The role of Plantaris Longus in Achilles tendinopathy: a biomechanical study.

BACKGROUND The Plantaris Longus Tendon (PLT) may be implicated in Achilles (AT) tendinopathy. Different mechanical characteristics may be the cause. This study is designed to measure these. METHODS Six PLT and six AT were harvested from frozen cadavers (aged 65-88). Samples were stretched to failure using a Minimat 2000™ (Rheometric Scientific Inc.). Force and elongation were recorded. Calculated tangent stiffness, failure stress and strain were obtained. Averaged mechanical properties were compared using paired, one-tailed t-tests. RESULTS Mean stiffness was higher (p<0.001) in the PLT, measuring 5.71 N/mm (4.68-6.64), compared with 1.73 N/mm (1.40-2.22) in AT. Failure stress was also higher (p<0.01) in PLT: 1.42 N/mm(2) (0.86-2.23) AT: 0.20 N/mm(2) (0.16-0.25). Failure strain was less (p<0.05) in PLT: 14.1% (11.5-16.8) than AT: 21.8% (14.9-37.9). CONCLUSIONS The PLT is stiffer, stronger than AT, demonstrating potential for relative movement under load. The stiffer PLT could tether AT and initiate an inflammatory response.