Associations between age and mechanical properties in skeletally immature human patellar tendons

The objective was to quantify the mechanical properties of the central region of skeletally immature human patellar tendons and the associations with age. Twenty-six patella-patellar tendon complexes were examined (range 0.1-9.9 years, 17 males, 9 females). The cross-sectional area at the midsubstance of the native and dog-boned patellar tendons were measured using a 3D laser scanning system. The patellar tendons underwent a mechanical testing protocol to failure with loading criteria normalized to cross-sectional area. Associations between mechanical properties, native cross-sectional area, and age were determined using Pearson or Spearman's correlations. The only association observed between age and mechanical properties was a positive association between age and ultimate stress (R² = 0.21, p = 0.02), thus as age increased, the ultimate stress increased. No association between age and modulus was found (p > 0.05). A positive association between age and native cross-sectional area was observed (R² = 0.64, p = 0.001). Furthermore, 46 % of specimens lacked a typical toe region of the stress-strain curve. Increased ultimate stress with age may indicate the patellar tendon adapts throughout maturation to increase the force per unit area withstood before failing. In combination with the increases in native cross-sectional area, the patellar tendon may adapt to increased loading occurring at the knee throughout maturation at the macrostructural and microstructural levels. The lack of a toe region in some patellar tendons may indicate additional differences in tissue architecture such as smaller collagen crimp angles, more collagen cross-linking, or lower elastin concentrations. Thus, the current study provides information on changes in tissue function throughout growth and development.