Chao Wan

The effect of boundary condition on the biomechanics of a human pelvic joint under an axial compressive load: a three-dimensional finite element model.

The finite element (FE) model of the pelvic joint is helpful for clinical diagnosis and treatment of pelvic injuries. However, the effect of an FE model boundary condition on the biomechanical behavior of a pelvic joint has not been well studied. The objective of this study was to study the effect of boundary condition on the pelvic biomechanics predictions. A 3D FE model of a pelvis using subject-specific estimates of intact bone structures, main ligaments and bone material anisotropy by computed tomography (CT) gray value was developed and validated by bone surface strains obtained from rosette strain gauges in an in vitro pelvic experiment. Then three FE pelvic models were constructed to analyze the effect of boundary condition, corresponding to an intact pelvic joint, a pelvic joint without sacroiliac ligaments and a pelvic joint without proximal femurs, respectively. Vertical load was applied to the same pelvis with a fixed prosthetic femoral stem and the same load was simulated in the FE model. A strong correlation coefficient (R(2)=0.9657) was calculated, which indicated a strong correlation between the FE analysis and experimental results. The effect of boundary condition changes on the biomechanical response depended on the anatomical location and structure of the pelvic joint. It was found that acetabulum fixed in all directions with the femur removed can increase the stress distribution on the acetabular inner plate (approximately double the original values) and decrease that on the superior of pubis (from 7 MPa to 0.6 MPa). Taking sacrum and ilium as a whole, instead of sacroiliac and iliolumber ligaments, can influence the stress distribution on ilium and pubis bone vastly. These findings suggest pelvic biomechanics is very dependent on the boundary condition in the FE model.

The effect of the variation in ACL constitutive model on joint kinematics and biomechanics under different loads: a finite element study.

The biomechanics and function of the anterior cruciate ligament (ACL) have been widely studied using both experimental and simulation methods. It is known that a constitutive model of joint tissue is a critical factor in the numerical simulation. Some different ligament constitutive models have been presented to describe the ACL material behavior. However, the effect of the variation in the ligament constitutive model on joint kinematics and biomechanics has still not been studied. In this paper, a three-dimensional finite element model of an intact tibiofemoral joint was reconstructed. Three ACL constitutive models were compared under different joint loads (such as anterior tibial force, varus tibial torque, and valgus tibial torque) to investigate the effect of the change of the ACL constitutive model. The three constitutive models corresponded to an isotropic hyperelasticity model, a transversely isotropic hyperelasticity model with neo-Hookean ground substance description, and a transversely isotropic hyperelastic model with nonlinear ground substance description. Although the material properties of these constitutive equations were fitted on the same uniaxial tension stress-strain curve, the change of the ACL material constitutive model was found to induce altered joint kinematics and biomechanics. The effect of different ACL constitutive equations on joint kinematics depended on both deformation direction and load type. The variation in the ACL constitutive models would influence the joint kinematic results greatly in both the anterior and internal directions under anterior tibial force as well as some other deformations such as the anterior and medial tibial translations under valgus tibial torque, and the medial tibial translation and internal rotation under varus torque. It was revealed that the transversely isotropic hyperelastic model with nonlinear ground substance description (FE model III) was the best representation of the realistic ACL property by a linear regression between the simulated and the experiment deformation results. But the comparison of the predicted and experiment force of ligaments showed that all the three ACL constitutive models represented similar force results. The stress value and distribution of ACL were also altered by the change in the constitutive equation. In brief, although different ACL constitutive models have been fitted using the same uniaxial tension curve and have the similar longitudinal material property, the ACL constitutive equation should still be carefully chosen to investigate joint kinematics and biomechanics due to the different transverse material behavior.

A quantitative study of the relationship between the distribution of different types of collagen and the mechanical behavior of rabbit medial collateral ligaments.

