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. Author manuscript; available in PMC: 2024 Jan 1.
Published in final edited form as: J Biomech. 2022 Dec 24;146:111424. doi: 10.1016/j.jbiomech.2022.111424

Acetabular Labrum and Cartilage Contact Mechanics During Pivoting and Walking Tasks in Individuals with Cam Femoroacetabular Impingement Syndrome

Lindsay L Schuring 1,2, Joseph D Mozingo 2, Amy L Lenz 1,2,3, Keisuke Uemura 2, Penny R Atkins 2,4, Niccolo M Fiorentino 5, Stephen K Aoki 2, Christopher L Peters 2, Andrew E Anderson 1,2,4,6
PMCID: PMC9869780  NIHMSID: NIHMS1862590  PMID: 36603366

Abstract

Femoroacetabular impingement syndrome (FAIS) is a motion-related pathology of the hip characterized by pain, morphological abnormalities of the proximal femur, and an elevated risk of joint deterioration and hip osteoarthritis. Activities that require deep flexion are understood to induce impingement in cam FAIS patients, however, less demanding activities such as walking and pivoting may induce pain as well as alterations in kinematics and joint stability. Still, the paucity of quantitative descriptions of cam FAIS has hindered understanding underlying hip joint mechanics during such activities. Previous in silico studies have employed generalized model geometry or kinematics to simulate impingement between the femur and acetabulum, which may not accurately capture the interplay between morphology and motion. In this study, we utilized models with participant-specific bone and articular soft tissue anatomy and kinematics measured by dual-fluoroscopy to compare hip contact mechanics of cam FAIS patients to controls during four activities of daily living (internal/external pivoting and level/incline walking). Averaged across the gait cycle during inclined walking, patients displayed increased strain in the anterior joint (labrum strain: p-value = 0.038, patients: 11.7±6.7%, controls: 5.0±3.6%; cartilage strain: p-value = 0.029, patients: 9.1±3.3%, controls: 4.2±2.3). Patients also exhibited increased average anterior cartilage strains during external pivoting (p-value = 0.039; patients: 13.0±9.2%, controls: 3.9±3.2%]). No significant differences between patient and control contact area and strain were found for level walking and internal pivoting. Our study provides new insights into the biomechanics of cam FAIS, including spatiotemporal hip joint contact mechanics during activities of daily living.

Keywords: Hip, Gait, Femoroacetabular Impingement Syndrome, Acetabular Labrum, Acetabular Cartilage, Dual-Fluoroscopy

1. Introduction

Femoroacetabular impingement syndrome (FAIS) is a painful, motion-related pathology of the hip characterized by the presence of abnormal osseous spurs found either on the femoral head-neck junction (cam FAIS), acetabular rim (pincer FAIS), or a combination of both (mixed FAIS) (Griffin et al., 2016). Cam morphology in particular is associated with rapid and severe joint degeneration, including articular cartilage damage, labral tears, and delamination at the chondrolabral junction; it has been identified as a leading etiological factor for hip osteoarthritis (Barros et al., 2010; Beaulé et al., 2018; Ganz et al., 2008). The proposed mechanism of deleterious joint mechanics in cam FAIS is that a reduction of the femoral head sphericity and head-neck offset causes the femoral head to lever out or abut prematurely against the anterosuperior acetabulum in certain joint orientations, adding additional strain to the cartilage and labrum that may not otherwise occur in hips with non-pathologic morphology (Ganz et al., 2003; Ito et al., 2001).

