How Variable Are Achilles Allografts Used for Anterior Cruciate Ligament Reconstruction? A Biomechanical Study

Abstract

Background:

Allograft tendon used in anterior cruciate ligament reconstruction (ACLR) requires sterilization before implantation. Low-dose gamma irradiation is a means of sterilization that may minimize tissue damage.

Purpose:

To quantify the variability in mechanical properties between low-dose irradiated Achilles tendon allografts used for ACLR.

Study Design:

Descriptive laboratory study.

Methods:

A total of 15 intact outer-third Achilles tendon allograft specimens were collected from the remains of full Achilles allografts used for intraoperative ACLR at a single hospital. All grafts were obtained from a single tissue bank and underwent proprietary disinfection and low-dose gamma irradiation (1.5-2.5 Mrad). Biomechanical testing was carried out to measure tendon elongation, failure location during tensile testing, maximum stress, maximum strain, and modulus of elasticity. The mean and standard deviation were calculated for each outcome measure, and the variability between specimens was calculated by the coefficient of variation (CV). The effect of donor age on graft material properties was examined by use of linear regression. One-way analysis of variance was performed to compare differences in the mechanical properties across failure locations.

Results:

During cyclic testing, tendon elongation averaged 1.4% ± 1.6% with a CV of 118%. During failure testing, the maximum stress averaged 12.2 ± 4.1 MPa, maximum strain averaged 21.0% ± 6.3%, and modulus of elasticity averaged 95.5 ± 30.8 MPa. The CVs for maximum stress, maximum strain, and modulus of elasticity were 34%, 30%, and 32%, respectively. Ten tendons failed in the midsubstance and 5 failed at the tendon-bone enthesis. No differences were noted in mechanical properties between grafts that failed in the midsubstance versus those that failed at the enthesis. Donor age did not correlate with allograft elongation during cyclic load or any of the material property measures during failure testing.

Conclusion:

The variabilities in the material properties and graft elongation during cyclic loading of Achilles tendon allografts used in ACLR fall within the range of properties reported in the literature for other ACLR allografts. Material properties do not differ by donor age or graft failure location observed during failure testing.

Clinical Relevance:

Surgeons should be aware that there exists considerable variation in the mechanical properties of Achilles allograft tendons used for ACLR. This variability is difficult to detect by tissue bank screening or the treating surgeon’s inspection and may contribute to the heterogeneity in outcomes of allograft ACLR.