Lumbar vertebropexy after unilateral total facetectomy

BACKGROUND CONTEXT Posterior decompression with spinal instrumentation and fusion is associated with well-known complications. Alternatives that include decompression and restoration of native stability of the motion segment without fusion continue to be explored, however, an ideal solution has yet to be identified. PURPOSE The aim of this study was to test two different synthetic lumbar vertebral stabilization techniques that can be used after unilateral total facetectomy. STUDY DESIGN Biomechanical cadaveric study. METHODS Twelve spinal segments were biomechanically tested after unilateral total facetectomy and stabilized with a FiberTape cerclage. The cerclage was pulled through the superior and inferior spinous process (interspinous technique) or through the spinous process and around both laminae (spinolaminar technique). The specimens were tested after (1) unilateral total facetectomy, (2) interspinous vertebropexy and (3) spinolaminar vertebropexy. The segments were loaded in flexion-extension (FE), lateral shear (LS), lateral bending (LB), anterior shear (AS) and axial rotation (AR). RESULTS Unilateral facetectomy increased native ROM in FE by 10.6% (7.6%-12.6%), in LS by 25.8% (18.7%-28.4%), in LB 7.5% (4.6%-12.7%), in AS 39.4% (22.6%-49.2%), and in AR by 27.2% (15.8%-38.6%). Interspinous vertebropexy significantly reduced ROM after unilateral facetectomy: in FE by 73% (p=.001), in LS by 23% (p=.001), in LB by 13% (p=.003), in AS by 16% (p=.007), and in AR by 20% (p=.001). In FE and LS the ROM was lower than in the baseline/native condition. In AS and AR, the baseline ROM was not reached by 17% and 1%, respectively. Spinolaminar vertebropexy significantly reduced ROM after unilateral facetectomy: in FE by 74% (p=.001), in LS by 24% (p=.001), in LB by 13% (p=.003), in AS by 28% (p=.004), and in AR by 15 % (p=.001). Baseline ROM was not reached by 9% in AR. CONCLUSION Interspinous vertebropexy seems to sufficiently counteract destabilization after unilateral total facetectomy, and limits range of motion in flexion and extension while avoiding full segmental immobilization. Spinolaminar vertebropexy additionally restores native anteroposterior stability, allowing satisfactory control of shear forces after facetectomy. CLINICAL SIGNIFICANCE Lumbar vertebropexy seems promising to counteract the destabilizating effect of facetectomy by targeted stabilization

Interspinous and spinolaminar synthetic vertebropexy of the lumbar spine

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

A novel approach for tetrahedral-element-based finite element simulations of anisotropic hyperelastic intervertebral disc behavior

Intervertebral discs are microstructurally complex spinal tissues that add greatly to the flexibility and mechanical strength of the human spine. Attempting to provide an adjustable basis for capturing a wide range of mechanical characteristics and to better address known challenges of numerical modeling of the disc, we present a robust finite-element-based model formulation for spinal segments in a hyperelastic framework using tetrahedral elements. We evaluate the model stability and accuracy using numerical simulations, with particular attention to the degenerated intervertebral discs and their likely skewed and narrowed geometry. To this end, 1) annulus fibrosus is modeled as a fiber-reinforced Mooney-Rivlin type solid for numerical analysis. 2) An adaptive state-variable dependent explicit time step is proposed and utilized here as a computationally efficient alternative to theoretical estimates. 3) Tetrahedral-element-based FE models for spinal segments under various loading conditions are evaluated for their use in robust numerical simulations. For flexion, extension, lateral bending, and axial rotation load cases, numerical simulations reveal that a suitable framework based on tetrahedral elements can provide greater stability and flexibility concerning geometrical meshing over commonly employed hexahedral-element-based ones for representation and study of spinal segments in various stages of degeneration

The biomechanical fundamentals of crosslink-augmentation in posterior spinal instrumentation

Posterior screw-rod constructs can be used to stabilize spinal segments; however, the stiffness is not absolute, and some motion can persist. While the effect of crosslink-augmentation has been evaluated in multiple studies, the fundamental explanation of their effectiveness has not been investigated. The aim of this study was to quantify the parameters "screw rotation" and "parallelogram deformation" in posterior instrumentations with and without crosslinks to analyze and explain their fundamental effect. Biomechanical testing of 15 posteriorly instrumented human spinal segments (Th10/11-L4/L5) was conducted in axial rotation, lateral bending, and flexion-extension with ± 7.5 Nm. Screw rotation and parallelogram deformation were compared for both configurations. Parallelogram deformation occurred predominantly during axial rotation (2.6°) and was reduced by 60% (-1.45°, p = 0.02) by the addition of a crosslink. Simultaneously, screw rotation (0.56°) was reduced by 48% (-0.27°, p = 0.02) in this loading condition. During lateral bending, 0.38° of parallelogram deformation and 1.44° of screw rotation was measured and no significant reduction was achieved by crosslink-augmentation (8%, -0.03°, -p = 0.3 and -13%, -0.19°, p = 0.7 respectively). During flexion-extension, parallelogram deformation was 0.4° and screw rotation was 0.39° and crosslink-augmentation had no significant effect on these values (-0.12°, -30%, p = 0.5 and -0°, -0%, p = 0.8 respectively). In axial rotation, crosslink-augmentation can reduce parallelogram deformation and with that, screw rotation. In lateral bending and flexion-extension parallelogram deformation is minimal and crosslink-augmentation has no significant effect. Since the relatively large screw rotation in lateral bending is not caused by parallelogram deformation, crosslink-augmentation is no adequate countermeasure. The fundamental understanding of the biomechanical effect of crosslink-augmentation helps better understand its potential and limitations in increasing construct stiffness

