Illustration of the hip joint. The two joint surfaces (grey) are visible. the pelvic joint cavity (acetabulum) and the femur proximal epiphysis (femoral head). The articular capsule is filled with synovial fluid (blue). surrounded by its membrane (pink). it is attached to the pelvis on the cotyloidal ring and surrounds the interarticular ligament (purple) linking the centre of the femoral head and pelvis.