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

Mirjam Neumann MD, Quanjun Cui MD, Klaus A. Siebenrock MD, Martin Beck MD
Symposium: Femoroacetabular Impingement: Current Status of Diagnosis and Treatment
Volume 467, Issue 3 / March , 2008

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Abstract

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 α, observed on a lateral crosstable radiograph, reflects the location where the femoral head becomes aspheric. Previous studies reported a mean angle α of 42° in asymptomatic patients. Currently, it is believed an angle α of 50° to 55° is normal. The aim of this study was to identify that angle α which allows impingement-free motion. In 45 patients who underwent surgical treatment for femoroacetabular impingement, we measured the angle α 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° hip flexion, an internal rotation of 20° to 25° was possible. The angle α was corrected from a preoperative mean of 66° (range, 45°–79°) to 43° (range, 34°–60°) postoperatively. Because the acetabulum is corrected to normal first, the femoral correction is tested against a normal acetabulum. We therefore concluded an angle α of 43° achieved surgically and with impingement-free motion, represents the normal angle α, an angle lower than that currently considered sufficient.

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