Hip pain is mostly seen in the elderly, mainly due to degenerative changes in the articular cartilage caused by decades of excessive joint axial loading of the hip joint. However, there are some young and middle-aged patients, especially those who exercise regularly, who also present with chronic hip pain, a condition that is difficult to explain by the pathogenesis described above. A classic case provided by Dr. Kaye from the Department of Imaging at the University of Pennsylvania may shed a new light on this issue. The patient, a 35-year-old male, had hip pain for several years. The results of a hip MRI were as follows. Imaging: oblique axial T1-weighted and coronal proton density-weighted fat-suppressed imaging, respectively, showed an anterior-superior tear of the acetabular glenoid labrum. The oblique axial image showed an abnormal femoral neck profile with an abnormal protrusion of bone impingement at the femoral head-neck joint. There was also cartilage damage and subchondral cystic changes in the lateral acetabulum. For the alpha angle measurement, the result was 58° (normal less than 50°) as shown in the figure. Diagnosis: femoral acetabular impingement syndrome (cam impingement type). CASE STUDY: Femoroacetabular impingement syndrome (FAI) is a series of clinical symptoms caused by congenital abnormal development of the acetabulum and proximal femur, and is a major cause of osteoarthritis of the hip in young patients. The pathogenesis is triggered by abnormal development of the hip joint resulting in altered biomechanics and perennial repetitive microtrauma to the acetabular glenoid labrum. The two currently recognized types are cam and pincer, and both types are often present together (86% of patients have both). The cam type, as in the present case of FAI, often presents with structural abnormalities at the femoral head-neck junction. In this type, the abnormal impingement of the proximal femur against the acetabular glenoid labrum, repeated over the years, leads to cartilage damage in the hip joint and eventually to osteoarthritis. The alpha angle can be determined on an oblique axial MR sequence, which should be parallel to the femoral neck plane on an oblique axial MRI scan. The circle should be delineated along the contour of the femoral head, excluding the femur and acetabular cartilage. The center of the circle is drawn perpendicular to the femoral neck and bisects the femoral neck plane. Another straight line is drawn between the center of the circle and the point where the contour of the femoral head begins to deviate from the circle. The angle between the two straight lines is the alpha angle, which is normally less than 50° and indicates the possibility of cam-type FAI when the angle is greater than 55°. FAI often occurs in young patients, between the ages of 15 and 50. It usually develops into osteoarthritis in later stages; therefore, it is more difficult to distinguish it from primary osteoarthritis in later stages if the diagnosis is not clear. Treatment: It consists mainly of surgical removal of developmentally abnormal bone and correction of biomechanics. If there is a possibility of secondary osteoarthritis, arthroscopic repair of cartilage damage or even hip replacement is required.