Foal 2
At 6 months post discharge, Foal 2 was lame at a walk on the LH (AAEP 4/5). There was palpable thickening and fibrosis over the LM (Figure 4 c). Left gluteal muscle atrophy was apparent with resentment of passive global flexion of the limb. US revealed synovial effusion within the LFTJ and SER (Figure 4 b). The LM was displaced laterally, with an irregular outline, and areas of increased echogenicity suggestive of mineralization (Figure 4 b). Radiographs revealed multiple coalescing radiolucencies within the lateral femoral condyle (LFC), which appeared flattened (Figure 4 d). Mineralization was visible in the soft tissues in the region of the LM. A small radiolucent area was evident on the lateral tibial plateau, surrounded by sclerosis (Figure 4 e). Arthroscopic exploration of the L LFTJ and femoropatellar joint (FPJ) was performed under general anesthesia. The cranial horn of the meniscus was enlarged and fused with the joint capsule. It was displaced cranially, rigid and fibrotic on palpation. Three subchondral bone cysts with deep cloacae could be appreciated within the LFC. The lateral tibial plateau lesion could not be visualized. After a further 7 weeks, the filly’s comfort levels had not improved any further. Repeat radiographs revealed significant deterioration. The filly was humanely euthanatized and submitted for post-mortem investigation.
Post-mortem macroscopic findings : Increased viscosity and volume of articular fluid were found in the left LFTJ. Edematous thickening of the joint capsule was observed with hypertrophy of synovial villi and increased yellow-ness of the synovial fluid (Figure 6 a). The LM showed marked roughness and edematous loosening/malacia and was significantly thinner than the medial meniscus (Figure 6 b). Examination of the proximal tibia revealed diffuse gray-white lesions in the cancellous bony tissue immediately below the articular cartilage, as well as multiple focal erosions on the caudal articular surface of the lateral condyle of the proximal tibia adjacent to the meniscus (Figure 6 c).
Post-mortem histological findings : Histological evaluation focused on the articular capsule of the left LFTJ, LM, articular cartilage of the proximal tibia, and the subchondral bone tissue beneath. Diffuse villous proliferation of the capsular synovium was observed. In the superficial layer of the synovium, slight hyperplasia of the epithelium covering the superficial layer of the synovium was noted (Figure 6 d). In the sub-synovial stroma, infiltration of inflammatory cells (neutrophils, lymphocytes, macrophages with hemosiderin deposition, and plasma cells), interstitial edema, and capillary angiogenesis were observed (Figure 6 d). In the LM, irregular arrangement of collagen fibers, multifocal coagulation necrosis, fibrin deposition in the collagen fiber tissue, infiltration of inflammatory cells (neutrophils, lymphocytes, and macrophages), and capillary angiogenesis, were detected (Figure 6 e).
In the lateral condyle of the proximal tibia, surface irregularities of the superficial layer of the articular cartilage were found, associated with the multifocal erosions seen grossly on the articular cartilage surface. In addition, septic cartilage canals, characterized by neutrophil/macrophage infiltration into the articular cartilage canals, and the presence of fibrin-like material within the canals were found (Figure 6 f). Multifocal-to-continuous bone necrosis/hemorrhage/inflammatory cell infiltration, and fibroblasts as well as angiogenesis, were present at the periphery of the bone tissue, immediately below the articular cartilage. These lesions were not detected within the deeper epiphysis.