Comparison of Different Bipolar Construct Configurations for the Correction of Adult Spine Deformity: A Finite Element Analysis

Abstract

Purpose A minimally invasive bipolar spinal fixation was recently developed to correct the deformity in pediatric neuromuscular scoliosis and has recently been adapted for adult scoliosis. Although the clinical results are promising, mechanical complications are still not negligible. In this work, alternative configurations of bipolar constructs were compared through numerical simulation, in order to evaluate stress distribution along the implant according to each configuration.

Methods The configurations included doubling the rods, adding lumbar screws to strengthen the distal anchorage, and combining two different materials (titanium and chromium-cobalt alloy). This resulted in seven different configurations, which were implemented in a subject-specific and experimentally validated finite element model, based on the geometry of an asymptomatic subject. Von Mises stresses were compared between configurations.

Results The results confirm that doubling the rods reduced mid-rod stresses, as expected, but also shifted some of the load from the distal anchorage to the rods, which is a common site of implant failure. The addition of pedicle screws also reduced the stress in the distal anchorage. The configuration showing the best compromise between stress reduction and the mini-invasive character of surgery included a doubling of both rods in titanium.

Conclusions These results should be confirmed by clinical results, but they already provide clear guidelines for the surgeon.