The influence of the acetabular labrum on hip joint cartilage consolidation: a poroelastic finite element model

The goal of this study was to investigate the influence of the acetabular labrum on the consolidation, and hence the solid matrix strains and stresses, of the cartilage layers of the hip joint. A plane-strain finite element model was developed, which represented a coronal slice through the acetabular and femoral cartilage layers and the acetabular labrum. Elements with poroelastic properties were used to account for the biphasic solid/fluid nature of the cartilage and labrum. The response of the joint over an extended period of loading (10,000s) was examined to simulate the nominal compressive load that the joint is subjected to throughout the day. The model demonstrated that the labrum adds an important resistance in the flow path of the fluid being expressed from the cartilage layers of the joint. Cartilage layer consolidation was up to 40% quicker in the absence of the labrum. Following removal of the labrum from the model, the solid-on-solid contact stresses between the femoral and acetabular cartilage layers were greatly increased (up to 92% higher), which would increase the friction between the joint surfaces. In the absence of the labrum, the centre of contact shifted towards the acetabular rim. Subsurface strains and stresses were much higher without the labrum, which could contribute to fatigue damage of the cartilage layers. Finally, the labrum provided some structural resistance to lateral motion of the femoral head within the acetabulum, enhancing joint stability and preserving joint congruity.     

Role of the acetabular labrum in load support across the hip joint

The relatively high incidence of labral tears among patients presenting with hip pain suggests that the acetabular labrum is often subjected to injurious loading in vivo. However, it is unclear whether the labrum participates in load transfer across the joint during activities of daily living. This study examined the role of the acetabular labrum in load transfer for hips with normal acetabular geometry and acetabular dysplasia using subject-specific finite element analysis. Models were generated from volumetric CT data and analyzed with and without the labrum during activities of daily living. The labrum in the dysplastic model supported 4-11% of the total load transferred across the joint, while the labrum in the normal model supported only 1-2% of the total load. Despite the increased load transferred to the acetabular cartilage in simulations without the labrum, there were minimal differences in cartilage contact stresses. This was because the load supported by the cartilage correlated with the cartilage contact area. A higher percentage of load was transferred to the labrum in the dysplastic model because the femoral head achieved equilibrium near the lateral edge of the acetabulum. The results of this study suggest that the labrum plays a larger role in load transfer and joint stability in hips with acetabular dysplasia than in hips with normal acetabular geometry.     

Specimen-specific predictions of contact stress under physiological loading in the human hip: validation and sensitivity studies

Hip osteoarthritis may be initiated and advanced by abnormal cartilage contact mechanics, and finite element (FE) modeling provides an approach with the potential to allow the study of this process. Previous FE models of the human hip have been limited by single specimen validation and the use of quasi-linear or linear elastic constitutive models of articular cartilage. The effects of the latter assumptions on model predictions are unknown, partially because data for the instantaneous behavior of healthy human hip cartilage are unavailable. The aims of this study were to develop and validate a series of specimen-specific FE models, to characterize the regional instantaneous response of healthy human hip cartilage in compression, and to assess the effects of material nonlinearity, inhomogeneity and specimen-specific material coefficients on FE predictions of cartilage contact stress and contact area. Five cadaveric specimens underwent experimental loading, cartilage material characterization and specimen-specific FE modeling. Cartilage in the FE models was represented by average neo-Hookean, average Veronda Westmann and specimen- and region-specific Veronda Westmann hyperelastic constitutive models. Experimental measurements and FE predictions compared well for all three cartilage representations, which was reflected in average RMS errors in contact stress of less than 25 %. The instantaneous material behavior of healthy human hip cartilage varied spatially, with stiffer acetabular cartilage than femoral cartilage and stiffer cartilage in lateral regions than in medial regions. The Veronda Westmann constitutive model with average material coefficients accurately predicted peak contact stress, average contact stress, contact area and contact patterns. The use of subject- and region-specific material coefficients did not increase the accuracy of FE model predictions. The neo-Hookean constitutive model underpredicted peak contact stress in areas of high stress. The results of this study support the use of average cartilage material coefficients in predictions of cartilage contact stress and contact area in the normal hip. The regional characterization of cartilage material behavior provides the necessary inputs for future computational studies, to investigate other mechanical parameters that may be correlated with OA and cartilage damage in the human hip. In the future, the results of this study can be applied to subject-specific models to better understand how abnormal hip contact stress and contact area contribute to OA.     

