Are precontoured cobalt-chromium spinal rods mechanically superior to manually contoured rods?

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Laboratory based study. To compare reduction force and plastic deformation of cobalt-chromium (Co-Cr) spinal rods using a rigid, thoracolumbar spinal deformity model. Pre-contoured spinal rods are growing in their utilization for spinal deformity. Although there are theoretical advantages to pre-contouring rods, no previous studies have compared pre-contoured and manually contoured rods for their ability to maintain sagittal contour and resist mechanical load. A spinal deformity model was utilized, simulating a rigid, thoracolumbar spinal deformity fixated with pedicle screws. Roll-formed pre-contoured and manually contoured 5.5mm and 6.0mm Co-Cr rods were reduced to the model with a load cell attached to the apical screw to measure corrective force. Rods remained reduced in the model for 20min and change in contour was assessed to characterize plastic deformation. Twenty-four rods were tested with six rods per group (Table 1). The load to reduction was significantly lower in the 5.5mm rods compared to the 6.0mm rods (95% CI -254.0 to -61.42; p = 0.008). Although there was no difference in the corrective forces for manual and pre-contoured 5.5mm rods (p = 0.722), the 6.0mm rod produced significantly less corrective force compared to the manually contoured 6.0mm rods (95% CI -134.42 to -5.317; p = 0.039). Additionally, rod contour for the manual group showed significantly less plastic deformation than the pre-contoured group in both 5.5mm and 6.0mm rods (5.5mm: 57.1% vs. 61.6%, p = 0.006; 6.0mm: 54.3% vs. 62.28%, p = 0.003). Roll formed, pre-contoured Co-Cr rods demonstrated significantly greater plastic deformation when compared with manually contoured rods of the same diameter. Furthermore, 6.0mm pre-contoured rods required significantly lower load for rod reduction, the equivalent of 15lb-force. Post-manufacturing, roll-formed pre-contouring of larger diameter Co-Cr may impair the rods mechanical properties.