Histopathology and computed tomography of age‐associated degeneration of the equine temporohyoid joint

Reasons for performing study: The aetiology of temporohyoid osteoarthropathy (THO) is unknown; both primary infectious and degenerative causes have been suggested. Hypothesis: There is a significant association between increasing age and severity of temporohyoid joint degeneration. To examine the histopathology of the temporohyoid articulation in aged horses and to compare the appearance of the joint with computed tomography (CT) and peripheral quantitative CT (pQCT). Methods: pQCT scans of the temporohyoid articulations were obtained bilaterally from 31 horses (range age 1–44 years) post mortem and images were graded by 2 blinded observers on 2 occasions for the presence of osteophytes, irregularity of the joint surface and mineralisation. Eight heads had been examined previously by CT, with the images similarly graded for the shape and density of the proximal stylohyoid bones, bone proliferation surrounding the joint, mineralisation of the tympanohyoid cartilage and the relationship of the petrous temporal bone to the stylohyoid bone. Sixteen temporohyoid joints were then evaluated histologically. Results: There was significant association between the mean pQCT degeneration score and age (rho = 0.75; P<0.0001), between the pQCT and CT score (rho = 0.63; P = 0.01) and between the degenerative changes identified within each temporohyoid joint within each horse (rho = 0.81; P<0.0001). Age‐associated changes included the development of a club shape by the proximal stylohyoid bone, rounding of the synostosis with the petrous temporal bone and extension of osteophytes from the petrous temporal bone to envelope the stylohyoid head and bridge the joint. In no horse was there any evidence of osteomyelitis within the petrous temporal bone, stylohyoid bone or tympanohyoid cartilage. Conclusions: This study provides evidence that age is associated with increasing severity of degenerative changes in the equine temporohyoid joint and that similar changes are commonly found bilaterally. Potential relevance: The changes identified appear similar, albeit milder to the changes reported in horses with THO, suggesting that degenerative, rather than infectious causes may underlie the aetiology of THO. Future work should be directed at examining the histopathology of clinical THO cases.