In vivo deformation of anatomically pre-bent rods in thoracic adolescent idiopathic scoliosis

Abstract

Some surgical strategies can maintain or restore thoracic kyphosis (TK); however, next-generation surgical schemes for adolescent idiopathic scoliosis (AIS) should consider anatomical corrections. A four-dimensional correction could be actively achieved by curving the rod. Thus, anatomically designed rods have been developed as notch-free, pre-bent rods for easier anatomical reconstruction. This study aimed to compare the initial curve corrections obtained using notch-free rods and manually bent, notched rods for the anatomical reconstruction of thoracic AIS. Two consecutive series of 60 patients who underwent anatomical posterior correction for main thoracic AIS curves were prospectively followed up. After multilevel facetectomy, except for the lowest instrumented segment, either notch-free or notched rods were used. Patient demographic data, radiographic measurements, and sagittal rod angles were analyzed within 1 week after surgery. Patients with notch-free rods had significantly higher postoperative TK than patients with notched rods (P < .001), but both groups achieved three-dimensional spinal corrections and significantly increased postoperative rates of patients with T6–T8 TK apex (P = .006 for notch-free rods and P = .008 for notched rods). The rod deformation angle at the concave side was significantly lower in the notch-free rods than in the notched rods (P < .001). The notch-free, pre-bent rod can maintain its curvature, leading to better correction or maintenance of TK after anatomical spinal correction surgery than the conventional notched rod. These results suggest the potential benefits of anatomically designed notch-free, pre-bent rods over conventional, manually bent rods.

Surgical Outcome Prediction Using a Four-Dimensional Planning Simulation System With Finite Element Analysis Incorporating Pre-bent Rods in Adolescent Idiopathic Scoliosis: Simulation for Spatiotemporal Anatomical Correction Technique

“…To our knowledge, this is the first study to show that the simulation model can predict the deformation of the implanted rod. The simulation model determined significant changes in the rod contours, especially on the concave side which has been clinically reported (Sudo et al, 2021). In addition, based on the changes in rod geometry and FEA, the highest stress was found at the apex of the rod curvature and the extremities of the instrumented levels, which is in agreement with previous results (Belmont et al, 2001;Aubin et al, 2008;Abe et al, 2015).…”

“…If the rod curvature does not match the patient’s deformity and does not allow for deformity correction, such situations will lead to an inadequate correction or implant-related complications due to the overstress on the implant and spine ( Sudo et al, 2018 ). These issues require some innovative systems to assist surgery or predict the most probable outcome of surgery ( Aubin et al, 2008 ; Sudo et al, 2021 ).…”

“…This insufficient correction resulting in a postoperative non-anatomical TK is thought to be, because the spinal rods are being bent to match the curvature of scoliosis. From the standpoint of spatiotemporal deformity correction, we recently showed that anatomical four-dimensional (4D) spinal correction could be accomplished by curving the rod ( Figure 1 ) ( Sudo et al, 2018 ; Sudo et al, 2021 ). In the surgical procedure, two rods are bent in a nearly identical fashion to confirm spinal anatomical alignment without reference to the intraoperative alignment of the deformity ( Sudo et al, 2018 ; Sudo et al, 2021 ).…”

“…From the standpoint of spatiotemporal deformity correction, we recently showed that anatomical four-dimensional (4D) spinal correction could be accomplished by curving the rod ( Figure 1 ) ( Sudo et al, 2018 ; Sudo et al, 2021 ). In the surgical procedure, two rods are bent in a nearly identical fashion to confirm spinal anatomical alignment without reference to the intraoperative alignment of the deformity ( Sudo et al, 2018 ; Sudo et al, 2021 ). Consequently, pre-bent rod geometries were obtained from intraoperative tracings of the rod shapes, and optimized rod shapes were derived using iterative closest point method followed by hierarchical cluster analysis ( Kokabu et al, 2018 ).…”

“…Consequently, pre-bent rod geometries were obtained from intraoperative tracings of the rod shapes, and optimized rod shapes were derived using iterative closest point method followed by hierarchical cluster analysis ( Kokabu et al, 2018 ). Currently, 11 types of pre-bent cobalt-chrome (CoCr) alloy rods are available based on the deformity types and its lengths in Japan that can guide anatomical spinal correction regardless of the surgeons’ experience ( Figure 1 , Sudo et al, 2021 ).…”