Abstract
Objectives:
In presence of cam-type femoroacetabular impingement syndrome (FAIS), secondary labral tears are common and can disrupt the functions of the acetabular labrum, leaving the articular cartilage vulnerable to potential accelerated degeneration. Currently, significant controversy exists regarding the indications for repair, reconstruction, and augmentation of the labrum. While others have suggested that labral reconstruction should be utilized in the setting of the ossified labrum, severely damaged labrum, irreparable labrum, and severe labral deficiency, others have suggested that indications should be expanded to include hypotrophic (< 3mm) or hypertrophic labra. Despite the fact that surgical techniques have already been developed to address the hypotrophic and hypertrophic labrum, the role of labral size in chondrolabral and femoroacetabular biomechanics is currently unknown. Therefore, the objective of this study was to assess the influence of labral size on the mechanical environment of the cam-type hip during walking and squatting using finite element (FE) modeling. We hypothesized that a variation in labral size will not further alter the already deleterious contact stresses and strains in cam-type hips, therefore mechanical indices reflecting the likelihood of cartilage damage would be unchanged.
Methods:
For our FE models, we adapted closed bounded surfaces of the bones, cartilage, and labrum segmented from CT image data of a volunteer with cam-type FAIS with an alpha angle of 78°. Three clinical morphologies were considered in the simulations: (1) anatomically normal labrum, corresponding to the segmented geometry; (2) a hypotrophic labrum (width ≤ 3mm); (3) a hypertrophic labrum (width ≥ 6mm). The width of the labrum was defined as the distance from the chondrolabral junction to the apex of the labrum (Fig. 1). The labrum, cartilage, and pelvis surfaces were discretized using quadratic tetrahedral elements in the ANSA software package (BETA CAE Systems SA). Cartilage layers were represented as anisotropic hyperelastic materials with tension-only collagen fibers that varied in primary orientation through the thickness of the cartilage, reflecting the physiological distribution in articular cartilage. The labrum was modeled as a transversely isotropic material with collagen fibers oriented circumferentially along the acetabular rim. The modulus of the pelvis was assigned to the FE mesh based on the CT image data intensity through an established relationship between CT scanner pixel intensity and calcium equivalent density. Patient-specific kinematics from skin-marker motion tracking during gait and squat were combined with joint reaction forces collected by Bergmann et al. to provide kinematic and force boundary conditions for the FE models. All models were analyzed in FEBio (www.febio.org). Labral load support and the labrum fiber strain were evaluated for each morphology. Additionally, the maximum contact pressure and first principal Lagrange strain on the articular surface, and maximum shear stress on the osteochondral surface were evaluated. All outcomes were analyzed at heel-strike for gait and deep flexion for the squat.
Results:
A larger labrum resulted in greater load transferred to the labrum during heel-strike of the gait cycle and during deep flexion of the squat. For both activities, labral load support was the greatest for the hypertrophic labrum compared to the hypotrophic and normotrophic labra (Fig. 2). The maximum circumferential labrum fiber strain varied from 11.0% to 12.6% at heel-strike, and between 7.0% to 7.5% during deep flexion across the three models. Despite differences in labral load support, there were no measurable changes in contact pressure between the three cases during heel-strike or deep flexion (Fig. 3). The first principal Lagrange strain at heel-strike reached a maximum of 28.3% in the normotrophic labrum model, with similar levels in the hypotrophic (28.5%) and hypertrophic (27.4%) labrum models, primarily located in the anterosuperior portion of the acetabular cartilage. There was minimal variation in the first principal Lagrange strain between labrum morphologies during deep flexion, with maximum strain values ranging from 25.3% to 25.6%. Peak maximum shear stress during gait showed minimal differences across all three modeling cases, and slightly increased during deep flexion compared to heel-strike (Fig. 4).
Conclusions:
The variance in labral load support between hypertrophic and hypotrophic morphologies suggests that labral size affects load distribution during walking and squatting, consistent with previous findings in dysplastic hips showing larger labra correlating with increased labral load support. Despite the differences in load transmission between the three labral morphologies, the contact pressure and first principal Lagrange strain across the articular cartilage was minimally affected, and were within similar ranges previously reported for cam-type hips. Similarly, the variation in maximum shear stress was minimal between the three cases, and was above the reported magnitude of shear stress within normal hips. These findings indicate that the mechanical loading on the acetabular cartilage in cam-type hips is left unchanged regardless of labral size during walking and deep flexion, suggesting similar levels of susceptibility to cartilage damage. In the context of clinical interventions, these results have important implications for clinical decision making during primary arthroscopic surgery for cam-type FAIS, particularly regarding management of the labrum. While additional clinical studies are warranted, labral reconstruction may be unnecessary during FAIS surgery, as repair using native fibers could potentially suffice to restore hip functionality, irrespective of labral morphology.
