Aging Decreases the Ultimate Tensile Strength of Bone-Patellar Tendon-Bone Allografts

Abstract Purpose The purpose of this study was to determine whether aging imparts a clinically significant effect on the: 1) mechanism of graft failure, and 2) structural, material, and viscoelastic properties of patellar tendon allografts by evaluating these properties in younger donors (≤30 years of age) and older donors (>50 years of age). Methods A total of 34 younger (≤30 years of age) and 34 older (>50 years of age) nonirradiated, whole bone-tendon-bone allografts were prepared for testing by isolating the central third of the patellar tendon using a double-bladed 10 mm width scalpel under 10 N load to ensure uniformity of harvest. Bone blocks were potted in polymethylmethacrylate within custom molds. Tendon length and cross-sectional area were measured using an area micrometer. A mechanical loading system was used to precondition the grafts for 100 cycles with a load between 50N and 250N (1 Hz). A creep load (500N) was then applied at a rate of 100 mm/min (10 min). Grafts were allowed to recover at 1N (10 min), followed by pull-to-failure at a rate of 100% strain per second. Mechanisms of failure (mid-substance versus avulsion) were noted and the structural, material, and viscoelastic properties calculated and compared between groups. Results There were 33 (97%) mid-substance tears in the younger group and 28 (82%) in the older group (p=0.034). Younger grafts showed greater ultimate load-to-failure (1,782N [1,533, 2,032] versus 1,319N [1,103, 1,533]) (p=0.006) and ultimate tensile stress (37.4MPa [32.4, 42.4] versus 27.5MPa [22.9, 32.0]) (p=0.006). There were no significant differences in displacement (p=0.595), stiffness (p=0.950), strain (p=0.783), elastic modulus (p=0.114), creep displacement (p=0.881), and creep strain (p=0.614). Conclusion This in vitro study suggests that aging weakens the bone-tendon junction and decreases the ultimate tensile strength of patellar tendon allografts. However, aging did not affect the displacement, strain, stiffness, elastic modulus, creep displacement, or creep strain of patellar tendon allografts. Clinical Relevance Surgeons should be aware that patellar tendon allografts from donors > 50 years of age have a lower ultimate tensile stress than donors ≤ 30 years of age.