Evaluating the iliotibial band as a biological internal brace in ACL repair surgery: a finite element analysis study.

Introduction: The anterior cruciate ligament (ACL) is essential for knee stability but has limited regeneration ability due to low cell density and poor vascularization. Current ACL repair and augmentation approaches aim to preserve native tissue and provide early mechanical stability; however, challenges remain, including variable healing, risk of retear, and inconsistent clinical outcomes. A promising alternative is biological reinforcement using the iliotibial (IT) band, which may provide structural support and biological advantages while permitting a smaller graft harvest and tunnel size. This study evaluates IT band graft biomechanics in ACL repair using finite element analysis (FEA), focusing on graft thickness and tunnel-diameter compatibility. Methods: FE models simulated 10 mm wide IT band grafts of three thicknesses (1, 1.5, 2 mm) in femoral and tibial tunnels of 5, 6, and 7 mm. Knee anatomy was derived from CT data and processed in Blender, Gmsh, and FEBio. Bones were modeled as rigid, cartilage as hyperelastic Mooney-Rivlin, menisci as orthotropic Fung, and ligaments/grafts as fiber-reinforced hyperelastic materials. Contact included tied and sliding elastic formulations. The tibia was fully constrained in all six degrees of freedom, while femoral rotation was prescribed to achieve 30° of knee flexion. This position was maintained as a 100 N posterior femoral force was progressively applied. Von Mises stress and effective Lagrange strain distributions were analyzed to assess graft-tunnel compatibility. Results: Stress and strain increased when thicker grafts were paired with smaller tunnels. The 2 mm graft in a 5 mm tunnel showed peak stress of 11.62 MPa and strain of 2.5%. However, the 1.5-mm graft-6-mm tunnel configuration exhibited a lower maximum effective Lagrange strain of approximately 1.1%, a maximum principal stress of 5.69 MPa, and AP laxity of 3.5 mm, which was close to the intact-knee value of 3.2 mm. Larger tunnels consistently reduced stress and strain for all graft sizes. Peak stress and strain were localized near the femoral tunnel, identifying the region where graft stress and strain were highest. Discussion: FE analysis shows graft-tunnel mismatch compromises ACL repair mechanics. The 1.5-mm IT band graft with a 6-mm tunnel showed comparatively low graft stress and strain while maintaining AP laxity close to the intact-knee value. Tunnel diameter strongly influenced graft stress and strain, with smaller tunnels generally producing higher values. Limitation: A single patient-specific FE model was used, which may not capture anatomical variability. These findings provide computational support for further evaluation of the IT band as a biological internal brace and may inform future graft-selection and tunnel-sizing strategies in ACL repair.