Search

Purpose: To analyze the effect of endplate weakness prior to PLIF or TLIF cage implantation and compare it to the opposite intact endplate of the same vertebral body. In addition, the influence of bone quality on endplate resistance was investigated., Methods: Twenty-two human lumbar vertebrae were tested in a ramp-to-failure test. One endplate of each vertebral body was tested intact and the other after weakening with a rasp (over an area of 200 mm 2 ). Either a TLIF or PLIF cage was then placed and the compression load was applied across the cage until failure of the endplate. Failure was defined as the first local maximum of the force measurement. Bone quality was assessed by determining the Hounsfield units (HU) on CT images., Results: With an intact endplate and a TLIF cage, the median force to failure was 1276.3N (693.1-1980.6N). Endplate weakening reduced axial endplate resistance to failure by 15% (0-23%). With an intact endplate and a PLIF cage, the median force to failure was 1057.2N (701.2-1735.5N). Endplate weakening reduced axial endplate resistance to failure by 36.6% (7-47.9%). Bone quality correlated linearly with the force at which endplate failure occurred. Intact and weakened endplates showed a strong positive correlation: intact-TLIF: r = 0.964, slope of the regression line (slope) = 11.8, p < 0.001; intact-PLIF: r = 0.909, slope = 11.2, p = 5.5E-05; weakened-TLIF: r = 0.973, slope = 12.5, p < 0.001; weakened-PLIF: r = 0.836, slope = 6, p = 0.003., Conclusion: Weakening of the endplate during cage bed preparation significantly reduces the resistance of the endplate to subsidence to failure: endplate load capacity is reduced by 15% with TLIF and 37% with PLIF. Bone quality correlates with the force at which endplate failure occurs., (© 2024. The Author(s).)

Purpose: To develop and test synthetic vertebral stabilization techniques ("vertebropexy") that can be used after decompression surgery and furthermore to compare them with a standard dorsal fusion procedure., Methods: Twelve spinal segments (Th12/L1: 4, L2/3: 4, L4/5: 4) were tested in a stepwise surgical decompression and stabilization study. Stabilization was achieved with a FiberTape cerclage, which was pulled through the spinous process (interspinous technique) or through one spinous process and around both laminae (spinolaminar technique). The specimens were tested (1) in the native state, after (2) unilateral laminotomy, (3) interspinous vertebropexy and (4) spinolaminar vertebropexy. The segments were loaded in flexion-extension (FE), lateral shear (LS), lateral bending (LB), anterior shear (AS) and axial rotation (AR)., Results: Interspinous fixation significantly reduced ROM in FE by 66% (p = 0.003), in LB by 7% (p = 0.006) and in AR by 9% (p = 0.02). Shear movements (LS and AS) were also reduced, although not significantly: in LS reduction by 24% (p = 0.07), in AS reduction by 3% (p = 0.21). Spinolaminar fixation significantly reduced ROM in FE by 68% (p = 0.003), in LS by 28% (p = 0.01), in LB by 10% (p = 0.003) and AR by 8% (p = 0.003). AS was also reduced, although not significantly: reduction by 18% (p = 0.06). Overall, the techniques were largely comparable. The spinolaminar technique differed from interspinous fixation only in that it had a greater effect on shear motion., Conclusion: Synthetic vertebropexy is able to reduce lumbar segmental motion, especially in flexion-extension. The spinolaminar technique affects shear forces to a greater extent than the interspinous technique., (© 2023. The Author(s).)

Purpose: The aim of this study was to elucidate segmental range of motion (ROM) before and after common decompression and fusion procedures on the lumbar spine., Methods: ROM of fourteen fresh-frozen human cadaver lumbar segments (L1/2: 4, L3/4: 5, L5/S1: 5) was evaluated in six loading directions: flexion/extension (FE), lateral bending (LB), lateral shear (LS), anterior shear (AS), axial rotation (AR), and axial compression/distraction (AC). ROM was tested with and without posterior instrumentation under the following conditions: 1) native 2) after unilateral laminotomy, 3) after midline decompression, and 4) after nucleotomy., Results: Median native ROM was FE 6.8°, LB 5.6°, and AR 1.7°, AS 1.8 mm, LS 1.4 mm, AC 0.3 mm. Unilateral laminotomy significantly increased ROM by 6% (FE), 3% (LB), 12% (AR), 11% (AS), and 8% (LS). Midline decompression significantly increased these numbers to 15%, 5%, 21%, 20%, and 19%, respectively. Nucleotomy further increased ROM in all directions, most substantially in AC of 153%. Pedicle screw fixation led to ROM decreases of 82% in FE, 72% in LB, 42% in AR, 31% in AS, and 17% in LS. In instrumented segments, decompression only irrelevantly affected ROM., Conclusions: The amount of posterior decompression significantly impacts ROM of the lumbar spine. The here performed biomechanical study allows creation of a simplified rule of thumb: Increases in segmental ROM of approximately 10%, 20%, and 50% can be expected after unilateral laminotomy, midline decompression, and nucleotomy, respectively. Instrumentation decreases ROM by approximately 80% in bending moments and accompanied decompression procedures only minorly destabilize the instrumentation construct., (© 2023. The Author(s).)

