Radiographic method to measure the inclination angle of the acetabulum.

BACKGROUND: Various attempts have been made to assess the inclination angle of the acetabulum utilizing new imageprocessing technologies to enable three-dimensional reconstruction of the acetabulum. However, reliability of these methods has not been estimated in comparison with anatomical measurements. This study developed a geometric method for measuring the acetabular inclination with radiograms and evaluated the reliability of this method by comparing the anatomically measured acetabular inclination angle of the same dry pelvic bone.

METHODS: One hundred and ten acetabulums from 55 human pelvis specimens were used. The uppermost, most frontal, and posterior lowermost points of the acetabular rim were determined, and an axis perpendicular to the triangular plane formed by these three points was defined as the anatomical inclination axis of the acetabulum. The anatomical lateral and anterior opening angles were directly measured using a tool we devised for this purpose. Posteroanterior and lateral radiograms of each pelvis were taken concurrently and three points were marked by small metal plates. Based on projections of these three points onto the posteroanterior and lateral images, we geometrically measured and calculated the lateral and anterior opening angles of the acetabulum.

RESULTS: Anatomical measurements of the lateral opening angles ranged from 38 degrees to 63 degrees (mean 51.0 degrees ), and anterior opening angles from 10 degrees to 36 degrees (mean 20.8 degrees ). Geometric measurements of the lateral opening angles ranged from 40 degrees to 61 degrees (mean 50.5 degrees ), and anterior opening angles ranged from 7 degrees to 35 degrees (mean 20.8 degrees ). Geometrically measured acetabular inclination angles were highly correlated with the anatomically measured ones for both the lateral and anterior opening angles, with correlation coefficients of 0.803 and 0.822, respectively.

CONCLUSIONS: Our geometric measurement method of the acetabular inclination angle enabled us to calculate the lateral and anterior opening angles, which were very close to the corresponding anatomical measurements. This method allows three-dimensional evaluation of the anatomical structure of the hip and may be useful in predicting progression of coxarthrosis by measuring the femoral neck shaft and anteversion angles.