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The Effect of Loading Rate on the Degree of Acute Injury and Chronic Conditions in the Knee After Blunt Impact
Technical Paper
2000-01-SC20
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English
Abstract
Lower extremity injuries due to automobile accidents are often
overlooked, but can have a profound societal cost. Knee injuries,
for example, account for approximately 10% of the total injuries.
Fracture of the knee is not only an acute issue but may also have
chronic, or long term, consequences. The criterion currently used
for evaluation of knee injuries in new automobiles, however, is
based on experimental impact data from the 70''s using
seated human cadavers. These studies involved various padded and
rigid impact interfaces that slightly alter the duration of
contact. Based on these data and a simple mathematical model of the
femur, it appears fracture tolerance increases as contact duration
shortens. In contrast, more recent studies have shown mitigation of
gross fractures of the knee itself using padded interfaces. The use
of padded interfaces, however, result in coincidental changes in
contact duration and knee contact area. Therefore, it is difficult
to extract the direct effect of loading rate on fracture tolerance
of the knee. The object of the current study was to isolate the
effect of loading rate alone on fracture tolerance of the human
knee joint. Paired experiments were conducted on eight pairs of
isolated cadaver knees impacted with a rigid interface to
approximately 5 kN at a high (5 ms to peak) or low (50 ms to peak)
rate of loading. Gross fracture and occult microfractures of the
knee joint were documented. A second part of the study examined
some chronic effects of loading rate on "subfracture"
injuries in an animal. Thirty-four rabbits were subjected to a
"subfracture" knee load at the same rates as used in the
human studies. Alterations in the mechanical properties of
retropatellar cartilage and thickening of subchondral bone were
documented out to one year post "subfracture" trauma to
the joint.
The current study documented an opposite effect than that
expected based on 70''s experiments with seated cadavers.
There was an increase in the number of gross fractures and occult
microfractures in high versus low rate of loading experiments. A
similar effect was also seen in the "subfracture" chronic
animal experiments, which showed relatively more degradative change
in the mechanical properties of cartilage following high versus low
rate of loading experiments. There was also a significant increase
in subchondral bone thickening underlying cartilage and increased
fissuring of cartilage in high versus low rate of loading
experiments. The current study suggests a relative decrease in
tolerance of the knee at high versus low rates of loading in acute
experiments with human cadavers and in the chronic setting with
animals. Therefore, it would appear that rate of knee loading may
be an important issue in establishing a future injury criterion for
the knee itself.
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