Biomechanical comparison of different anchors (foundations) for the pediatric dual growing rod technique

Abstract

Background context Children with early onset scoliosis benefit from early operative treatment with dual growing rods as they provide an “internal brace” for the spine and allow curve correction and sequential lengthenings to maximize thoracic cage development. The foundations that provide anchor points for the dual growing rods may incorporate hooks, screws, or a hybrid construct. It is unclear how stable different types of foundation constructs are with regard to pullout. Purpose This study was to determine the differences in stability between four foundation configurations used in the pediatric dual growing rod technique. Eight porcine spines were sectioned and randomly assigned to one of four foundation groups: 1) hook-hook with cross-link; 2) hook-screw with cross-link; 3) screw-screw with cross-link; and 4) screw-screw without cross-link. After instrumentation, biomechanical pullout tests were conducted. Ultimate failure loads, modes of failure, and level of instrumentation were analyzed. Study design/setting Controlled in vitro laboratory investigation. Results Screw-screw with cross-link constructs demonstrated the greatest failure load but this was not statistically significant compared with the screw-screw without cross-link constructs. Both screw-screw constructs were statistically stronger than either construct containing hooks. There was no difference between the hook-screw and hook-hook constructs that was statistically significant. Hook-containing constructs sustained higher failure loads in lumbar versus thoracic vertebrae. Conclusions A foundation composed of four pedicle screws implanted in two adjacent vertebral bodies provides the strongest construct in pullout testing. A cross-link does not seem to enhance fixation. Hook constructs are stronger in lumbar versus thoracic laminae.