Impingement-free Hip Motion: The ‘Normal’ Angle Alpha after Osteochondroplasty

Femoroacetabular impingement is considered a cause of hip osteoarthrosis. In cam impingement, an aspherical head-neck junction is squeezed into the joint and causes acetabular cartilage damage. The anterior offset angle alpha, observed on a lateral crosstable radiograph, reflects the location where the femoral head becomes aspheric. Previous studies reported a mean angle alpha of 42 degrees in asymptomatic patients. Currently, it is believed an angle alpha of 50 degrees to 55 degrees is normal. The aim of this study was to identify that angle alpha which allows impingement-free motion. In 45 patients who underwent surgical treatment for femoroacetabular impingement, we measured the angle alpha preoperatively, immediately postoperatively, and 1 year postoperatively. All hips underwent femoral correction and, if necessary, acetabular correction. The correction was considered sufficient when, in 90 degrees hip flexion, an internal rotation of 20 degrees to 25 degrees was possible. The angle alpha was corrected from a preoperative mean of 66 degrees (range, 45 degrees - 79 degrees) to 43 degrees (range, 34 degrees - 60 degrees) postoperatively. Because the acetabulum is corrected to normal first, the femoral correction is tested against a normal acetabulum. We therefore concluded an angle alpha of 43 degrees achieved surgically and with impingement-free motion, represents the normal angle alpha, an angle lower than that currently considered sufficient.