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.