A Musculoskeletal Model for Estimating Hip Contact Pressure During Walking.

Cartilage contact pressures are major factors in osteoarthritis etiology and are commonly estimated using finite element analysis (FEA). FEA models often include subject-specific joint geometry, but lack subject-specific joint kinematics and muscle forces. Musculoskeletal models use subject-specific kinematics and muscle forces but often lack methods for estimating cartilage contact pressures. Our objective was to adapt an elastic foundation (EF) contact model within OpenSim software to predict hip cartilage contact pressures and compare results to validated FEA models. EF and FEA models were built for five subjects. In the EF models, kinematics and muscle forces were applied and pressure was calculated as a function of cartilage overlap depth. Cartilage material properties were perturbed to find the best match to pressures from FEA. EF models with elastic modulus = 15 MPa and Poisson's ratio = 0.475 yielded results most comparable to FEA, with peak pressure differences of 4.34 ± 1.98 MPa (% difference = 39.96 ± 24.64) and contact area differences of 3.73 ± 2.92% (% difference = 13.4 ± 11.3). Peak pressure location matched between FEA and EF for 3 of 5 subjects, thus we do not recommend this model if the location of peak contact pressure is critically important to the research question. Contact area magnitudes and patterns matched reasonably between FEA and EF, suggesting that this model may be useful for questions related to those variables, especially if researchers desire inclusion of subject-specific geometry, kinematics, muscle forces, and dynamic motion in a computationally efficient framework.

How Does Chondrolabral Damage and Labral Repair Influence the Mechanics of the Hip in the Setting of Cam Morphology? A Finite-Element Modeling Study.

BACKGROUND: Individuals with cam morphology are prone to chondrolabral injuries that may progress to osteoarthritis. The mechanical factors responsible for the initiation and progression of chondrolabral injuries in these individuals are not well understood. Additionally, although labral repair is commonly performed during surgical correction of cam morphology, the isolated mechanical effect of labral repair on the labrum and surrounding cartilage is unknown. QUESTION/PURPOSES: Using a volunteer-specific finite-element analysis, we asked: (1) How does cam morphology create a deleterious mechanical environment for articular cartilage (as evaluated by shear stress, tensile strain, contact pressure, and fluid pressure) that could increase the risk of cartilage damage compared with a radiographically normal hip? (2) How does chondrolabral damage, specifically delamination, delamination with rupture of the chondrolabral junction, and the presence of a chondral defect, alter the mechanical environment around the damage? (3) How does labral repair affect the mechanical environment in the context of the aforementioned chondrolabral damage scenarios? METHODS: The mechanical conditions of a representative hip with normal bony morphology (characterized by an alpha angle of 37°) and one with cam morphology (characterized by an alpha angle of 78°) were evaluated using finite-element models that included volunteer-specific anatomy and kinematics. The bone, cartilage, and labrum geometry for the hip models were collected from two volunteers matched by age (25 years with cam morphology and 23 years with normal morphology), BMI (both 24 kg/m2), and sex (both male). Volunteer-specific kinematics for gait were used to drive the finite-element models in combination with joint reaction forces. Constitutive material models were assigned to the cartilage and labrum, which simulate a physiologically realistic material response, including the time-dependent response from fluid flow through the cartilage, and spatially varied response from collagen fibril reinforcement. For the cam hip, three models were created to represent chondrolabral damage conditions: (1) "delamination," with the acetabular cartilage separated from the bone in one region; (2) "delamination with chondrolabral junction (CLJ) rupture," which includes separation of the cartilage from the labrum tissue; and (3) a full-thickness chondral defect, referred to throughout as "defect," where the acetabular cartilage has degraded so there is a void. Each of the three conditions was modeled with a labral tear and with the labrum repaired. The size and location of the damage conditions simulated in the cartilage and labrum were attained from reported clinical prevalence of the location of these injuries. For each damage condition, the contact area, contact pressure, tensile strain, shear stress, and fluid pressure were predicted during gait and compared. RESULTS: The cartilage in the hip with cam morphology experienced higher stresses and strains than the normal hip. The peak level of tensile strain (25%) and shear stress (11 MPa) experienced by the cam hip may exceed stable conditions and initiate damage or degradation. The cam hip with simulated damage experienced more evenly distributed contact pressure than the intact cam hip, as well as decreased tensile strain, shear stress, and fluid pressure. The peak levels of tensile strain (15% to 16%) and shear stress (2.5 to 2.7 MPa) for cam hips with simulated damage may be at stable magnitudes. Labral repair only marginally affected the overall stress and strain within the cartilage, but it increased local tensile strain in the cartilage near the chondrolabral junction in the hip with delamination and increased the peak tensile strain and shear stress on the labrum. CONCLUSION: This finite-element modeling pilot study suggests that cam morphology may predispose hip articular cartilage to injury because of high shear stress; however, the presence of simulated damage distributed the loading more evenly and the magnitude of stress and strain decreased throughout the cartilage. The locations of the peak values also shifted posteriorly. Additionally, in hips with cam morphology, isolated labral repair in the hip with a delamination injury increased localized strain in the cartilage near the chondrolabral junction. CLINICAL RELEVANCE: In a hip with cam morphology, labral repair alone may not protect the cartilage from damage because of mechanical overload during the low-flexion, weightbearing positions experienced during gait. The predicted findings of redistribution of stress and strain from damage in the cam hip may, in some cases, relieve disposition to damage progression. Additional studies should include volunteers with varied acetabular morphology, such as borderline dysplasia with cam morphology or pincer deformity, to analyze the effect on the conclusions presented in the current study. Further, future studies should evaluate the combined effects of osteochondroplasty and chondrolabral treatment.

