Occurrence of a recently described and validated subchondral defect in the distal tarsus of Icelandic horses

Sammanfattning: This study sought to retrospectively investigate the presence of a subchondral bone defect and its relation to lesions associated with osteoarthritis (OA) in the distal tarsal joints of Icelandic horses. The subchondral bone defect has earlier been described and validated in this breed, and is a potential biomarker for early signs of OA. The initial material used for this study was analogue radiographic projections from 788 joints in 394 adult skeletally mature horses. All the material was initially evaluated with respect to image quality, number of projections taken for each joint, if the structures of interest can be seen in the projections, if the radiographs showed open physes at the distal tibia or proximal metatarsus and if the joints had advanced stages of OA which might obscure the subchondral bone defect. Out of the initial 788 joints, 130 joints from 117 different horses were selected for further evaluation. Three projections were used for each joint in order to assess the presence of the subchondral bone defect and OA lesions in the centrodistal (CD) and tarsometatarsal (TMT) joint. The joints were assessed by the student (SW) and supervisor (KH) by grading each lesion 0 - 3, including the subchondral bone defect depending on the severity of the findings. The lesions associated with OA that were graded in the study were periarticular osteophytes, narrowing/lack of joint width space, subchondral sclerosis, entheseophytes and cyst-like lesions. All joints were also given a subjective OA grading between normal, suspected mild, mild, moderate and severe (0 - 4). Out of the 130 joints evaluated SW identified the subchondral bone defect in 38 (29.2%) of the joints, while KH saw the defect in 35 joints (26.9%). The defect was detected by both KH and SW in 18 joints with or without concurrent OA, and in 7 out of these 18 joints the defects were seen as the only finding. In a majority of the cases, the defect was observed in the CD joint, which is in agreement with earlier research. However, in a number of projections, the subchondral bone defect could also be identified in the TMT joint by both assessors. One may speculate that the different levels of experience with interpreting distal tarsal joints on analogue radiographs could explain why there was a notable difference in which joints the defect was identified. The result show that the subchondral bone defect is present and could be identified in (combination with concurrent OA and as the only finding) in a material consisting of analogue radiographs taken of adult Icelandic horses. As for the subchondral bone defects in relation to clinically manifest and significant OA, longitudinal studies are needed in order to further evaluate the defect as a possible biomarker for early OA.