Background: The purpose of this investigation is to determine the optimum position of the prosthesis in total hip arthroplasty for reducing neck impingement using a mathematical formula.
Methods: We calculated the cup inclination, cup anteversion, and stem antetorsion in cases with various sizes of femoral head (28, 32, 36, and 44 mm in diameter) to fulfill severe range of motion criteria: (1) flexion more than 120°, (2) extension more than 30°, (3) internal rotation at 90° flexion more than 60°, and (4) external rotation at neutral more than 40°.
Results: When the areas to fulfill the severe range of motion criteria were compared by femoral head diameter, the area for 28 mm was extremely small relative to those of 32, 36, and 44 mm. Theoretically, the optimum position of the prosthesis in total hip arthroplasty without neck impingement should be oriented at a cup inclination of 45° combined with the cup anteversion and stem antetorsion so that the sum of the cup anteversion plus 0.7 times the stem antetorsion equals 42° with a head diameter more than 32 mm. This study also recommends the optimum position of the prosthesis as 45° cup inclination, 25° cup anteversion, and 25° stem antetorsion when the surgeon can choose a freely adjustable modular stem system. However, this theory assumes that the pelvic inclination has no changes caused by aging and can be validated in the lying, sitting, and standing positions.
Conclusions: The prosthesis in total hip arthroplasty without neck impingement should be oriented at a cup inclination of 45° combined with cup anteversion and stem antetorsion determined by the formula: cup anteversion + 0.7 × stem antetorsion = 42°. A range of acceptable positions would be more helpful and realistic to a surgeon trying to ensure adequate prosthesis positions.