Pull out strength of pedicle screw in normal and osteoporotic cancellous bone models

Lower back pain is treated by bone fusion between adjacent vertebrae typically using instrumentation to immobilize and stabilize the spine till bony fusion takes place. Instrumentation is effected surgically using pedicle screws. However breakage and loosening of pedicle screws from the vertebral body are two main clinical concerns particularly for osteoporotic patients. The objective of the present study is to evaluate the effect of bone density, insertion depth and insertion angle of pedicle screw on the pull out strength and insertion torque using rigid polyurethane foam constructs. Insertion torque was measured during the insertion of the screw and pull-out tests were performed on instrumented pedicle screws and foam construct. These rigid foams represented the range of extremely osteoporotic to normal bone densities. The insertion angle represented the range of angle of pedicle and the insertion depth represented the depth to which these pedicle screws are inserted in the vertebra. The results showed that the pull out strength and insertion torque increased with increase in density and insertion depth (p<;0.05) whereas pull out strength decreased with increase in insertion angle. There was significant interaction effect of the various factors on the pull out strength and insertion torque. The findings from the current study suggest that holding power of the pedicle screws can be increased by increasing the purchase depth and reducing the angle of insertion. A statistical model was developed to predict the holding power of the screw which can assist surgeon in pre surgical planning.