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, posterol ateral spinal fusion (L-4-L-5) were randomly selected to receive stainless steel (9) or titanium pedicle screw divices (9). In both groups, the divice s were CCD (Sofamore Danek) type with the same size and shape. The postoper ative observation time was 3 months. Screws from L-4 were harvested along t heir long axis of pedicle for histomorphometric study. Done-screw interface and bone volume from thread were examined using linear intercept technique s. Mechanical testing (torsional test and pullout test) was performed on th e screws from 5. Results The titanium screw group had a significantly higher maximum torque (P < 0.05) and angle related stiffness (P < 0.05) measured by torsional tes t. 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 bo ne contact with the screw in percentage was 29.4% for stainless steel and 4 3.8% for titanium (P < 0.05). No differences in the bone purchase between t he vertebral body part and pedicle part were Found. Conclusion Pedicle screws made of titanium have a better bone-screw interfa ce binding than screws made of stainless steel. Torsional tests are more in formative for bone-screw interface study. Pull-out tests seem less valuable when comparing bone purchase of screws made from different materials.