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Are precontoured cobalt-chromium spinal rods mechanically superior to manually contoured rods?
K Aaron Shaw1, Dennis P Devito2, Michael L Schmitz2
1Department of Orthopedic Surgery, Dwight D. Eisenhower Army Medical Center, 300 East Hospital Road, Fort Gordon, GA, 30905, USA. kenneth.a.shaw34.mil@mail.mil.
Study Design:
Laboratory based study.
Objective:
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.
Methods:
A spinal deformity model was utilized, simulating a rigid, thoracolumbar spinal deformity fixated with pedicle screws. Roll-formed pre-contoured and manually contoured 5.5 mm and 6.0 mm 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 20 min and change in contour was assessed to characterize plastic deformation.
Results:
Twenty-four rods were tested with six rods per group (Table 1). The load to reduction was significantly lower in the 5.5 mm rods compared to the 6.0 mm 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 5.5 mm rods (p = 0.722), the 6.0 mm rod produced significantly less corrective force compared to the manually contoured 6.0 mm 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 5.5 mm and 6.0 mm rods (5.5 mm: 57.1% vs. 61.6%, p = 0.006; 6.0 mm: 54.3% vs. 62.28%, p = 0.003).
Conclusions:
Roll formed, pre-contoured Co-Cr rods demonstrated significantly greater plastic deformation when compared with manually contoured rods of the same diameter. Furthermore, 6.0 mm pre-contoured rods required significantly lower load for rod reduction, the equivalent of 15 lb-force. Post-manufacturing, roll-formed pre-contouring of larger diameter Co-Cr may impair the rods mechanical properties.
