Influence of anthropometric and mechanical variations on functional instability in the ACL-deficient knee

Individual variations in joint anatomy, tissue mechanical properties, and muscle strength of the knee are believed to affect the clinical outcome of ACL-deficient patients, but the effects have not been studied systematically. The impact of individual anthropometric and mechanical variation on functional stability of the ACL-deficient knee was investigated in this study using a two-dimensional mathematical knee model. The model included the tibiofemoral and the patellofemoral articulations, four ligaments, the medial capsule, and four muscle units surrounding the knee. Simulations were conducted to determine tibial anterior translation as well as tibiofemoral and patellofemoral joint loading at a single selected position during early stance phase of gait. Incremental hamstring muscle forces were applied to the modeled ACL-deficient knee in order to examine the level of the hamstring muscle forces required to prevent abnormal tibial anterior translation relative to the femur. Simulations were repeated using incremental variations in the selected anthropometric and mechanical properties of the ACL-deficient knee. It was found that bony geometry of the knee joint, especially the slope of tibial plateau, strongly affected both the tibial translations in the ACL-deficient knee and the effectiveness of the hamstring muscles to compensate for the ACL deficiency. For instance, simulations indicated that, due to ACL deficiency, the tibial anterior displacement increased by 9.1 mm for a tibial slope angle of 4 degrees compared to 15.2 mm for a tibial slope angle of 12 degrees. Future outcome studies for ACL-deficient knee may be required to include individual anthropometric and mechanical parameters of the knee as covariants.