Comprehensive Evaluation of Accessory Rod Position, Rod Material and Diameter, Use of Cross-connectors, and Anterior Column Support in a Pedicle Subtraction Osteotomy Model

Abstract

Study Design. In silico finite element study. Objective. The aim of this study was to evaluate the effect of six construct factors on apical rod strain in an in silico pedicle subtraction osteotomy (PSO) model: traditional inline and alternative Ames-Deviren-Gupta (ADG) multi-rod techniques, number of accessory rods (three- vs. four-rod), rod material (cobalt-chrome [CoCr] or stainless steel [SS] vs. titanium [Ti]), rod diameter (5.5 vs. 6.35 mm), and use of cross-connectors (CC), or anterior column support (ACS). Summary of Background Data. Rod fracture following lumbar PSO is frequently reported. Clinicians may modulate reconstructs with multiple rods, rod position, rod material and diameter, and with CC or ACS to reduce mechanical demand or rod contouring. A comprehensive evaluation of these features on rod strain is lacking. Methods. A finite element model (T12–S1) with intervertebral discs and ligaments was created and validated with cadaveric motion data. Apical rod strain of primary and accessory rods was collected for 96 constructs across all six construct factors, and normalized to the Ti two-rod control. Results. Regardless of construct features, CoCr and SS material reduced strain across all rods by 49.1% and 38.1%, respectively; increasing rod diameter from 5.5 mm to 6.35 mm rods reduced strain by 32.0%. Use of CC or lumbosacral ACS minimally affected apical rod strain (<2% difference from constructs without CC or ACS). Compared to the ADG technique, traditional inline reconstruction reduced primary rod strain by 32.2%; however, ADG primary rod required 14.2° less rod contouring. The inline technique produced asymmetrical loading between left and right rods, only when three rods were used. Conclusion. The number of rods and position of accessory rods affected strain distribution on posterior fixation. Increasing rod diameter and using CoCr rods was most effective in reducing rod strain. Neither CC nor lumbosacral ACS affected apical rod strain. Level of Evidence: N/A

Biomechanical comparison of multi-rod constructs by satellite rod configurations (in-line vs. lateral) and screw types (monoaxial vs. polyaxial) spanning a lumbar pedicle subtraction osteotomy (PSO): is there an optimal configuration?

“…Gupta et al reported that when the osteotomy site is closed by satellite rods independent from the primary rods, a sharp curvature at the apex of the primary rods is avoided, which eventually will lead to less primary rod stresses and theoretically lower rates of rod breakage [4]. This was demonstrated by Gelb et al who found that in-line satellite rods across a PSO decreased primary rod stresses, as the satellite rods allowed the primary rods to be undercontoured by 14.2° [17,28]. As the vast majority of rod fractures occur at the level of the PSO [6], this effect of satellite rods on reducing primary rod stresses is important.…”

Effects of Revision Rod Position on Spinal Construct Stability in Lumbar Revision Surgery: A Finite Element Study

“…The SRC has been applied in clinical practice to deal with spinal deformity involving osteotomy and long segment instrumentation and the rescue of previous rod fracture ( Buell et al, 2018 ; Zhu et al, 2018 ; Jung et al, 2019 ; El Dafrawy et al, 2020 ). The reported advantages of the SRC include dispersing the rod stress concentrated at the osteotomy site, enhancing the stability of constructs and reducing the occurrence of PJK ( Hyun et al, 2014 ; Zhu et al, 2018 ; Gelb et al, 2021 ). However, extensive surgical site exposure and related complications, rod fracture at the rod-connecter site, and additional medical expenses are still concerns when using SRCs ( Hyun et al, 2014 ; Jung et al, 2019 ).…”

Does number of rods matter? 4-, 5-, and 6-rods across a lumbar pedicle subtraction osteotomy: a finite element analysis

“…Other comparisons that were beyond the scope of this study, but would also be important avenues for future investigations, were determination of the relative biomechanics between different PSO levels (L2 vs. L3 vs. L4 vs. L5 vs. S1) and the biomechanical effects on “super” multi-rod constructs with anterior column support adjacent to the PSO site. We used L3 as the osteotomy site, given the majority of prior biomechanical studies have used L3 as the PSO site [ 8 , 10 – 13 , 16 ]; however, it is important to note that level of osteotomy is important for restoring the appropriate lumbar shape and lordosis based on Roussouly types [ 26 , 27 ]. As such, L4 or L5 PSO may be more appropriate for patients without high pelvic incidences.…”

“…Multi-rod configurations can be created using “satellite” rods (not connected to primary rod) and/or “accessory” rods (connected to primary rod) [ 15 ]. While biomechanical properties of 4-rods (two primary rods + two satellite rods or two accessory rods) [ 10 , 11 , 13 , 16 – 19 ], have previously been reported, there is limited understanding of the relative biomechanical behavior of “super” multi-rod constructs (5-, 6-rods) spanning a lumbar PSO. As such, the aim of this study is to evaluate the biomechanical characteristics of increasing number of rods (4-, 5-, 6-rod) across a lumbar PSO.…”