Biomechanics of posterior instrumentation in L1-L3 lateral interbody fusion: Pedicle screw rod construct vs. transfacet pedicle screws

Biomechanical Effects of a Novel Standalone Posterior Lumbar Facet Joint Stabilization Device: An In Vitro Cadaveric Study

OBJECTIVE

Although pedicle screw and rod instrumentation remains the gold standard method of posterior rod fixation, it is associated with complications, including pedicle breach and facet joint violation. There is current interest in facet joint stabilization with the potential to create a less invasive, natural arch of fixation that may avoid the complications associated with pedicle screw and rod instrumentation. This study examined the stabilizing potential of a novel facet joint fixation device for single-level (L4-L5) fixation in a human cadaveric model.

METHODS

Six L3-S1 specimens were tested multidirectionally under pure moment loading (7.5 Nm) in 3 conditions: 1) intact, 2) L4-L5 facet fixation without screws, and 3) L4-L5 facet fixation with screws. L4-L5 intervertebral disc angles were measured radiographically. Range of motion (ROM) and disc angles were compared using repeated-measures analysis of variance, with statistical significance set at P < 0.05.

RESULTS

Compared with the intact condition, L4-L5 bilateral facet fixation without or with screw fixation significantly reduced L4-L5 angular ROM in all directions (P ≤ 0.003). No significant differences were observed in cranial and caudal adjacent-segment ROM (P ≥ 0.08) except for L3-L4 fixation in extension, which exhibited small motion increases (0.12° without screws, 0.1° with screws) versus the intact condition (P ≤ 0.003). No statistically significant differences were observed in disc angle values between the conditions (P = 0.87).

CONCLUSIONS

Bilateral lumbar facet fixation with and without supplemental transfacet screw fixation provided significant stability. Cranial and caudal adjacent-level ROM was not influenced by facet fixation except for a slight increase in cranial segment motion during extension. Facet fixation did not alter the lordotic intervertebral disc angle at the instrumented level.

Optimization of Spinal Reconstructions for Thoracolumbar Burst Fractures to Prevent Proximal Junctional Complications: A Finite Element Study

The management strategies of thoracolumbar (TL) burst fractures include posterior, anterior, and combined approaches. However, the rigid constructs pose a risk of proximal junctional failure. In this study, we aim to systemically evaluate the biomechanical performance of different TL reconstruction constructs using finite element analysis. Furthermore, we investigate the motion and the stress on the proximal junctional level adjacent to the constructs. We used a T10-L3 finite element model and simulated L1 burst fracture. Reconstruction with posterior instrumentation (PI) alone (U2L2 and U1L1+(intermediate screw) and three-column spinal reconstruction (TCSR) constructs (U1L1+PMMA and U1L1+Cage) were compared. Long-segment PI resulted in greater global motion reduction compared to constructs with short-segment PI. TCSR constructs provided better stabilization in L1 compared to PI alone. Decreased intradiscal and intravertebral pressure in the proximal level were observed in U1L1+IS, U1L1+PMMA, and U1L1+Cage compared to U2L2. The stress and strain energy of the pedicle screws decreased when anterior reconstruction was performed in addition to PI. We showed that TCSR with anterior reconstruction and SSPI provided sufficient immobilization while offering additional advantages in the preservation of physiological motion, the decreased burden on the proximal junctional level, and lower risk of implant failure.

An In Vitro Biomechanical Evaluation of a Lateral Lumbar Interbody Fusion Device With Integrated Lateral Modular Plate Fixation

STUDY DESIGN

In vitro cadaveric biomechanical study.

OBJECTIVE

Biomechanically characterize a novel lateral lumbar interbody fusion (LLIF) implant possessing integrated lateral modular plate fixation (MPF).

METHODS

A human lumbar cadaveric (n = 7, L1-L4) biomechanical study of segmental range-of-motion stiffness was performed. A ±7.5 Nċm moment was applied in flexion/extension, lateral bending, and axial rotation using a 6 degree-of-freedom kinematics system. Specimens were tested first in an intact state and then following iterative instrumentation (L2/3): (1) LLIF cage only, (2) LLIF + 2-screw MPF, (3) LLIF + 4-screw MPF, (4) LLIF + 4-screw MPF + interspinous process fixation, and (5) LLIF + bilateral pedicle screw fixation. Comparative analysis of range-of-motion outcomes was performed between iterations.

RESULTS

Key biomechanical findings: (1) Flexion/extension range-of-motion reduction with LLIF + 4-screw MPF was significantly greater than LLIF + 2-screw MPF (P < .01). (2) LLIF with 2-screw and 4-screw MPF were comparable to LLIF with bilateral pedicle screw fixation in lateral bending and axial rotation range-of-motion reduction (P = 1.0). (3) LLIF + 4-screw MPF and supplemental interspinous process fixation range-of-motion reduction was comparable to LLIF + bilateral pedicle screw fixation in all directions (P ≥ .6).

CONCLUSIONS

LLIF with 4-screw MPF may provide inherent advantages over traditional 2-screw plating modalities. Furthermore, when coupled with interspinous process fixation, LLIF with MPF is a stable circumferential construct that provides biomechanical utility in all principal motions.