The mechanical properties of ligaments are key contributors to the stability and function of musculoskeletal joints. Ligaments are generally composed of ground substance, collagen (mainly type I and III collagen), and minimal elastin fibers. However, no consensus has been reached about whether the distribution of different types of collagen correlates with the mechanical behaviors of ligaments. The main objective of this study was to determine whether the collagen type distribution is correlated with the mechanical properties of ligaments. Using axial tensile tests and picrosirius red staining-polarization observations, the mechanical behaviors and the ratios of the various types of collagen were investigated for twenty-four rabbit medial collateral ligaments from twenty-four rabbits of different ages, respectively. One-way analysis of variance was used in the comparison of the Youngs modulus in the linear region of the stress-strain curves and the ratios of type I and III collagen for the specimens (the mid-substance specimens of the ligaments) with different ages. A multiple linear regression was performed using the collagen contents (the ratios of type I and III collagen) and the Youngs modulus of the specimens. During the maturation of the ligaments, the type I collagen content increased, and the type III collagen content decreased. A significant and strong correlation () was identified by multiple linear regression between the collagen contents (i.e., the ratios of type I and type III collagen) and the mechanical properties of the specimens. The collagen content of ligaments might provide a new perspective for evaluating the linear modulus of global stress-strain curves for ligaments and open a new door for studying the mechanical behaviors and functions of connective tissues.

A review on research on development of ligament constitutive relations on macro, meso, and micro levels

Ligaments are densely connective soft tissues capable of maintaining stability and function of knee joint. As an important factor, the constitutive relation of ligament would affect its biomechanics and further play an essential role in the research on ligament injury, healing and treatment. The objective of this paper is to provide an overview of the current research on ligament constitutive relations on the macro, meso, and micro levels as well as the anatomy and histological structure of ligament. Some studies of biomechanical behaviors during ligament injury and healing periods have also been investigated. Based on the research on ligament constitutive relation in the past three decades, a discussion of some research perspectives is also presented, such as a validated accurate measuring method of in situ strain in ligament, a new constitutive relation involving the distribution of ultra-structural properties, and a rational estimation of ligament injury and healing process by the change of its ultra-structural or histological characteristics.

The effect of healing in the medial collateral ligament of human knee joint: A three-dimensional finite element analysis.

The medial collateral ligament (MCL) is one of the main ligaments that provide knee joint with major restraints against valgus, internal, and external torque loads. The MCL injury most frequently occurs near its femoral attachment but can be healed spontaneously. Hence, the usual clinical treatment for MCL injury is conservative therapy with early controlled rehabilitation motion. However, the effect of the variations in the healing conditions of the MCL portion (i.e. near the femoral insertion) is still unclear. In this study, finite element tibiofemoral joint models with three different MCL healing conditions were analyzed under six kinds of joint loads, such as 10 and 20 N·m valgus tibial torques, 5 and 10 N·m internal tibial torques, and 5 and 10 N·m external tibial torques. The three healing conditions corresponded to the early, medium, and final (i.e. healthy) stages of the healing period, respectively. It was found that different MCL healing conditions greatly affected the main joint kinematics under valgus tibial torques, but neither the reaction force nor stress results of the MCL. The peak strain values in the MCL healing portion changed greatly under all the six loads. Moreover, all the joint kinematics, strain results, and reaction force of the MCL at the medium stage were similar to those in the healthy joint, that is, at the final healing stage. These imply that the partially healed MCL might be enough for providing the restraints for knee joints and would not lead to some high strains occurring in the MCL.

An update on the constitutive relation of ligament tissues with the effects of collagen types

The musculoskeletal ligament is a kind of multiscale composite material with collagen fibers embedded in a ground matrix. As the major constituent in ligaments to bear external loads, collagens are composed mainly of two collagen contents with different mechanical properties, i.e., types I and III collagen. The constitutive relation of ligaments plays a critical role in the stability and normal function of human joints. However, collagen types have not been distinguished in the previous constitutive relations. In this paper a constitutive relation for ligament tissues was modified based on the previous constitutive relation by considering the effects of collagen types. Both the collagen contents and the mechanical properties of sixteen ligament specimens from four cadaveric human knee joints were measured for determining their material coefficients in the constitutive relation. The mechanical behaviors of ligaments were obtained from both the uniaxial tensile and simple shear tests. A linear regression between joint kinematic results from in vitro and in silico experiments was made to validate the accuracy of this constitutive relation. The high correlation coefficient (R(2)=0.93) and significance (P<0.0001) of the regression equation revealed that this modified constitutive relation of ligaments was accurate to be used in studying joint biomechanics. Another finite element analysis with collagen contents changing demonstrated that the effect of variations in collagen ratios on both joint kinematics and ligament biomechanics could be simulated by this constitutive relation.