Effects of idealized joint geometry on finite element predictions of cartilage contact stresses in the hip

Computational models may have the ability to quantify the relationship between hip morphology, cartilage mechanics and osteoarthritis. Most models have assumed the hip joint to be a perfect ball and socket joint and have neglected deformation at the bone-cartilage interface. The objective of this study was to analyze finite element (FE) models of hip cartilage mechanics with varying degrees of simplified geometry and a model with a rigid bone material assumption to elucidate the effects on predictions of cartilage stress. A previously validated subject-specific FE model of a cadaveric hip joint was used as the basis for the models. Geometry for the bone-cartilage interface was either: (1) subject-specific (i.e. irregular), (2) spherical, or (3) a rotational conchoid. Cartilage was assigned either a varying (irregular) or constant thickness (smoothed). Loading conditions simulated walking, stair-climbing and descending stairs. FE predictions of contact stress for the simplified models were compared with predictions from the subject-specific model. Both spheres and conchoids provided a good approximation of native hip joint geometry (average fitting error ～0.5 mm). However, models with spherical/conchoid bone geometry and smoothed articulating cartilage surfaces grossly underestimated peak and average contact pressures (50% and 25% lower, respectively) and overestimated contact area when compared to the subject-specific FE model. Models incorporating subject-specific bone geometry with smoothed articulating cartilage also underestimated pressures and predicted evenly distributed patterns of contact. The model with rigid bones predicted much higher pressures than the subject-specific model with deformable bones. The results demonstrate that simplifications to the geometry of the bone-cartilage interface, cartilage surface and bone material properties can have a dramatic effect on the predicted magnitude and distribution of cartilage contact pressures in the hip joint.     

The low permeability of healthy meniscus and labrum limit articular cartilage consolidation and maintain fluid load support in the knee and hip

The knee meniscus and hip labrum appear to be important for joint health, but the mechanisms by which these structures perform their functions are not fully understood. The fluid phase of articular cartilage provides compressive stiffness and aids in maintaining a low friction articulation. Healthy fibrocartilage, the tissue of meniscus and labrum, has a lower fluid permeability than articular cartilage. In this study we hypothesized that an important function of the knee meniscus and the hip labrum is to augment fluid retention in the articular cartilage of a mechanically loaded joint. Axisymmetric hyperporoelastic finite element models were analyzed for an idealized knee and an idealized hip. The results indicate that the meniscus maintained fluid pressure and inhibited fluid exudation in knee articular cartilage. Similar, but smaller, effects were seen with the labrum in the hip. Increasing the fibrocartilage permeability relative to that of articular cartilage gave a consolidation rate and loss of fluid load support comparable to that predicted by meniscectomy or labrectomy. The reduced articular cartilage fluid pressure that was calculated for the joint periphery is consistent with patterns of endochondral ossification and osteophyte formation in knee and hip osteoarthritis. High articular central strains and loss of fluid load support after meniscectomy could lead to fibrillation. An intact low-permeability fibrocartilage is important for limiting fluid exudation from articular cartilage in the hip and knee. This may be an important aspect of the role of fibrocartilage in protecting these joints from osteoarthritis.     

