Affiliations: Spine Research Laboratory, Cleveland Clinic Center for Spine Health, Cleveland Clinic, Cleveland, OH, USA | Department of Neurosurgery, Medical School of Trakya University, Edirne, Turkey | Cleveland Clinic Center for Spine Health, Cleveland Clinic, Cleveland, OH, USA
Abstract: Stress relaxation during pullout of a pedicle screw decreases the peak load and stiffness of the bone–screw interface. However, it is unknown whether this can be generalized to all types of screw designs. This study aimed to show whether screw design altered the effects of stress relaxation on the mechanical performance of the pedicle screw during pullout. Twelve calf vertebrae were obtained: six vertebrae were instrumented with 7.5×40 mm conical pedicle screws and the other six with 5.0×40 mm cylindrical pedicle screws. The screws with two different designs were pulled out using either a standard pullout or a stress relaxation pullout protocol. Both bone–screw interfaces had lower stiffness in the stress relaxation pullout model than in the standard pullout model, but it was significant in only the cylindrical design group (P<0.05). However, the stress relaxation and standard pullout models did not yield any difference in peak loads in either screw type. Although stress relaxation at the bone–screw interface can alter the mechanical performance of the screw, this may be eliminated by modifying the screw design. A better understanding of viscoelastic properties of the bone–screw interface may help improve implant design and thus, clinical outcomes.
