Factors affecting the pullout strength of self-drilling and self-tapping anterior cervical screws

Study design. A biomechanical comparison of 12-, 14-, and 16-mm self-tapping and self-drilling screws in cadaveric cervical spines was performed. Objective. This study compared self-drilling and self-taping screw designs used in anterior cervical locking plates in terms of insertion torque and pullout strength. In addition to screw design, the effects to screw length (12, 14, and 16 mm) and bone mineral density were examined. Methods. A total of 201 screws were implanted and tested. Nineteen cadaver spine specimens (C2 through C7) were radiographed and their bone mineral density measured. In each vertebral level, two screws of the same length were implanted and their insertion torque measured. One of each screw design (self-drilling or self-tapping) was randomly placed in each vertebra for side-by-side comparison. Vertebrae then were individually potted and mounted on an MTS machine for accurate measurement of maximum pullout strength. Results. Pullout strength was strongly related to screw length (P=0.0002). There was no significant difference in pullout strength between the self-drilling and self-tapping screw designs at any length (P=0.9352). A significant correlation (Pless than or equal to0.0077) exists between insertion torque and bone mineral density for the self-drilling and self-tapping 14- and 16-mm screws. A significant correlation exists between pullout strength and bone mineral density for 14- and 16-mm screw lengths of both screw types. (P<0.001). Conclusions. It would appear that the longest screw feasible should always be used clinically for maximum pullout strength. There is no difference in pullout strength between self-drilling and self-tapping screws. Bone mineral density significantly influences screw performance.