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.