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. The alpha angle is formed by a line drawn from the center of the femoral head through the center of the femoral neck, and a line from the center of the femoral head to the femoral head/neck junction, found by the point by which the femoral neck diverges from a circle drawn around the femoral head.  At present, the upper end of normal is an alpha angle of 50 - 55 degrees.


Femoroacetabular impingement


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Femoroacetabular impingement is a suggested diagnosis for pain in the hip in the absence of other apparent causes, in which abnormal anatomy or superphysiologic motion produces eccentric loading of the joint or actual impingement on the femur by the acetabular rim. It is thought to be a cause of progressive athrosis of the hip.


Standard anterior-posterior pelvic, frog-leg lateral and cross-table lateral radiographs should be obtained. In certain situations a false-profile radiograph can asses anterior femoral head coverage.



The AP pelvis X-ray must be well centered and well developed as to show a clear outline of the acetabulum.  The coccyx should point toward the symphysis pubis, and there should be about 1-2 cm between them. The sourcil, the anterior and posterior walls, the tear drop and the lateral edge of the acetabulum should be noted. Measurements may be taken to evaluate for hip dysplasia including the Tönnis angle (abnormal > 10 degrees), the lateral center-edge angle of Wiberg (abnormal < 25 degrees), and the anterior center-edge angle of Lequesne (abnormal < 25 degrees) as measured on a false-profile radiograph. The neck shaft angle of the proximal femur is considered normal between 120 and 140 degrees.


Alterations of the proximal femoral anatomy, such as head neck offset and bump formation can be observed in addition to acetabular and labral pathology. A pistol grip deformity of the femoral head is often seen in Cam Type impingement.  In this situation the superior-lateral head neck junction is convex instead of concave.  A high fovea can also indicate asphericity of the femoral head that is not able to be appreciated on the AP films. The alpha angle is a useful radiographic measurement for quantifying the head-neck junction deformity. This angle is most accurate when measured on MRI, however it can also be measured using plain films. The Dunn view (AP of hip in neutral rotation, 45 degrees hip flexion, and 20 degrees abduction) is the most sensitive x-ray view for detecting femoral head-neck asphericity


If closer examination of the radiographs indicates dysplasia, CT scanning will allow accurate observation of the bony anatomy and more precise quantification of the degree of dysplasia.  If impingement or labral pathology is still suspected, special MRI with specialized radial sequences perpendicular to the true plane of the acetabulum have been found to be useful.


Reorientation of the articulating surfaces of the hip joint is an attractive procedure in the patient with hip dysplasia. Increased joint congruity after reorientation of the osteotomized fragment allows load transmission through a broader area subjected to less pressure. These changes can be expected to reduce pain and possibly protect the articular cartilage from degenerative changes. Joint reorientation, particularly of the femur, also may result in a more functional arc of motion.  Osteotomies should be offered to young patients who have symptomatic hip dysplasia without excessive proximal migration of the center of rotation, reasonably well preserved range of motion, and no more than mild degenerative changes of the articular surface. The primary abnormality in most patients with hip dysplasia is located on the acetabular side of the joint. Thus, pelvic osteotomy corrects the major anatomic abnormality and has the further advantage over femoral osteotomy of not creating a secondary femoral deformity. Femoral osteotomy may be added to pelvic osteotomy when coexistent femoral anatomic abnormalities are significant. The Bernese periacetabular osteotomy is indicated for patients with hip symptoms of mechanical overload, impingement, or hip instability as a result of insufficient acetabular coverage.  It is performed with a series of straight, relatively reproducible extra-articular cuts. This osteotomy allows large corrections of the osteotomized acetabular fragment in all directions, including lateral rotation, anterior rotation, and medialization of the hip joint. The posterior column of the hemipelvis remains intact; thus, minimal internal fixation is required, and early ambulation with no external immobilization with a cast or a brace is possible.

Related Resources


  1. Burnett RS. Della Rocca GJ. Prather H. Curry M. Maloney WJ. Clohisy JC. Clinical presentation of patients with tears of the acetabular labrum. Journal of Bone & Joint Surgery - American Volume. 88(7):1448-57, 2006 Jul.
  2. Ganz R. Parvizi J. Beck M. Leunig M. Notzli H. Siebenrock KA. Femoroacetabular impingement: a cause for osteoarthritis of the hip. Clinical Orthopaedics & Related Research. (417):112-20, 2003 Dec.
  3. Wenger DE. Kendell KR. Miner MR. Trousdale RT. Acetabular labral tears rarely occur in the absence of bony abnormalities. Clinical Orthopaedics & Related Research. (426):145-50, 2004 Sep.
  4. Trousdale RT. Acetabular osteotomy: indications and results. Clinical Orthopaedics & Related Research. (429):182-7, 2004 Dec.
  5. Garbuz DS. Masri BA. Haddad F. Duncan CP. Clinical and radiographic assessment of the young adult with symptomatic hip dysplasia. Clinical Orthopaedics & Related Research. (418):18-22, 2004 Jan.
  6. Sanchez-Sotelo J, Trousdale RT, Berry DJ, Cabanela ME. Surgical Treatment of DDH in Adults: I Non-Arthroplasty Options and II Arthroplasty Options  JAAOS 2002; 10(5): 321-344.