Biomechanical Evaluation of Anterior Thoracic Salvage Screws in the Osteoporotic Thoracic Spine

Abstract

Controlled, cadaveric implantation trial. Evaluation of revision thoracic screw fixation: with revision from unicortical screws to bicortical screws, to larger diameter screws, and the addition of bone cement. Limited data is available regarding the effect of salvage screws on fixation quality in the anterior thoracic spine. Biomechanical studies in the cervical spine and the lumbar spine demonstrate dramatic decreases in fixation in salvage situations. Seventy-two cadaveric thoracic vertebrae from 6 specimens were DEXA scanned at T1-T12. A control screw and a second identical screw were placed in each segment. One screw was then removed and replaced with a revision screw. Varying screw diameter, the number of cortices, and the addition of 1.5 cm of bone cement was evaluated in a pairwise fashion. Comparisons were made using descriptive statistical analysis and a general linear statistical model. Bone mineral density had a significant effect on the pullout force. Pullout force did not vary significantly with control screw diameter. Revision of a smaller diameter bicortical to a larger diameter bicortical screw resulted in a decrease in pullout strength for 4-5 mm screws by 40.1% (P=0.02) and 5-7 mm screws by 63.7% (P=0.05). When a 4 mm bicortical screw is revised to a 5 mm unicortical screw, the pullout force decreases by 67.7% (P<0.001). There was a nonsignificant increase (44%) in pullout with revision of a unicortical 4 mm screw to a bicortical 4 mm screw. If a bicortical screw is revised to a unicortical screw with 1.5 cm of cement, the pullout strength is increased by 240% (P<0.001). When the use of salvage screws is required, the surgeon should anticipate a significant decrease in the holding force compared with the original screw regardless of screw size unless a unicortical screw is revised to a bicortical screw or cement is added to the construct.