The Effect of Axial Load in the Tibia on the Response of the 90 degrees Flexed Knee to Blunt Impacts with a Deformable Interface.
Lower extremity injuries are a frequent outcome of automotive accidents. While the lower extremity injury criterion is based on fracture of bone, most injuries are of less severity. Recent studies suggest microscopic, occult fractures that have been shown to be precursors of gross bone fractures, may occur in the kneecap (patella) for impacts with rigid and deformable interfaces due to excessive levels of patello-femoral contact pressure. One method of reducing this contact pressure for a 90 degrees flexed knee is to provide a parallel pathway for knee impact loads into the tibial tuberosity. Yet, blunt loads onto the tibial tuberosity can cause posterior drawer motion of the tibia, leading to injury or rupture of the posterior cruciate ligament (PCL). Recently studies have shown that axial loads in the tibia, which are measured during blunt loading on the knee in typical automobile crashes, can induce anterior drawer motion of the tibia and possibly help unload the PCL. The purpose of the current study was to explore the effect of combined anterior knee loading (AKL) and axial tibia loading (ATL), on response and injury for the 90 degrees flexed human knee. In repeated impacts with increasing ATL the stiffness of the knee to an AKL impact increased. For a 3 kN AKL, the stiffness of the knee increased approximately 26% when the ATL was increased from 0 kN to 2 kN. For 6 kN and 9 kN AKL, the stiffness was increased approximately 17% and 20%, respectively, when the ATL was increased from 0 kN (uniaxial) to 4 kN (biaxial). The effect, however, was not statistically significant at the 9 kN AKL level. The posterior tibial drawer was shown to increase with increased AKL and decrease with increased levels of ATL at an average of 0.3 mm per kN ATL for both the 3 kN and 6 kN ATL scenarios. For 9 kN AKL this drawer displacement was significantly reduced for biaxial versus uniaxial impacts, from 7.4+/-1.4 mm to 5.8+/-0.6 mm, respectively. Additionally, the percentage of the load carried by the tibial tuberosity increased with an ATL. For AKL impacts of 3, 6, and 9 kN, the percentage of load carried by the tibial tuberosity increased from 2.1% (range 0-19%) to 4.9% (0-36%), 2.1% (0-15%) to 6.9% (0-36%), and 8.7% (0-25%) to 12.7% (0-33%), respectively, between uniaxial and biaxial tests. The biaxial loading scenario also resulted in a reduction of the patello-femoral (PF) contact force as the ATL was increased. Ten knee impacts resulted in PCL tears at an average peak load of 12.7+/-2.4 kN in biaxial impacts (n=5) and 12.0+/-3.1 kN for uniaxial impacts (n=5). These PCL injured specimens had an average age of 62+/-11.3 years. The remaining specimens (n=11, 78+/-12.9 years of age) had bone fractures at approximately 8.9+/-3.1 kN. This study showed that combinations of compressive ATL and AKL reduced the PF contact force and had a stiffening effect on the response of the knee impacting a stiff but deformable interface. Furthermore, ATL reduced the posterior drawer of the tibia, which is the current basis for PCL injury in the knee, although it did not reduce the incidence of PCL injury in this study. While the current injury tolerance criterion reflects the vulnerability of the PCL to injury by limiting tibial drawer to 15 mm, the current dummy design does not incorporate the stiffening effect of an ATL that may occur at the same time as knee contact with an instrument panel during a typical automotive crash.