The hip is one of the body's largest weight-bearing joints. It is called a ball-and-socket joint because the spherical head of the thighbone (femoral head) moves inside the cup-shaped hollow of the pelvis (acetabulum). Ligaments connect the ball of the thighbone to the pelvic socket and stabilize the joint.

The surfaces of the femoral head and the acetabulum are covered with articular cartilage, which cushions the bones and enables them to move easily. The synovial membrane covers all of the remaining surfaces of the hip joint. The fluid it produces lubricates the joint and nearly eliminates friction.

Normally, all of the parts of the hip work in harmony, allowing it to move easily and without pain. When problems do occur, the most common cause is arthritis. Other conditions include fractures, strains, dislocation, bursitis, transient osteoporosis of the hip, snapping hip, osteonecrosis of the hip, and meralgia paresthetica.

Arthritis–A disease characterized by joint inflammation. The most common types of inflammatory arthritis are osteoarthritis, Rheumatoid arthritis, ankylosing spondylitis, and systemic lupus erythematosus. Traumatic arthritis is another common form of the disease. Joint pain is a symptom of all types of arthritis. Inflammatory arthritis is characterized by dull, aching pain in the groin, outer thigh, or buttocks.

• Osteoarthritis–Usually caused by wear and tear, although sometimes the result of an injury. In this form of arthritis, the articular cartilage wears away. The bones rub against each other, causing pain and stiffness.
• Rheumatoid arthritis–An autoimmune disease in which the synovial membrane becomes inflamed, produces too much fluid, and damages the articular cartilage, leading to pain and stiffness.
• Ankylosing spondylitis–Chronic inflammation of the spine and the sacroiliac joint (the point where the spine meets the pelvic bone).
• Systemic lupus erythematosus–An autoimmune disease in which the body harms its own healthy cells and tissues.
• Traumatic arthritis–This condition usually follows a serious hip injury or fracture. The articular cartilage is damaged, causing hip pain and stiffness to develop over time.

Fracture–A break, which may occur in the top of the thighbone just below the hip, in the thighbone shaft, or in the pelvis.

• Hip Fracture–A break in the top of the thighbone (femur) where it angles into the hip socket. Usually results from a fall. The risk of a broken hip increases with advanced age, female gender, family history, poor nutrition, smoking, excessive alcohol use, and physical or mental impairment. When a hip fracture occurs, the pain is usually too intense to stand and the leg may turn outward.
• Thighbone Fracture–A break in the thighbone shaft, usually resulting from high-force trauma, such as that caused by a motor vehicle accident or a fall from a high place. Severe pain, inability to move the leg, deformity, and swelling are characteristic of a broken thighbone. In addition, the injured thigh may be shorter than the uninjured one, and there may be extensive bruising.
• Pelvis Fracture–The pelvis is a ring-like structure of bones at the lower end of the trunk. A broken pelvis is painful, swollen, and bruised. Most pelvic fractures are caused by high-energy forces, such as those generated in a motor vehicle accident, a crush accident, or a fall. Growing teens involved in sports and elderly people with osteoporosis are at greater risk for pelvic fractures.

Strain–Stretching or tearing of the muscle fibers caused by overuse or injury. Symptoms include swelling, bruising, tenderness, pain over the injured muscle that increases with use, and loss of strength. Muscle strains are particularly common among people who participate in sports.

Dislocation–A painful injury in which the head of the thighbone is popped out of the pelvic socket. Motor vehicle and other high-force accidents are the most common cause.

Bursitis–Irritation or inflammation of the bursae, small, fluid-filled sacs that act as cushions between bones and overlying soft tissues. Symptoms of hip bursitis include pain in the point of the hip or in the groin area, depending on where the bursa is located.

Transient osteoporosis of the hip–A type of osteoporosis that is painful but reversible. Pregnant women and middle-aged men are at greatest risk of developing the condition. Symptoms include pain in the front of the thigh, the side of the hip, the buttocks, or the groin. The pain usually occurs suddenly and without injury, intensifies with turning movements and weight-bearing activities, and increases over a period of weeks or months to a level that may be disabling.

Snapping hip–A "snapping" sensation in the hip resulting from the movement of muscles or tendons over bony structures, a tear in the cartilage, or bone debris in the hip joint. When caused by muscle or tendon movement, the condition is usually harmless and painless, though it can sometimes lead to hip bursitis. When caused by a cartilage tear or bone debris, the condition can be painful and disabling.

Osteonecrosis of the hip–A disabling condition in which the blood vessels gradually cut off nourishment to the top of the thighbone (femur) where it fits into the hip socket. Without blood, the head of the femur eventually dies and collapses. A dull ache or throbbing pain in the side of the hip, the groin, or the buttock is characteristic of the condition. Symptoms may begin without warning.

Meralgia paresthetica–A condition caused by compression of the lateral femoral cutaneous nerve, one of the large sensory nerves in the leg. Characterized by pain that starts in the outer side of the thigh and occasionally extends to the outer side of the knee and burning, tingling, or numbness in the same areas. Restrictive clothing and weight gain are two common causes.

In children and young adults, other hip problems can include developmental dislocation (dysplasia) of the hip, Perthes disease, and slipped capital femoral epiphysis.

• Developmental dislocation (dysplasia) of the hip–Abnormal formation of the hip joint in which the femoral head is not stable in the pelvic socket. The ligaments of the hip joint may be loose and stretched. Hip dysplasia is usually noted in the newborn exam; it has a familial tendency and is predominant in girls, first-born children, and infants born in the breech position.
• Perthes disease–A condition characterized by a temporary loss of blood supply to the hip, resulting in the death of the femoral head. Intense inflammation and irritation follow. Usually strikes children between the ages of two and twelve years of age; more common in boys than in girls. Symptoms include mild pain in the hip, groin, thigh, or knee and limping.
  Slipped capital femoral epiphysis–A disorder of the adolescent hip in which the femoral epiphysis–located at the top of the thighbone–slips off the thighbone in a backward direction. Although the cause is unknown, most cases occur in obese children. Symptoms include hip or knee pain and walking with an intermittent limp.

Source: American Association of Orthopaedic Surgeons