Because the purpose of this problem is to show the anisotropic, hyperelastic, and viscoelastic behavior of the ACL, a simplified model is used. The problem focuses on the ACL part of the knee only. Instead of using a complex model with the femur, tibia, and fibula parts, only the boundary conditions are considered accordingly.

The femur is modeled with a rigid contact surface to enable modeling of the bending and twisting behavior of the ACL. In the analyses, the tibia (bottom) side of the ACL model remain fixed and is constrained with all degrees of freedom.

The knee joint works in flexion, extension, and rotation. The simulations therefore show the behavior of the ACL under the uniaxial flexion, tension, and rotation.

In the first case, a uniaxial displacement loading is applied at the top surface of the ACL with total stretch of 1.12. A high constant strain rate of 50 percent per second with an elongation rate of 15.5 mm/sec is maintained. The following example input specifies the loading:

Ly = 31         ! length of the ACL (mm)
erat = 15.5     ! elongation rate (mm/s)
ts = 1.12       ! total stretch
dl = (ts-1)*Ly  ! total displacement (mm)
tt = dl/erat    ! total time (sec)

In the second case, a flexion of 45 degrees is applied.

In the third case, a rotation of 11.25 degrees is applied using the femur surface in a small time period of 1.5 seconds.