Vertebropexy as a semi-rigid ligamentous alternative to lumbar spinal fusion

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

Biomechanical limitations of partial pediculectomy in endoscopic spine surgery

BACKGROUND CONTEXT Transforaminal endoscopic decompression is an emerging minimally invasive surgical technique in spine surgery. The biomechanical effects and limitations of resections associated with this technique are scarce. PURPOSE The objective of this study was to analyze the effects of three different extents of reduction at the craniomedial pedicle (10%, 25%, and 50%) and to compare them with the intact native side. In addition, the influence of bone quality on the resistance of the pedicle after reduction was investigated. STUDY DESIGN Biomechanical cadaveric study. METHODS Thirty lumbar vertebrae originating from six fresh frozen cadavers were tested under uniaxial compression load in a ramp-to-failure test: (1) the reduced pedicle on one side, and (2) the native pedicle on the other side. Of the 30 lumbar vertebrae, ten were assigned to each reduction group (10%, 25%, and 50%). RESULTS On the intact side, the median axial compression force to failure was 593 N (442.4-785.8). A reduction of the pedicle by 10% of the cross-sectional area resulted in a decrease of the axial load resistance by 4% to 66% compared to the intact opposite side (p=.046). The median compression force to failure was 381.89 N (range: 336-662.1). A reduction by 25% resulted in a decrease of 7% to 71% (p=.001). The median compression force to failure was 333 N (265.1-397.3). A reduction by 50% resulted in a decrease of 39% to 90% (p<.001). The median compression force to failure was 200.9 N (192.3-283.9). At 10% pedicle reduction, the Hounsfield units (HU) value and the absolute force required to generate a pedicle fracture showed significant correlations (ρ=.872; p=.001). At 25%, a positive correlation between the two variables could still be identified (ρ=.603; p=.065). At 50%, no correlation was found (ρ=-.122; p=.738). CONCLUSION Resection of the inner, upper part of the pedicle significantly reduces the axial resistance force of the pedicle until a fracture occurs. CLINICAL SIGNIFICANCE The extent of pedicle reduction itself plays only a limited role: once the cortical bone in the pedicle region is compromised, significant loss of resistance to loading must be anticipated

Bone density optimized pedicle screw insertion

Background: Spinal fusion is the most common surgical treatment for the management of degenerative spinal disease. However, complications such as screw loosening lead to painful pseudoarthrosis, and are a common reason for revision. Optimization of screw trajectories to increase implant resistance to mechanical loading is essential. A recent optimization method has shown potential for determining optimal screw position and size based on areas of high bone elastic modulus (E-modulus). Aim: The aim of this biomechanical study was to verify the optimization algorithm for pedicle screw placement in a cadaveric study and to quantify the effect of optimization. The pull-out strength of pedicle screws with an optimized trajectory was compared to that of a traditional trajectory. Methods: Twenty-five lumbar vertebrae were instrumented with pedicle screws (on one side, the pedicle screws were inserted in the traditional way, on the other side, the screws were inserted using an optimized trajectory). Results: An improvement in pull-out strength and pull-out strain energy of the optimized screw trajectory compared to the traditional screw trajectory was only observed for E-modulus values greater than 3500 MPa cm$^{3}$. For values of 3500 MPa cm$^{3}$ or less, optimization showed no clear benefit. The median screw length of the optimized pedicle screws was significantly smaller than the median screw length of the traditionally inserted pedicle screws, p < 0.001. Discussion: Optimization of the pedicle screw trajectory is feasible, but seems to apply only to vertebrae with very high E-modulus values. This is likely because screw trajectory optimization resulted in a reduction in screw length and therefore a reduction in the implant-bone interface. Future efforts to predict the optimal pedicle screw trajectory should include screw length as a critical component of potential stability

A novel approach for tetrahedral-element-based finite element simulations of anisotropic hyperelastic intervertebral disc behavior

Intervertebral discs are microstructurally complex spinal tissues that add greatly to the flexibility and mechanical strength of the human spine. Attempting to provide an adjustable basis for capturing a wide range of mechanical characteristics and to better address known challenges of numerical modeling of the disc, we present a robust finite-element-based model formulation for spinal segments in a hyperelastic framework using tetrahedral elements. We evaluate the model stability and accuracy using numerical simulations, with particular attention to the degenerated intervertebral discs and their likely skewed and narrowed geometry. To this end, 1) annulus fibrosus is modeled as a fiber-reinforced Mooney-Rivlin type solid for numerical analysis. 2) An adaptive state-variable dependent explicit time step is proposed and utilized here as a computationally efficient alternative to theoretical estimates. 3) Tetrahedral-element-based FE models for spinal segments under various loading conditions are evaluated for their use in robust numerical simulations. For flexion, extension, lateral bending, and axial rotation load cases, numerical simulations reveal that a suitable framework based on tetrahedral elements can provide greater stability and flexibility concerning geometrical meshing over commonly employed hexahedral-element-based ones for representation and study of spinal segments in various stages of degeneration

Residual motion of cortical versus pedicle screw constructs after decompression, interbody fusion and cross-link augmentation

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

Residual motion of different posterior instrumentation and interbody fusion constructs

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