Reasons for performing study: Back pain is well recognised as a cause of poor performance in horses, but the role of lesions of the thoracolumbar synovial intervertebral articulations (facet joints) has not been well documented. Objectives: To describe the clinical features, radiographic appearance and location of facet,joint lesions and determine if there was any breed, gender, age, bodyweight or work discipline predilection. Methods: Data from 77 horses examined at the Animal Health Trust January 1997-September 2007 with evidence of thoracolumbar pain and radiographic changes of the facet joints were reviewed. The presence of either other osseous abnormalities of the thoracolumbar region or other problems potentially contributing to poor performance were recorded. Facet joint lesions were graded radiographically and their location determined. Influence of breed, gender and discipline on the presence of lesions, effect of location on the type of lesion and the influence of impinging dorsal spinous processes on the clinical features were assessed using Chi-squared tests. Results: There was no effect of breed, gender, age or bodyweight on occurrence of facet joint lesions. Showjumpers were significantly less affected than horses from other disciplines. There were commonly 2-5 affected facet joints, usually in the caudal thoracic and cranial lumbar spine (T15-LI). Sclerosis, periarticular new bone and narrowing of the joint space were the most frequent radiographic lesion types. Clinical features were significantly different between horses with and without impinging dorsal spinous processes. Severity of clinical signs was related to the presence of other osseous abnormalities, not the number of facet joints involved or the lesion grade. Conclusions: Osteoarthritis of the facet joints of the thoracolumbar spine can occur alone, in horses with back pain, or in association with other osseous abnormalities. Potential clinical relevance: Osteoarthritis of the thoracolumbar facet joints probably contributes to back pain, but further investigation of the prevalence of osteoarthritis in horses without clinical signs of back pain is merited.
