Finite Element Analysis of Graft Stress in Various Tunnel Positions for ACL Reconstruction

The anterior cruciate ligament (ACL) is one of the most important ligaments in stabilizing the knee and injury of the ACL is one of the most common athletic injuries. The ACL has limited ability to heal due to its avascular environment; therefore, the primary treatment is to replace the ACL with a tissue graft. There is debate among clinicians on appropriate reconstruction techniques, such as the position and orientation of the graft. Improper graft placement may result in elevated forces and stresses which could lead to graft failure. This study examines the forces and stresses present in grafts when placed in five common tunnel positions, four anatomic and one non-anatomic. This was accomplished using computational models and experimental testing. Through the use of the finite element (FE) method a computational model was developed. Three-dimensional solid models of the tibia and femur were obtained through medical imaging in order to obtain the actual geometry of the bones and precise location of tunnel positions. An isotropic hyperelastic material was used to model the cylindrical grafts, placed in their respective positions. Three-dimensional joint kinematics were obtained via a six degree-of-freedom robotic manipulator and were used for input into the computational model. Graft stress and tension were calculated after the kinematics were applied for two initial conditions: stress-free and pre-stress that resulted from a 40N initial tension. Graft tension for each case was compared to experimentally determined graft force, obtained from a separate study of 16 cadaveric knee specimens. Results suggest that the FE estimation of graft tension in the 40 N tension initial condition more closely resembled experimental data and the non-anatomic graft placement resulted in elevated graft tension and stress