Purpose: To develop ligamentous vertebral stabilization techniques ("vertebropexy") that can be used after microsurgical decompression (intact posterior structures) and midline decompression (removed posterior structures) and to elaborate their biomechanical characteristics., Methods: Fifteen spinal segments were biomechanically tested in a stepwise surgical decompression and ligamentous stabilization study. Stabilization was achieved with a gracilis or semitendinosus tendon allograft, which was attached to the spinous process (interspinous vertebropexy) or the laminae (interlaminar vertebropexy) in form of a loop. The specimens were tested (1) in the native state, after (2) microsurgical decompression, (3) interspinous vertebropexy, (4) midline decompression, and (5) interlaminar vertebropexy. In the intact state and after every surgical step, the segments were loaded in flexion-extension (FE), lateral shear (LS), lateral bending (LB), anterior shear (AS) and axial rotation (AR)., Results: Interspinous vertebropexy significantly reduced the range of motion (ROM) in all loading scenarios compared to microsurgical decompression: in FE by 70% (p < 0.001), in LS by 22% (p < 0.001), in LB by 8% (p < 0.001) in AS by 12% (p < 0.01) and in AR by 9% (p < 0.001). Interlaminar vertebropexy decreased ROM compared to midline decompression by 70% (p < 0.001) in FE, 18% (p < 0.001) in LS, 11% (p < 0.01) in LB, 7% (p < 0.01) in AS, and 4% (p < 0.01) in AR. Vertebral segment ROM was significantly smaller with the interspinous vertebropexy compared to the interlaminar vertebropexy for all loading scenarios except FE. Both techniques were able to reduce vertebral body segment ROM in FE, LS and LB beyond the native state., Conclusion: Vertebropexy is a new concept of semi-rigid spinal stabilization based on ligamentous reinforcement of the spinal segment. It is able to reduce motion, especially in flexion-extension. Studies are needed to evaluate its clinical application., (© 2023. The Author(s).)

Purpose: To elucidate residual motion of cortical screw (CS) and pedicle screw (PS) constructs with unilateral posterior lumbar interbody fusion (ul-PLIF), bilateral PLIF (bl-PLIF), facet-sparing transforaminal lumbar interbody fusion (fs-TLIF), and facet-resecting TLIF (fr-TLIF)., Methods: A total of 35 human cadaver lumbar segments were instrumented with PS (n = 18) and CS (n = 17). Range of motion (ROM) and relative ROM changes were recorded in flexion/extension (FE), lateral bending (LB), axial rotation (AR), lateral shear (LS), anterior shear (AS), and axial compression (AC) in five instrumentational states: without interbody fusion (wo-IF), ul-PLIF, bl-PLIF, fs-TLIF, and fr-TLIF., Results: Whereas FE, LB, AR, and AC noticeably differed between the instrumentational states, AS and LS were less prominently affected. Compared to wo-IF, ul-PLIF caused a significant increase in ROM with PS (FE + 42%, LB + 24%, AR + 34%, and AC + 77%), however, such changes were non-significant with CS. ROM was similar between wo-IF and all other interbody fusion techniques. Insertion of a second PLIF (bl-PLIF) significantly decreased ROM with CS (FE -17%, LB -26%, AR -20%, AC -51%) and PS (FE - 23%, LB - 14%, AR - 20%, AC - 45%,). Facet removal in TLIF significantly increased ROM with CS (FE + 6%, LB + 9%, AR + 17%, AC of + 23%) and PS (FE + 7%, AR + 12%, AC + 13%)., Conclusion: bl-PLIF and TLIF show similarly low residual motion in both PS and CS constructs, but ul-PLIF results in increased motion. The fs-TLIF technique is able to further decrease motion compared to fr-TLIF in both the CS and PS constructs., (© 2023. The Author(s).)

