Summary: The present study analyzed the corrective forces applied to the spine resulting from “over contouring” of scoliosis rod instrumentation. The deformation and the forces exerted by the instrumented spine were determined by combining 3 research approaches: 1) Mechanical analysis of the force‐deformation characteristics of straight 5.5mm Ultra Strength Steel (USS) rods; 2) forces required to unbend pre‐contoured rods; and 3) pre‐ and post‐op rod shape changes for extraction of in vivo forces.
Introduction: Instrumentation for scoliosis correction and fusion commonly employs surgical rods. However, there is a lack of understanding of the forces applied to the spine from those implants.
Methods: Initially, 4‐point bending tests of nine 5.5mm USS rods were performed using MTS858 test frame. Next, pre‐bent rods were straightened via the same method. This testing defined the force‐deformation curves during both bending and unbending allowing mathematical equations to be derived by curve fitting. Lastly, 29 patients with surgical scoliosis correction had pre‐op (rod tracings) to post‐op (sagittal x‐rays) rod contour changes measured extracting in vivo rod deflections. In vivo corrective forces were calculated using the derived equations (from the 4 pt rod bending/unbending tests).
Results: The force‐deformation relationship shows the typical linear region of elastic deformation as well as the typical unloading curve parallel to the original elastic portion of the curve, with an Ultimate Load (UL) for this material of 2080N. For the 29 clinical cases examined, the surgeons majority (97%) of the rods on the concave (left) side were contoured to a large kyphotic deflection of 25.5±5.1mm (kyphosis of 34°) which then unbent after implantation to 54±18% of the pre‐bent deflection with a corresponding bending force in the sagittal plane of 1761 ±718N. Contrarily, on the convex side, rods were pre‐bent to only 15.1±2.7mm (kyphosis of 20°, p<0.001) and 66% of them were bent further after implantation to 16.6±3.9mm (kyphosis of 22°, p=0.01). The resultant convex rod bending force averaged 1249N (p=0.001).
Conclusion: Concave rod forces were significantly larger than forces in the convex rod (p=0.025) and were more likely to further unbend after implantation (p=0.001) resulting in high pull‐out forces for the screws. For the first time, in vivo spinal rod forces have been determined using biomechanical and clinical information. This study will help surgeons select the ideal pre‐operative spinal rod shape and screw dimensioning for optimal and safe surgical correction.