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Hip impingement or femoroacetabular impingement (FAI) is a condition in which there is an abnormal relationship between the top of the femur, especially the head and the neck, with the rim of the hip socket or acetabulum. This results in abnormal contact between them, which wears down the cartilage over the head of the femur, the labral cartilage at the rim, may cause acetabular fracture, and leads to osteoarthritis of the hip over the long term.
There are two types of hip impingement, the cam type and the pincer type. The first type is due to abnormal morphology of the junction between the femoral head and neck, and the second is due to an excessively deep acetabulum. Both coexist to varying degrees in most patients. It is important to understand the manner in which different forms of these conditions contribute to impingement and later on to the development of arthritis, so that it may be prevented.
FAI of the cam type is found in cases where there is a reduced angle between the femoral neck and head, a good example being the pistol-grip deformity following a slipped capital femoral epiphysis (SCFE).
FAI of the pincer type may occur in a femur with a normal head-neck junction morphology, but with an overly deep rim which brings it into contact with the head before the joint has achieved normal movement in that direction. This is unlike the physiological impingement which occurs at the extremes of movement. One instance of this is severe retroversion of the acetabulum.
There are several putative risk factors for this condition but definite evidence is yet to be gathered with respect to their causative roles.
Some studies have shown that siblings are twice as likely to have shared pincer type impingement and have 2.8 times the risk of having shared cam type impingement as controls, as documented by clinical features and radiography. The risk of cam impingement of the hip is also higher among men. This may indicate the presence of inherited characteristics.
It is well known that repeatedly subjecting the hip joint to a supraphysiologic range of motion in the growing years may cause the bone architecture to undergo reactive changes in both soft and bony tissues. This explains the relationship between sports such as basketball, lacrosse, soccer, tennis, football, dancing and golf, which require extreme flexion and rotation of the hips, and the occurrence of hip impingement.
The heavy and repetitive stress applied to these joints induces remodeling of the bone, according to this model, which eventually leads to the changes of bony structure seen in FAI with the characteristic clinical features. In this way FAI may be a result of specific activity during a specific period of time.
Pathology of the hip joint may occur in conditions such as SCFE or Perthe’s disease (osteonecrosis of the head of the femur between the ages of 4 and 12 years). In both, the femoral head, or the neck-head junction, may undergo deformity because the joint no longer functions well over the full range of motion.
In some children, the femoral neck may undergo fracture which heals by malunion, thereby altering the relative anatomy and positions of the head, neck and acetabulum, as well as the way in which the joint functions thereafter. This could also lead to hip impingement.
A few cases of pediatric FAI may be due to structural anomalies of the femoroacetabular joint following corrective surgery, such as osteotomy of the joint to treat hip dysplasia.