Pedicle screw fixation is a standard procedure in spine surgery. A secure anchoring in deficient (e.g., osteoporotic) bone or in a secondary supply after a revision is a major challenge. This study aimed to test the primary stability of a new self-expanding shape memory actuator screw concept with the traditional screw design. The actuator part braces itself against the osseous environment after implantation by heating to body temperature. Thirty screws and twenty-four actuator sheets were manufactured and tested in vertebrae from seven human cadavers in vitro. The pullout force and mechanical work for the pullout were evaluated. Bone quality was determined from μCT. The mean actuator sheet compression force of the used shape memory alloy (SMA) sheets averaged - 8.2 ± 0.6 N. Seven of the SMA screws activated correctly after the implantation, four activated unilaterally only. The pullout force averaged 868 ± 392 N for the standard screws and 828 ± 353 N for the SMA screws. The mechanical work was conducted after the first 100 N, and the loss of strain resistance of 40% of the pullout force averaged 2.2 ± 1.6 Nm for the standard screws, and 1.7 ± 0.9 Nm for the SMA screws. Consequently, the novel concept showed non-inferiority compared with the traditional screw designs. Graphical abstract.
Keywords: Expandable implant; Mechanical strength; Pedicle screw; Shape memory alloy.