Abstract
Introduction
One of the objectives of total hip arthroplasty is to restore femoral and acetabular combined anteversion. It is desirable to reproduce both femoral and acetabular antevesions to maximize the acetabular cup fixation coverage and hip joint stability. Studies investigated the resultant of implanted femoral stem anteversion in western populations showed that the implanted femoral stems had only a small portion can meet the desirable femoral anteversion angle1, and anteversion angle increases after the implantation of an anatomical femoral stem with anteverted stem neck comparing to anatomical femoral neck2. The purpose of this study was to anatomically measure the anteversion angular difference between metaphyseal long axis and femoral neck in normal Chinese population. The metaphyseal long axis represents the coronal fixation plane of modern cementless medial-lateral cortical fitting taper stem. This angular difference or torsion Δ angle provides the estimation of how much the neck antevertion angle of femoral stem would be needed to match for desirable anatomical femoral neck version.
Methods
140 (77 male and 63 female) anonymous normal adult Chinese CT data with average age of 54.6 (male 54.6, female 54.5, P=0.95) were segmented and reconstructed to 3D models in Trauson Orthopeadic Modeling and Analytics (TOMA) program. Femoral head center, femoral neck axis and center point of diaphyseal canal 100mm bellow calcar formed the femoral neck plane. The metaphyseal stem implantation plane was determined by the center point of medial calcar, proximal canal central axis formed by femoral neck plane and the center point of diaphyseal canal 100mm bellow calcar. [Fig. 1] The angle between two planes was the torsion Δ angle between femoral placement plane and anatomical femoral neck. [Fig. 2] The torsion Δ angles were measured for all 140 cases. The traditional anteversion angle for anatomical femoral neck was also measured by Murphy's method. Student T test was perform to compare the angles for male and female. The 98% confidence level was assumed.
Results
The average torsion Δ angle for whole population was 4.9°(0.04°-15.6°), SD=3.52°, male: 4.6° (0.42°-13.9°), SD=3.09°; female: 5.3° (0.04°-15.6°), SD=3.98°. There was no statistical significant difference between genders. P=0.28. All metaphyseal stem placement planes were less anteverted than anatomical femoral neck plane. [Fig. 3] The average anatomical femoral neck anteversion angle for total population was 18.6° (0.27°-42.6°), SD=7.54°; male: 18.6° (0.27°-32.9°), SD=7.37°; female: 18.7° (1.74°-42.6°), SD=7.81°. There was no statistical significance between male and female P=0.92. Only 26% of study population or 37 cases with unadjusted implant neck version had normal anteversion angle of 10°-15° (Tönnis).
Discussion
The study suggested femoral stem neck anteversion angle adjustments up to 11° was necessary to match anatomical femoral neck for 94% of cases in Chinese population. And the adjustments of 0°-7° represented the 76% majority of population. This finding was in agreement with the published data in western population2.
Significance
Variable femoral stem neck anteversion angles up to 11° are necessary to reproduce the anatomical anteversions for 94% of normal Chinese population.
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