Lumbar Disc Degeneration Affects the Risk of Rod Fracture Following PSO; A Finite Element Study

Abstract

Study Design Finite element (FE) study. Objective Pedicle subtraction osteotomy (PSO) is a surgical method to correct sagittal plane deformities. In this study, we aimed to investigate the biomechanical effects of lumbar disc degeneration on the instrumentation following PSO and assess the effects of using interbody spacers adjacent to the PSO level in a long instrumented spinal construct. Methods A spinopelvic model (T10-pelvis) with PSO at the L3 level was used to generate 3 different simplified grades of degenerated lumbar discs (mild (Pfirrmann grade III), moderate (Pfirrmann grade IV), and severe (Pfirrmann grade V)). Instrumentation included eighteen pedicle screws and bilateral primary rods. To investigate the effect of interbody spacers, the model with normal disc height was modified to accommodate 2 interbody spacers adjacent to the PSO level through a lateral approach. For the models, the rods’ stress distribution, PSO site force values, and the spine range of motion (ROM) were recorded. Results The mildly, moderately, and severely degenerated models indicated approximately 10%, 26%, and 40% decrease in flexion/extension motion, respectively. Supplementing the instrumented spinopelvic PSO model using interbody spacers reduced the ROM by 22%, 21%, 4%, and 11% in flexion, extension, lateral bending, and axial rotation, respectively. The FE results illustrated lower von Mises stress on the rods and higher forces at the PSO site at higher degeneration grades and while using the interbody spacers. Conclusions Larger and less degenerated discs adjacent to the PSO site may warrant consideration for interbody cage instrumentation to decrease the risk of rod fracture and PSO site non-union.

High-Demand Spinal Deformity With Multi-Rod Constructs and Porous Fusion/Fixation Implants: A Finite Element Study

“…6 For example, a PSO at L4 leaves two caudal levels available for fixation, ie, L5 and S1, allowing in most cases for a solid fixation without the involvement of the SIJs or pelvis. Several papers describe in vitro 8,10,25,26 and finite element models 11,12,14,27,28 in which spine specimens with a PSO and concomitant instrumentation were investigated, either by means of standard posterior fixation or with multi-rod constructs. Interestingly, in a study evaluating PSO at L3 and L4 Luca et al 12 calculated stress values in the posterior rods in close agreement with those of the present study, with maximal values in correspondence with the level of the osteotomy under flexion loading, as well as a very similar impact of the use of multi-rod constructs.…”

“…8 Complications include pseudarthrosis and rod breakage. [9][10][11][12][13][14] Though less common, PSO at L5 has also been performed to treat sagittal imbalance (Figure 1). Although clinical outcomes evidence is scarce, one clinical case series concluded that PSOs in L5 are an effective means of treating sagittal imbalance 15 while a recent paper advocated the use of PSO in L5 in select cases of patients showing kyphosis with the apex at L4-L5.…”