Osteoarthritis of the equine temporomandibular joint

Degeneration of the temporomandibular joint (TMJ) is a well reported condition in several mammalian species, including man. Yet, for the length of time that horses have been domesticated, we understand very little about the conditions affecting their TMJs. Up to this point, equine temporomandibular joint disease (TMJD) has been infrequently reported, with cases occurring secondary to a traumatic event, one that results in overt inflammation, either due to facture, joint sepsis, or both. With various etiologic causes of degenerative joint disease (DJD) in other joints, the apparent absence of these causes within equine TMJD is an enigma. As such, the focus of this work is to characterize the clinical effect of TMJ inflammation on mastication and histologically evaluate the changes seen within equine TMJs as horses age. The first work is a retrospective case report describing the occurrence of bilateral, non- traumatic, DJD in the TMJs, of a geriatric horse. The focus-patient had no history of trauma, but slowly developed signs of headshaking and began suffering from several bouts of impaction colic. When eating, the horse would consistently drop partially chewed feed (quid) and the jaw would make an audible ‘clicking’ sound. Computed tomography of the TMJs demonstrated bilateral mineralization of the rostral aspect of both intra-articular discs. Intraarticular injection of corticosteroid resulted in temporary resolution of the quidding behavior, the ‘clicking’ sound, and the recurrent episodes of colic. The occurrence of bilateral TMJD in an elderly horse, without evidence of trauma or sepsis, is unique. It suggests that, like many other joints, age-related changes occur within the TMJ, predisposing the joint to degeneration. The second study examines the relationship between the onset of acute unilateral inflammation of the TMJ and the development of clinical signs; distortion of the masticatory cycle, effusion of the affected TMJ, and aversion behavior suggestive of pain. Using a three-dimensional (3-D) motion tracking system, the authors examined the masticatory cycle of six horses, free of dental abnormalities. Horses were observed chewing grass hay over three minute intervals. Regardless of the side of mastication, all horses were injected in the left TMJ with lipopolysaccharide. Six hours post-injection the horses were re-assessed. All data were compared using paired t-tests. iii Four of six horses developed effusion of the injected TMJs; two also began quidding. All horses injected on the “chewing side”, switched sides post-injection; the two injected contralateral to the “chewing side” did not. All horses showed reduced vertical pitch of the mandible (mouth opening), but not lateral movement, post-injection. Overall rostrocaudal movement of the mandible did not change, but timing relative to the opening and closing phases of the cycle was different. Although horses demonstrated aversion behavior, all subjects continued to eat. The third, and final, study analyses the histologic presence of age-related change within the TMJs of 11 clinically normal horses. Each TMJ was frozen in-situ and sectioned in 5 mm slices, at random (transverse or sagittal direction). A histologic assessment for age-related change was made using a modified Mankin scoring system. The horses were divided into three age groups. The overall joint score was not significantly different between left and right joints within each age group. As the age of the horses in each group increased, there was a significant increase in the total joint score (accumulation of age-related changes); the youngest group of horses were significantly different than those in the oldest group, but neither were significantly different from the middle- aged group of horses