Common clinical understanding is that activities requiring deep flexion induce impingement in individuals with cam morphology, especially when deep flexion is combined with internal rotation. However, other activities with submaximal hip flexion encountered during daily life, such as walking and pivoting, may also elicit pain in patients with FAIS (Griffin et al., 2016; Groh and Herrera, 2009), which is indicative that true bony impingement is likely not the cause of symptoms in these patients. Recent research using dual fluoroscopy has shown that hip biomechanics are altered in patients with cam FAIS compared to controls during incline walking on a shallow slope of five degrees (Atkins et al., 2019). In a study of the same participants, Lewis et al. showed that patients had greater variability in minimum bone-to-bone distance between the femoral head and acetabulum during both level and incline walking compared to controls (Lewis et al., 2022). The differences in bone-to-bone distance were theorized to indicate greater movement of the femoral head within the acetabulum, which could have downstream effects on soft tissue mechanics at the cartilage and labrum. Finite element studies evaluating hip biomechanics during subject-specific gait have reported that hips with cam FAIS displayed increased tensile strains and increased shear stresses in anterolateral and posterior cartilage compared to control hips, as well as a less even distribution of contact pressure across the joint (Ng et al., 2017; Todd et al., 2022). Collectively, these findings suggest that articular biomechanics of the hip, including increased soft-tissue stresses and strains, during low-range of motion activities may be involved with the pathophysiology of cam FAIS.

Soft tissue overlap (STO) modeling provides an approach to estimate participant-specific joint contact mechanics from kinematically-driven 3D reconstructions of the tissue anatomy. STO methods have been used to estimate contact area and strains in the ankle and knee (Akpinar et al., 2019; Bingham et al., 2008; Bischof et al., 2010; Shin et al., 2011; Thorhauer and Tashman, 2015; Wan et al., 2006; Yin et al., 2017; Zheng et al., 2018), and contact deformation depth in the hip (Arbabi et al., 2010). Applied to the study of cam FAIS, the STO approach could provide a computationally-efficient method to estimate cartilage and labrum contact mechanics.

The purpose of this study was to utilize participant-specific STO models to compare cartilage and labrum contact area and strain between patients with cam FAIS and a cohort of controls with non-pathologic hip joint morphology and no significant history of hip pain. Kinematics from four common activities of daily living were evaluated: level walking, incline walking, external pivoting, and internal pivoting. Based on the findings of previous studies (Atkins et al., 2019; Lewis et al., 2022; Ng et al., 2017; Todd et al., 2022), we hypothesized that contact strains and contact area would be increased in cam FAIS hips when compared to control hips. To improve the interpretation of our results, contact area and peak contact strains were compared by anatomical region (anterior, superior, and posterior joint) and tissue type (labrum or cartilage).

2. Methods

2.1. Participants

Six cam FAIS participants (4M/2F, age [mean ± standard deviation]: 28.0±7.3 years, BMI: 23.5±2.2 kg/m2) and eleven control participants (6M/5F, age: 23.3±2.2 years, BMI: 21.1±1.9 kg/m2) provided informed consent prior to participation in this Institutional Review Board approved study. Participant recruitment, screening, and imaging procedures were described previously (Atkins et al., 2019; Lewis et al., 2022). Briefly, cam FAIS patients were recruited from the clinic of an orthopaedic surgeon (SKA) with diagnoses following the Warwick agreement definition (Griffin et al., 2016) with 1) patient-reported symptoms of impingement, 2) a positive anterior impingement exam (Klaue et al., 1991), and 3) radiographic confirmation of cam morphology in the modified false profile, frog-leg lateral, and anteroposterior (AP) positions (Atkins et al., 2019). All participants were screened with an AP plain film radiograph to measure the alpha angle (to confirm the presence or absence of cam morphology) and to ensure no signs of pincer morphology, dysplasia, acetabular retroversion, or Legg-Calve-Perthes disease were present. Oblique-axial magnetic resonance (MRI) images from a prior study using the same control population (Atkins et al., 2017) were further used to confirm the AP alpha angle measurements (Nötzli et al., 2002). Herein, an alpha angle larger than 55.5° indicated cam morphology (Allen et al., 2009) and resulted in exclusion from the control cohort. The mean ± standard deviation MRI-measured alpha angle for the groups contained herein were 61±4°, and 40±5° for the cam FAIS and control groups respectively. After initial screening, CT arthrogram data were acquired for each study candidate; any signs of tissue degeneration or osteoarthritis found in CT images resulted in exclusion.

