A Histological and Ultrastructural Study of a Loose Body Retrieved from a Cadaveric Knee Joint

Introduction: Loose bodies (LBs) are fragments of cartilage or bone that remain free-floated in the synovial fluid of knee joints. The various cell types present in LBs are not fully described. Objectives: We examined the histological contents of an LB retrieved from a cadaveric knee joint. Materials and Methods: An LB located in the posterior-medial side of the knee joint of an embalmed male cadaver (age: 75 years) was retrieved during dissection of a lower limb in anatomy demonstration class for undergraduate teaching. Its diameter was determined using a screw gauge: the supero-inferior dimension was 1.5 cm, and the medio-lateral dimension ranged between 0.6 and 0.9 cm. It was fixed and processed for histology, Masson’s trichrome staining, scanning and transmission electron microscopy, and fibroblast growth factor 23 (FGF-23) immunolabeling. Results: The LB was present deep in the popliteal fossa between the femur and tibia, inferior to the lateral condyle. It contained hypertrophic chondrocytes and bone cells. Immunofluorescence labeling with FGF-23 revealed that most cells appeared morphologically preserved; however, functional viability cannot be inferred in embalmed cadaveric tissue. Stages of proliferation and hypertrophy were present in the LB. Osteoclasts were more abundant than the osteocytes. The osteoclasts contained phagocytosed materials from hypertrophied chondrocytes lying above the bone core. Bundled collagen fibers were seen in the cartilage matrix more than in the bone core, having a mineralized matrix and less scattered collagen, evident from DAPI labeling. Conclusion: The present sample was a case of osteochondral growth of the fracture and reflects bone regeneration through the interaction of hypertrophic chondrocytes. Given the descriptive nature of this single cadaveric observation, no therapeutic or clinical management recommendations are drawn.