Hip joint degeneration due to cam impingement: a finite element analysis

The goal of this study was to investigate the impact of cam impingement, a biomechanical risk factor, on hip joint degeneration and ultimately coxarthrosis. 3D finite element solid models of a healthy and a pathologic hip were developed based on clinical reports. The biphasic characteristics of cartilaginous tissues were considered to identify localised solid matrix overloading during normal walking and sitting down (SD). Localised femoral intrusion at the anterior-superior pelvic horn was revealed in the pathologic hip during SD, where the radial and meridional solid stresses in the acetabular cartilage and circumferential solid stresses within the acetabular labrum increased by 3.7, 1.5 and 2.7 times, respectively. The increased solid-on-solid stresses, reduction in fluid-load support and associated higher friction during articulation may result in joint wear and other degenerative changes in the hip.     

The importance of femur/acetabulum cartilage in the biomechanics of the intact hip: experimental and numerical assessment

Experimental studies have been made to study and validate the biomechanics of the pair femur/acetabulum considering both structures without the presence of cartilage. The main goal of this study was to validate a numerical model of the intact hip. Numerical and experimental models of the hip joint were developed with respect to the anatomical restrictions. Both iliac and femur bones were replicated based on composite replicas. Additionally, a thin layer of silicon rubber was used for the cartilage. A three-dimensional finite element model was developed and the boundary conditions of the models were applied according to the natural physiological constrains of the joint. The loads used in both models were used just for comparison purposes. The biomechanical behaviour of the models was assessed considering the maximum and minimum principal bone strains and von Mises stress. We analysed specific biomechanical parameters in the interior of the acetabular cavity and on femur's surface head to determine the role of the cartilage of the hip joint within the load transfer mechanism. The results of the study show that the stress observed in acetabular cavity was 8.3 to 9.2 MPa. When the cartilage is considered in the joint model, the absolute values of the maximum and minimum peak strains on the femur's head surface decrease simultaneously, and the strains are more uniformly distributed on both femur and iliac surfaces. With cartilage, the cortex strains increase in the medial side of the femur. We prove that finite element models of the intact hip joint can faithfully reproduce experimental models with a small difference of 7%.     

The mechanics of focal chondral defects in the hip

There is a mean incidence of osteoarthritis (OA) of the hip in 8% of the overall population. In the presence of focal chondral defects, defined as localized damage to the articular cartilage, there is an increased risk of symptomatic progression toward OA. This relationship between chondral defects and subsequent development of OA has led to substantial efforts to develop effective procedures for surgical cartilage repair. This study examined the effects of chondral defects and labral delamination on cartilage mechanics in the dysplastic hip during the gait cycle using subject-specific finite element analysis. Models were generated from volumetric CT data and analyzed with simulated chondral defects at the chondrolabral junction on the posterior acetabulum during five distinct points in the gait cycle. Focal chondral defects increased maximum shear stress on the osteochondral surface of the acetabular cartilage, when compared to the intact case. This effect was amplified with labral delamination. Additionally, chondral defects increased the first principal Lagrange strain on the articular surface of the acetabular cartilage and labrum. Labral delamination relieved some of this tensile strain. As defect size was increased, contact stress increased in the medial zone of the acetabulum, while it decreased anteriorly. The results suggest that in the presence of chondral defects and labral delamination the cartilage experiences elevated tensile strains and shear and contact stress, which could lead to further damage of the cartilage, and subsequent arthritic progression. The framework presented here will serve as the procedure for future finite element studies on cartilage mechanics in hips with varying disease states with simulated chondral defects and labral tears.     