2.2. Imaging

CT images of the hip and knee were obtained for each participant in supine position using a SOMATOM Definition 128-slice CT scanner (Siemens AG, Munich, Germany). To facilitate differentiation of soft tissues (femoral cartilage, acetabular cartilage, and acetabular labrum), contrast agent was injected into the hip joint by a board-certified Interventional Radiologist and traction was applied during imaging (Henak et al., 2014a). The CT scan included both hips and was acquired at 120 kVp, 200 – 400 mAs, with a raw in-plane resolution of ~0.7 mm (range:0.61–0.77 mm) and 1.0 mm slice thickness. To define the femoral coordinate system, the distal femur was also scanned at 120kVp, 150 mAs, with the same in-plane resolution and 3.0 mm slice thickness.

Following a validated protocol (Kapron et al., 2014), participants were imaged at 100 Hz during external/internal pivoting and level/incline walking using a custom DF system (Fiorentino et al., 2016b; Kapron et al., 2014). None of the participants reported pain or discomfort as they performed the activities. Two trials of each activity were captured and the best trial (determined based on task performance and image quality) was used in subsequent analysis. Participants walked at a self-selected speed on an instrumented treadmill set to 0° (level walking) and 5° incline (incline walking). Treadmill speed for both level and incline walking was set based on the participants’ self-selected level ground walking speed determined prior to data collection (Fiorentino et al., 2017, 2016a). The cam FAIS patients walked with a speed of 1.29±0.16 m/s, and controls 1.29±0.07 m/s (mean ± 95% confidence interval). For the pivoting activities, participants were instructed to plant both feet on the ground hip width apart and then rotate their body until they reached the end range of motion (ROM) of hip external or internal rotation. Only the end ROM of the pivoting activities were processed in further steps. Femur and pelvis kinematics were quantified using CT and a validated model-based markerless tracking method (Bey et al., 2008; Kapron et al., 2014). Kinematics of the walking trials were normalized to 101 frames per trajectory via linear interpolation in MATLAB (v2017b; MathWorks, Natick, MA). Initial heel-strike was aligned to the first frame (0% of gait), toe-off to the 61st frame (60% of gait) (Whittle, 1991), and final heel-strike was aligned to the last frame (100% of gait). The total radiation per participant, including both CT and DF procedures, was estimated to be 10.72 mSv (Atkins et al., 2019).

2.3. Mesh Generation and Partitioning

Segmentations of the pelvis, proximal femur, femoral cartilage, and acetabular soft tissue (labrum and cartilage) from resampled (3x the original resolution) CT arthrograms were created semi-automatically in Amira (v.5.6.1 and 6.0.0; FEI, Hillsboro, OR). Surface meshes were created from the segmentations following smoothing and decimation procedures utilized in previous studies (Fiorentino et al., 2016a; Harris et al., 2012) (Supplemental Material, Section 1.2). Acetabular labrum and cartilage tissues were separated manually using PreView (v 2.1; FEBio, SLC, UT) (Maas et al., 2012) based on anatomical features as performed in previous studies (Henak et al., 2014b, 2011). More information on the labrum and cartilage separation can be found in the Supplemental Materials, Section 1.1.

Labrum and cartilage meshes were subdivided into three anatomic regions (anterior, superior, and posterior) for region-specific analyses (Figure 1). Anatomic partitions were standardized by projecting the labrum and cartilage meshes onto a plane fit to the acetabular rim (Atkins et al., 2018; Kapron et al., 2014; Uemura et al., 2019). To create the acetabular plane, the acetabular rim and lunate surfaces were first delineated using 2nd principal curvature thresholding in PostView (v 2.2; FEBio, SLC, UT) (Maas et al., 2012). Singular value decomposition of acetabular rim mesh nodes provided an acetabular plane, with the center of the plane set to the projection of the lunate center on the plane. The acetabular plane was divided into three equally-spaced regions, oriented such that the anterior/posterior boundary aligned with the lunate center and midpoint between the anterior and posterior acetabular rim edges on either side of the acetabular notch (Figure 1).

