Range of Motion Testing of a Novel 3D-Printed Synthetic Spine Model.

STUDY DESIGN: Biomechanical model study. OBJECTIVE: The Barrow Biomimetic Spine (BBS) project is a resident-driven effort to manufacture a synthetic spine model with high biomechanical fidelity to human tissue. The purpose of this study was to investigate the performance of the current generation of BBS models on biomechanical testing of range of motion (ROM) and axial compression and to compare the performance of these models to historical cadaveric data acquired using the same testing protocol. METHODS: Six synthetic spine models comprising L3-5 segments were manufactured with variable soft-tissue densities and print orientations. Models underwent torque loading to a maximum of 7.5 N m. Torques were applied to the models in flexion-extension, lateral bending, axial rotation, and axial compression. Results were compared with historic cadaveric control data. RESULTS: Each model demonstrated steadily decreasing ROM on flexion-extension testing with increasing density of the intervertebral discs and surrounding ligamentous structures. Vertically printed models demonstrated markedly less ROM than equivalent models printed horizontally at both L3-4 (5.0° vs 14.0°) and L4-5 (3.9° vs 15.2°). Models D and E demonstrated ROM values that bracketed the cadaveric controls at equivalent torque loads (7.5 N m). CONCLUSIONS: This study identified relevant variables that affect synthetic spine model ROM and compressibility, confirmed that the models perform predictably with changes in these print variables, and identified a set of model parameters that result in a synthetic model with overall ROM that approximates that of a cadaveric model. Future studies can be undertaken to refine model performance and determine intermodel variability.

Dimensional Characterization of the Human Cervical Interlaminar Space as a Guide for Safe Application of Minimally Invasive Dilators.

BACKGROUND: The risk of interlaminar passage of a dilator into the cervical spinal canal in minimally invasive approaches is currently unknown. Among the various anthropometric data reported in the literature, there is no report of the interlaminar dimensions in the cervical spine. OBJECTIVE: To report the cervical interlaminar dimensions in neutral, flexion, and extension. METHODS: A total of 8 spines were sectioned into cervical (C2-T1) segments. Digitized coordinate data defining the locations and movements of chosen anatomic points on the laminar edges at a given spinal level were used to compute the dimensions during a static neutral posture, flexion, and extension positions to mimic the positions during surgery. Interlaminar dimensions were averaged and categorized for each vertebral level and spinal posture. RESULTS: Based on the reported measurements, the smallest diameter dilator in commonly used dilator sets has the potential to traverse the interlaminar space at all levels in flexion. In a neutral posture, the average interlaminar distance at C2-3, C6-7, and C7-T1 was still greater than 2.0 mm, the smallest diameter of the initial dilator. The largest interlaminar distance was at C6-7 in flexion (7.68 ± 1.60 mm). CONCLUSION: Because dilators pass directly onto the cervical lamina without visualization of the midline structures, the interlaminar distances have increased relevance in the minimally invasive cervical approaches of foraminotomy and laminectomy. The data in this report demonstrate the theoretical risk of interlaminar passage with small diameter dilators in posterior minimally invasive approaches to the cervical spine.

Optimizing biomechanics of anterior column realignment for minimally invasive deformity correction.

