gms | German Medical Science

The purpose of the current study was to verify whether uncemented screws with thicker core diameters increase fixation strength in the cervical spine.

Bone mineral density (BMD) was determined for each vertebral body from six human cervical spine segments C4-C7. According to bone mineral density, the specimens were assigned to two groups in which torque and pullout were tested. Two burr holes were drilled into the vertebral bodies and tests were first performed using a standard screw. The test was repeated using a thicker osteoporosis screw, which was inserted into the same burr hole. Mean value of peak torque resulting from the single measurements left/ right was used for statistics. A T-test was used to determine the effect of screw design on peak torque and pullout force. Moment correlation coefficients were calculated to determine the effect of BMD on peak torque and pullout force.

Mean insertional peak torque for the standard screw was 82.1 Ncm and 47.6 Ncm for the osteoporosis screw (p < 0.001). There was a strong correlation between insertional peak torque and bone mineral density for both standard (r = 0.71, p = 0.02), and osteoporosis screw (r = 0.59, p = 0.07). Mean pullout force for standard screws was 464.7 N and 164.5 N for osteoporosis screws (p < 0.001). There was a strong correlation between pullout force and bone mineral density for both standard (r = 0.75, p = 0.0081), and osteoporosis screws (r = 0.7, p = 0.025).

Uncemented osteoporosis screws that have been inserted into a fatigued burr hole of the c- spine vertebral bodies do not strengthen the screw- bone interface when compared to the interface as achieved initially by a standard screw. This is an important finding that will help to improve implants which need to be fixed within osteoporotic bone in elderly patients.