Figure 1:

Figure 1:

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Anatomic partitioning of the acetabular cartilage and labrum based on bony acetabular landmarks for a left hemi-pelvis. A sphere-fit to the lunate surface identified the lunate center. Singular value decomposition of the acetabular rim surface defined the plane from which the divisions were projected, with the projection of the lunate center defining the center of divisions. The anterior/posterior rim edges were used to orient the three equal divisions of the plane, with the anterior/posterior division centered between the anterior and posterior rim edges.

2.4. Contact Analysis

Kinematics were applied to each of the surface meshes using PostView, such that soft tissue structures moved rigidly with contiguous bones. Engineering strain in the acetabular labrum and cartilage was computed using the distance variables shown in Figure 2. Nodes from the apex of the labrum were identified via 2nd principal curvature thresholding and used to separate the articular and non-articular nodes for the calculation of strain. For each time point of the kinematics, the distance variables outlined in Figure 2 were computed at each node of the articular meshes. Peak strain per tissue and anatomic region was determined for each participant and frame of activity. Contact area was calculated by summing the surface area of the acetabular mesh faces contained within overlap regions and reported as a percentage of total area (per tissue, and anatomical division).

Figure 2:

Figure 2:

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Schematic of a hip model demonstrating soft tissue overlap calculations. Distance variables used to compute labral and cartilage strains included dO: distance of overlap between the two soft tissue surfaces, dF: femoral cartilage thickness, dL: acetabular labrum thickness, and dC: acetabular cartilage thickness. Note that distance variables were computed per acetabular cartilage and labrum mesh node (although figure shows each starting at different locations) to the minimum Euclidian distance on the opposing surface. The labral apex nodes defined the boundary between articular and non-articular labrum surfaces.

2.5. Supplementary Analyses

In accordance with recommendations (Anderson et al., 2007), we performed a sensitivity analysis on the position of the chondrolabral boundary, as this can be difficult to identify from CT arthrogram images. In the sensitivity analysis, the original chondrolabral boundaries of the acetabular meshes were shifted up to ±4 mm. Incline walking contact area and strain estimates were recalculated with the adjusted chondrolabral boundary and the mean percent change from original results was calculated. It was determined that calculations of both contact area and strain were relatively insensitive to medial shifts of the chondrolabral boundary for both the cartilage and labrum and lateral shifts for the labrum (errors less than 7% per mm of shift); however, for the cartilage, changes of 20% strain and 40% contact area per mm of lateral shift were observed (Supplemental Materials, 1.3, Table S1). Still, because the same methods were used to define the chondrolabral boundary for all participants, a bias on STO modeling predictions would not be expected for either of the two groups.

Prior to interpreting our study results, we compared STO predictions to those of a quasi-static finite element (FE) model. STO and FE predictions were qualitatively similar in the location and magnitude of contact area and peak strain, providing confidence in the use of STO modeling. Still, there were some differences in predictions that were likely attributed to the fact that STO cannot account for tissue compression and deflection (Supplementary Materials, 2.3, Figures S2 and S3).

2.6. Statistical Analysis

Percent contact area and peak strain for the anterior, superior, and posterior regions of the cartilage and labrum were compared between cam FAIS and control cohorts. Right-tailed (i.e.: cam FAIS > control), unequal variance Student’s T-tests (α = 0.05) were used to compare cam FAIS and control pivoting results, as well as the mean of the of walking results. Additionally for the walking trials, statistical parametric mapping (SPM) tests (right-tailed, α = 0.05) were used to compare the discrete timepoints across gait using an open-source SPM package (www.spm1d.org, 1d v 0.4) (Pataky, 2010). Unless otherwise noted, all results were presented in the format of mean ± 95% confidence interval. The Cohen’s d effect size of our study was calculated to be 1.32 using an open-source G*Power computation tool (v 3.1.9.6, gpower.hhu.de) for a one-tailed T-test of unequal variance using both an α=0.05 and 80% power (Faul et al., 2009).

