Anterior thoracic scoliosis constructs: Effect of rod diameter and intervertebral cages on multi-segmental construct stability

Background context: Many studies have reported on the use of anterior instrumentation for thoracolumbar scoliosis and more recently thoracic scoliosis. However, the optimal construct design remains an issue of debate. Purpose: To optimize construct design and enhance implant survival until a successful spinal arthro- desis is achieved. Study design: This study evaluated the effect of rod diameter and intervertebral cages on construct stiffness and rod strain using a long-segment, anterior thoracic scoliosis model with varying levels of intervertebral reconstruction. Methods: Sixteen fresh-frozen calf spine specimens (T1 to L1) were divided into two groups based on rod diameter reconstruction (4 mm and 5 mm). Testing included axial compression, anterior flexion, extension and lateral bending with variations in the number and level of intervertebral cage reconstructions: apical disc (one), end discs (two), apical and end discs (three), all seven levels (seven). Multisegmental construct stiffness and rod strain were determined and normalized to the intact specimen for analysis. Results: The seven-level intervertebral cage construct showed significantly greater stiffness in axial compression for both the 4-mm (366% increased stiffness) and 5-mm (607% increased stiffness) rod groups (p<.001). The remaining constructs were not significantly different from each other (p>.05). In flexion, similar results were obtained for the 4-mm construct (p<.001) but not the 5-mm construct, because the reconstruction-alone, one-, two- and three-cage constructs were all significantly stiffer than the intact specimen (p<.05). Multisegmental construct stiffness under extension loading, as well as right and left lateral bending, also exhibited significant differences between the seven-level interbody cage reconstructions and the remaining constructs. Apical rod strain for both the 4-mm-rod and 5-mm-rod groups were significantly higher for the two cage constructs (a cage at either end but not the apex where the strain gauges were located) as compared with the other constructs (p<.05). These differences were more pronounced in the 4-mm-rod group. Similar results were obtained in anterior flexion, extension and lateral bending. Conclusions: Intervertebral cages at every level significantly improved construct stiffness compared with increasing rod diameter alone. Moreover, cages markedly decreased rod strain, and when structural interbody supports were not used, axial compression created the greatest rod strain.