Characterization and finite element validation of transchondral strain in the human hip during static and dynamic loading.

Distribution of strain through the thickness of articular cartilage, or transchondral strain, is highly dependent on the geometry of the joint involved. Excessive transchondral strain can damage the solid matrix and ultimately lead to osteoarthritis. Currently, high-resolution transchondral strain distribution is unknown in the human hip. Thus, knowledge of transchondral strain patterns is of fundamental importance to interpreting the patterns of injury that occur in prearthritic hip joints. This study had three main objectives. We sought to 1) quantify high-resolution transchondral strain in the native human hip, 2) determine differences in transchondral strain between static and dynamic loading conditions to better understand recovery and repressurization of cartilage in the hip, and 3) create finite element (FE) models of the experimental testing to validate a modeling framework for future analysis. The transchondral strain patterns found in this study provide insight on the localization of strain within cartilage of the hip. Most notably, the chondrolabral junction experienced high tensile and shear strain across all samples, which explains clinical data reporting it as the most common region of damage in cartilage of the hip. Further, the representative FE framework was able to match the experimental static results and predict the dynamic results with very good agreement. This agreement provides confidence for both experimental and computational measurement methods and demonstrates that the specific anisotropic biphasic FE framework used in this study can both describe and predict the experimental results.

Patient-Specific Parameters Associated With Traction in Primary and Revision Hip Arthroscopic Surgery.

BACKGROUND: Distraction of the hip joint is a necessary step during hip arthroscopic surgery. The force of traction needed to distract the hip is not routinely measured, and little is known about which patient factors may influence this force. PURPOSE: To quantify the force of traction required for adequate distraction of the hip during arthroscopic surgery and explore the relationship between hip joint stiffness and patient-specific demographics, flexibility, and anatomy. STUDY DESIGN: Case series; Level of evidence, 4. METHODS: A total of 101 patients (61 female) undergoing primary hip arthroscopic surgery were prospectively enrolled. A load cell attached to the traction boot continuously measured traction force. Fluoroscopic images were obtained before and after traction to measure joint displacement. The stiffness coefficient was calculated as the force of traction divided by joint displacement. Relationships between the stiffness coefficient and patient demographics and clinical parameters were investigated using a univariable regression model. The regression analysis was repeated separately by patient sex. Variables significant at P < .05 were included in a multivariable regression model. RESULTS: The instantaneous peak force averaged 80 ± 18 kilogram-force (kgf), after which the force required to maintain distraction decreased to 57 ± 13 kgf. In univariable regression analysis, patient sex, alpha angle, hamstring flexibility, and Beighton hypermobility score were each correlated to stiffness. However, patient sex was the only significant variable in the multivariable regression model. Intrasex analysis demonstrated that increased hamstring flexibility correlated with decreased final holding stiffness in male patients and that higher Beighton scores correlated with decreased maximal stiffness in female patients. CONCLUSION: Male patients undergoing primary arthroscopic surgery have greater stiffness to hip distraction during arthroscopic surgery compared with female patients. In male patients, stiffness increased with decreasing hamstring flexibility. In female patients, increased Beighton scores corresponded to decreased stiffness. The presence of a labral tear was not correlated with stiffness to distraction. These data may be used to identify patients in whom a specific focus on capsular repair and/or plication may be warranted.

Hip chondrolabral mechanics during activities of daily living: Role of the labrum and interstitial fluid pressurization.

