Mechanical and histological analysis of bone-pedicle screw interface in vivo: Titanium versus stainless steel

Objective To investigate the differences in bone interface between titanium and stainless steel pedicle screws in the lumbar spine. Methods Eighteen adult mini-pigs that underwent total laminectomy, posterolateral spinal fusion (L₄-L₅) were randomly selected to receive stainless steel (9) or titanium pedicle screw divices (9). In both groups, the divices were CCD (Sofamore Danek) type with the same size and shape. The postoperative observation time was 3 months. Screws from L₄ were harvested along their long axis of pedicle for histomorphometric study. Bone-screw interface and bone volume from thread were examined using linear intercept techniques. Mechanical testing (torsional test and pull-out test) was performed on the screws from L₅. Results The titanium screw group had a significantly higher maximum torque (P < 0.05) and angle related stiffness (P < 0.05) measured by torsional test. In the pull-out tests, no differences were found between the two groups in relation to the maximum load, stiffness and energy to failure. Direct bone contact with the screw in percentage was 29.4% for stainless steel and 43.8% for titanium (P < 0.05). No differences in the bone purchase between the vertebral body part and pedicle part were found. Conclusion Pedicle screws made of titanium have a better bone-screw interface binding than screws made of stainless steel. Torsional tests are more informative for bone-screw interface study. Pull-out tests seem less valuable when comparing bone purchase of screws made from different materials.