Biomaterials in spinal fixation. An experimental animal study to improve the performance

Different pedicle screws were biomechanically and morphologically studied a nd compared through the use of an animal model to determine their efficacy and resistance in spinal fixation. The principal objective was to compare b iomechanical and histomorphological aspects of HA-coated screws to uncoated ones. Fourty-eight cylindrical transpedicular self-tapping screws divided into th ree groups of sixteen each were employed; Group A: stainless steel screws; Group B: titanium screws; Group C: HA-coated Titanium screws. The screws we re implanted bilaterally and randomly into the L3, L4, and L5 pedicles of e ight adult mongrel sheep. The final insertion torque was measured in all th e implants. After one and four months, upon euthanization, four samples per group were extracted from the surrounding bone and the screw extraction to rque was measured. The remaining samples were examined and processed for hi stological and histomorphological evaluations. No differences were observed at one month among the extraction torque of th e three groups. After four months the only significance between insertion a nd extraction values was for the HA group, i.e. p=0.001. Comparing the extr action torque values of the three groups after four months of healing, the HA-coated group showed a greater than twofold increase (p<0.0005). No diffe rences were observed at one month among the percentages of bone-implant con tact in the three groups. After four months the percentage was significant only for the C group (p<0.0005). Ar four months a correlation was found bet ween the morphological and the biomechanical data of group C (p<0.0005). The use of hydroxyapatite-coated screws could act as an effective method to improve the bone-implant interface, thus obtaining a strong fixation of th e implant independently of the arthrodesis achieved with bone graft.