According to the National Accident Sampling System (NASS), 10% of all automobile accident injuries involve the knee. These injuries involve bone fracture and/or “soft tissue” injury. Previous investigators have determined the tibial-femoral (TF) joint failure load for an experimentally constrained human knee at 90° flexion. In these experiments bone fractures have been documented. During TF joint compression, however, anterior motion of the tibia has been noted by others. It was therefore the objectives of this study to document effects of flexion angle and anterior-posterior joint constraint on the nature and severity of knee injury during TF compression loading via axial loads in the tibia.

The effect of flexion angle was examined using 10 unconstrained human knees from 5 cadavers aged 73.2±9.4 years. The tibial-femoral joint was loaded in compression as a result of axial loading along the tibia using a servo-hydraulic testing machine until gross failure with the knee flexed 60° or 120°. Pressure sensitive film measured the distribution of internal TF joint loads. Both 60° and 120° flexed preparations failed by rupture of the anterior cruciate ligament (ACL) at 4.6±1.2 kN, and the internal joint loads were significantly higher (2.6±1.5 kN) on the medial versus the lateral (0.4±0.5 kN) aspect of the tibial plateau.

The effect of anterior-posterior (AP) constraint of the femur along the longitudinal axis of the femur was investigated in a second series of tests using the same TF joint loading protocol on 6 pairs of human joints (74.3±10.5 years) flexed at 90°. The primary mode of failure for the AP constrained joints was fracture of bone via the femoral condyle at a maximum load of 9.2±2.6 kN. The mode of failure for unconstrained joints was primarily due to rupture of the ACL at a maximum load of 5.8±2.9 kN. Again, the pressure film indicated an unequal internal TF load distribution for the unconstrained knee (medial plateau 4.1±1.9 kN versus lateral plateau 0.8±0.8 kN). However, there was a more equal distribution of internal loads between the medial (4.4±1.8 kN) and lateral (2.8±1.9 kN) aspects of the tibial plateau in the constrained joints.

This study showed that the mechanism of tibial-femoral knee joint injury and internal TF joint load distribution depends on the degree of AP constraint offered by the test apparatus. Flexion angle did not significantly affect failure load or the mechanism of failure for the unconstrained knee. The findings from this study may be useful in understanding the complex failure mechanisms for an unconstrained knee under axial compression loads in the tibia during automobile crashes.