The mechanical properties of the Richards polyethylene ligament implant
were studied and compared with the strength of human cruciate ligament
preparations. The implant yields and plastically deforms at a force of 420
newtons (ninety-four pounds) and approximately 10 per cent elongation when
tested at a strain rate of 100 per second. This is one-fourth the average
tensile strength of the human anterior curciate ligament preparations. When
tested at a strain rate of 1 per cent per second the implant's yield-point
force was lowered by 15 per cent. The implant exhibits considerable
viscoelastic behavior and will progressively elongate under repetitive
loads when insufficient time is allowed for it to return to its original
length. In bending, the implant did not fail after eighty million stress
reversals (forty million revolutions) when tested while immersed in saline
solution at body temperature. The tests indicate that the tensile strength
of the implant is low in comparison with both the strength of human
preparations and estimates of actual in vivo functional loads. We conclude
that caution is required in the use of this implant.