BACKGROUND CONTEXT: Anterior column realignment (ACR) is a powerful but destabilizing minimally invasive technique for sagittal deformity correction. Optimal biomechanical design of the ACR construct is unknown. PURPOSE: Evaluate the effect of ACR design on radiographic lordosis, range of motion (ROM) stability, and rod strain (RS) in a cadaveric model. STUDY DESIGN/SETTING: Cadaveric biomechanical study. PATIENT SAMPLE: Seven fresh-frozen lumbar spine cadaveric specimens (T12-sacrum) underwent ACR at L3-L4 with a 30° implant. OUTCOME MEASURES: Primary outcome measure of interest was maximum segmental lordosis measured using lateral radiograph. Secondary outcomes were ROM stability and posterior RS at L3/4. METHODS: Effect of grade 1 and grade 2 osteotomies with single-screw anterolateral fixation (1XLP) or 2-screw anterolateral fixation (2XLP) on lordosis was determined radiographically. Nondestructive flexibility tests were used to assess ROM and RS at L3-L4 in flexion, extension, lateral bending, and axial rotation. Conditions included (1) intact, (2) pedicle screw fixation and 2 rods (2R), (3) ACR+1XLP with 2R, (4) ACR+2XLP+2R, (5) ACR+1XLP with 4 rods (4R) (+4R), and (6) ACR+2XLP+4R. RESULTS: Segmental lordosis was similar between ACR+1XLP and ACR+2XLP (p>.28). ACR+1XLP+2R was significantly less stable than all other conditions in flexion, extension, and axial rotation (p<.014); however, adding an extra screw improved stability to levels equal to 4R conditions (p>.36). Adding 4R to ACR+1XLP reduced RS in all directions of loading (p<.048), whereas adding a second screw did not (p>.12). There was no difference in strain between ACR+1XLP+4R and ACR+2XLP+4R (p>.55). CONCLUSIONS: For maximum stability, ACR constructs should contain either fixation into both vertebral bodies (2XLP) or accessory rods (4R). 2XLP can be used without compromising the maximal achievable lordosis but does not provide the same RS reduction as 4R. CLINICAL SIGNIFICANCE: ACR is a highly destabilizing technique that is increasingly being used for minimally invasive deformity correction. These biomechanical data will help clinicians optimize ACR construct design.

Evaluation of abnormal styloid anatomy as a cause of internal jugular vein compression using a 3D-printed model: a laboratory investigation.

This study used a 3-dimensional (3D) craniocervical junction model of styloidogenic jugular venous compression (SJVC) syndrome to simulate and evaluate intracranial pressure (ICP) after internal jugular vein (IJV) compression by an elongated styloid process during axial rotation. The 3D-printed model created using data from an SJVC-syndrome patient included an articulating occipital-cervical junction, simplified arteriovenous system, gauge to measure simulated ICP, fixed obstruction simulating left-sided venous occlusion, and right-sided vascular tubing to simulate IJV compression. The model was rotated axially to its extreme right and left; maximum degree of motion and pressure were recorded for 3 cycles. Measurements were repeated after styloid resection in 25% increments. The extreme right rotation (11°) of the intact styloid condition yielded a mean pressure of 15.34 ±â€¯2.85 mmHg. After 25% styloid resection, extreme rotation (11°) yielded 13.96 ±â€¯2.88 mmHg. After 50%, extreme rotation increased to 16° yielding 17.41 ±â€¯3.52 mmHg; 11° rotation was 2.76 ±â€¯1.96 mmHg. After 75%, extreme rotation increased to 19° yielding -0.86 ±â€¯1.08 mmHg; 16° and 11° rotation yielded -0.69 ±â€¯1.19 and -0.86 ±â€¯1.08 mmHg, respectively. After 100%, extreme rotation to 19° yielded -1.21 ±â€¯0.60 mmHg; 16° and 11° rotation yielded -0.34 ±â€¯0.30 and 0.00 ±â€¯0.00 mmHg, respectively. Extreme left rotations (11°) yielded mean pressures of -0.17 ±â€¯0.00 (intact), -0.17 ±â€¯0.30 (25%), 2.24 ±â€¯0.79 (50%), 0.34 ±â€¯0.30 (75%), and 0.17 ±â€¯0.30 mmHg (100%). Simulated ICP increased proportionally to maximum ipsilateral axial rotation, and was highest after 50% styloid resection. Contralateral axial rotation did not increase pressure. IJV compression was relieved at 75% resection, suggesting that partial (75%) or complete styloidectomy is a potentially efficacious treatment for SJVC syndrome.

Comparing the Biomechanical Stability of Cortical Screw Trajectory Versus Standard Pedicle Screw Trajectory for Short- and Long-Segment Posterior Fixation in 3-Column Thoracic Spinal Injury.

