Background: The use of allograft tendons has increased for primary and revision anterior cruciate ligament reconstruction, but allograft supply is currently limited to a narrow range of tendons and donors up to the age of 65 years. Expanding the range of donors and tendons could help offset an increasing clinical demand. Purpose: To investigate the effects of donor age, sex, height, and specific tendon on the mechanical properties of a range of human lower leg tendons. Study Design: Descriptive laboratory study. Methods: Nine tendons were retrieved from 39 fresh-frozen human cadaveric lower legs (35 donors [13 female, 22 male]; age, 49-99 years; height, 57-85 inches [145-216 cm]) including: Achilles tendon, tibialis posterior and anterior, fibularis longus and brevis, flexor and extensor hallucis longus, plantaris, and flexor digitorum longus. Tendons underwent tensile loading to failure measuring cross-sectional area (CSA), maximum load, strain at failure, ultimate tensile strength, and elastic modulus. Results from 332 tendons were analyzed using mixed-effects linear regression, accounting for donor age, sex, height, and weight. Results: Mechanical properties were significantly different among tendons and were substantially greater than the effects of donor characteristics. Significant effects of donor sex, age, and height were limited to specific tendons: Achilles tendon, tibialis posterior, and tibialis anterior. All other tendons were unaffected. The Achilles tendon was most influenced by donor variables: greater CSA in men (β = 15.45 mm2; Šidák adjusted P < .0001), decreased maximum load with each year of increased age (β = −17.20 N per year; adjusted P = .0253), and increased CSA (β = 1.92 mm2 per inch; adjusted P < .0001) and maximum load (β = 86.40 N per inch; adjusted P < .0001) with each inch of increased height. Conclusion: Mechanical properties vary significantly across different human tendons. The effects of donor age, sex, and height are relatively small, are limited to specific tendons, and affect different tendons uniquely. The findings indicate that age negatively affected only the Achilles tendon (maximum load) and challenge the exclusion of donors aged >65 years across all tendon grafts. Clinical Relevance: The findings support including a broader range of tendons for use as allografts for anterior cruciate ligament reconstruction and reviewing the current exclusion criterion of donors aged >65 years.

Increasing anterior cruciate ligament (ACL) rupture rates are driving the need for new graft materials which undergo testing to characterize material properties and function. The in vitro collagenase assay is routinely used to determine the degradation rate of collagenous materials. At times, it is used to screen new biomaterials on the basis that results reflect breakdown rates in vivo. However, its predictive potential is inconclusive with no guidelines for acceptable in vitro rates of degradation. Reference data from target tissue/s or existing clinical materials are needed to determine appropriate thresholds. From a summary of reported protocols, the most common bench conditions (bacterial collagenase; unloaded samples) were used to evaluate the in vitro degradation of human tendons used as ACL allografts: patellar, semitendinosus, gracilis, Achilles, tibialis anterior and posterior. Tendons were sectioned in equal volumes and exposed to 100 U collagenase for 1, 2, 4 or 8 h. The change in dry weight was analysed using mixed linear regression. All tendon samples demonstrated a significant reduction in mass over time but the patellar tendon degraded significantly faster than all other tendons (P ≤ 0.004). As all tendons used in this study are clinically accepted, this study provides a range of human tendon reference data for comparative assessment of new tendon and ligament biomaterials. However, the more rapid degradation of the patellar tendon, one of the most successful ACL graft materials, also highlights the limitations of common collagenase assay conditions for predicting in vivo performance, particularly in the absence of suitable comparative controls.