2015 ISAKOS Biennial Congress ePoster #306

High-Load Preconditioning of Soft Tissue Grafts: An In Vitro Biomechanical Bovine Tendon Model

Jeffrey R. Jaglowski, MD, MSc, Vail, CO UNITED STATES
Brady Thomas Williams, BS, Vail, CO UNITED STATES
Travis Lee Turnbull, PhD, Vail, CO UNITED STATES
Robert F. LaPrade, MD, PhD, Edina, MN UNITED STATES
Coen Abel Wijdicks, PhD, Naples, Florida UNITED STATES

Steadman Philippon Research Institute, Vail, Colorado, USA

FDA Status Not Applicable

Summary: Current preconditioning protocols for soft tissue grafts do not minimize time zero cyclic displacement nor create a graft biomechanically equivalent to the native ACL; however, this study demonstrated that increased preconditioning force significantly decreased the subsequent elongation during simulated early rehabilitation and created grafts with equivalent stiffness to the native ACL.

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Abstract:

Background

Tendon grafts used for ACL reconstruction are viscoelastic which can lead to graft laxity or failure following reconstruction. Current preconditioning protocols are surgical techniques that minimize graft viscoelasticity, and thus decrease the susceptibility for knee laxity in the initial postoperative period. However, no consensus exists regarding the optimal preconditioning protocol that will minimize postoperative elongation while creating a graft that is biomechanically equivalent to the native ACL.

Purpose

The purpose was to define the optimal preconditioning protocol to minimize elongation while creating a graft with equivalent stiffness to the native ACL under simulated early rehabilitation loading. The inherent biological characteristics of the graft tissue itself were isolated in this study, as the viscoelastic contribution could be independent of the optimal solution of other variables (e.g., fixation method). It was hypothesized that a preconditioning protocol of specific mode and magnitude would create a graft with equivalent stiffness to the native ACL and minimize laxity resulting from a simulated early rehabilitation protocol.

Methods

Thirty-six bovine extensor tendon grafts were sized to 9 mm in diameter, trimmed to be 180 mm in length, doubled over, and 30 mm of each free end was whipstitched. This created a graft 60 mm in length which approximated the amount of free graft (graft subject to elongation) in a routine ACL reconstruction. Grafts were randomly allocated among six preconditioning groups including three cyclic (10 cycles at 0.5 Hz between 10-80 N, 100-300 N, and 300-600 N) and three static loading protocols (20 seconds at 80 N, 300 N, and 600 N). Following preconditioning, grafts were immediately cyclically loaded for 500 cycles between 50-250 N at 0.5 Hz to simulate forces in the ACL during early rehabilitation. Equivalence tests determined whether cyclic stiffness values could be considered equivalent to previously published data for ACL stiffness.

Results

Cyclic 300-600 N and static 600 N loading protocols both demonstrated significantly less elongation during simulated early rehabilitation when compared to lower, current clinical standard preconditioning levels of 10-80 N (62% delta) and 80 N (69% delta). Additionally, stiffness values resulting from cyclic 300-600 N and static 600 N load preconditioning protocols demonstrated statistical equivalence (p = .042 and .024, respectively) when compared to the stiffness of the native ACL.

Conclusion

The most important finding of this study was that current preconditioning protocols for soft tissue grafts do not minimize time zero cyclic displacement nor create a graft biomechanically equivalent to the native ACL. This study demonstrated that increased force applied to soft tissue grafts during preconditioning significantly decreased the subsequent elongation during simulated early rehabilitation. Furthermore, a static load of 600 N resulted in the most graft elongation during preconditioning, had the least amount of cyclic displacement, and was statistically equivalent to the previously reported native ACL stiffness.

CLINICAL RELEVANCE: The development of a validated preconditioning protocol capable of producing grafts with structural properties equivalent to the intact ACL may reduce postoperative graft elongation.