BACKGROUND: Information on the performance of posterior fixation with cortical screw (CS) versus pedicle screw (PS) trajectories for stabilizing thoracolumbar burst fractures is limited. Therefore, we sought to analyze stability with CS versus PS in short- and long-segment fixations using a 3-column spinal injury model. METHODS: Nondestructive flexibility tests: (1) intact, (2) intact + short fixation, (3) intact + long fixation, (4) after burst fracture, (5) short fixation + burst fracture, and (6) long fixation + burst fracture using thoracic spine segments (7 CS, 7 PS). RESULTS: With CS, the range of motion (ROM) was significantly greater with short-segment than with long-segment fixation in all directions, with and without burst fracture (P ≤ .008). With PS and burst fracture, ROM was significantly greater with short fixation during lateral bending and axial rotation (P < .006), but not during flexion-extension (P = .10). Groups with CS versus PS were not significantly different after burst fracture during flexion-extension and axial rotation, with short (P ≥ .58) or long fixation (P ≥ .17). During lateral bending, ROM was significantly greater with CS versus PS, without burst fracture (long fixation, P = .02) and with burst fracture (short and long fixation, P ≤ .001). CONCLUSIONS: CS trajectory is a valid alternative to PS trajectory for thoracic spine fixation in 3-column spinal injuries, and long-segment fixation is superior to short-segment fixation with either.

Biomechanical implications of unilateral facetectomy, unilateral facetectomy plus partial contralateral facetectomy, and complete bilateral facetectomy in minimally invasive transforaminal interbody fusion.

OBJECTIVE: Minimally invasive transforaminal interbody fusion techniques vary among surgeons. One decision point is whether to perform a unilateral facetectomy (UF), a unilateral facetectomy plus partial contralateral facetectomy (UF/PF), or a complete bilateral facetectomy (CBF). The authors therefore compared the biomechanical benefits of all 3 types of facetectomies to determine which approach produces improved biomechanical outcomes. METHODS: Seven human cadaveric specimens (L3-S1) were potted and prepped for UF, with full facet removal, hemilaminectomy, discectomy, and pedicle screw placement. After distraction, a fixed interbody spacer was placed, and compression was performed. A final fixation configuration was performed by locking the rods across the screws posteriorly with bilateral compression. Final lordosis angle and change and foraminal height were measured, and standard nondestructive flexibility tests were performed to assess intervertebral range of motion (ROM) and compressive stiffness. The same procedure was followed for UF/PF and CBF in all 7 specimens. RESULTS: All 3 conditions demonstrated similar ROM and compressive stiffness. No statistically significant differences occurred with distraction, but CBF demonstrated significantly greater change than UF in mean foraminal height after bilateral posterior compression (1.90 ± 0.62 vs 1.00 ± 0.45 mm, respectively, p = 0.04). With compression, the CBF demonstrated significantly greater mean ROM than the UF (2.82° ± 0.83° vs 2.170° ± 1.10°, p = 0.007). The final lordosis angle was greatest with CBF (3.74° ± 0.70°) and lowest with UF (2.68° ± 1.28°). This finding was statistically significant across all 3 conditions (p ≤ 0.04). CONCLUSIONS: Although UF/PF and CBF may require slightly more time and effort and incur more risk than UF, the potential improvement in sagittal balance may be worthwhile for select patients.

Tailoring selection of transforaminal interbody spacers based on biomechanical characteristics and surgical goals: evaluation of an expandable spacer.

OBJECTIVE: Transforaminal lumbar interbody fusion (TLIF) is commonly used for lumbar fusion, such as for foraminal decompression, stabilization, and improving segmental lordosis. Although many options exist, surgical success is contingent on matching design strengths with surgical goals. The goal in the present study was to investigate the effects of an expandable interbody spacer and 2 traditional static spacer designs in terms of stability, compressive stiffness, foraminal height, and segmental lordosis. METHODS: Standard nondestructive flexibility tests (7.5 N⋅m) were performed on 8 cadaveric lumbar specimens (L3-S1) to assess intervertebral stability of 3 types of TLIF spacers at L4-5 with bilateral posterior screw-rod (PSR) fixation. Stability was determined as range of motion (ROM) in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Compressive stiffness was determined with axial compressive loading (300 N). Foraminal height, disc height, and segmental lordosis were evaluated using radiographic analysis after controlled PSR compression (170 N). Four conditions were tested in random order: 1) intact, 2) expandable interbody cage with PSR fixation (EC+PSR), 3) static ovoid cage with PSR fixation (SOC+PSR), and 4) static rectangular cage with PSR fixation (SRC+PSR). RESULTS: All constructs demonstrated greater stability than the intact condition (p < 0.001). No significant differences existed among constructs in ROM (FE, AR, and LB) or compressive stiffness (p ≥ 0.66). The EC+PSR demonstrated significantly greater foraminal height at L4-5 than SRC+PSR (21.1 ± 2.6 mm vs 18.6 ± 1.7 mm, p = 0.009). EC+PSR demonstrated higher anterior disc height than SOC+PSR (14.9 ± 1.9 mm vs 13.6 ± 2.2 mm, p = 0.04) and higher posterior disc height than the intact condition (9.4 ± 1.5 mm vs 7.1 ± 1.0 mm, p = 0.002), SOC+PSR (6.5 ± 1.8 mm, p < 0.001), and SRC+PSR (7.2 ± 1.2 mm, p < 0.001). There were no significant differences in segmental lordosis among SOC+PSR (10.1° ± 2.2°), EC+PSR (8.1° ± 0.5°), and SRC+PSR (11.1° ± 3.0°) (p ≥ 0.06). CONCLUSIONS: An expandable interbody spacer provided stability, stiffness, and segmental lordosis comparable to those of traditional nonexpandable spacers of different shapes, with increased foraminal height and greater disc height. These results may help inform decisions about which interbody implants will best achieve surgical goals.

