Articular cartilage response to blunt vs sharp lesions in an in vivo equine carpal groove model

Abstract

Purpose: Chondral defects are common in humans and horses and may initiate the development of osteoarthritis. However, if, when and how they should be approached therapeutically is still subject of discussion. In the equine carpal joint, critical size of chondral lesions has been determined at 2mm diameter, beyond which no spontaneous healing occurs. However, the progression of degeneration is likely determined by other factors besides the size of a lesion. In bovine cartilage explants, sharp and blunt trauma lead to different responses in the tissue adjacent to the lesion. A better understanding of the progression of various forms of chondral lesions could help in determining the best (time of) intervention. Therefore, we investigated the long-term response of articular cartilage to artificially created blunt and sharp grooves in the equine carpal joint. We hypothesized that disruption of the collagen network of articular cartilage, combined with intensified loading would always lead to progressive degeneration, but that the presence of initial tissue loss (blunt grooves) would substantially accelerate this process. Methods: In one randomly assigned front limb of nine adult female Shetland ponies the cartilage layers at the proximal surface of the intermediate carpal bone and at the radial facet of the third carpal bone were grooved via arthrotomy. Blunt and sharp grooves were randomly assigned to either of the two joints. Blunt grooves were made using a hooked arthroscopic probe with a sharpened tip. Sharp grooves were made with a surgical blade that was clamped in a custom-made device. The contra-lateral joints were sham-operated. After 3 weeks of box rest, the ponies were trained for 8 weeks on a treadmill, followed by free exercise in a group shed or at pasture until the end of the study. Radiographs of both carpal joints, taken at baseline and 38 weeks, were scored for osteoarthritic changes. After euthanasia at 39 weeks, osteochondral samples were harvested. Cartilage thickness was measured using micro-CT and biomechanical properties were measured by indentation testing. Histopathological OARSI scoring was performed on Safranin-O/Fast-green stained sections and collagen type-I and II staining was evaluated qualitatively. Results: Grooved joints showed higher OARSI scores compared with the contra-lateral control joints (p< 0.0001) at 39 weeks. Blunt lesions were inherently larger and caused more tissue loss than sharp lesions. This resulted in a more extensive loss of proteoglycans and more pronounced cell cluster formation and focal cell loss in the adjacent cartilage (Fig1). Consequently, higher OARSI scores were observed in blunt-grooved than in sharp-grooved cartilage (p = 0.007). Cartilage thickness did not differ between grooved and control joints, but the cartilage stiffness trended lower in grooved joints. The lower stiffness was significant for the equilibrium modulus of sharp-grooved cartilage and the instantaneous modulus of blunt-grooved cartilage, in the radiocarpal joint. In the middle carpal joint, the ‘kissing’ site of blunt grooves displayed a significantly lower equilibrium modulus than cartilage in the control joint (Fig2). Radiographic scores of the carpal joints increased mildly but significantly in both grooved (p = 0.0003) and control (p = 0.163) joints, although the highest mean increase was observed in the blunt-grooved radiocarpal joint. Conclusions: Both blunt and sharp articular cartilage grooves in the equine carpal joint were not spontaneously repaired and led to various degrees of degenerative changes within the surrounding tissue over a 39-week follow-up period. The tissue response was different for blunt and sharp injuries. These findings underline the importance of lesion morphology in combination with the loading profile in relation to tissue response to trauma and progression of damage. This is also important when considering or testing treatment strategies because biochemical composition, cell viability, structure, and mechanical properties of the lesion margin may affect the integration of cartilage repair tissue. Furthermore, this data could be highly valuable for the validation of computational models aiming at prediction of the progression of focal defects and development of post-traumatic OA. [Formula presented] [Formula presented]