Lower strength of the human posterior patellar tendon seems unrelated to mature collagen cross-linking and fibril morphology

Abstract

The human patellar tendon is frequently affected by tendinopathy, but the etiology of the condition is not established, although differential loading of the anterior and posterior tendon may be associated with the condition. We hypothesized that changes in fibril morphology and collagen cross-linking would parallel differences in material strength between the anterior and posterior tendon. Tendon fascicles were obtained from elective ACL surgery patients and tested micromechanically. Transmission electron microscopy was used to assess fibril morphology, and collagen cross-linking was determined by HPLC and calorimetry. Anterior fascicles were markedly stronger (peak stress: 54.3 +/- 21.2 vs. 39.7 +/- 21.3 MPa; P < 0.05) and stiffer (624 +/- 232 vs. 362 +/- 170 MPa; P < 0.01) than posterior fascicles. Notably, mature pyridinium type cross-links were less abundant in anterior fascicles (hydroxylysylpyridinoline: 0.859 +/- 0.197 vs. 1.416 +/- 0.250 mol/mol, P = 0.001; lysylpyridinoline: 0.023 +/- 0.006 vs. 0.035 +/- 0.006 mol/mol, P < 0.01), whereas pentosidine and pyrrole concentrations showed no regional differences. Fibril diameters tended to be larger in anterior fascicles (7.819 +/- 2.168 vs. 4.897 +/- 1.434 nm(2); P = 0.10). Material properties did not appear closely related to cross-linking or fibril morphology. These findings suggest region-specific differences in mechanical, structural, and biochemical properties of the human patellar tendon.