Augmentation of an anterior solid rod construct with threaded cortical bone dowels - A biomechanical study

Study Design. This static, nondestructive, in vitro biomechanical study exa mines anterior solid rod construct stiffness following the addition of mult ilevel, threaded cortical bone dowels in a bovine model. A comparison is ma de with a clinically relevant posterior construct with and without an anter ior release. Objectives. To determine if the addition of solid, multilevel disc space im plants will increase construct rigidity, while maintaining or enhancing ant erior column length. Summary of Background Data. Anterior instrumentation for thoracolumbar and lumbar scoliosis has achieved greater correction and preserved distal motio n segments; however, kyphosis over the instrumented segments and nonunion h ave been observed more frequently than with posterior segmental spinal inst rumentation. Method. Fifteen calf spines underwent mechanical testing. Group A (n=7) inc luded anterior constructs: 1) intact, 2) anterior release/rod/rib graft (L2 -L5), and 3) anterior release/rod/dowels (L2-L5). Group a (n=8) included po sterior constructs: 1) intact, 2) posterior rod without anterior release (T 13-L5), 3) posterior rod (T13-L5)/anterior release/rib graft (L2-L5). The p rotocol included axial compression (-600N), axial rotation (+ 7 Nm), flexio n/ extension (+ 7.5 Nm), and lateral bending (+ 7.5 Nm). An anterior extens ometer measured segmental displacements to calculate construct stiffness. L ateral radiographs evaluated alignment for the anterior constructs. Statist ical analysis involved a one way analysis of variance (ANOVA) and a Student -Newman-Keuls post hoc test. Results. All reconstructions restored stiffness to intact values with the e xception of the dowels alone in axial rotation. The rod/dowel construct was stiffer than all other groups in axial compression, flexion/extension, and lateral bending, with the exception of the posterior rod without discectom y, which was superior in flexion and statistically similar in extension, la teral bending, and axial rotation. The anterior construct with rib graft wa s equivalent to the posterior construct with rib graft in all modes of test ing. The dowels created greater lordosis than the bicortical rib grafts. Conclusions. Disc space augmentation increased stiffness except in axial ro tation, in which values were restored to the intact level. Stiffness was su perior to a clinically relevant posterior instrumentation comparison group following anterior release, and was equivalent to a posterior construct wit hout anterior release except in anterior-flexion. In addition, the implants enhanced lordosis. Increased rigidity should improve rates of arthrodesis, while maintenance of sagittal alignment may prevent pathologic compensator y curves in adjacent spinal segments. Further research is required to deter mine the optimal method of: achieving structural interspace support.