The hip joint is the largest weight-bearing joint in the human body. It is also referred to as a ball and socket joint and is surrounded by muscles, ligaments, and tendons. The thigh bone or femur and the pelvis join to form the hip joint.
Any injury or disease of the hip will adversely affect the joint's range of motion and ability to bear weight.
The hip joint is made up of the following:
- Bones and joints
- Ligaments of the joint capsule
- Muscles and tendons
- Nerves and blood vessels that supply the bones and muscles of the hip
Bones & Joints
The hip joint is the junction where the hip joins the leg to the trunk of the body. It is comprised of two bones: the thigh bone or femur and the pelvis which is made up of three bones called ilium, ischium, and pubis. The ball of the hip joint is made by the femoral head while the socket is formed by the acetabulum. The Acetabulum is a deep, circular socket formed on the outer edge of the pelvis by the union of three bones: ilium, ischium and pubis. The lower part of the ilium is attached by the pubis while the ischium is considerably behind the pubis. The stability of the hip is provided by the joint capsule or acetabulum and the muscles and ligaments which surround and support the hip joint.
The head of the femur rotates and glides within the acetabulum. A fibrocartilaginous lining called the labrum is attached to the acetabulum and further increases the depth of the socket.
The femur or thigh bone is one of the longest bones in the human body. The upper part of the thigh bone consists of the femoral head, femoral neck, and greater and lesser trochanters. The head of the femur joins the pelvis (acetabulum) to form the hip joint. Next to the femoral neck, there are two protrusions known as greater and lesser trochanters which serve as sites of muscle attachment.
Articular cartilage is the thin, tough, flexible, and slippery surface lubricated by synovial fluid that covers the weight-bearing bones of the body. It enables smooth movements of the bones and reduces friction.
Ligaments are fibrous structures that connect bones to other bones. The hip joint is encircled with ligaments to provide stability to the hip by forming a dense and fibrous structure around the joint capsule. The ligaments adjoining the hip joint include:
- Iliofemoral ligament – This is a Y-shaped ligament that connects the pelvis to the femoral head at the front of the joint. It helps in limiting over-extension of the hip.
- Pubofemoral ligament – This is a triangular shaped ligament that extends between the upper portion of the pubis and the iliofemoral ligament. It attaches the pubis to the femoral head.
- Ischiofemoral ligament – This is a group of strong fibers that arise from the ischium behind the acetabulum and merge with the fibers of the joint capsule.
- Ligamentum teres – This is a small ligament that extends from the tip of the femoral head to the acetabulum. Although it has no role in hip movement, it does have a small artery within that supplies blood to a part of the femoral head.
- Acetabular labrum – The labrum is a fibrous cartilage ring which lines the acetabular socket. It deepens the cavity increasing the stability and strength of the hip joint.
Muscles & Tendons
A long tendon called the iliotibial band runs along the femur from the hip to the knee and serves as an attachment site for several hip muscles including the following:
- Gluteals – These are the muscles that form the buttocks. There are three muscles (gluteus minimus, gluteus maximus, and gluteus medius) that attach to the back of the pelvis and insert into the greater trochanter of the femur.
- Adductors – These muscles are in the thigh which help in adduction, the action of pulling the leg back towards the midline.
- Iliopsoas: This muscle is in front of the hip joint and provides flexion. It is a deep muscle that originates from the lower back and pelvis, and extends up to the inside surface of the upper part of the femur.
- Rectus femoris – This is the largest band of muscles located in front of the thigh. They also are hip flexors.
- Hamstring muscles- These begin at the bottom of the pelvis and run down the back of the thigh. Because they cross the back of the hip joint, they help in extension of the hip by pulling it backwards.
Nerves & Arteries
Nerves of the hip transfer signals from the brain to the muscles to aid in hip movement. They also carry the sensory signals such as touch, pain, and temperature back to the brain.
The main nerves in the hip region include the femoral nerve in the front of the femur and the sciatic nerve at the back. The hip is also supplied by a smaller nerve known as the obturator nerve.
In addition to these nerves, there are blood vessels that supply blood to the lower limbs. The femoral artery, one of the largest arteries in the body, arises deep in the pelvis and can be felt in front of the upper thigh.
All the anatomical parts of the hip work together to enable various movements. Hip movements include flexion, extension, abduction, adduction, circumduction, and hip rotation.
Hip Injuries & Tears
A gluteus medius tear is a condition characterised by severe strain on the gluteus medius muscle that results in partial or complete rupture of the muscle.
The gluteus medius is one of the major muscles of the hip and is essential for movement of the lower body and keeping the pelvis level during ambulation.
Hamstring injuries are common in athletes who participate in sports activities such as track, soccer, and basketball that involve running. The three hamstring muscles namely semitendinosus, semimembranosus and biceps femoris are at the back of the thigh and helps you bend (flex) your knee and extend your leg.
Osteoarthritis, also called degenerative joint disease is the most common form of arthritis. It occurs most often in older people. This disease affects the tissue covering the ends of bones in a joint (cartilage). In a person with osteoarthritis, the cartilage becomes damaged and worn out causing pain, swelling, stiffness and restricted movement in the affected joint. Although osteoarthritis may affect various joints including hips, knees, hands, and spine, hip joint is most commonly affected.
Inflammation of the joints is referred to as arthritis. The inflammation arises when the smooth covering (cartilage) at the end surfaces of the bones wears away. In some cases, the inflammation is caused when the lining of the joint becomes inflamed as part of an underlying systemic disease. These conditions are referred to as inflammatory arthritis.
Transient osteoporosis of the hip is a rare condition that causes bone loss temporarily in the upper part of the thighbone (femur). It is mostly found in young or middle-aged men between the ages of 30 and 60, and women in their later stages of pregnancy or early postpartum period (following childbirth). It is characterised by abrupt onset of pain that increases with activity.
Total hip replacement is a surgical procedure in which the damaged cartilage and bone is removed from the hip joint and replaced with artificial components. The hip joint is one of the body's largest weight-bearing joints, located between the thigh bone (femur) and the pelvis (acetabulum). It is a ball and socket joint in which the head of the femur is the ball and the pelvic acetabulum forms the socket. The joint surface is covered by a smooth articular cartilage which acts as a cushion and enables smooth movements of the joint.
Gluteus medius is one of 3 muscles in the buttocks and is situated on the outer surface of the hip. The function of the gluteus medius is to assist with pelvis stability, hip abduction, along with internal and external rotation of the hip. Tears of the gluteus medius usually occur where the tendon inserts at the greater trochanter, causing lateral hip pain.
The hamstring muscles are three muscles at the back of the thigh that are commonly injured during sports that involve running, jumping and sudden changes in speed. Injury usually occurs at the proximal tendinous origin of the muscles which attach to the ischium, a pelvic bone. The risk of injury increases with age, prior injury, lack of flexibility and being overweight.