Application of polylactides in spinal cages: studies in a goat model

Spinal cages are currently made of non-resorbable materials, but they only have a temporary function: after fusion, resorption is desirable both from a biological and mechanical point of view. We studied different polylactides in stand-alone condition in a goat model. Cages were made of 100% poly(L-lactic acid) (PLLA) or 70/30 poly(L/DL-Lactic acid) (PLDLLA); titanium served as control. After six months, all titanium cages showed non-unions comparable to that observed in a clinical retrieval, thus showing validity of the goat model. PLLA cages maintained their mechanical integrity for six months, enough to allow fusion. After that, the material resorbed within 48 months without adverse tissue reactions. Bone formation was faster in PLDLLA cages, but these already failed within three months, thus losing their stabilising function: 50% ended in pseudo-arthrosis. Additional internal fixation provided enough stability for fusion (83%). Biocompatibility of both PLLA and PLDLLA was excellent. The long-term results show that PLLA cages can be used for stand-alone interbody fusion, and that PLLA is an improvement over titanium in terms of fusion rate. PLDLLA showed enhanced bone formation, but also earlier failure of the implant. Chances for spinal fusion were better with additional internal fixation.