Biomechanical Effects of an Oblique Lumbar PEEK Cage and Posterior Augmentation.

OBJECTIVE: Lumbar interbody spacers are widely used in lumbar spinal fusion. The goal of this study is to analyze the biomechanics of a lumbar interbody spacer (Clydesdale Spinal System, Medtronic Sofamor Danek, Memphis, Tennessee, USA) inserted via oblique lumbar interbody fusion (OLIF) or direct lateral interbody fusion (DLIF) approaches, with and without posterior cortical screw and rod (CSR) or pedicle screw and rod (PSR) instrumentation. METHODS: Lumbar human cadaveric specimens (L2-L5) underwent nondestructive flexibility testing in intact and instrumented conditions at L3-L4, including OLIF or DLIF, with and without CSR or PSR. RESULTS: OLIF alone significantly reduced range of motion (ROM) in flexion-extension (P = 0.005) but not during lateral bending or axial rotation (P ≥ 0.63). OLIF alone reduced laxity in the lax zone (LZ) during flexion-extension (P < 0.001) but did not affect the LZ during lateral bending or axial rotation (P ≥ 0.14). The stiff zone (SZ) was unaffected in all directions (P ≥ 0.88). OLIF plus posterior instrumentation (cortical, pedicle, or hybrid) reduced the mean ROM in all directions of loading but only significantly so with PSR during lateral bending (P = 0.004), without affecting the compressive stiffness (P > 0.20). The compressive stiffness with the OLIF device without any posterior instrumentation did not differ from that of the intact condition (P = 0.97). In terms of ROM, LZ, or SZ, there were no differences between OLIF and DLIF as standalone devices or OLIF and DLIF with posterior instrumentation (CSR or PSR) (P > 0.5). CONCLUSIONS: OLIF alone significantly reduced mobility during flexion-extension while maintaining axial compressive stiffness compared with the intact condition. Adding posterior instrumentation to the interbody spacer increased the construct stability significantly, regardless of cage insertion trajectory or screw type.

Supplemental rods are needed to maximally reduce rod strain across the lumbosacral junction with TLIF but not ALIF in long constructs.

