Biomechanical analysis of biodegradable interbody fusion cages augmented With poly(propylene glycol-co-fumaric acid)

Three different types of biodegradable poly(L-lactide-co-D,L-lactide) cages with and without augmentation of a biodegradable poly(propylene glycol-cofumaric acid) scaffold were compared with autograft and metallic cages of the same design and size by determining the stiffness and failure load of the L4-L5 motion segment of cadaveric human spines.

To determine how these devices limit the range of motion in the lumbar spine compared with a metallic cage. If biomechanically equivalent, biodegradable spinal fusion systems ultimately could reduce local stress shielding and diminish the incidence of clinical complications, including device-related osteopenia, implant loosening, and breakage.

Previous studies in dogs and humans have demonstrated vertebral body osteopenia as a result of instrumented spine fusions. To the authors' knowledge, neither an in vitro nor an in vivo biomechanical analysis of a biodegradable interbody fusion system has been performed.

Forty-eight L4-L5 motion segments were isolated from 22 male and 26 female human donors with an average age of 49.6 +/- 2.7 years (range 36-55 years). Cages of similar dimensions and design, including a threaded, hollow, porous titanium BAK cage and three different BIO cages (BIO cage 1, pure polymer; BIO cage 2, polymer plus hydroxyapatite buffer; BIO cage 3, polymer plus nano-sized hydroxyapatite), produced from the same poly(L-lactide-co-D,L-lactide) polymer were tested in a comparative analysis to intact motion segment, interbody implantation of autograft, and a BIO cage augmented with an expandable biodegradable foam-scaffold fashioned from poly(propylene glycol-cofumaric acid).

All cages were able to increase stiffness and failure load of the unstable motion segment significantly (P < 0.01). In comparison with the bone graft, the BAK cage (P < 0.01) and BIO cages 1 and 3 (P < 0.05) were able to increase stiffness and failure load. There was no significant difference between BIO cage 2 and the bone graft. Augmentation of BIO cage 1 with the foaming PPF scaffold resulted in higher stiffness and similar failure load as seen with the BAK cage.

By comparison, the in vitro lumbar spinal motion segment stiffness and failure load produced by implantation of a biodegradable interbody fusion cage augmented with an expandable PPF scaffold is similar to that of the titanium BAK cage. This suggests that biodegradable anterior interbody fusion systems could be further developed for clinical applications.