gms | German Medical Science

Introduction: Loose bodies are free floating elements of bone and cartilage. Loose bodies can be caused by degenerative joint disease, a fragment resulting from fracture or possibly a torn piece of cartilage. They can cause symptoms such as: pain and swelling, the inability to straighten the joint. The hypothesis of the study was that loose bodies undergo endochondral ossification.

Methods: Loose bodies were harvested during surgery. They were fixed in 4% PFA over night and embedded in paraffin. Bone architecture was analysed using X-Ray and µCT analysis. For analysis of histological composition sections were stained using toluidine blue staining was performed. Immunohistological stainings for collagen I, II, VI and X were performed. Microscopy using polarized light was used to analyse collagen fibril orientation. Marker for hypertrophic chondrocytes (MMP13), osteoblasts (osterix) and fibroblasts (fibronectin) were stained. Osteoclasts were detected using TRAP staining.

Results: We found 4 stages of loose body differentiation: a fibrous, a cartilage, a bone/ cartilage and a bone stage. Morphological analysis of different stages of loose bodies. X-Ray and µCT showed increasing amounts of bone at higher stages. Toluidine blue staining was used to identify cellular structures. We found that a synovium like tissue surrounds stages 1-3, but is not present any more at the bone stage. All stages are marked by expression of specific marker proteins. Stage 1 expresses collagen II, no collagen X and MMP13. Stage 2 expresses high levels of collagen II, VI as well as some collagen X and MMP13. These staining indicate that the loose body is formed mainly of hyaline cartilage, which chondrocytes differentiating to hypertrophic state. Stage 3 also expresses all collagens, showing increased amounts of MMP13 and collagen X. Furthermore, osterix positive cells can be found indicating a differentiation to an osteoblastic phenotype. Stage 4 expresses mainly collagen I. No osteoclasts were found in all stages.

Conclusion: Loose bodies seem to derive from a cartilage like source and then undergo endochondral ossification. A transdifferentiation into a more fibroblast like phenotype, as well as an osteoblast like phenotype can be observed. The differentiation to osteoclasts, however, seems not to be possible.