Review Question - QID 210161

This patient likely has femoroacetabular impingement (FAI). Figures A & E demonstrate the alpha angle and the crossover sign which are both used in the diagnosis of FAI.

The alpha angle is a measurement for quantifying the head-neck junction deformity. On the frog-leg lateral, it is formed by a line drawn from the center of the femoral head, through the center of the femoral neck. A second line is drawn from the center of the femoral head to the point on the anterolateral head-neck junction where the radius of the femoral head begins to increase beyond the radius found more centrally in the acetabulum where the head is more spherical. The intersection of these two lines forms the alpha angle, and values of >60° are suggestive of a head-neck offset deformity. The crossover sign is a sensitive and specific indicator of native acetabular version. It is seen on AP radiograph of the hip or pelvis. Normally, the anterior and posterior wall shadows should meet superiorly and laterally. In cases of acetabular retroversion, the crossover of the shadows occurs more distally. If the crossover sign is present it signifies acetabular retroversion and possible pincer impingement.

Clohisy et al. performed a systematic approach to the plain radiographic evaluation of the adult hip. They report that many radiographic measurements have been described as indicators of structural disease. They report that the lateral center-edge angle, the anterior center-edge angle, the acetabular index of depth to width, the femoral head extrusion index, and the Tönnis angle have all been used in the diagnosis of acetabular dysplasia. Measurements of acetabular version, the head-neck offset, and the alpha angle have been used in the diagnosis of FAI.

Nötzli et al. developed a method to describe concavity at the femoral head-neck junction. MRI scans of 39 patients with groin pain and positive impingement tests were compared to asymptomatic controls. They found an alpha angle of 74.0 degrees for symptomatic patients and 42.0 degrees for the control group. They conclude that using MRI symptomatic hips of patients who have impingement have significantly less concavity at the femoral head-neck junction than normal hips.

Figure A is a lateral of the hip demonstrating the calculation of the alpha angle. Figure B is an AP pelvis radiograph demonstrating the Tönnis angle. Acetabula having a Tönnis angle of 0° to 10° are considered normal, whereas those having an angle of >10° or <0° are considered to have increased and decreased inclination, respectively. Figure C is an AP radiograph of the hip demonstrating the lateral center-edge angle. Values of <25° may indicate inadequate coverage
of the femoral head. Figure D is a false-profile radiographic view of the hip demonstrating the anterior center-edge angle. Values of <20° can be indicative of structural instability. Figure E is an AP radiograph of the hip demonstrating the crossover sign.

Incorrect Answers:
Answers 2, 3, 4, & 5: The Tönnis angle, lateral center-edge angle, and the anterior center-edge angle are used in diagnosing and quantifying acetabular dysplasia. The alpha angle and the crossover sign are used in the diagnosis of FAI.