3. Results

3.1. Level Walking

No significant differences between cam FAIS and control contact area or peak strain averaged across the entire walking trajectory could be found in any regions of the joint using T-tests (Figures 3–4). Likewise, SPM analysis identified no differences in percent contact area or peak strain in any region of the labrum or cartilage during any time-point of level walking (Figures 3–4).

Figure 3:

Figure 3:

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Contact area across the gait cycle in the labrum (top row) and cartilage (bottom row) of cam FAIS (red) and control (blue) cohorts during level walking. No significant differences in any comparisons of cam FAIS and control results were found with SPM tests or T-tests of the mean values of the trajectories.

Figure 4:

Figure 4:

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Peak contact strain across the gait cycle in the labrum (top row) and cartilage (bottom row) of cam FAIS (red) and control (blue) cohorts during level walking. No significant differences in any comparisons of cam FAIS and control results were found with SPM tests or T-tests of the mean values of the trajectories.

3.2. Incline Walking

No significant differences between patient and control mean contact area results were found for any tissue or anatomic region during incline walking (Figure 5). Mean peak contact strain was significantly greater in cam FAIS patients in the anterior labrum (p-value = 0.038; cam FAIS: 11.7±6.7%, control: 5.0±3.6%) and anterior cartilage (p-value = 0.029; cam FAIS: 9.1±3.3%, control: 4.2±2.3) (Figure 6). For evaluation over the gait cycle, SPM tests showed that cam FAIS patients displayed significantly greater contact area in the superior labrum just before toe-off in 52–56% of gait (minimum p-value = 0.045, corresponding contact area: [cam FAIS: 23.2±13.2%, control: 3.5±2.8%]) (Figure 6). Cam FAIS patients also displayed significantly greater peak strain in the anterior labrum during 27–31% (mid-stance) of gait (minimum p-value = 0.029, corresponding strain: [cam FAIS: 26.0±12.9%, control: 2.9±3.6%]) and anterior cartilage than controls during 68–74% (mid-swing) of gait (minimum p-value = 0.015, corresponding strain: [cam FAIS: 22.51±11.3%, control: 2.9±1.6%]).

Figure 5:

Figure 5:

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Contact area across the gait cycle in the labrum (top row) and cartilage (bottom row) of cam FAIS (red) and control (blue) cohorts during incline walking. Significant differences in comparisons found in SPM tests are indicated with an asterisk (*). No significant differences in the mean values of the trajectories were found with T-tests.

Figure 6:

Figure 6:

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Peak contact strain across the gait cycle in the labrum (top row) and cartilage (bottom row) of cam FAIS (red) and control (blue) cohorts during incline walking. Significant differences in comparisons found in SPM tests are indicated with an asterisk (*). Comparisons where significant differences in the mean values of the trajectories found with T-tests are indicated with a dagger (†).

3.3. External and Internal Pivoting

In comparison to control subjects, cam FAIS patients showed significantly greater anterior cartilage peak strain during external pivoting (p-value = 0.039; cam FAIS: 13.0±9.2%, control: 3.9±3.2%) (Figure 7). No significant differences between cam FAIS and control groups were found in comparisons of contact area and peak strain predictions for internal pivoting (Figure 8).

Figure 7:

Figure 7:

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Comparison of cam (red) and control (blue) averaged contact area and strain during the external pivot, with error bars displaying the upper bounds of 95% confidence intervals. Results are organized by tissue regions (solid: labrum, striped: cartilage) and the three anatomical regions (anterior, superior, and posterior) with p-values reported in parenthesis under the x-axis. Significant differences found with T-tests are indicated with an asterisk (*).

Figure 8:

Figure 8:

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Comparison of cam (red) and control (blue) averaged contact area and strain during internal pivot, with error bars displaying the upper bounds of 95% confidence intervals. Results are organized by tissue regions (solid: labrum, striped: cartilage) and the three anatomical regions (anterior, superior, and posterior) with p-values reported in parenthesis under the x-axis. No significant differences in means were found using T-tests.

