The use of poly-L-lactic acid in lumbar interbody cages: design and biomechanical evaluation in vitro. - van Dijk M; Smit TH; Arnoe MF; Burger EH; Wuisman PI (European spine journal : official publication of the European Spine Society, the European Spinal Deformity Society, and the European Section of the Cervical Spine Research Society, 2003)