Tensile properties of human tendo Achillis: effect of donor age and strain rate.

Two sets of tensile tests were conducted on tendo Achillis taken from 16 embalmed cadavers (donor ages, 36 to 100 years). From the stress, sigma-versus-strain, epsilon results, values of the following properties were obtained: linear stiffness, ST (which is defined as the slope of the linear zone of the post-toe region of the load-versus-extension plot), ultimate tensile strength [UTS], the strain at the UTS point (herein called the ultimate tensile strain) [epsilon max], and the tangent modulus of elasticity, [ET] (which is the slope of the linear zone of the post-toe region of the sigma-epsilon curve). In the first set of tests, the loading rate was 10%/sec. The results are thus: linear stiffness: 685 +/- 262 N/mm.; UTS = 59 +/- 18 MPa; epsilon max = 22 +/- 7%; and ET = 375 +/- 102 MPa. It was found that donor age exerts an insignificant effect on linear stiffness, a marked effect on UTS, an insignificant effect on epsilon max, and a moderate effect on ET (Student's t-test; p < 0.05). In the second set of tests, the test tendons were taken from the cadavers of "middle-aged" donors (52 to 67 years), and the strain rate was 100%/sec. By combining the results of the first and second sets of tests for a subset of this group (i.e., those with matched counterparts), it was found that strain rate has a statistically significant effect on ST and ET but not on UTS and epsilon max (paired Student's t-test; p < 0.05). Based on all results and findings, donor age is suggested as one of the considerations when selecting tendo Achillis to be used as allografts for the repair of severely damaged or ruptured tendo Achillis or anterior cruciate ligament.