Articular invasion by malignant bone tumours around the knee is one of the most important criterions to determine prior surgery. MR imaging is the most accurate exam in staging bone sarcomas. Although, past studies showed that when MRI shows evidence of intra articular involvement by tumour, the incidence of false positive diagnosis and subsequent excessively radical surgery is as high as 50%. The aim of this study is to determine growth pattern of bone sarcomas into the joint in order to assess which are the limits of the joint compartment. We reviewed retrospectively 18 cases of primary intra medullary sarcomas with epiphyseal extension located around the knee. The tumour was located in the distal femur in 11 cases and in the proximal tibia in 7 cases. In tumours located in the distal femur, two distinct modes of extension towards synovium and joint space were identified. The most common pattern was tumour growth along the anterior and intra articular part of the distal femur. This pattern was observed in 10 cases. The tumour displaced anteriorly soft tissues and remained extra synovial in 6 cases. Only in 4 cases, tumour contaminated the joint space. The extension was in all cases marginally close to the cartilage of the trochlea in the transitional zone between cartilage and synovial membrane. The second pattern was extension through the inter condylar notch which was observed in three cases. Growth was around the osseous-tendinous junction of the cruciate ligaments and never within the ligament. In tumors in the proximal tibia, although tumour was close to the osteochondral junction, cartilage was not breached anyway. Tumour got around the cartilage. Extension of the tumour to the articular joint was marginally under the posterior capsule insertions making contact with the edge of the articular cartilage. This pattern was observed in two cases. We didn’t observe an erosion of cartilage layer, in the limits of the sections done. Our study, demonstrated that cartilage and synovial membrane, since they are not breached, represent reliable margins for intra articular resections. We identified in the current study, one mode of tumour extension towards synovium and joint space. In all cases, extension was in junctional zones between cartilage and synovial membrane or cartilage and articular capsule. The articular cartilage was the most resistant barrier, having no vascular perforations and probably an intrinsic resistance to tumour.
