Control of material stiffness during degradation for constructs made of absorbable polymer fibers

Abstract

Augmentation devices for cruciate ligament surgery should provide gradually decreasing mechanical properties with a half-time strength of at least 6 months to temporarily protect healing tendon grafts or sutured ligaments against high tensile loads during the postoperative healing period. The absorbable material of choice that shows such slow degradation kinetics is poly(L-lactide). However, previous studies have shown that poly(L-lactide) fulfills the requirement of a long half-time strength, while the corresponding stiffness decreases at a much slower rate. An augmentation stiffness that does not change much versus time cannot provide a gradual increase in graft load, which is important to stimulate the orientation of the collagenous tissue. Therefore a new augmentation device was designed, which should decrease both in strength and stiffness during degradation. The cord was braided out of two fibers made of poly(L-lactide) and poly(Llactide-co-glycolide), which degrade at different rates. The cord prototype was degraded in vitro and the rupture force and stiffness was tested at eight different time points up to 60 weeks. The initial rupture force and stiffness was 522.7 ؎ 2.8 N and 104.1 ؎ 3.8 N/%, respectively. Both strength and stiffness decreased continuously with a half-time strength of 18 weeks and a half-time stiffness of 39 weeks. The gradually decreasing stiffness was achieved by the breakdown of the faster-degrading fiber component made of poly(Llactide-co-glycolide). Thus the new augmentation device can provide a continuous increase of forces in a tendon graft or in a healing ligament.

Biological response to a new composite polymer augmentation device used for cruciate ligament reconstruction

“…So far, no experimental study in sheep achieved a stability of the knee joint similar to the stability of a nonoperated knee, either by augmented repair or by patellar tendon graft 12, 37, 38. In an in vitro test, the half‐life of the new composite cord was 4 months with a remaining rupture force of 400 N using a double‐strand cord 27. Assuming the half‐life is reached after 3 months due to a faster degradation in vivo , the rupture force of the double‐strand composite cord was still sufficient to meet the minimum requirements of 200 N for ligament protection 27.…”

“…In an in vitro test, the half‐life of the new composite cord was 4 months with a remaining rupture force of 400 N using a double‐strand cord 27. Assuming the half‐life is reached after 3 months due to a faster degradation in vivo , the rupture force of the double‐strand composite cord was still sufficient to meet the minimum requirements of 200 N for ligament protection 27. In most cases, the PLA cord ruptured early because of abrasion, friction, and bending at the edges of the drill tunnels, although they showed superior fatigue behavior in vitro compared with PDS (our own unpublished results).…”

“…PLA was slowly degrading and LG 90/10 was fast degrading. The exact composition, manufacture protocol, mechanical properties, and degradation kinetics have already been characterized in detail in previous studies 27, 28. In group II, a doubled 2‐mm resorbable PDS cord (Ethicon, Norderstedt, Germany) was used for ligament augmentation.…”

“…Furthermore, the stress protection effect should gradually decrease with progressing ACL healing, adapting the tissue to an increasing mechanical load. To achieve a decreasing stress protection during ligament healing, an augmentation construct braided out of two fiber materials with different degradation rates was designed in a prior study 27. The augmentation cord was braided out of a faster degrading poly(L‐lactide‐co‐glycolide) fiber and a slower degrading poly (L‐lactide) fiber 28…”

Comparison of Achilles Tendon Repair Techniques in a Sheep Model Using a Cross-linked Acellular Porcine Dermal Patch and Platelet-rich Plasma Fibrin Matrix for Augmentation

“…These findings indicated fibroblast infiltration into the scaffold, thereby incorporating the implanted material with native tendon and surrounding tissues before its eventual degradation, which may serve to inhibit fibrosis (scar formation) and clinical tendon contracture. Surgeons should keep in mind, too, that an augmentation scaffold should act as a load-sharing device during the course of tendon healing, thereby minimizing the risk of recurrent tendon injury during the healing phase and allowing the tendon to heal under conditions of protected load (20,21).…”