response in the TMJ changes not just the kinematics of mastication. During performance, the TMJ is under a load through bit control. This could tie in the presence of pain with movement with bit and bridle.  As a dynamic joint, the TMJ might be affected by developmental orthopedic disease early on to create abnormalities, or it might accrue pathologic changes with age. 

Rein lameness in the horse is defined as a lameness that is appreciated (felt or visible) only at exercise under saddle or when wearing a bit and bridle. Generally, practitioners rule out other issues first and then begin looking at the TMJ as a reason for poor performance, says Carmalt. Previous studies have not found a correlation of dental abnormalities with CT-apparent TMJ disease.

Studies on TMJ Disease

In one study of a horse with TMJ disease, hindlimb lameness worsened when the side reins were tightened, adding pressure on the TMJ. Following with an anesthetic block of the TMJ, the hindlimb lameness didn’t change but the horse did not respond to tightening of the side reins. Similar studies in humans have demonstrated that dentures and bite plates contribute to the ability to walk with a normal posture. Elite human athletes have also improved their scores when using a bite plate that modifies occlusal biomechanics and proprioception.

TMJ Injections in Horses with Rein Lameness

Carmalt describes a study in five horses that determined the effect of TMJ inflammation on rein tension when the horse is long-reined on a treadmill. In this study, six horses were free-walked and trotted on long reins on a treadmill. Then, they were asked to work into the bit to achieve a baseline at each gait. A low dose of lipopolysaccharide (LPS) was injected into the TMJ to achieve maximum inflammation in six hours. The long-lined free walk and trot and work into the bit on the treadmill were repeated following LPS injection. After a washout period of 10 days, the opposite TMJ was injected with the same protocol. 

The researchers wanted to see if the dominant rein changed, and how the horses responded when worked into the bit. An objective assessment was obtained through a Bluetooth-enabled device placed between the reins and the bit that reports continuous rein tension data to an iPad. Data was collected over 60 seconds from both sides. Investigators were blinded as to which side was injected with LPS.

Dominant Side vs. Non-Dominant Side Injections

When the dominant side was injected, the horse switched its dominance to the originally non-dominant side. When the non-dominant side was injected, the dominant side became very heavy in the handler’s hands. The horse overcompensated and overreacted to rein inputs on the injected side. The injected TMJ side needed less rein tension to affect it at the walk. The non-injected side at trot needed much less rein tension. Trot effects on bit acceptance were substantially different than the walk.

The rein lameness could not be seen objectively or subjectively, and there was no appreciation of change in movement. However, the blinded handler could “feel” which joint was injected. Objective data additionally revealed the effects of LPS injection and corresponding inflammation in the joint. All horses reduced rein tension on the side with the inflamed TMJ. This suggests that TMJ disease might be a component of poor performance.

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