According to the previous papers, it was demonstrated that anterior cruciate ligament (ACL) played an important role in resisting the coupled anterior-posterior laxity rather than the rotation laxity under axial torque load of knee joint. In the biomechanics simulation research of knee joint, some different ligament constitutive models were presented to describe the ACL material behavior. However, there is few published paper to study the effect of variable ligament constitutive model on the joint biomechanics under axial torque load.

In this paper, a 3-dimension finite element model of an intact tibiofemoral joint including all the main anatomical structures was reconstructed and two ACL constitutive models were compared under 10 Nm femur external torque load. The two ACL constitutive models corresponded to an isotropic hyperelastic model and a transversely isotropic hyperelastic model considering fiber effect, respectively. All the ACL material properties of the two constitutive models were defined by fitting the same stress-strain data. Another model with ACL resected was also analyzed under the same load to estimate the function ACL played under joint axial torque load.

It was found that the resection of ACL changed the knee joint deformations significantly in all directions except the distal-proximal translation. In the ACL resected joint model, the internal-external rotation, anterior-posterior and medial-lateral translations increased by about 20%, 500% and 600%, respectively. Comparing to the ACL intact joint model, the Mises stress values of medial collateral ligament decreased while that on lateral collateral ligament increased greatly (from 35 MPa to 61 MPa). In the comparison of the two different ACL constitutive models, the internal-external rotation, as the highest deformation of the knee joint, changed by about 11% and the maximal deformation alteration was obtained in the anterior-posterior translation (about 80%). Both the highest stress value and distribution on ACL have altered mostly while the Mises stress distributions of other ligaments and menisci have changed slightly.

The alteration of joint kinematics and ligament biomechanics by different ACL constitutive models would be due to the different descriptions of the material transverse behavior and the real complex ACL stress distribution under an axial torque load, although the longitudinal material behaviors described by different ACL constitutive models were almost the same based on the same experiment data.

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