Influence of bone mineral density, age, and strain rate on the failure mode of human Achilles tendons

Objective, To examine the influence of strain rate. bone mineral density, a nd age in determining the mode by which human Achilles tendons fail. Design. Dual-energy X-ray absorptiometry and mechanical testing of excised Achilles tendon-calcaneus specimens. Background. The Achilles tendon can fail by tendon rupture or bony avulsion . These injuries are caused by similar loading mechanisms and can present s imilar symptoms. It is important to understand when each mode of injury is likely to occur so that accurate diagnoses can be made and appropriate trea tments selected. Methods, Excised human Achilles tendons were loaded to failure at strain ra tes of 1% s(-1) and 10% s(-1) following dual-energy Xray absorptiometry exa mination to determine bone mineral density near the tendon insertion. Calca neal bone mineral density, donor age, and strain rate were compared between specimens that failed by avulsion and those that failed by tendon rupture. Results. While strain rate was not observed to affect failure mode, the cal caneal bone mineral density of specimens that failed by avulsion was signif icantly lower than the bone mineral density of specimens that failed by ten don rupture (P = 0.004). There was a significant decrease in bone mineral d ensity with age (P = 0.004), and the difference in age between the avulsed and ruptured specimens was close to statistical significance (P = 0.058). F or the avulsed specimens, there was a significant linear relationship betwe en failure load and bone mineral density squared (P = 0.002). Logistic regr ession indicated that the effect of age on failure mode is secondary to the primary effect of bone mineral density. Conclusions. The avulsions were primarily "premature" failures associated w ith low bone mineral density. Since bone mineral density decreases with age , older individuals are more likely to experience avulsions while younger i ndividuals are more likely to experience tendon ruptures.