Pedicular fixation in the osteoporotic spine: a pilot in vivo study on long-term ovariectomized sheep

Spinal instrumentation success is greatly affected by the presence of osteoporosis. To date, however, no data exist on in vivo investigations on biomaterial and surgical techniques in the osteoporotic spine. In the present study 24 uncoated and 24 HA-coated screws were implanted in the L3, L4 and L5 pedicles of eight sheep (four ovariectomized, OVX Group; four sham-operated, Control Group). At four months, uncoated screws showed a significant decrease of about -22% in the extraction torque of the OVX Group as compared to the Control Group (p < 0.005). The extraction torque of HA-coated screws significantly (p < 0.0005) improved in both groups when compared to that of uncoated screws and showed increases ranging from 133% to 157%. Pedicle trabecular bone of OVX sheep showed a significant decrease in BV/TV (-30%; p < 0.05) and Tb.Th (-33%; p < 0.01). The affinity index (AI) results revealed significant (p < 0.0005) differences between uncoated and HA-coated screws for both groups: values were lower for uncoated than HA-coated screws by about -35%. A significant difference was also found for the AI data of uncoated screws between the OVX and Control Groups (-13%, p < 0.005). The current findings have demonstrated that long-term ovariectomized sheep can be used to study in vivo osteointegration in the osteoporotic spine. The HA coating has proven to improve bone purchase and bone-screw interface strength in healthy and osteopenic animals.