Microstructural changes at the osteochondral junction in naturally occurring osteoarthritis of the equine medial femorotibial joint

Abstract

Purpose: Osteoarthritis (OA) is a progressive degenerative disease of the joint organ and affects all species. The medial femorotibial (MFT) joint is the knee compartment most commonly affected with OA in humans, dogs, and horses. We hypothesized that microCT (μCT) assessment of specimens with naturally occurring equine MFT-OA will provide new insight into the pathophysiology of OA in this joint. The aim of this study was to compare healthy and MFT-OA joint microarchitectural changes in both 2 and 3D μCT images. Methods: Equine medial femoral condyles (MFC; n = 24) and tibial plateaus (MTP; n = 24) were available in a tissue bank. Regions of interest (cranial and caudal) were scored for lesions. Articular surfaces were macroscopically scored for OA (cartilage fibrillation and erosion) and categorized into 2 groups (healthy and OA) for comparative purposes. MFCs and MTPs were scanned with μCT (Xtek HMXST 225, Nexus Metrology, Canada) and 2D and 3D images were assessed. Hyaline articular cartilage (HAC) was scored for fibrillation (0-3) and high density mineralized protrusions (HDMP) (0-3). The calcified cartilage/subchondral bone plate (ACC/SCP) was also assessed for radiolucencies and the subchondral trabecular bone (SCB) for sclerosis. Results: Seventeen healthy MFTs and 7 OA MFTs were studied. HAC fibrillation was more prevalent in the MFC OA group (100%) compared with the healthy MFC group (71%) and the scores (s) were also more severe: OA MFCs (s2- 29%, s3- 71%); healthy MFCs (s1- 29%, s2- 24%, s3- 18%). Although HAC fibrillation was almost equally observed in the MTP OA group (100%) and the healthy MTP group (94%), the scores were worse in the MTP OA group (s2- 29%, s3- 71%) compared with MTP healthy group (s1- 12%, s2- 43%, s3- 43%). HDMPs were observed frequently in both groups: in 57% OA MFCs and in 47% healthy MFCs. However, the HDMP scores were more severe in MFC OA (s1- 14%, s3- 43%) compared with healthy MFCs (s1- 12%, s2- 12%, s3- 23%). They were located almost exclusively in the cranial MFC (100% OA, 87% healthy) (Figure 1). Although HDMPs were more prevalent in the MTP OA group (43%) compared with the healthy MTP group (18%), the severity of the lesions was similar in both groups: OA MTPs (s1- 29%, s3- 14%); healthy MTPs (s1- 6%, s3- 12%). ACC/SCP radiolucency was never observed in either healthy or OA MFCs. However, ACC/SCP radiolucency was observed in the MTP: MTP OA (86%) and healthy (65%) and almost all were located in the caudal part of the plateau in both healthy (91%) and OA (100%) MTPs. SCB sclerosis had a similar prevalence in both healthy (82%) and OA (86%) MFCs but the distribution pattern was different. In the healthy MFCs, sclerosis was more frequent caudally (71%) whereas in OA MFCs, it was distributed equally in both regions (57%). At the tibial plateau, sclerosis was most frequently observed in MTP OA specimens (86%) and predominantly at the caudal site (86%). In healthy MTPs it was identified in 65% with a more balanced distribution between cranial (41%) and caudal (59%) sites. Conclusions: High density mineralized protrusions (HDMP) were observed protruding into macroscopically normal appearing cartilage and OA cartilage. However, they were more severe in OA joints. This confirms our previous identification of HDMP in equine post traumatic osteoarthritis (PTOA) of the carpal bones and extends them to the equine MFT where racing-induced PTOA is not a characteristic feature and suggests that HDMP is feature all forms of OA. ACC/SCP radiolucency was never observed in the MFCs but occurred in the caudal MTP of both groups, but more frequently in OA.