Cadaveric biomechanical analysis of multilevel lateral lumbar interbody fusion with and without supplemental instrumentation

BACKGROUND

This study was to evaluate and compare the biomechanical features of multilevel lateral lumbar interbody fusion (LLIF) with or without supplemental instrumentations.

METHODS

Six human lumbar specimens were tested under multidirectional nondestructive moments (7.5 N·m), with a 6 degree-of-freedom spine simulator. The overall and intervertebral range of motion (ROM) were measured optoelectronically. Each specimen was tested under the following conditions at L2-5 levels: intact; stand-alone; cage supplemented with lateral plate (LP); cage supplemented with unilateral or bilateral pedicle screw/rod (UPS or BPS).

RESULTS

Compared with intact condition, the overall and intersegmental ROM were significantly reduced after multilevel stand-alone LLIF. The ROM was further reduced after using LP instrumentation. In flexion-extension (FE) and axial rotation (AR), pedicle screw/rod demonstrated greater overall ROM reduction compared to LP (P < 0.01), and bilateral greater than unilateral (P < 0.01). In lateral bending (LB), BPS demonstrated greater overall ROM reduction compared to UPS and LP (P < 0.01), however, UPS and LP showed similar reduction (P = 0.245). Intervertebral ROM reductions showed similar trend as the overall ones after using different types of instrumentation. However, at L2/3 (P = 0.57) and L3/4 (P = 0.097) levels, the intervertebral ROM reductions in AR were similar between UPS and LP.

CONCLUSIONS

The overall and intervertebral stability increased significantly after multilevel LLIF with or without supplemental instrumentation. BPS provided the greatest stability, followed by UPS and LP. However, in clinical practice, less invasive adjunctive fixation methods including UPS and LP may provide sufficient biomechanical stability for multilevel LLIF.

Biomechanical Analysis of an Interspinous Process Fixation Device with In Situ Shortening Capabilities: Does Spinous Process Compression Improve Segmental Stability?

OBJECTIVE

The objective of this study was to characterize the biomechanical implications of spinous process compression, via in situ shortening of a next-generation interspinous process fixation (ISPF) device, in the context of segmental fusion.

METHODS

Seven lumbar cadaveric spines (L1-L4) were tested. Specimens were first tested in an intact state, followed by iterative instrumentation at L2-3 and subsequent testing. The order followed was 1) stand-alone ISPF (neutral height); 2) stand-alone ISPF (shortened in situ from neutral height; shortened); 3) lateral lumbar interbody fusion (LLIF) + ISPF (neutral); 4) LLIF + ISPF (shortened); 5) LLIF + unilateral pedicle screw fixation; 6) LLIF + bilateral pedicle screw fixation. A 7.5-Nm moment was applied in flexion/extension, lateral bending, and axial rotation via a kinematic test frame. Segmental range of motion (ROM) and lordosis were measured for all constructs. Comparative analysis was performed.

RESULTS

Statistically significant flexion/extension ROM reductions: all constructs versus intact condition (P < 0.01); LLIF + ISPF (neutral and shortened) versus stand-alone ISPF (neutral and shortened) (P < 0.01); LLIF + USPF versus ISPF (neutral) (P = 0.049); bilateral pedicle screw fixation (BPSF) versus stand-alone ISPF (neutral and shortened) (P < 0.01); LLIF + BPSF versus LLIF + unilateral pedicle screw fixation (UPSF) (P < 0.01). Significant lateral bending ROM reductions: LLIF + ISPF (neutral and shortened) versus intact condition and stand-alone ISPF (neutral) (P < 0.01); LLIF + UPSF versus intact condition and stand-alone ISPF (neutral and shortened) (P < 0.01); LLIF + BPSF versus intact condition and all constructs (P < 0.01). Significant axial rotation ROM reductions: LLIF + ISPF (shortened) and LLIF + UPSF versus intact condition and stand-alone ISPF (neutral) (P ≤ 0.01); LLIF + BPSF versus intact condition and all constructs (P ≤ 0.04).

CONCLUSIONS

In situ shortening of an adjustable ISPF device may support increased segmental stabilization compared with static ISPF.

Value of single-level circumferential fusion: a 10-year prospective outcomes and cost-effectiveness analysis comparing posterior facet versus pedicle screw fixation

PURPOSE

To compare the clinical and economic outcomes of facet versus pedicle screw instrumentation for single-level circumferential lumbar spinal fusion.

METHODS

Outcomes included self-assessment of back and leg pain, pain drawing, ODI, pain medication usage, and procedure success. The CEA was based on the 10-year data collected, and the base-case was from a US payer perspective. Costs included the index surgery, additional surgeries, outpatient/ED visits, and medications. To determine quality-adjusted life years (QALYs), ODI scores were used to predict SF-6D utilities. Sensitivity analyses were performed from a modified payer perspective including device costs and from a societal perspective including productivity loss. Discounted and undiscounted incremental costs and QALYs were calculated. Bootstrapping was performed to estimate the distribution of incremental costs and effects.