An in vitro investigation of the acetabular labral seal in hip joint mechanics

Labrum pathology may contribute to early joint degeneration through the alteration of load transfer between, and the stresses within, the cartilage layers of the hip. We hypothesize that the labrum seals the hip joint, creating a hydrostatic fluid pressure in the intra-articular space, and limiting the rate of cartilage layer consolidation. The overall cartilage creep consolidation of six human hip joints was measured during the application of a constant load of 0.75 times bodyweight, or a cyclic sinusoidal load of 0.75+/-0.25 times bodyweight, before and after total labrum resection. The fluid pressure within the acetabular was measured. Following labrum resection, the initial consolidation rate was 22% greater (p=0.02) and the final consolidation displacement was 21% greater (p=0.02). There was no significant difference in the final consolidation rate. Loading type (constant vs. cyclic) had no significant effect on the measured consolidation behaviour. Fluid pressurisation was observed in three of the six hips. The average pressures measured were: for constant loading, 541+/-61kPa in the intact joint and 216+/-165kPa following labrum resection, for cyclic loading, 550+/-56kPa in the intact joint and 195+/-145kPa following labrum resection. The trends observed in this experiment support the predictions of previous finite element analyses. Hydrostatic fluid pressurisation within the intra-articular space is greater with the labrum than without, which may enhance joint lubrication. Cartilage consolidation is quicker without the labrum than with, as the labrum adds an extra resistance to the flow path for interstitial fluid expression. However, both sealing mechanisms are dependent on the fit of the labrum against the femoral head.     

A finite element study of the effect of diametral interface gaps on the contact areas and pressures in uncemented cylindrical femoral total hip components

Uncemented femoral total hip components rely entirely on contact with the prepared femur for their initial fixation. The contact areas and stresses between a straight tubular bone and a metal cylindrical prosthesis 12.5 cm long and 13 mm in diameter were calculated in a finite element model which includes uniform diametral gaps varying from 20 to 500 microns, using transverse loads from 100 to 2000 N. Frictionless three-dimensional contact elements were used between the bone and the prosthesis. Contact stresses were high and irregular in all cases, and the contact areas were small. Two regions of contact were apparent for lower loads and larger gaps. A third region of contact occurred near the distal tip of the implant at higher loads. This region of contact markedly increased the contact stresses at the distal tip of the prosthesis. A 20 microns overlap between bone and implant was modelled to assess a slight interference fit. The contact stress distribution in this case was markedly different from the stress distribution with a 20 microns diametral gap. The data collectively indicates that gaps of less than 20 microns between bone and implant can substantially change contact stress distributions.     

An augmented Lagrangian finite element formulation for 3D contact of biphasic tissues

Biphasic contact analysis is essential to obtain a complete understanding of soft tissue biomechanics, and the importance of physiological structure on the joint biomechanics has long been recognised; however, up to date, there are no successful developments of biphasic finite element contact analysis for three-dimensional (3D) geometries of physiological joints. The aim of this study was to develop a finite element formulation for biphasic contact of 3D physiological joints. The augmented Lagrangian method was used to enforce the continuity of contact traction and fluid pressure across the contact interface. The biphasic contact method was implemented in the commercial software COMSOL Multiphysics 4.2^® (COMSOL, Inc., Burlington, MA). The accuracy of the implementation was verified using 3D biphasic contact problems, including indentation with a flat-ended indenter and contact of glenohumeral cartilage layers. The ability of the method to model multibody biphasic contact of physiological joints was proved by a 3D knee model. The 3D biphasic finite element contact method developed in this study can be used to study the biphasic behaviours of the physiological joints.     

A contact-coupled finite element analysis of the natural adult hip

A non-linear two-dimensional finite element model was used to study phenomena of stress redistribution in the natural adult hip resulting from parametric material property variations in the juxtarticular regions of the femoral head. Despite the geometrical simplifications employed, the intra-articular contact stresses (computed using the FEAP program) were found to be in reasonable qualitative agreement with previous in vitro data for the case of a normal hip. Generalized sclerotic changes in the subchondral plate, as reflected either in apparent modulus increases or in plate thickening, were found to have only minor effects on the computed contact stress distribution, although stress levels within the plate itself were markedly influenced. Localized subchondral plate sclerosis, by contrast, led to marked stress elevations in the cartilage immediately overlying the stiffened bone. Cartilage modulus increases caused increased load uptake for a given imposed deformation, but involved stress distribution increases which were very nearly linearly proportional to the increases in resultant load magnitude. Friction coefficient elevations had no noticeable effects on normal contact stress or upon overall load transmission, but involved complex, possibly slip-related, changes in intra-articular and cartilaginous shear stresses.     

