Comparative in vitro, biomechanical study.
Compare the effect of rod curvature and material properties on rod flattening
and correctional forces
Summary of Background Data:
Traditional methods of correction for large progressive deformities involve 3-dimensional correction, performed with an attempt to reach a balanced correction in all planes, spinal instrumentation
, and fusion. Increasing attention to the transverse plane correction has developed after the introduction of segmental pedicle screws into the treatment of idiopathic scoliosis. Approximation of the spine (pedicle screws or hooks) to the rods remains the heart of many deformity procedures. Therefore, it is crucial that the instrumentation used provide and maintain the initial correction of the spinal deformity while minimizing potential intraoperative failures.
Two experiments were performed using 80 rods made from 4 different materials namely: stainless steel (SS), titanium (Ti), cobalt chromium (CoCr), and ultrahigh strength stainless steel (UHSS). Half of the rods were contoured to 20 degrees, whereas the reaming contoured to 30 degrees. Half of the rods were approximated to a synthetic spine models to measure the flattening of the rods when approximated to highly rigid spine. The other half was used to measure the correctional forces
produced by each rod type and curvature.
For the 20-degree pre-bend rods, Ti was the best in maintaining its original shape followed by UHSS, SS, and CoCr of 90%, 77%, 62.5%, and 54.4%, respectively. The 30-degree pre-bend showed exactly a similar trend with 80.7% for Ti, 71% for UHSS, 54.6% for SS, and 48.1% for the CoCr rods. For 30-degree pre-bend CoCr and UHSS rods, the intraoperative reduction forces were almost 42% and 10% higher than the Ti and SS rods, respectively. The correctional force produced by the Ti 30-degree pre-bend rod was approximately 67% that of a CoCr and UHSS rods.
CoCr and UHSS rods have the ability to produce the highest correction forces, however, both can plastically deform in a very rigid curves. Therefore, it is critical to have sense of the quality of the bone fixation as well as the curve flexibility when selecting for appropriate rod size material and contouring the rod to the desired shape.