A biomechanical and histological evaluation of a bioresorbable lumbar interbody fusion cage

Novel spinal interbody fusion cages made of bioactive and bioresorbable composites by a unique forging process were developed. Previous in vitro study demonstrated that these cages marked excellent biomechanical values. The purpose of the present in vivo study was to evaluate the viability and advantage of this forged composite of uncalcined hydroxyapatite/poly L-Lactide (F-u-HA/PLLA) cage radiographically, biomechanically, and histologically, when compared to conventional autologous iliac bone (AIB) and carbon fiber cage (CFC). Twenty-five mature sheep underwent posterior lumbar interbody fusion at L2-3 level with pedicle screws system made of titanium. Three types of interbody fusion implants were grafted: AIB (n=7), CFCs (n=9), F-u-HA/PLLA cages (n=9). Two types of cages were packed with autologous fragmented cancellous bone harvested locally. All animals were euthanized at 120 days after surgery. The fusion scoring using the coronal view CT scans was designed to three-dimensionally evaluate fusion quality within and around cages. The mean CT scores of three groups were 33.3 points, 35.0 points, and 33.6 points in AIB, CFC, and F-u-HA/PLLA cage groups, respectively (full-score: 56 points). Statistical differences were not detected among the three groups. The mean range of motion values among fused groups had no significant difference under all pure loadings. The range of motion showed strong and significant correlation with the CT fusion scores. Histologic results demonstrated that F-u-HA/PLLA cages contacted with the surrounding bone directly, and CFC was encircled with thick fibrous tissue layers without any sign of inflammation around cages. The fusion quality of fused spinal segment using F-u-HA/PLLA cages was equal to that of AIB or CFCs both radiographically and biomechanically. In the histological observation, biocompatibility of F-u-HA/PLLA cage was obviously superior to CFC. It has been confirmed that the novel bioactive and bioresorbable cages had valuable advantages over existing CFC for use in spinal reconstructive surgery.