Patient-specific knee joint finite element model validation with high-accuracy kinematics from biplane dynamic Roentgen stereogrammetric analysis

Little is known about in vivo menisci loads and displacements in the knee during strenuous activities. A new method that combines high-speed kinematics measured with biplane dynamic Roentgen stereogrammetric analysis (DRSA) and a subject-specific finite element (FE) model for studying in vivo meniscal behavior is presented here. Further model calibration in a very controlled uniaxial low and high-rate compression loading condition is presented by comparing the model behavior against the measured high-accuracy menisci DRSA kinematics and direct tibio-femoral pressure measurement from a K-scan sensor. It is apparent that certain model aspects such as removing of the pressure sensor from the model can result in relatively large errors (14%) in contact parameters that are not reflected in the change of the measured meniscal kinematics. Changing mesh size to 1mm by 1mm elements increased the magnitude of all but one of the contact variables by up to 45%. This local validation using accurate localized patient-specific geometry and meniscal kinematics was needed to enhance model fidelity at the level of contact between menisci and cartilage.     

Prediction of strength and strain of the proximal femur by a CT-based finite element method

Hip fractures are the most serious complication of osteoporosis and have been recognized as a major public health problem. In elderly persons, hip fractures occur as a result of increased fragility of the proximal femur due to osteoporosis. It is essential to precisely quantify the strength of the proximal femur in order to estimate the fracture risk and plan preventive interventions. CT-based finite element analysis could possibly achieve precise assessment of the strength of the proximal femur. The purpose of this study was to create a simulation model that could accurately predict the strength and surface strains of the proximal femur using a CT-based finite element method and to verify the accuracy of our model by load testing using fresh frozen cadaver specimens. Eleven right femora were collected. The axial CT scans of the proximal femora were obtained with a calibration phantom, from which the 3D finite element models were constructed. Materially nonlinear finite element analyses were performed. The yield and fracture loads were calculated, while the sites where elements failed and the distributions of the principal strains were determined. The strain gauges were attached to the proximal femoral surfaces. A quasi-static compression test of each femur was conducted. The yield loads, fracture loads and principal strains of the prediction significantly correlated with those measured (r=0.941, 0.979, 0.963). Finite element analysis showed that the solid elements and shell elements in undergoing compressive failure were at the same subcapital region as the experimental fracture site.     

应用三维有限元分析髋臼骨结核软骨下骨塌陷的风险

目的建立人体髋臼骨结核三维有限元模型,探讨不同部位髋臼骨结核软骨下骨塌陷的风险。方法通过正常髋关节CT数据,利用Mimics软件和ANSYS有限元软件,建立正常髋关节三维有限元模型（模型A）、髋臼顶部骨结核（模型B）、髋臼中心部骨结核（模型C）、髋臼前部骨结核（模型D）、髋臼后部骨结核（模型E）三维有限元模型,模拟人体单脚站立进行加载,分析髋臼软骨下骨峰值Von Mises应力和初始微动值。结果建立了正常髋关节和不同部位髋臼骨结核三维有限元模型,各模型含节点269284,三维四面体单元184786个。通过加载分析结果显示：与正常髋关节相比,峰值Von Mises应力,依次增加84%、3%、21%、67%;髋臼软骨下骨初始微动值依次增加66%、11%、17%、29%。结论髋臼顶部结核软骨下骨峰值Von Mises应力和初始微动值最大,塌陷的风险最大。     

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.     

