Analysis of scoliosis rod deformation after cutting with a surgical rod cutter.

PURPOSE

Scoliosis is a complex multi-dimensional deformity of the spine that is common in children and adults. Of the various treatments for scoliosis, one is posterior spinal fusion with instrumentation. The rods typically used are composed of titanium or cobalt-chrome. Rods are cut during surgery, which causes rods to be deformed on the cut end. Inducing rod deformation raises concerns about deformed end influencing the stability of the rod-tulip-set screw interface. This study examines rod deformation from the rod cutter.

METHODS

This study was performed using photogrammetry, a technique allowing the creation of three-dimensional (3D) models from photographs. Rod materials included titanium (Ti) and cobalt chrome (CoCr). Three different diameters, 4.75 mm, 5.5 mm, and 6.0 mm, were analyzed for each rod material. Five rods of each material and diameter were used for these groups, totaling 30 rods for the study. Photogrammetry was used to create a 3D rendering of the cut end of the rods. The parameters measured included local angle of deformation at each mm away from the cut, as well as roundness of the cross section. Means and standard deviations were taken for each measurement. A two-way ANOVA analysis and a Tukey post-hoc analysis were used for statistical analysis.

RESULTS

Five rods in each rod group resulted in the analysis of 30 rods. Deformation from the rod cutter resulted in more angular deformation in the CoCr rods than the Ti rods. The CoCr rods also had lower cross-sectional roundness measurements. The 6.0-mm rods had significantly more angular deformation as well as lower roundness measurements compared to the smaller diameter rods. The 4.75-mm and 5.5-mm diameter Ti rods showed deformation up to 4 mm from the cut end, while the 6.0-mm Ti rods, and all the CoCr rods, showed deformation 5 mm from the cut end.

CONCLUSIONS

The data from this study offer information about the amount of deformation present at the cut end of spinal rods. There was a difference in the angle of deformation as well as roundness along the length of the rod's cut end. Placing the deformed portion of the rods within a screw tulip theoretically increases the risk of failure of the rod-screw interface. Based on these data, to decrease the risk of construct failure, we recommend leaving at least 4 mm of rod between the cut end in 4.75 mm and 5.5-mm Ti rods, and 5 mm in CoCr rods and 6.0-mm Ti rods.