BACKGROUND CONTEXT: Rod fracture at the lumbosacral (LS) junction remains challenging in long segment fusions and likely stems from increased LS strain. Reduction of LS instrumentation strain may help reduce fracture rates. PURPOSE: The goal of this investigation was to assess the effect of supplemental posterior 4-rod (4R) construction on LS stability and rod strain compared with standard 2-rod (2R) construction in a long segment fusion model. STUDY DESIGN/SETTING: Cadaveric biomechanical study. OUTCOME MEASURES: Range of motion (ROM), rod strain, and sacral screw (SS) bending moments during flexion, extension, compression, lateral bending, and axial rotation. METHODS: Standard nondestructive flexibility tests (7.5 Nm) were performed on 14 cadaveric specimens (L1-ilium) to assess ROM stability, rod strain, and SS bending moment of a supplemental 4R construction versus standard 2R construction. Specimens were equally divided into L5-S1 anterior lumbar interbody fusion (ALIF) or L5-S1 transforaminal lumbar interbody fusion (TLIF) groups. Three conditions were tested in each group: (1) no lumbar interbody fusion (No LIF)+2R, (2) ALIF or TLIF+2R, and (3) ALIF or TLIF+4R. Data were analyzed using repeated measures analysis of variance (ANOVA) or ANOVA. RESULTS: No differences were observed between groups 1 and 2 for age, sex, bone mineral density, or baseline ROM (p>.09). Overall, TLIF+2R demonstrated greater ROM than ALIF+4R in extension (p=003), with greater rod strain in flexion, extension, and compression (p<.001), and greater SS in compression and AR (p<.04). Compared with TLIF+2R, TLIF+4R resulted in reduced rod strain in flexion, extension, compression, and LB (p<.04), as well as SS in AR (p<.001). The TLIF+4R yielded biomechanics comparable to ALIF+2R in ROM and rod strain but SS inflexion, extension, compression, and AR remained elevated (p<001). The ALIF+4R did not significantly improve ROM, rod strain, or SS (p>.11). CONCLUSIONS: The use of ALIF and adding accessory rods with TLIF significantly reduced LS rod strain in a long segment cadaveric model with iliac fixation. CLINICAL SIGNIFICANCE: Reducing strain could decrease the risk of failure associated with long segment fixation.

Iliac screws may not be necessary in long-segment constructs with L5-S1 anterior lumbar interbody fusion: cadaveric study of stability and instrumentation strain.

BACKGROUND CONTEXT: Lumbosacral pseudoarthrosis and instrumentation failure is common with long-segment constructs. Optimizing lumbosacral construct biomechanics may help to reduce failure rates. The influence of iliac screws and interbody type on range of motion (ROM), rod strain (RS), sacral screw strain (SS) is not well-established. PURPOSE: Investigate the effects of transforaminal lumbar interbody fusion (TLIF), anterior lumbar interbody fusion (ALIF), and iliac screws on long-segment lumbosacral construct biomechanics. STUDY DESIGN: Biomechanical study. PATIENT SAMPLE: Fourteen human cadaveric spine specimens. OUTCOME MEASURES: Lumbosacral ROM, RS, and SS. METHODS: Specimens were potted at L1 and the ilium. Specimens were equally divided into either an L5-S1 ALIF or TLIF group and underwent testing in the following conditions: (1) intact (2) L2-S1 pedicle screw rod fixation (PSR-S) (3) L2-ilium (PSR-I) (4) PSR-S+ALIF (ALIF-S) or TLIF (TLIF-S) (5) PSR-I + ALIF (ALIF-I) or TLIF (TLIF-I). Pure moment bending (7.5 Nm) in flexion, extension, lateral bending, axial rotation, and compressive loads (400N) were applied and ROM, SS, and RS were measured. Comparisons were performed using a one-way ANOVA (p<.05). RESULTS: ALIF-S and TLIF-S provided similar decreases in ROM as TLIF-I (p>.05). Compared to PSR-S, PSR-I significantly decreased SS during bending in all directions (p<.02) but increased RS in flexion and extension (p≤.02). Anterior lumbar interbody fusion-S provided similar decreases in SS as TLIF-I in all directions (p>.40) but had significantly less RS than TLIF-I in flexion, extension, compression (p<.01). TLIF-S had more SS than TLIF-I in flexion, extension, axial rotation (p<.02), while TLIF-S had less RS only in flexion (p=.03). Compared to PSR-I, ALIF-I decreased the RS (p<.02) but TLIF-I did not (p>.67). CONCLUSIONS: Iliac screws were protective of SS but increased RS at the lumbosacral junction. Constructs with ALIF and no iliac screws result in comparable SS as constructs with TLIF and iliac screws with significantly reduced RS. If iliac screws are utilized, ALIF but not TLIF reduces the iliac screw-induced RS. CLINICAL SIGNIFICANCE: There is a relatively high incidence of lumbosacral instrumentation failure in adult spinal deformity. Optimizing lumbosacral construct biomechanics may help to reduce failure rates. Iliac screws induce lumbosacral rod strain and may be responsible for instrumentation failure. Constructs with lumbosacral ALIF reduce iliac-screw induced rod strain and may obviate the need for fixation to the ilium.

Impact of Connector Placement and Design on Bending Stiffness of Spinal Constructs.

