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Study Design. A finite element analysis. Objective. To investigate the association between the position of an inserted pedicle screw and the corresponding facet contact force or intradiscal pressure. Summary of Background Data. Although superior facet joint violation by pedicle screws is not an uncommon occurrence in instrumented lumbar fusion surgery, its actual biomechanical significance is not well understood. Furthermore, the association between the position of the pedicle screw and the stress on the corresponding disc/facet joint has yet to be investigated. Methods. According to the positions of pedicle screws in L4 of the L4–L5 lumbar fusion, 4 L4–L5 fusion models were simulated. These models included the violation of both L3–L4 superior facet joints by pedicle screws (facet joint violation [FV] model), the nonencroachment of both L3–L4 superior facet joints by pedicle screws (facet joint preservation [FP] model), and the removal state of pedicle screws in the FV model (removal of violated pedicle screws [rFV] model). The facet joint contact [FC] model represented the scenario in which the pedicle screws did not encroach upon either facet joint but were inserted close to the L3–L4 facet joint surface. Moreover, the uninstrumented fusion [UF] model represented the uninstrumented L4–L5 fusion. In each scenario, the intradiscal pressures and facet contact forces at the L2–L3 and L3–L4 segments were analyzed under extension and torsion moments. Results. The FV model yielded the greatest increases in facet contact force and intradiscal pressure at the L3–L4 segment under extension and torsion moments. Following the FV model, the increases in intradiscal pressure and facet contact force were the second highest in the FC model followed by the FP model. Furthermore, the rFV model represented prominent reductions of previously increased facet contact force and intradiscal pressure at the L3–L4 segment. Conclusion. In models of 1-segment lumbar fusion surgery, the positions of pedicle screws were closely linked with corresponding disc stresses and facet contact forces. However, even in cases of facet violation by pedicle screws, removal of the pedicle screw after fusion completion can reduce facet contract forces and disc stresses under both extension and torsional moments.

Study design. A prospective case control study. Objectives: To investigate the risk of a fall using functional mobility tests in patients with lumbar spinal stenosis (LSS) via a comparison with patients with knee osteoarthritis (KOA). Summary of Background Data: LSS is a degenerative arthritic disease in the spine that results in decreasing function, impaired balance and gait deficit with increased levels of leg and back pain. This physical impairment might result in an increased risk of fall later in the disease process as shown in knee osteoarthritis. However, there has been no study regarding the association between the risk of a fall and LSS. Methods: The study was an age- and weight-matched case control study consisting of two groups, one group consisting of 40 patients with LSS who were scheduled to undergo spine surgery (LSS group), the other group consisting of 40 patients with advanced OA in both knees, scheduled to undergo TKA on both knees (KOA group). For both groups, four functional mobility tests such as a six meter walk test (SMT), sit-to-stand test (STS), alternative step test (AST), and timed up and go test (TUGT) were performed. Results: There was no difference in demographic data between both groups except for BMI. For the SMT and STS, the patients in the LSS spent significantly more time performing these tests than the patients in the KOA. For the AST, however, patients in the KOA group presented a statistically worse performance in functional mobility, compared to the LSS group. The mean TUGT time was not statistically different between the two groups. Conclusions: The current study highlights that patients with symptomatic LSS have a comparable risk of a fall to patients who had degenerative KOA based on the results of functional mobility tests (SMT, STS, AST, and TUGT). Language: en

The purpose of this study is to investigate the change in biomechanical milieu following removal of pedicle screws in instrumented single level lumbar arthrodesis. Using a validated finite element (FE) model of the intact lumbar spine (L2-5), two scenarios of L3-4 lumbar fusion were simulated: posterolateral fusion (PLF) at L3-4 using pedicle screws (PLF with pedicle screws; WiP) and L3-4 lumbar posterolateral fusion state after removal of pedicle screws (PLF without pedicle screws; WoP). The WiP model had greater range of motion (ROM) at each adjacent segment than the WoP model. This phenomenon became pronounced at the proximal adjacent segment under flexion moment. Similarly, removal of pedicle screws (the WoP model) relieved the maximal von Mises stress at adjacent segments under 4 moments compared to the WiP model. This study demonstrated that removal of pedicle screws could decrease stiffness of fusion segments, which would reduce the disk stress of adjacent segments.

Study Design. Finite element method. Objective. To investigate the changes in the disc stress and range of motion (ROM) at adjacent segments after lumbar fusion based on whether or not pedicle screws are removed and whether or not the continuity of the proximal posterior ligament complex (PLC) is preserved. Summary of Background Data. The ablation of proximal PLC continuity and the presence of pedicle screws have been reported to affect the biomechanics at adjacent segments after lumbar fusion. However, there have been few studies regarding the quantitative assessment of their contribution to overstress at adjacent segments after lumbar fusion. Methods. In the validated intact lumbar finite element model (L2–L5), four types of L3–L4 fusion models were simulated. These models included the preservation of the PLC continuity with pedicle screws (Pp WiP), the preservation of PLC continuity without pedicle screws (Pp WoP), the sacrifice of PLC with pedicle screws (Sp WiP), and the sacrifice of PLC without pedicle screws (Sp WoP). In each scenario, the ROM, maximal von Mises stress of discs, and the facet joint contract force at adjacent segments were analyzed. Results. Among the four models, the Sp WiP yielded the greatest increase in the ROM and the maximal von Mises stress of the disc at adjacent segments under four moments. Following the SP WiP, the order of increase of the ROM and the disc stress was Pp WiP, Sp WoP, and Pp WoP. Furthermore, the increase of ROM and disc stress at the proximal adjacent segment was more than at the distal adjacent segment under all four moments in each model. The facet joint contact was also most increased in the Sp WiP under extension and torsion moment. Conclusion. The current study suggests that the preservation of the PLC continuity or the removal of pedicle screws after complete fusion could decrease the stress at adjacent segments, and their combination could act synergistically.