A comparison of material characterizations in frequently used constitutive models of ligaments

Longitudinal tensile and simple shear stress-strain curves of human medial collateral ligaments (MCL) were fitted by six frequently used constitutive relations of ligaments using two different fitting methods for determining which was the best fitting method and the most preferable constitutive model for describing the ligament properties. According to the results of fitting goodness, two typical constitutive models were further analyzed by FEM to investigate the effect of the variation in MCL constitutive models under some physiological loads (i.e., 4.5 Nm external tibial and 10 Nm valgus tibial torques). It was found that different fitting methods induced great variations in describing the simple shear behavior whereas no obvious difference in the longitudinal tensile behavior. The most accurate description of both the longitudinal tensile and simple shear behaviors was obtained from the constitutive model with ground substance defined by an exponential function when the parameters were fitted by the two test data, respectively. Although the distributions of maximal principal stress were almost the same, the variation in MCL constitutive models affected the highest value of the stress greatly when MCL was under the complex physiological loads.

The effect of the material property change of anterior cruciate ligament by ageing on joint kinematics and biomechanics under tibial varus/valgus torques

It is known that the anterior cruciate ligament (ACL) plays a role in providing joint stabilities under tibial varus/valgus torques and the material behavior of the ACL has changed with ageing. However, the effect of this variation of the ACL material property on joint kinematics and biomechanics under tibial varus/valgus torques has still not been clarified.In this paper, three finite element (FE) models of an intact tibiofemoral joint were reconstructed with different ACL material properties, corresponding to the ACL on the younger, middle and older ages, respectively. The joint kinematics, the stress distribution and resultant force of the ACL were obtained under a tibial varus or valgus torque load. It was found that the variation in the ACL material property would result in great changes in some joint displacements (i.e., the tibial anterior translation and external rotation). The maximal stress value in the ACL had also altered while the stress distribution did not varied obviously. The great change in the tibial anterior translation illustrated that ACL played an important role against varus/valgus torques by controlling the coupled tibial anterior translation//external rotation rather than the corresponding varus/valgus rotation.

A quantitative comparison of morphological and histological characteristics of collagen in the rabbit medial collateral ligament

Collagen fiber is one of the critical factors in determining mechanical properties of ligaments and both the morphological and histological characteristics of collagen have been widely studied. However, there was still no consensus about whether the morphological characteristics of collagen correlated with its histological characteristics in physiological ligaments. Rabbit medial collateral ligaments (MCLs) were measured under a transmission electron microscope and a polarized light microscope plus picrosirius red-staining to obtain the distributions of collagen fibril diameters and types at different anatomical sites of rabbit MCLs, respectively. The correlation between the fibril diameter and type was determined by a correlation analysis. The collagen fibril diameters at the different anatomical sites had different distributions (unimodal or bimodal) and mean fibril diameters were found to increase significantly from the anterior part to the posterior part (P=0.0482) as well as from the proximal to the distal sections (P=0.0208). Type I collagen in the core portion of MCLs was significantly less than at the other four peripheral areas (P<0.005) but no significant variation was found in each respective portion (P>0.05). The low coefficient in the correlation analysis (r=0.3759) demonstrated collagen fibril diameters had no correlation with collagen types. This may provide a new view of collagen types in studying the mechanical behavior of ligaments.

The effect of screw fixation type on a modular hemi-pelvic prosthesis: a 3-D finite element model

In this article, a 3-D finite element (FE) model of human pelvic with a modular hemi-pelvic prosthesis was constructed to study the effect of screw fixation type on the biomechanics of the prosthesis. The results showed that the elimination of the screw far away from the pelvic arcuate line did not induce the instability and stress increase in the prosthesis. On the contrary, some stress in the sustain and acetabular parts decreased by 26.4% and 11.4%, respectively. In conclusion, the optimization of screw fixation can maintain the prosthesis stability and reduced stress concentration on some prosthesis parts. It was deduced that the optimization of the prosthesis could help surgeon reconstruct the pelvic joint function better and diminish the clinical time and cost. Implications for Rehabilitation Modular hemi-pelvic protheses are effective and popular treatments for pelvic type II resection. The elimination of screw far away from the pelvic arcuate line would induce stress descrease in some prothesis components and did not increase the instability of pelvic joint. This new prothesis could be optimized by removing the screw far away from the pelvic arcuate line and futher contribute to reduce the clinical time and cost.