OBJECTIVE: To evaluate the stability of multiple rod-connector construct designs using a mechanical 4-point bending testing frame. METHODS: A mechanical study was used to evaluate the bending stiffness of 3 connectors across 12 different configurations of rod-connector-rod constructs. Stability was evaluated in flexion-extension and lateral bending. Combinations of rods having 1 of 3 diameters (4.0 mm, 5.5 mm, and 6.0 mm) connected by 1 of 3 connector types (parallel open, snap-on, and hinged) were compared. Configurations with single connectors and with double connectors with variable spacing were also compared to simulate revision surgery conditions. RESULTS: Constructs consisting of 4.0-mm rods connected to 4.0-mm rods were significantly less stiff as the total number of connectors used in a series exceeded 2. When single-connector configurations were compared, parallel open rod connectors demonstrated greater stiffness in flexion-extension than hinged open connectors, whereas hinged open connectors demonstrated greater stiffness in lateral bending. Using double connectors increased stiffness of 4.0- to 4.0-mm rod configurations in flexion-extension and lateral bending, 4.0- to 6.0-mm rod configurations in flexion-extension, and 5.5- to 6.0-mm rod configurations in lateral bending. Spacing the double connectors significantly improved lateral bending stiffness of 4.0- to 4.0-mm and 5.5- to 6.0-mm rod configurations. CONCLUSIONS: Our data indicate that the design, number, and placement of rod connectors have a significant impact on the bending stiffness of a surgical construct. Such mechanical data may influence construct design in primary and revision surgeries of the cervical spine and cervicothoracic junction.

Biomechanical Evaluation of Cervicothoracic Junction Fusion Constructs.

OBJECTIVE: We studied the effect of different cervicothoracic construct design variables on biomechanical stability in vitro. METHODS: Six fresh-frozen human cadaveric spines (C5-T4) were used. After intact analysis, each specimen was destabilized and reconstructed, with all groups having 4.0-mm pedicle screws placed at T1-T3. The 2 hook-rod constructs included interlaminar hooks at C6 and C7, with either 3.5-mm or 4.0-mm rods (C6-T3). The 2 screw-rod constructs tested included lateral mass screws at C6 and C7, with either 3.5-mm or 4.0-mm rods (C6-T3). The 2 screw-connector-rod constructs tested included lateral mass screws at C6 and C7, with either 3.5-mm or 4.0-mm rods; 1 rod spanned C6-C7 with a connector to a second rod of the same size spanning T1-T3. Global (C6-T3) and intervertebral (C6-C7, C7-T1, T1-T2, and T2-T3) ranges of motion were compared for each construct. RESULTS: In terms of global (C6-T3) stability, 3.5-mm versus 4.0-mm rod constructs were not significantly different, regardless of whether the construct was hook-rod, screw-rod, or screw-connector-rod. The hook-rod constructs provided less stability compared with the screw-rod and screw-connector-rod constructs in lateral bending (P < 0.04) and axial rotation (P < 0.001). The screw-rod constructs demonstrated a similar range of motion to that of the screw-connector-rod constructs, except for significantly less axial rotation at the C6-C7 level with 3.5-mm rods (P = 0.04). CONCLUSIONS: We found that the rod diameter of a construct does not appear to significantly influence the biomechanical stability of subaxial constructs. The screw-rod construct resulted in certain biomechanical advantages compared with the screw-connector-rod construct, and both were significantly superior to the hook-rod construct.

In Vitro Biomechanical Evaluation of a Novel, Minimally Invasive, Sacroiliac Joint Fixation Device.

