Summary

This study describes a new biomechanical cadaveric model for hip microinstability. After cyclic stretching of the anterior capsule, an increase in hip rotation and femoral head displacement was found. Labral insufficiency increased femoral head translation when associated with a lax capsule, but not when the capsule is intact and not lax

Abstract

Introduction

Hip microinstability is a recently described source of pain in the non-arthritic hip. It is caused by laxity of the hip ligaments, especially the iliofemoral, which may lead to chondro-labral damage.
The goal of this study was to develop a biomechanical cadaveric model of hip microinstability. We hypothesized that internal/external rotation and femoral head displacements would increase following cyclic stretching of the capsule.

Methods

12 hips from 6 cadaveric pelvii (average age 29, range 18-41) were dissected, potted, and aligned on a custom jig. An Instron machine cyclically stretched the hip capsule in extension and external rotation. Motion capture analysis recorded motion of the femoral head relative to the acetabulum under a 50N force in the medial-lateral (M-L), anterior-posterior (A-P), and superior-inferior (S-I) directions. One hip of each pelvis was randomized to the capsular laxity group, while the matched pair served as the control, without capsular stretching. Both groups underwent a limited capsulotomy and creation of a complete radial tear at the 12’o clock position to simulate labral insufficiency.

Results

Capsular stretching increased the internal-external arc of rotation by 7.6 + 1.7 degrees (p< 0.01), and femoral head translation in the M-L plane by 1.2mm + 0.9mm (p= 0.02), A-P direction by 0.5+ 0.5mm (p=0.02), and S-I direction by 0.4 + 0.7mm (p=0.18). Labral insufficiency significantly increased femoral head translation in the M-L and S-I planes in the capsular laxity group, but not the capsule intact group.

Discussion/Conclusion
It is established that the hip joint may be unstable in cases of bony abnormalities such as hip dysplasia. However, hip microinstability is still an evolving concept. Sources of instability may be traumatic such as prior dislocation events or iatrogenic due to an incomplete capsular closure after hip arthroscopy. Atraumatic instability may happen due to repetitive microtraumatic activities such as ballet and gymnastics, which produce stress to the anterior hip capsule by constant external rotation and extension forces.
This biomechanical model replicates atraumatic microinstability in a physiologic manner by cyclic stretching the anterior capsule. One strength of the current model is the young age of the cadaveric donors, which more closely approximates the typical microinstability patient. The findings confirm our hypothesis that a stretched anterior capsule would increase the rotational arc and femoral head displacement. Interestingly, labral insufficiency only increased femoral head displacement in the capsular laxity group, further demonstrating the importance of the hip capsule in normal joint mechanics.
In conclusion, stretching of the anterior hip capsule leads to altered hip biomechanics.