Experimental Measurement and Applied Modelling of Patellar Tendon Strain

Abstract

Patellar tendinopathy is an overuse injury that occurs from repetitive loading of the patellar tendon in a scenario resembling mechanical fatigue. Tendon strain is correlated with the number of repetitive loading cycles that can be withstood prior to fatigue failure, and therefore represents a potential metric to infer patellar tendinopathy risk. To this end, I first quantified the mechanical fatigue behaviour of the patellar tendon in a cyclic loading scenario. I discovered that initial peak nominal strain, initial peak median strain, creep rate, and damage rate all displayed significant power law relationships with fatigue life. Based on the non-linear relationship between strain and fatigue life, I examined the effect of shoe outsole stiffness and surface construction on in vivo estimates of patellar tendon strain with the aim of exponentially increasing the number of jumps that could be performed prior to fatigue failure. Unfortunately, I discovered that neither intervention altered patellar tendon strain during jumping, potentially because the differences in shoe stiffness and surface construction were not large enough to require kinetic and/or kinematic changes at the knee. Finally, I assessed the effect of two strain estimation methodologies with varied levels of subject-specificity on fatigue-life estimates. I discovered that nominal strain models overestimated patellar tendon fatigue life compared to finite element models as they do not account for strain concentrations both within the tendon midsubstance and near the entheses, while fatigue-life estimates from both finite element and nominal strain models were sensitive to the use of generic material properties but not generic geometry. The results of these studies form a principled basis for the estimation of patellar tendon strain and additional time-dependent metrics to characterize patellar tendinopathy risk from a mechanical fatigue perspective, and further illustrate appropriate methodological approaches to do so.