Effects of repetitive multiple freeze–thaw cycles on the biomechanical properties of human flexor digitorum superficialis and flexor pollicis longus tendons

BackgroundDeep fresh freezing is the most commonly used preservation method for allografts, which is followed by a routine procedure before application—the thawing of the grafts. This work was to explore the biomechanics effects of repetitive freezing–thawing on human tendons.MethodsThirty tendons (flexor digitorum superficialis and flexor pollicis longus tendons) were taken from three fresh cadavers. We assigned these tendons into six groups randomly. After the preparation and measurement of cross-sectional area, Fresh Group was tested immediately. The remaining five groups, before biomechanical testing, underwent multiple cycles (1, 2, 3, 5 or 10 cycles) of freezing (− 80 °C for 4 days) and thawing (room temperature for 6 h). Using a material-testing machine, we performed a stress-relaxation test, a creep test and a load to failure test to estimate structural and mechanical properties as well as viscoelastic changes of each group.FindingsThe results of fewer cycles (< 3) of freezing–thawing groups were similar to Fresh Group. The tendons that underwent more than five freezing–thawing cycles demonstrated a significantly smaller ultimate load, stiffness, ultimate stress and Young's modulus. Moreover, the 5 and 10 Cycle Groups showed a significant increase of elongation in creep test and a decrease of load in stress-relaxation test compared with other groups.InterpretationRepetitive freezing–thawing (> 5 cycles) declines the structural, mechanical and viscoelastic properties of human tendons. From our results, it is recommended that for deep-frozen preservation of tendon allografts the cycles of freezing–thawing should be less than five times with caution.