The hip joint is a ball and socket type joint. It is composed of the femoral head and the acetabulum. Femoral head is the top part of the thigh bone or the femur and acetabulum is the socket formed by the pelvic bones. Muscles, tendons and ligaments do their part in holding the head of the femur (the ball) in the acetabulum (the socket).

The femoral head and the acetabulum are covered by a smooth shiny material called the articular cartilage. This cartilage serves as a covering for bony surfaces in places where they come in contact with one another. It allows the femoral head to move smoothly inside the acetabulum as and when the leg moves. The synovial fluid provides lubrication and nourishment to the hip joint thereby facilitating easy movement of the joint.

Stability

Joint capsule is a strong bag surrounding the hip joint. Ligaments act like strong ropes connecting the bones and providing stability to the hip joint. Ligaments reinforce the joint capsule and help in connecting the femoral head to the acetabulum; they also help prevent the ball from coming out of its socket. Stronger and larger ligaments make the hip joint stable. The labrum, a ring of tissue surrounding the acetabulum, also provides stability to the hip.

Tendons, muscles and bones

Muscles and tendons also offer stability to the joint when the leg is both moving or stationery. Muscles are connected to bones by the tendons. Many muscles surround the hip joint and they play a significant part in case of activities like walking, jumping and running.

The hamstring muscles are present in the hind part of the leg and they act along with the gluteus maximus in order to help move the leg backwards at the hip region. Iliopsoas and rectus femoris are hip flexors that move the leg forward at the hip joint. Adductor magnus and adductor longus are the groin muscles that move the leg towards the midline of the body. The gluteus medius, gluteus minimus and the tensor fascialata are muscles that move the leg away from body and help stabilize the hip joint during activities like weight bearing.

All these muscle groups are necessary to maintain the normal functioning of the hip.

Bursa

There is a small sac of fluid called the bursa that reduces friction between the tendons, muscles and bones. Bursa of the greater trochanter is the key bursa of the hip joint. It is present outside of the hip joint.