The use of poly-L-lactic acid in lumbar interbody cages: design and biomechanical evaluation in vitro.

Cage design and cage material may play a crucial role in the incidence of postoperative complications reported with current non-absorbable interbody cage devices. Bioabsorbable poly-L-lactic acid cage devices may have potential benefits. The purpose of this study was to determine the required strength of poly-L-lactic acid cages for use in experimental goat studies and to evaluate the mechanical properties of different cage designs in situ. The yield and ultimate strength of native goat motion segments (L1-L6) were determined; the yield strength was used as a design parameter for the cages. The mechanical behaviour of two types of poly-L-lactic acid cages, the influence of endplate perforation, differences between toothed and smooth cages, and the influence of cage filling were biomechanically tested and compared to native motion segments. Only axial compression until failure of the motion segments was performed. Dual energy X-ray absorptiometry was used to determine bone mineral content. The yield and ultimate strength of the native motion segments were 3.5 and 7.0 kN, respectively. Based on these data, flexible and stiff poly-L-lactic acid cages were designed with strengths of 3.5 and 7 kN, respectively. Poly-L-lactic acid cages, whether with or without bone graft and perforating the endplates, did not reduce the compressive strength of motion segments as compared to native segments. However, toothed titanium cages, with the same geometry, negatively influenced the segments' compressive strength, which effect was reduced using smooth titanium cages.