4. Discussion

The objective of this study was to utilize participant-specific STO models and kinematics to compare labrum contact area and strain between patients with cam FAIS and controls with non-pathologic hip joint morphology and no significant history of hip pain. No significant differences between cam FAIS and control participants were found in mean values of contact area and peak strain for the level walking activity. Conversely, during incline walking, patients demonstrated significantly greater mean contact strain in the anterior labrum and cartilage. Additionally, short periods of the gait cycle displayed elevated anterior labrum peak strain, superior labrum contact area, and anterior cartilage peak strain in patients, summing to a total of 17% of the inclined gait cycle. Finally, significantly greater peak strain was found in the anterior cartilage of patients in the external pivoting task, but no significant group differences were found for internal pivoting.

Walking elicits or exacerbates symptoms in the majority of patients with cam FAIS, thus it is an important activity for clinicians and researchers to understand the pathophysiology of cam FAIS (Griffin et al., 2016). Many studies have investigated the kinematics of gait in the cam FAIS population; however, few have related the kinematics to contact mechanics. Consistent with the findings of Atkins et al. (2019), which evaluated the same patient and control cohorts, differences between cam FAIS and controls hips were more pronounced during incline walking in comparison to level walking. Notably, Atkins et al. found that participants exhibited increased anterior pelvic tilt throughout incline walking as compared to level walking (Atkins et al., 2019). Herein, we observed cam FAIS hips to have increased anterior contact strain and superior labrum contact area during incline walking. The differences in level versus incline walking contact mechanics, together with the associated hip kinematics reported by Atkins et al. (2019) for the same participants, suggest that cam FAIS patients may employ a gait strategy during incline walking that results in increased articular contact area and strains. The increased anterior pelvic tilt in cam FAIS patients may position the acetabulum such that earlier and more frequent abutment of anterosuperior acetabular soft tissues with the cam lesion result in increased soft tissue contact and strain anterosuperiorly (Ross et al., 2014). Additionally, previous findings of increased joint forces in the anterior and superior directions during mid-late stance phase of gait (Lewis and Garibay, 2015) align with our observation of increased contact area and strain in the anterior and superior joint during the same portion of gait in incline walking. In contrast to recent finite element analysis studies by Ng et al. (2017) and Todd et al. (2022) of patients with cam morphology using subject-specific level walking kinematics, we did not find any statistically significant differences in cam FAIS and control contact mechanics during level walking. Ng et al. reported increased shearing stresses in the anterolateral and posterior joint given end-stance phase kinematics and loading conditions (Ng et al., 2017), and Todd et al. reported increased tensile strains and shear stresses across the joint during gait, with the cam FAIS patient displaying less evenly distributed contact pressures than the control participant (Todd et al., 2022). However, these findings identified differences in either tensile strains and/or shear stresses, which cannot be accounted for when using STO modeling.

Patients with cam FAIS often have reduced internal rotation ROM of the hip (Clohisy et al., 2009; Griffin et al., 2016; Kapron et al., 2012). A cadaver study investigating the torsional loading and ROM in cam FAIS hips before and after cam resection surgery reported reduced contact and torque required to achieve the same ROM for internal rotation after cam resection surgery, but no reduction for external rotation (Ng et al., 2019). With this in mind, any differences between patient and control contact area and strain would be anticipated to be more pronounced in internal pivoting than external pivoting. Instead, the contact area and strain mean and variance of both cohorts was nearly identical for internal pivoting for each anatomical division and tissue, whereas external pivoting displayed significant differences in anterior cartilage strain. Previously, no differences were found between cam FAIS and control hip kinematics for either external or internal rotation during pivoting (Atkins et al., 2019). Considering the kinematics and the fact that external rotations of the joint would position the cam lesion further from the anterior region of the joint, our results suggest that factors not directly related to the cam lesion such as capsular tightness or muscular compensation may play a larger role in contact mechanics during pivoting. Future studies could attempt to control for these factors by passively applying rotations to end ROM or to specific target torques.