BACKGROUND: Sacroiliac (SI) joint pathology may result in low-back pain, which causes substantial disability. Treatment failure with operative management of SI pain may be related to incomplete fusion of the joint and to fixation failure. The objective of this study was to evaluate the initial biomechanical stability of SI joint fixation with a novel implantable device in an in vitro human cadaveric model. METHODS: The right and left sides of 3 cadaveric L4-pelvis specimens were tested (1) intact, (2) destabilized, and (3) instrumented with an implantable SI joint fixation device using a simulated single-stance load condition. Right-leg and left-leg stance data were grouped together for a sample size of 6, and angular range of motion (ROM) was determined during application of flexion-extension, lateral bending, and axial rotation bending moments to a limit of 7.5 Nm. RESULTS: Following intact testing, destabilization by severing the posterior SI joint capsule and ligaments and the pubic symphysis reliably produced a significantly destabilized joint with the mean angular ROM more than doubling in flexion-extension and lateral bending and more than tripling in axial rotation (P ≤ .003) compared to the intact condition. Instrumentation with the SI screw fixation device significantly reduced mean joint ROM compared to the destabilized condition in all 3 anatomic planes tested (P < .001). When compared to the intact condition, the SI-instrumented condition significantly reduced lateral bending (P = .01) and had a similar ROM in flexion-extension (P = .14) and axial rotation (P = .85). CONCLUSIONS: Instrumentation with the SI screw fixation device significantly reduced mean joint ROM compared to the destabilized condition, with similar ROM in flexion-extension and axial rotation, and it significantly reduced ROM in lateral bending compared to that for the intact joint. The ROM values observed with the instrumented condition were comparable to levels of mobility considered favorable for spinal fusion.

Biomechanical evaluation of interbody fixation with secondary augmentation: lateral lumbar interbody fusion versus posterior lumbar interbody fusion.

BACKGROUND: Many approaches to the lumbar spine have been developed for interbody fusion. The biomechanical profile of each interbody fusion device is determined by the anatomical approach and the type of supplemental internal fixation. Lateral lumbar interbody fusion (LLIF) was developed as a minimally invasive technique for introducing hardware with higher profiles and wider widths, compared with that for the posterior lumbar interbody fusion (PLIF) approach. However, the biomechanics of the interbody fusion construct used in the LLIF approach have not been rigorously evaluated, especially in the presence of secondary augmentation. METHODS: Spinal stability of 21 cadaveric lumbar specimens was compared using standard nondestructive flexibility studies [mean range of motion (ROM), lax zone (LZ), stiff zone (SZ) in flexion-extension, lateral bending, and axial rotation]. Non-paired comparisons were made among four conditions: (I) intact; (II) with unilateral interbody + bilateral pedicle screws (BPS) using the LLIF approach (referred to as the LLIF construct); (III) with bilateral interbody + BPS using the PLIF approach (referred to as the PLIF construct); and (IV) with no lumbar interbody fusion (LIF) + BPS (referred to as the no-LIF construct). RESULTS: With bilateral pedicle screw-rod fixation, stability was equivalent between PLIF and LLIF constructs in lateral bending and flexion-extension. PLIF and LLIF constructs had similar biomechanical profiles, with a trend toward less ROM in axial rotation for the LLIF construct. CONCLUSIONS: LLIF and PLIF constructs had similar stabilizing effects.

Biomechanical Analysis of an Expandable Lumbar Interbody Spacer.

OBJECTIVE: Recently developed expandable interbody spacers are widely accepted in spinal surgery; however, the resulting biomechanical effects of their use have not yet been fully studied. We analyzed the biomechanical effects of an expandable polyetheretherketone interbody spacer inserted through a bilateral posterior approach with and without different modalities of posterior augmentation. METHODS: Biomechanical nondestructive flexibility testing was performed in 7 human cadaveric lumbar (L2-L5) specimens followed by axial compressive loading. Each specimen was tested under 6 conditions: 1) intact, 2) bilateral L3-L4 cortical screw/rod (CSR) alone, 3) WaveD alone, 4) WaveD + CSR, 5) WaveD + bilateral L3-L4 pedicle screw/rod (PSR), and 6) WaveD + CSR/PSR, where CSR/PSR was a hybrid construct comprising bilateral cortical-level L3 and pedicle-level L4 screws interconnected by rods. RESULTS: The range of motion (ROM) with the interbody spacer alone decreased significantly compared with the intact condition during flexion-extension (P = 0.02) but not during lateral bending or axial rotation (P ≥ 0.19). The addition of CSR or PSR to the interbody spacer alone condition significantly decreased the ROM compared with the interbody spacer alone (P ≤ 0.002); and WaveD + CSR, WaveD + PSR, and WaveD + CSR/PSR (hybrid) (P ≥ 0.29) did not differ. The axial compressive stiffness (resistance to change in foraminal height during compressive loading) with the interbody spacer alone did not differ from the intact condition (P = 0.96), whereas WaveD + posterior instrumentation significantly increased compressive stiffness compared with the intact condition and the interbody spacer alone (P ≤ 0.001). CONCLUSIONS: The WaveD alone significantly reduced ROM during flexion-extension while maintaining the axial compressive stiffness. CSR, PSR, and CSR/PSR hybrid constructs were all effective in augmenting the expandable interbody spacer system and improving its stability.

