The accuracy of the use of functional hip motions on localization of the center of the hip

Technical and surgical causes of outliers after computer navigated total knee arthroplasty

Background

Navigated total knee arthroplasty (TKA) improves implant and limb alignment but outliers continue to exist. This study aimed to determine the technical and surgical causes of outliers.

Methods

This retrospective cohort study included 208 patients who had undergone navigated TKA. Limb and implant alignment indices were measured on post-operative CT scans: mechanical femoro-tibial angle (MFTA); coronal femoral angle (CFA); coronal tibial angle (CTA); sagittal femoral angle (SFA); and sagittal tibial angle (STA). Values outside 0°±3° for MFTA and SFA, 90°±3° for CFA, CTA and STA were considered outliers. Intra-operative navigation data and CT scans were evaluated to categorize the causes of sagittal and coronal plane outliers into hip centre error; ankle centre error; heterogeneous tibial cement mantle; malalignment accepted by surgeon; suboptimal knee balance; and no obvious explanation.

Results

Of the 1040 measurements (five per TKA), the overall incidence of outliers was 10.4% (n = 108). Femoral component outliers (CFA + SFA, n = 51) were all attributable to hip centre error. Tibial component outliers (CTA + STA, n = 43) were attributable to ankle centre error (n = 6), heterogeneous cement mantle (n = 20), malalignment accepted by the surgeon (n = 6) and no obvious cause (n = 11). MFTA outliers were attributable to hip centre error (n = 4) or suboptimal knee balance (n = 10).

Conclusions

Surgeon related errors can be minimized by a meticulous operative technique. These results indicate scope for additional technical improvement, especially in hip centre acquisition, which may further reduce the incidence of outliers.

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.