Mechanical Analysis of Notch-Free Pre-Bent Rods for Spinal Deformity Surgery

Experimental study of spinal rod as per the American Society for Testing Materials (ASTM) F2193 methodology for static and dynamic four-point bending.

The hypotheses underlying this study were that the notch-free, curved rod would have a significantly higher ultimate load and fatigue strength compared with conventional notched curved rods. This study aimed to analyze the mechanical properties of notch-free curved rods compared with conventional notched rods.

The goal of instrumented spinal fusion in the management of spinal deformities is to realign the spine and maintain the correction and stability in order to obtain arthrodesis. Although rod curvature could play an important role, intraoperative contouring of the straight rod induces notches into the rod, leading to decreased fatigue strength.

Commercially produced titanium alloy (ϕ6.0 mm) and cobalt chromium alloy (ϕ5.5 mm) spinal rods were assessed by four-point bending tests in accordance with the ASTM F2193.

Static four-point bending tests for the curved spinal rods showed that cobalt chromium alloy rods had significantly higher stiffness compared with titanium alloy rods. Notch-free cobalt chromium alloy rods had a significantly higher ultimate load than the conventional notched cobalt chromium alloy and titanium alloy rods. The dynamic four-point bending test showed that force/displacement at a minimum force at 2,500,000 cycles was larger in the notch-free cobalt chromium alloy rod than in the notched cobalt chromium alloy rod.

The notch-free curved cobalt chromium alloy rod is likely to maintain its curvature after spinal deformity surgery with a decreased risk of breakage and could overcome the problems of the conventional notched rod such as breakage and spring-back.

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