Abstract
Combined anteversion angle of acetabular component and femeral neck is an important factor for total hip arthroplasty (THA) as it may affect impingement and dislocation. Previous studies have collected data mainly from direct measurements of bone morphology or manual measurements from 2D or 3D radiolographic images. The purpose of this study was to electronically measure the version angles in native acetabulum and femur in matured normal Caucasion population using a novel virtual bone database and analysis environment named SOMA™.
221 CT scans from a skeletally mature, normal Caucasian population with an age range of 30–95 years old. The population included 135 males and 86 females. CT data was converted to virtual bones with cortical and cancellous boundaries using custom CT analytical sofware. (SOMA™ V.3.2) Auxillary reference frames were constructed and measurements were performed within the SOMA™ design environment. Acetabular Anteversion (AA) angle as defined by Murray1 was measured. The acetabular rim plane was constructed by selecting 3 bony land marks from pubis, ilium and ischium. A vector through acetabular center point and normal to the rim plane defined the plane for the AA measurement. The AA was defined as the angle of this plane relative to the frontal (Coronal) plane of the pelvis. The Femoral Neck Anteversion (FNA) angle was measured from the neck axis plane to the frontal (Coronal) plane as defined by the posterior condyles. The neck axis plane was constructed to pass through femoral neck axis perpendicular to the transverse plane. The combined anteversion angle was computed as the summation of acetabular and femoral anteversion angles. Student's t tests were performed to compare gender difference with an assumed 95% confidence level.
The mean AA angle for total population was 25.8°, SD=7.95°. The mean AA for male was 24.8°, SD=5.93° and for female was 27.3°, SD=7.14°. P=0.009.
The mean FNA angle for total population was 14.3°, SD=6.52°. The mean FNA for male was 13.5°, SD=7.97° and for female was 15.5°, SD=7.80°. P=0.058.
The mean combined anteversion angle for total population was 40.1°, SD=10.76°. The mean combined anteversion angle for male was 38.3° SD=10.39 ° and for female was 42.8° SD=10.83 °. P=.0002. The plot of AA as a function of FNA shows weak correlation for both male and female. (Figure 1) The frequency distribution is shown in Figure 2.
The results showed the both AA, FNA and combined anteversion angles were significantly smaller in male population than that in female population. The FNA angle of the cementless femoral stem can be smaller than with the natural femur, therefore a higher AA or higher posterior build up may be required for the acetabular component for optimal function of a THA.