A Novel C2 Screw Trajectory: Preliminary Anatomic Feasibility and Biomechanical Comparison.

BACKGROUND: Pedicle screw and translaminar screw fixation in C2 may not be applicable in many patients with anatomic abnormalities or narrow laminar thickness and spinous process height. The aim of this study was to assess morphometric and mechanical feasibilities of a novel alternative screw trajectory that pierces the bifid base of C2. METHODS: Anatomic measurements that determined the feasibility of spinous process bifid base (SPB) screw fixation were assessed in 14 cadaveric C2 vertebrae. Pullout tests to assess ultimate fixation strength for 3 screw trajectories (transpedicular, translaminar, and SPB) were performed in cadaveric vertebrae for comparison. RESULTS: Anatomic measurements included mean spinous process height (10.4 ± 4.2 mm) and mean bilateral bifid base length (10.1 ± 2.2 mm) and thickness (left, 4.4 ± 1.0 mm; right, 4.3 ± 0.9 mm). In 64% (9/14) of specimens, bifid base length was ≥9 mm. Mean pullout strength for transpedicle, translaminar, and SPB screws in 9 viable specimens was 648 ± 305 N, 628 ± 417 N, and 755 ± 279 N. CONCLUSIONS: SPB screw fixation may be viable anatomically and mechanically for C2 fixation. Feasibility of SPB screw fixation is determined by length, thickness, and mutual angle of the bilateral bifid bases. Patients with thin (<4 mm) and short (<9 mm) bifid bases are not likely to be suitable candidates. SPB screw fixation shows potential as an alternative approach or a salvage technique for patients with high-riding vertebral arteries or severely thin C2 lamina and warrants further investigation.

Biomechanical Stability Afforded by Unilateral Versus Bilateral Pedicle Screw Fixation with and without Interbody Support Using Lateral Lumbar Interbody Fusion.

OBJECTIVE: To determine the stability of fusion constructs with unilateral pedicle screw (UPS) or bilateral pedicle screw (BPS) fixation with and without an interbody implant using the lateral lumbar interbody (LLIF) approach. METHODS: Standard nondestructive flexibility tests were performed on 13 cadaveric lumbar specimens to assess spinal stability of intact specimens and 5 configurations of posterior and interbody instrumentation. Spinal stability was determined as mean range of motion in flexion-extension, lateral bending, and axial rotation. Nonpaired comparisons were made for these 6 conditions: 1) intact; 2) unilateral interbody via the LLIF approach (LLIF construct); 3) unilateral interbody + unilateral pedicle screws (UPS) via the LLIF approach (LLIF + UPS); 4) unilateral interbody + bilateral pedicle screws (BPS) using the LLIF approach (LLIF+BPS); 5) UPS alone; and 6) BPS alone. RESULTS: UPS and BPS, with and without interbody support, significantly reduced range of motion during the majority of directions of loading. BPS alone provided greater stability than UPS alone and LLIF alone in all directions of motion except axial rotation. With interbody support, there was no significant difference in stability between BPS and UPS across all movement directions. CONCLUSIONS: These biomechanical results suggest that fixation in the lumbar spine with an interbody support using an LLIF approach with UPS is a promising alternative to BPS. Although BPS provides greater immediate stability compared with UPS, in the presence of a lateral interbody implant, UPS and BPS provide equivalent stability. In addition, LLIF does not appear to contribute significantly to immediate stability when BPS is used.

Effect of screw position on load transfer in lumbar pedicle screws: a non-idealized finite element analysis.

Angled screw insertion has been advocated to enhance fixation strength during posterior spine fixation. Stresses on a pedicle screw and surrounding vertebral bone with different screw angles were studied by finite element analysis during simulated multidirectional loading. Correlations between screw-specific vertebral geometric parameters and stresses were studied. Angulations in both the sagittal and axial planes affected stresses on the cortical and cancellous bones and the screw. Pedicle screws pointing laterally (vs. straight or medially) in the axial plane during superior screw angulation may be advantageous in terms of reducing the risk of both screw loosening and screw breakage.