Effect of Decreasing the Anterior Pelvic Tilt on Range of Motion in Femoroacetabular Impingement: A Computer-Simulation Study

Background: The influence of pelvic tilt mobility, which can be reproduced in computer-simulation models, is an important subject to be addressed in the understanding of femoroacetabular impingement (FAI) pathophysiology. Purpose: To use computer-simulation models of FAI cases to evaluate the optimum improvement in hip range of motion (ROM) achieved by decreasing the anterior pelvic tilt and compare the results with the improvement in ROM achieved after cam resection surgery. Study Design: Controlled laboratory study. Methods: The pre- and postoperative computed tomography (CT) images from 28 patients with FAI treated with arthroscopic cam resection were evaluated. Using a dynamic computer-simulation program, 3-dimensional models with a 5° and a 10° decrease in anterior pelvic tilt from the supine functional pelvic plane (baseline) were created from the preoperative CT scans. Similar models were constructed for hips before (at baseline) and after cam resection. Improvements from baseline in maximum internal rotation at 45°, 70°, and 90° of flexion were assessed for the 5° change in pelvic tilt, 10° change in pelvic tilt, and cam resection models, and the results were compared for all conditions. Results: The combination of a 10° change in pelvic tilt and cam resection showed the largest ROM improvement from baseline ( P < .001). Improvement in internal rotation in the cam resection model was significantly higher compared with the 5° pelvic tilt change model ( P < .001), while there was no significant difference between the cam resection model and the 10° pelvic tilt change model. Conclusion: Decreasing anterior pelvic tilt by 10° in the preoperative computer simulation model resulted in an equivalent effect to cam resection, while a 5° change in pelvic tilt was inferior to cam resection in terms of ROM improvement. Clinical Relevance: Enough of a decrease in anterior pelvic tilt may contribute to ROM improvement that is as effective as that of cam resection surgery.

Accurate Arthroscopic Cam Resection Normalizes Contact Stresses in Patients With Femoroacetabular Impingement

Background: Femoroacetabular impingement (FAI) is increasingly recognized as a cause of hip pain in young adults. The condition leads to chondrolabral separation and chondral delamination and eventually predisposes to osteoarthritis of the hip. FAI that inflicts cartilage damage has been observed in hips with abnormal morphological characteristics and is related to a long-term evolution toward osteoarthritis. Arthroscopic surgery, which allows for correction of morphological characteristics and restores impingement-free motions, is the current standard of treatment. Hypothesis: Arthroscopic cam resection can restore the normal mechanical environment of the hip joint in cam-type FAI. Study Design: Descriptive laboratory study. Methods: Patient-specific discrete element models from 10 patients with cam-type FAI (all male; age, 18-40 years) were defined based on preoperative computed tomography scans and postoperative magnetic resonance imaging (MRI) scans. Complete cam resection postoperatively on MRI was confirmed with alpha angles <55°. The preoperative and postoperative peak contact stress findings during impingement testing were compared against a matched control group. Results: Peak contact stress was significantly elevated in patients with cam-type FAI during impingement testing, with increasing amounts of internal hip rotation (26.6 ± 11.64 MPa in cam patients preoperatively, 12.1 ± 4.62 MPa in those same patients postoperatively, and 11.4 ± 1.72 MPa in the virtual control group during impingement testing at 20° of internal hip rotation; P < .01). This effect was normalized after arthroscopic cam resection and loading patterns matched those of the control group. Conclusion: Accurate arthroscopic cam resection restored the normal peak joint contact stresses in the hip joint. This highlights the importance of early and complete cam resections in the face of a positive diagnosis of cam-type FAI. Clinical Relevance: Treatment of cam-type FAI effectively normalizes hip joint contact mechanics.

Hip Joint Torsional Loading Before and After Cam Femoroacetabular Impingement Surgery

Background: Surgical management of cam femoroacetabular impingement (FAI) aims to preserve the native hip and restore joint function, although it is unclear how the capsulotomy, cam deformity, and capsular repair influence joint mechanics to balance functional mobility. Purpose: To examine the contributions of the capsule and cam deformity to hip joint mechanics. Using in vitro, cadaveric methods, we examined the individual effects of the surgical capsulotomy, cam resection, and capsular repair on passive range of motion and resistance of applied torque. Study Design: Descriptive laboratory study. Methods: Twelve cadaveric hips with cam deformities were skeletonized to the capsule and mounted onto a robotic testing platform. The robot positioned each intact hip in multiple testing positions: (1) extension, (2) neutral 0°, (3) flexion 30°, (4) flexion 90°, (5) flexion-adduction and internal rotation (FADIR), and (6) flexion-abduction and external rotation. Then the robot performed applicable internal and external rotations, recording the neutral path of motion until a 5-N·m of torque was reached in each rotational direction. Each hip then underwent a series of surgical stages (T-capsulotomy, cam resection, capsular repair) and was retested to reach 5 N·m of internal and external torque again after each stage. During the capsulotomy and cam resection stages, the initial intact hip’s recorded path of motion was replayed to measure changes in resisted torque. Results: Regarding changes in motion, external rotation increased substantially after capsulotomies, but internal rotation only further increased at flexion 90° (change +32%, P = .001, d = 0.58) and FADIR (change +33%, P < .001, d = 0.51) after cam resections. Capsular repair provided marginal restraint for internal rotation but restrained the external rotation compared with the capsulotomy stage. Regarding changes in torque, both internal and external torque resistance decreased after capsulotomy. Compared with the capsulotomy stage, cam resection further reduced internal torque resistance during flexion 90° (change −45%, P < .001, d = 0.98) and FADIR (change −37%, P = .003, d = 1.0), where the cam deformity accounted for 21% of the intact hip’s torsional resistance in flexion 90° and 27% in FADIR. Conclusion: Although the capsule played a predominant role in joint constraint, the cam deformity provided 21% to 27% of the intact hip’s resistance to torsional load in flexion and internal rotation. Resecting the cam deformity would remove this loading on the chondrolabral junction. Clinical Relevance: These findings are the first to quantify the contribution of the cam deformity to resisting hip joint torsional loads and thus quantify the reduced loading on the chondrolabral complex that can be achieved after cam resection.