Biomechanical evaluation of relationship of screw pullout strength, insertional torque, and bone mineral density in the cervical spine

Background: Understanding of implant failure mechanisms is important in the successful utilization of anterior cervical plates. Many variables influence screw purchase, including the quality of the bone. The purpose of this study was to assess the relationship of screw pull-out and screw insertional torque across a wide range of bone mineral densities (BMDs). Methods: A total of 54 cervical vertebrae in 12 cervical spines were evaluated for BMD using dual-energy x-ray absorptiometry scanning. Actual and perceived peak torques of 3.5-mm anterior cervical screws were determined at each level followed by screw pullout strength testing. Results: A high correlation was observed between screw pullout strength and BMD. However, there was a low correlation of peak insertional torque to pullout strength. Conclusion: These findings suggest the quality of the bone is more instrumental in the success or failure of anterior cervical screws than is the insertional torque with which the screws are placed.