CT-based simulation of impingement-free mechanical hip motion after unilateral total hip arthroplasty
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Bando Kazuma1, Nakamura Junichi2, Hagiwara Shigeo3,4
1Center for Frontier Medical Engineering, Chiba University, 1-33 Yayoi-cho, CFME room#B201, Inage-ku, Chiba, 263-8522, Japan.
Background:
Range of motion (ROM) after total hip arthroplasty (THA) is influenced by implant-related parameters such as femoral head diameter, neck length, and stem offset. However, the quantitative effects of these parameters on ROM, leg length discrepancy (LLD), and femoral offset discrepancy remain unclear. This study aimed to quantify the effects of implant-related parameters on simulated impingement-free mechanical ROM after unilateral THA.
Methods:
A computed tomography (CT) -based three-dimensional (3D) simulation was performed in 100 patients who underwent unilateral THA with a clinically and radiographically healthy contralateral hip. The contralateral healthy hip was also evaluated as a patient-specific comparator. Implant configurations were simulated by varying femoral head diameter, neck length, and stem offset. Femoral head diameters of 36 mm and 32 mm, neck lengths from - 3.5 mm to + 7 mm, and standard-offset and lateral-offset stems were evaluated. Simulated impingement-free mechanical ROM was assessed in maximum flexion, maximum extension, internal rotation at 90° of flexion, and external rotation at 10° of extension. Simulated LLD and femoral offset discrepancy relative to the contralateral healthy hip were also quantified.
Results:
Increasing neck length improved simulated mechanical ROM in all evaluated directions, with median increases of 6.3° in flexion and 5.3° in extension for every 3.5-mm increment. Lateral offset stems further increased flexion by 16.3° compared with standard stems. Femoral head enlargement from 32 mm to 36 mm modestly improved simulated mechanical ROM, increasing flexion by approximately 2.5°. However, neck lengthening increased LLD by a median of 2.3 mm per 3.5-mm increment. The contralateral healthy hips showed median simulated ROM values of 119° in flexion, 66° in extension, 31° in internal rotation, and 48° in external rotation.
Conclusions:
Increasing neck length and lateral offset improved simulated impingement-free mechanical ROM after unilateral THA, whereas neck lengthening increased LLD. Femoral head enlargement also improved simulated mechanical ROM, but its effect was limited compared with those of neck length and stem offset. The contralateral healthy hip may provide contextual patient-specific information for interpreting simulated mechanical ROM in unilateral THA.
