The effect of impact angle on knee tolerance to rigid impacts

While the number of deaths from vehicle accidents is declining, probably because of mandatory seat belt laws and air bags, a high frequency of lower extremity injuries from frontal crashes still occurs. For the years 1979-1995 the National Accident Sampling System (NASS) indicates that knee injuries (AIS 1-4) occur in approximately 10% of cases. Patella and femur fractures are the most frequent knee injuries. Current literature suggests that knee fractures occur in seated cadavers for knee impact forces of 7.3 to 21.0 kN. Experimental data shown in a study by Melvin et al. (1975) further suggests that fracture tolerance of the knee may be reduced for an impact directed obliquely to the axis of the femur. The current study hypothesized that the patella is more vulnerable to fracture from an oblique versus an axial impact (directed along the femoral axis), and that the fracture pattern would vary with impact direction. Isolated, 90 degrees flexed, paired human knee joints (73 -/+ 16.9 years) were impacted at sequentially higher loads either axially or obliquely from the medial aspect with a rigid interface on the patella. The peak load at fracture for each case was recorded, and a detailed description of the fracture pattern was documented. For axial impacts all nine knees failed by linear and comminuted patella fracture with an average peak load of 5.9 -/+ 1.4 kN. Seven of nine obliquely impacted knees also failed by linear and comminuted patella fracture with a significantly lower peak load of 3.5 -/+ 1.4 kN. The peak load data from fracture experiments for all knees showed a strong correlation with age and direction of the impact. Additionally, the fracture pattern for the axial impacts was generally oriented along a horizontal plane on the patella, while the fractures for oblique impacts were generally oriented along a vertical plane. In two oblique experiments patella fracture did not occur, as the patella dislocated at a load of 3.4 -/+ 0.2 kN. In one of these cases the medial aspect of the patello-femoral joint capsule was visibly torn, and in the other case surface damage was noted on the articular cartilage covering the lateral femoral condyle. In addition to the acute injuries described in this study, these data may suggest a potential for chronic diseases of the knee in cases where similar injuries are produced. Clinical studies have shown an increased risk of osteoarthritis in patients suffering patella fractures and damage to joint cartilage. Also, acute dislocation of the patella may cause soft tissue injury of the knee leading to chronic mal-tracking of the patella. These data may be particularly relevant in cases where occupants sit with abducted lower limbs prior to a frontal crash.