Biomechanical Characterization of Human Amniotic Membrane Preparations for Ocular Surface Reconstruction

PURPOSE: To investigate the tensile and elastic properties of both commercially available and experimental human amniotic membrane preparations. METHOD: Nine preparations of human amniotic membrane were studied. The four dry preparations were untreated (nonirradiated, n = 20), and gamma (n = 25), low-dose (AmbioDry, Okto Ophtho Inc., Costa Mesa, Calif., USA, n = 20) and high-dose (n = 20) electron beam sterilized. The same dry membranes were moistened with balanced salt solution (n = 20, 34, 20 and 20, respectively). The ninth group consisted of thawed medium-frozen amniotic membrane (AmnioGraft, Bio-Tissue Inc., Miami, Fla., USA, n = 20). The membranes were cut into thin strips, loaded on a gram range load sensor, and stretched incrementally to the point of rupture. The modulus of elasticity, displacement until rupture and maximum tolerated stress were recorded and compared. RESULTS: The dry preparations exhibited higher moduli of elasticity when compared with the moist samples, with the low-dose electron beam-irradiated samples having the greatest mean modulus of elasticity overall and maintaining a high modulus of elasticity as a moist sample (p < 0.05). Moist nonirradiated preparations and thawed medium-frozen preparations stretched the farthest before rupture and experienced the greatest mean stresses at the point of rupture. While 3 of 4 membranes had greater stretch when moistened as compared to their dry counterparts, there was no difference in the membrane stiffness between dry and moistened low-dose electron beam-irradiated samples (p > 0.8). CONCLUSIONS: Low-dose electron beam-irradiated amnion appeared to maintain desirable elastic characteristics in transition from a dry to rehydrated state and may thus provide an easy-to-manipulate transplant tissue for ocular surface reconstruction. Moist nonirradiated and thawed medium-frozen tissues, however, may provide surgical advantages as they required greater forces to rupture.