Pain in the groin or buttocks with a loss of hip motion requires special attention. Early diagnosis and treatment is imperative to avoid degenerative changes in the hip joint later in life. There are many possible causes of this type of hip pain. In this article, surgeons from the Rochester, Minnesota Mayo Clinic focus on femoroacetabular impingement (FAI) as a cause of hip pain leading to hip osteoarthritis.
Femoroacetabular impingement (FAI) describes a condition where the top of the femur (thigh bone) pinches the rim of the hip socket. The area that gets compressed is referred to as the acetabular rim. This type of impingement occurs most often when the hip is flexed and internally rotated.
For a long time, it was believed that FAI only occurred in people with some kind of abnormal anatomy of the hip. There was either a backward tilted angle of the hip socket called retroversion, a larger socket than the ball (head of the femur) inside the socket, or flattening of the femoral head. One type of abnormal shape of the hip was labeled a pistol grip because of the resemblance to the grip of a handgun.
More recent studies have shown it's possible to develop FAI even when the hip structure and anatomy are essentially normal. But, in general, more people with acetabular retroversion end up with hip replacements because of osteoarthritis than any other anatomical abnormality.
To get to the bottom of the cause of hip pain, a thorough history and examination are required. The surgeon looks at foot position, leg angles, leg length differences from one side to the other, hip motion, and muscle strength. Gait (walking) patterns are evaluated. Special tests such as the impingement test are done to identify the presence of an underlying FAI as the cause of the painful symptoms and restricted motion.
In the impingement test, the patient is lying on his or her back. The examiner flexes the involved hip and internally rotates the leg while at the same time moving the foot away from the body. This last motion is called hip abduction.
Limited joint motion and/or painful motion are signs of FAI. The further the hip is flexed, the more it hurts. Comparing hip motion from side to side may be helpful, but only if the patient doesn't have the same anatomical changes on both sides.
A second test for posterior impingement can also be done as part of the exam. This test is used to identify pinching of the cartilage along the back side of the acetabulum (hip socket). It is performed by quickly moving the leg into extension and external rotation while the patient is lying on his or her back. The leg to be tested starts in a position dangling off the end of the table. The patient holds the other leg up against the chest in a flexed position. A positive test is indicated by pain in the groin during the test movement.
X-rays, CT scans and/or MRIs may be used to confirm the diagnosis. A special type of MRI called magnetic resonance arthrography (MRA) is becoming very popular. A contrast dye is injected into the hip joint. The dye is absorbed by any areas of damage or degeneration of the cartilage. The surgeon can see if the femoral head has shifted position and/or is stable. Sometimes, a defect in the cartilage is large enough that the femoral head falls into the hole created by the lesion. An MRA would show this type of change.