RESULTS

Clinical improvement was significant from pre-op to 10-year follow-up for both groups (p < 0.01 for all outcomes scales). Outcomes were significantly better for back pain and ODI for the facet versus pedicle group at all follow-up periods > 1 year (p < 0.05). In the CEA base-case, facets had more QALYs (0.68) and lower costs (- $8650) per person compared with pedicle screws. Therefore, facets were dominant (i.e., provided cost savings and greater QALYs) compared with pedicle screws. Facets had a 97% probability of being below a willingness-to-pay threshold of $20,000 per QALY gained and were estimated to be dominant over pedicle screws in 84% of the simulations.

CONCLUSION

One-level circumferential spinal fusion using facet screws was clinically superior and provided cost savings compared with pedicle screw instrumentation in the USA.

Anterior and Lateral Lumbar Interbody Fusion With Supplemental Interspinous Process Fixation: Outcomes from a Multicenter, Prospective, Randomized, Controlled Study

Background

Rigid interspinous process fixation (ISPF) has received consideration as an efficient, minimally disruptive technique in supporting lumbar interbody fusion. However, despite advantageous intraoperative utility, limited evidence exists characterizing midterm to long-term clinical outcomes with ISPF. The objective of this multicenter study was to prospectively assess patients receiving single-level anterior (ALIF) or lateral (LLIF) lumbar interbody fusion with adjunctive ISPF.

Methods

This was a prospective, randomized, multicenter (11 investigators), noninferiority trial. All patients received single-level ALIF or LLIF with supplemental ISPF (n = 66) or pedicle screw fixation (PSF; n = 37) for degenerative disc disease and/or spondylolisthesis (grade ≤2). The randomization patient ratio was 2:1, ISPF/PSF. Perioperative and follow-up outcomes were collected (6 weeks, 3 months, 6 months, and 12 months).

Results

For ISPF patients, mean posterior intraoperative outcomes were: blood loss, 70.9 mL; operating time, 52.2 minutes; incision length, 5.5 cm; and fluoroscopic imaging time, 10.4 seconds. Statistically significant improvement in patient Oswestry Disability Index scores were achieved by just 6 weeks after operation (P < .01) and improved out to 12 months for the ISPF cohort. Patient-reported 36-Item Short Form Health Survey and Zurich Claudication Questionnaire scores were also significantly improved from baseline to 12 months in the ISPF cohort (P < .01). A total of 92.7% of ISPF patients exhibited interspinous fusion at 12 months. One ISPF patient (1.5%) required a secondary surgical intervention of possible relation to the posterior instrumentation/procedure.

Conclusion

ISPF can be achieved quickly, with minimal tissue disruption and complication. In supplementing ALIF and LLIF, ISPF supported significant improvement in early postoperative (≤12 months) patient-reported outcomes, while facilitating robust posterior fusion.

Timing of PMMA cement application for pedicle screw augmentation affects screw anchorage

INTRODUCTION

Cement augmentation is an established method to increase the pedicle screw (PS) anchorage in osteoporotic vertebral bodies. The ideal timing for augmentation when a reposition maneuver is necessary is controversial. While augmentation of the PS before reposition maneuver may increase the force applied it on the vertebrae, it bears the risk to impair PS anchorage, whereas augmenting the PS after the maneuver may restore this anchorage and prevent early screw loosening. The purpose of the present study was to evaluate the effect of cement application timing on PS anchorage in the osteoporotic vertebral body.

METHODS

Ten lumbar vertebrae (L1-L5) were used for testing. The left and right pedicles of each vertebra were instrumented with the same PS size and used for pairwise comparison of the two timing points for augmentation. For the reposition maneuver, the left PS was loaded axially under displacement control (2 × ±2 mm, 3 × ±6 mm, 3 × ±10 mm) to simulate a reposition maneuver. Subsequently, both PS were augmented with 2 ml PMMA cement. The same force as measured during the left PS maneuver was applied to the previously augmented right hand side PS [2 × F (±2 mm), 3 × F (±6 mm), 3 × F (±10 mm)]. Both PS were cyclically loaded with initial forces of +50 and -50 N, while the lower force was increased by 5 N every 100 cycles until total failure of the PS. The PS motion was measured with a 3D motion analysis system. After cyclic loading stress, X-rays were taken to identify the PS loosening mechanism.

RESULTS

In comparison with PS augmented prior to the reposition maneuver, PS augmented after the reposition maneuver showed a significant higher number of load cycles until failure (5930 ± 1899 vs 3830 ± 1706, p = 0.015). The predominant loosening mechanism for PS augmented after the reposition maneuver was PS toggling with the attached cement cloud within the trabecular bone. While PS augmented prior to the reposition, maneuver showed a motion of the screw within the cement cloud.

CONCLUSION

The time of cement application has an effect on PS anchorage in the osteoporotic vertebral body if a reposition maneuver of the instrumented vertebrae is carried out. PS augmented after the reposition maneuver showed a significant higher number of load cycles until screw loosening.