We created an experimental model to evaluate the effects of strain rate on
the mechanism of combined cruciate ligament injuries in knee hyperextension
. Using straight knee hyperextension to rupture the anterior and posterior
cruciates, two strain rates (approximately 100% per second and 5400% per se
cond) were applied to reproduce two clinical injury patterns of the knee: l
ow energy (sporting) and high energy (pedestrian-motor vehicle accident). T
en pairs of fresh-frozen cadaveric knees were injured to 45 degrees of hype
rextension. Strain rate sensitivity of the posterior cruciate ligament was
shown in this model, with midsubstance tears occuring in specimens tested a
t a low rate and avulsion "stripping" injuries from the femoral side occuri
ng at a high rate. A variable pattern of anterior cruciate ligament tears a
t both high and low rates suggests that the specific injury mechanism may a
lso involve other factors including notch morphology. We present a simplifi
ed mathematic model used to estimate posterior cruciate Ligament strain dur
ing knee hyperextension.