A dynamic finite element analysis of impulsive loading of the extension-splinted rabbit knee

A dynamic nonlinear finite element model was developed to study juxtarticular stresses in the splinted rabbit knee, an established laboratory model for creating osteoarthrosis due to impulsive loading. Plane strain finite element results were validated by comparison with corresponding experimental data. Parametric effects studied included the input tibial displacement speed, the local bone density distribution, and the modulus of cartilage and subchondral bone. While the computed resultant contact force magnitude was sensitive to a number of model parameters, the stress patterns, when normalized to a given resultant force magnitude, were not. Despite comparable force peaks, the finite element results showed approximately six-fold higher effective strain rate levels for a severely impulsive loading protocol known to induce rapid osteoarthrosis, versus those for a mildly impulsive loading protocol not usually associated with cartilage damage. A propensity for elevated shear in the deep cartilage layer near the contact periphery, observed in nearly all computed stress distributions, is consistent with previous experimental findings of fissuring at that level in the impulsively loaded rabbit knee.     

Articular cartilage friction increases in hip joints after the removal of acetabular labrum

The acetabular labrum is believed to have a sealing function. However, a torn labrum may not effectively prevent joint fluid from escaping a compressed joint, resulting in impaired lubrication. We aimed to understand the role of the acetabular labrum in maintaining a low friction environment in the hip joint. We did this by measuring the resistance to rotation (RTR) of the hip, which reflects the friction of the articular cartilage surface, following focal and complete labrectomy. Five cadaveric hips without evidence of osteoarthritis and impingement were tested. We measured resistance to rotation of the hip joint during 0.5, 1, 2, and 3 times body weight (BW) cyclic loading in the intact hip, and after focal and complete labrectomy. Resistance to rotation, which reflects articular cartilage friction in an intact hip was significantly increased following focal labrectomy at 1-3 BW loading, and following complete labrectomy at all load levels. The acetabular labrum appears to maintain a low friction environment, possibly by sealing the joint from fluid exudation. Even focal labrectomy may result in increased joint friction, a condition that may be detrimental to articular cartilage and lead to osteoarthritis.     

Shape optimization of a cementless hip stem for a minimum of interface stress and displacement

The primary stem stability is an essential factor for success of cementless hip stems. A correct choice of the stem geometry can improve the stem stability and, consequently, increase the life time of a hip implant. In this work, it is proposed a computational model for shape optimization of cementless hip stems. The optimization problem is formulated by the minimization of relative displacement and stress on bone/stem interface using a multi-criteria objective function. Also multiple loads are considered to incorporate several daily life activities. Design variables are parameters that characterize the geometry of selected cross sections, which are subject to geometric constraints to ensure a clinically admissible shape. The stem/bone set is considered a structure in equilibrium with contact conditions on interface. The contact formulation allows us to analyze different lengths of porous coating. The optimization problem is solved numerically by a steepest descent method. The interface stress and relative displacement are obtained solving the contact problem by the finite element method. Numerical examples are presented for a two-dimensional model of a hip stem, however, the formulation is general and can be applied to the three-dimensional case. The model gives indications about the relation between shape, porous coating and prosthesis stability.     

The effect of muscle loading on the simulation of bone remodelling in the proximal femur

A large number of finite element analyses of the proximal femur rely on a simplified set of muscle and joint contact loads to represent the boundary conditions of the model. In the context of bone remodelling analysis around hip implants, muscle loading affects directly the spatial distribution of the remodelling signal. In the present study we performed a sensitivity analysis on the effect of different muscle loading configurations on the outcome of the bone remodelling simulation. An anatomical model of the femur with the implanted stem in place was constructed using the CT data of the Visible Human Project dataset of the National Institute of Health. The model was loaded with three muscle force configurations with increasing level of complexity. A strain adaptive remodelling rule was employed to simulate the post-operative bone changes around the implant stem and the results of the simulation were assessed quantitatively in terms of the bone mineral content changes in 18 periprosthetic regions of interest. The results showed considerable differences in the amount of bone loss predicted between the three cases. The simplified models generally predicted more pronounced bone loss. Although the overall remodelling patterns observed were similar, the bone conserving effect of additional muscle forces in the vicinity of their areas of attachment was clear. The results of this study suggest that the loading configuration of the FE model does play an important role in the outcome of the remodelling simulation.