Dynamics of the suprapatellar bursa during knee joint extension

Purpose The suprapatellar bursa is located in the proximal deep layer of the patella and is thought to reduce tissue friction by changing from a single-membrane structure to a double-membrane structure during knee joint motion. However, the dynamics of the suprapatellar bursa have only been inferred from positional relationships, and the actual dynamics have not been confirmed. Methods Dynamics of the suprapatellar bursa during knee joint motion were observed in eight knees of four Thiel-fixed cadavers and the angle at which the bursa begins to show a double membrane was revealed. The flexion angles of knee joints were measured when the double-membrane structure of the suprapatellar bursa began to appear during knee joint extension. Results The suprapatellar bursa changes from a single membrane to a double-membrane structure at 91 ± 4° of flexion, when the knee joint is moved from a flexed position to an extended position. Conclusion The suprapatellar bursa may be involved in limitations to knee joint range of motion and pain at an angle of approximately 90°. Further studies are needed to verify whether the same dynamics are observed in living subjects.