Three-year follow-up of bioabsorbable PLLA cages for lumbar interbody fusion: in vitro and in vivo degradation.

Introduction: Current cage devices considerably exceed the stiffness of the spinal segment, which may lead to stress-shielding, migration of the cage, and pseudarthrosis. In order to avoid these material related complications, a poly (L-lactic acid) cage device (PLLA cage) has been designed with limited stiffness compared to current cage devices and which will be absorbed on the long term. Biomechanical in vitro testing demonstrated that the PLLA cage is mechanically sufficient directly after implantation [1] and radiographic evaluation in an in vivo investigation disclosed that interbody fusion could be obtained after 6 months follow-up [2]. In the last three decades the use of biodegradable PLLA osteosynthesis devices has gained an increasing interest and have mainly been used in non weightbearing conditions due to their attributed limited mechanical properties [3]. The use of PLLA spinal cages to induce lumbar interbody fusion under high load bearing conditions has not yet been investigated. Under the high load bearing conditions in the current investigation, the degradation kinetics of the PLLA cage may be increased as the rate of polymer degradation is influenced by the degree of dynamic loading. The aim of this investigation was therefore to describe the in vitro degradation of bioabsorbable PLLA cages and the in vivo degradation under high load bearing conditions in a lumbar interbody fusion goat model. In addition, the segments with the PLLA cages were subjected to radiologic, macroscopic and microscopic evaluation after 3, 6, 12, and 24 months of implantation.