Osteoarthritis of the hip can result from mechanical factors, which can be studied using finite element (FE) analysis. FE studies of the hip often assume there is no significant loss of fluid pressurization in the articular cartilage during simulated activities and approximate the material as incompressible and elastic. This study examined the conditions under which interstitial fluid load support remains sustained during physiological motions, as well as the role of the labrum in maintaining fluid load support and the effect of its presence on the solid phase of the surrounding cartilage. We found that dynamic motions of gait and squatting maintained consistent fluid load support between cycles, while static single-leg stance experienced slight fluid depressurization with significant reduction of solid phase stress and strain. Presence of the labrum did not significantly influence fluid load support within the articular cartilage, but prevented deformation at the cartilage edge, leading to lower stress and strain conditions in the cartilage. A morphologically accurate representation of collagen fibril orientation through the thickness of the articular cartilage was not necessary to predict fluid load support. However, comparison with simplified fibril reinforcement underscored the physiological importance. The results of this study demonstrate that an elastic incompressible material approximation is reasonable for modeling a limited number of cyclic motions of gait and squatting without significant loss of accuracy, but is not appropriate for static motions or numerous repeated motions. Additionally, effects seen from removal of the labrum motivate evaluation of labral reattachment strategies in the context of labral repair.

Biomechanical evaluation of internal fixation techniques for unstable meso-type os acromiale.

BACKGROUND: Several internal fixation surgical techniques have been described for the treatment of symptomatic os acromiale. The purpose of this study was to compare the biomechanical characteristics of different internal fixation techniques for the operative treatment of unstable meso-type os acromiale in a cadaveric model. METHODS: Testing was performed on 12 matched pairs of cadaveric acromia with simulated meso-type os acromiale. Twelve specimens were prepared with 2 cannulated 4.0-mm screws only (SO group), inserted in the anterior-posterior direction. Contralateral specimens were repaired with screws and a tension band (TB group). An inferiorly directed load to the anterior acromion was applied at a rate of 60 mm/min until failure. Ultimate failure load, stiffness, and fracture pattern were recorded and analyzed. RESULTS: Ultimate failure load was significantly higher for the TB group (mean, 336 N ± 126 N; range, 166-623 N; P = .01) than for the SO group (mean, 242 N ± 57 N; range, 186-365 N). In contrast, no significant difference in stiffness was found between the SO group (mean, 22.1 N/mm ± 4.7 N/mm; range, 13.0-33.3 N/mm; P = .94)) and the TB group (mean, 22.2 N/mm ± 2.9 N/mm; range, 18.2-26.6 N/mm). CONCLUSION: Surgical repair of simulated unstable meso-type os acromiale by a combination of cannulated screws with a tension band leads to significantly higher repair strength at time zero in a cadaveric model compared with cannulated screws alone.

Biomechanical Comparison of Arthroscopic Single- and Double-Row Repair Techniques for Acute Bony Bankart Lesions.

BACKGROUND: Single- and double-row arthroscopic reconstruction techniques for acute bony Bankart lesions have been described in the literature. HYPOTHESIS: The double-row fixation technique would provide superior reduction and stability of a simulated bony Bankart lesion at time zero in a cadaveric model compared with the single-row technique. STUDY DESIGN: Controlled laboratory study. METHODS: Testing was performed on 14 matched pairs of glenoids with simulated bony Bankart fractures with a defect width of 25% of the glenoid diameter. Half of the fractures were repaired with a double-row technique, while the contralateral glenoids were repaired with a single-row technique. The quality of fracture reduction was measured with a coordinate measuring machine. To determine the biomechanical stability of the repairs, specimens were preconditioned with 10 sinusoidal cycles between 5 and 25 N at 0.1 Hz and then pulled to failure in the anteromedial direction at a rate of 5 mm/min. Loads at 1 mm and 2 mm of fracture displacement were determined. RESULTS: The double-row technique required significantly higher forces to achieve fracture displacements of 1 mm (mean, 60.6 N; range, 39.0-93.3 N; P = .001) and 2 mm (mean, 94.4 N; range, 43.4-151.2 N; P = .004) than the single-row technique (1 mm: mean, 30.2 N; range, 14.0-54.1 N and 2 mm: mean, 63.7 N; range, 26.6-118.8 N). Significantly reduced fracture displacement was seen after double-row repair for both the unloaded condition (mean, 1.1 mm; range, 0.3-2.4 mm; P = .005) and in response to a 10-N anterior force applied to the defect (mean, 1.6 mm; range, 0.5-2.7 mm; P = .001) compared with single-row repair (unloaded: mean, 2.1 mm; range, 1.3-3.4 mm and loaded: mean, 3.4 mm; range, 1.9-4.7 mm). CONCLUSION: The double-row fixation technique resulted in improved fracture reduction and superior stability at time zero in this cadaveric model. CLINICAL RELEVANCE: This information may influence the surgical technique used to treat large osseous Bankart fractures and the postoperative rehabilitation protocols implemented when such repair techniques are used.