Purpose: To compare the residual range of motion (ROM) of cortical screw (CS) versus pedicle screw (PS) instrumented lumbar segments and the additional effect of transforaminal interbody fusion (TLIF) and cross-link (CL) augmentation., Methods: ROM of thirty-five human cadaver lumbar segments in flexion/extension (FE), lateral bending (LB), lateral shear (LS), anterior shear (AS), axial rotation (AR), and axial compression (AC) was recorded. After instrumenting the segments with PS (n = 17) and CS (n = 18), ROM in relation to the uninstrumented segments was evaluated without and with CL augmentation before and after decompression and TLIF., Results: CS and PS instrumentations both significantly reduced ROM in all loading directions, except AC. In undecompressed segments, a significantly lower relative (and absolute) reduction of motion in LB was found with CS 61% (absolute 3.3°) as compared to PS 71% (4.0°; p = 0.048). FE, AR, AS, LS, and AC values were similar between CS and PS instrumented segments without interbody fusion. After decompression and TLIF insertion, no difference between CS and PS was found in LB and neither in any other loading direction. CL augmentation did not diminish differences in LB between CS and PS in the undecompressed state but led to an additional small AR reduction of 11% (0.15°) in CS and 7% (0.05°) in PS instrumentation., Conclusion: Similar residual motion is found with CS and PS instrumentation, except of slightly, but significantly inferior reduction of ROM in LB with CS. Differences between CS and PS in diminish with TLIF but not with CL augmentation., (© 2023. The Author(s).)

Purpose: Pedicle screw loosening is a frequent complication in lumbar spine fixation, most commonly among patients with poor bone quality. Determining patients at high risk for insufficient implant stability would allow clinicians to adapt the treatment accordingly. The aim of this study was to develop a computational model for quantitative and reliable assessment of the risk of screw loosening., Methods: A cohort of patient vertebrae with diagnosed screw loosening was juxtaposed to a control group with stable fusion. Imaging data from the two cohorts were used to generate patient-specific biomechanical models of lumbar instrumented vertebral bodies. Single-level finite element models loading the screw in axial or caudo-cranial direction were generated. Further, multi-level models incorporating individualized joint loading were created., Results: The simulation results indicate that there is no association between screw pull-out strength and the manifestation of implant loosening (p = 0.8). For patient models incorporating multiple instrumented vertebrae, CT-values and stress in the bone were significantly different between loose screws and non-loose screws (p = 0.017 and p = 0.029, for CT-values and stress, respectively). However, very high distinction (p = 0.001) and predictability (R 2 Pseudo  = 0.358, AUC = 0.85) were achieved when considering the relationship between local bone strength and the predicted stress (loading factor). Screws surrounded by bone with a loading factor higher than 25% were likely to be loose, while the chances of screw loosening were close to 0 with a loading factor below 15%., Conclusion: The use of a biomechanics-based score for risk assessment of implant fixation failure might represent a paradigm shift in addressing screw loosening after spondylodesis surgery., (© 2022. The Author(s).)

Purpose: The cortical bone trajectory (CBT) is an alternative to the traditional pedicle screw trajectory (TT) in posterior spinal instrumentation, enhancing screw contact with cortical bone and therefore increasing fixation strength. Additional to the trajectory, insertion depth (pericortical vs. bicortical placement) could be a relevant factor affecting the fixation strength. However, the potential biomechanical benefit of a bicortical placement of CBT screws is unknown. Therefore, the aim of this study was to quantify the fixation strength of pericortical- versus bicortical-CBT (pCBT versus bCBT) screws in a randomized cadaveric study., Methods: Pedicle screws were either placed pericortical or bicortical with a CBT in 20 lumbar vertebrae (2 × 20 instrumented pedicles) from four human spine cadavers by using patient-specific templates. Instrumented specimens underwent physiological cyclic loading testing (1'800'000 cycles, 10 Hz), including shear and tension loads as well as bending moments. Translational and angular displacements of the screws were quantified and compared between the two techniques., Results: There was a slight decrease in translational (0.2 mm ± 0.09 vs. 0.24 mm ± 0.11) and angular displacements (0.06° ± 0.05 vs. 0.13° ± 0.11) of bCBT screws when compared with pCBT screws after 1'800'000 cycles. However, the results were non-significant (p > 0.05)., Conclusion: The authors do not recommend placing CBT screws bicortically, as no relevant biomechanical advantage is gained while the potential risk for iatrogenic injury to structures anterior to the spine is increased., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)