The hip joint connects the hind limb to the pelvic girdle. The hip joint is a ball (top of femur) and socket (acetabulum of the pelvis) joint that is surrounded by large muscle groups for stabilization. Detection of pain in this joint is uncommon except in horses with fractures of the acetabulum, often caused by trauma. Even in cases of fracture, the lameness can be subtle. Other causes of hip discomfort, while uncommon, include osteoarthritis, hip dislocation, and injury to the proximal femur.