To our knowledge, this is the largest study to investigate contact mechanics of individuals with cam FAIS using patient-specific geometries and kinematics. However, it is important to interpret our results in light of the limitations of this study. First, while we made efforts to homogenize hip morphology within our two cohorts such that observed differences would not be confounded by other hip pathology, our sample size was small. Although we did not conduct an a-priori sample size estimate, our convenience sample (6 patients and 11 controls) provided a Cohen’s d effect size of 1.32, which is only sensitive enough to detect a very large effect. Considering this, we believe our study to represent a methodological proof of concept that provides potential insight into the pathomechanics of cam FAIS. Second, there likely exists large variation in hip kinematics and kinetics as well as considerable variation in bone and soft tissue morphology in both pathologic and non-pathologic hips (Atkins et al., 2019; Petersen et al., 2016; Steppacher et al., 2021). It is therefore unlikely that we captured the full spectrum of cartilage and labrum mechanics in this clinical population. Third, although intra-participant variability in kinematics could occur for the selected activities, only a single trial of kinematics was evaluated per participant. This was done to drive simulations with only high-quality DF trajectories because accuracy of the kinematics was critical to the accuracy of subsequent contact predictions. The high accuracy and low bias of DF (Kapron et al., 2014) minimized the potential errors that could propagate from kinematic measurements to STO analysis.

STO modeling assumptions and their effect on predictive reliability are also important to consider in this study. Because STO modeling only considers regions of overlap between rigid bodies to approximate deformation strains, labral deflection cannot be captured by this approach; this could result in an underestimation of contact area and overestimation of contact strains at the labrum as well as a potential overestimation of peak contact strains near the labral apex (Supplemental Material, Section 2.4; Figure S2–S3). While we used methods previously described in the literature to define the chondrolabral boundary (Henak et al., 2014b, 2011; Ng et al., 2017; Todd et al., 2022, 2018), our analysis demonstrated that STO predictions were sensitive to its location. For this reason, we chose to focus our study on comparisons between participant cohorts, activities, and anatomical divisions rather than differences between labrum and cartilage results. We do however recognize the difference in structure and function of the cartilage and labrum; thus, we chose to include these tissue delineations in our results. Finally, we note that additional studies need to be performed to evaluate the meaningfulness of STO-derived contact strains and contact area from a tissue pathology standpoint.

In summary, our results suggest abnormalities in joint mechanics in patients with cam FAIS may be found in certain activities of daily living. Of the four activities examined, incline walking results were most consistent with clinical descriptions of impingement in patients with cam FAIS, with slightly increased contact area and strain seen in both the anterior and superior regions of acetabular labrum and cartilage during mid-late stance, and mid-swing phases of gait. Patients also displayed increased peak strain in the anterior cartilage of the external pivoting task. Collectively, our results support the concept that hip joint contact mechanics may be altered in cam FAIS patients even during simple activities of daily living with minimal hip flexion. Nonetheless, we believe caution should be exercised when interpreting and extrapolating the results of the exploratory study herein. To reduce the computational burden of mechanical analyses, STO methods could be considered in future studies of a larger patient population that would have a broader spectrum of pathomorphology, pathokinematics and pathomechanics. Future studies utilizing STO methods could help to identify activities where contact mechanics from individuals with impingement deviate from the general population, and how treatment, such as surgical intervention to improve femoral head sphericity and femoral head-neck offset, may alter the soft tissue contact mechanics of the hip.

Supplementary Material

1

Acknowledgements

Financial support was provided by the National Institutes of Health (R01-AR077636, R56-AR074416, R21-AR063844, F32-AR067075, F32-AR078019, S10-RR026565, R01-GM083925) and the LS Peery Discovery Program in Musculoskeletal Restoration. The contents herein are solely the responsibility of the authors and do not necessarily represent the official views of these agencies.

Footnotes

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