Biomechanical evaluation of stand-alone lumbar polyether-ether-ketone interbody cage with integrated screws

Impact of bone density and integrated screw configuration on standalone anterior lumbar interbody construct strength

Background

In anterior lumbar interbody fusion (ALIF), the use of integrated screws is attractive to surgeons because of the ease of implantation and no additional profile. However, the number and length of screws necessary for safe and stable implantation in various bone densities is not yet fully understood. The current study aims to determine how important both length and number of screws are for stability of ALIFs.

Methods

Three bone models with densities of 10, 15, and 20 pounds per cubic foot (PCF) were chosen as surrogates. These were instrumented using the Z-Link lumbar interbody system with either 2, 3, or 4 integrated 4.5 × 20 mm screws or 4.5 × 25 mm screws (Zavation, LLC, Flowood, MS). The bone surrogates were tested with loading conditions resulting in spine extension to measure construct stiffness and peak force.

Results

The failure load of the construct was influenced by the length of screws (p=.01) and density of the bone surrogate (p<.01). There was no difference in failure load between using 2 screws and 3 screws (p=.32) or when using four 20 mm screws versus three 25 mm screws (p=.295).

Conclusion

In our study, both bone density and length of screws significantly affected the construct's load to failure. In certain cases where a greater number of screws are unable to be implanted, the same stability can potentially be conferred with use of longer screws. Future clinical studies should be performed to test these biomechanical results.

Application of self-anchored lateral lumbar interbody fusion in lumbar degenerative diseases

STUDY DESIGN

This is a retrospective study.

OBJECTIVE

The aim of the study was to evaluate the efficacy of self-anchored lateral lumbar interbody fusion (SA-LLIF) in lumbar degenerative diseases.

METHODS

Forty-eight patients with lumbar degenerative disease between January 2019 and June 2020 were enrolled in this study. All patients complained of low back and leg pain, which were aggravated during standing activities and alleviated or disappeared during lying. After general anesthesia, the patient was placed in the right decubitus position. The anterior edge of the psoas major muscle was exposed through an oblique incision of approximately 6 cm, using an extraperitoneal approach. The psoas major muscle was then properly retracted dorsally to expose the disc. After discectomy, a suitable cage filled with autogenous bone graft from the ilium was implanted. Two anchoring plates were inserted separately into the caudal and cranial vertebral bodies to lock the cage. Clinical efficacy was evaluated using the visual analog scale (VAS) and Oswestry Disability Index (ODI). Lumbar lordosis, intervertebral disc height, spondylolisthesis rate, cage subsidence and fusion rate were also recorded.

RESULTS

A total of 48 patients were enrolled in this study, including 20 males and 28 females, aged 61.4 ± 7.3 (range 49-78) years old. Surgery was successfully performed in all patients. Lumbar stenosis and instability were observed in 22 cases, disc degenerative disease in eight cases, degenerative spondylolisthesis in nine cases, degenerative scoliosis in six cases, and postoperative revision in three cases. In addition, five patients were diagnosed with osteoporosis. The index levels included L2-3 in three patients, L3-4 in 13 patients, L4-5 in 23 patients, L2-4 in three patients, and L3-5 in six patients. The operation time was 81.1 ± 6.4 (range 65-102) min. Intraoperative blood loss was 39.9 ± 8.5 (range 15-72) mL. No severe complications occurred, such as nerve or blood vessel injuries. The patients were followed up for 11.7 ± 2.3 (range 4-18) months. At the last follow-up, the VAS decreased from 6.2 ± 2.3 to 1.7 ± 1.1, and the ODI decreased from 48.4% ± 11.2% to 10.9% ± 5.5%. Radiography showed satisfactory postoperative spine alignment. No cage displacement was found, but cage subsidence 2-3 mm was found in five patients without obvious symptoms, except transient low back pain in an obese patient. The lumbar lordosis recovered from 36.8° ± 7.9° to 47.7° ± 6.8°, and intervertebral disc height recovered from 8.2 ± 2.0 mm to 11.4 ± 2.5 mm. The spondylolisthesis rate decreased from 19.9% ± 4.9% to 9.4% ± 3.2%. The difference between preoperative and last follow-up was statistically significant (P<0.05).

CONCLUSION

SA-LLIF can provide immediate stability and good results for lumbar degenerative diseases with a standalone anchored cage without posterior internal fixation.

Biomechanical stability of oblique lateral interbody fusion combined with four types of internal fixations: finite element analysis

Objective: Using finite element analysis to identify the optimal internal fixation method for oblique lateral lumbar interbody fusion (OLIF), providing guidance for clinical practice. Methods: A finite element model of the L4 - L5 segment was created. Five types of internal fixations were simulated in the generated L4-L5 finite element (FE) model. Then, six loading scenarios, i.e., flexion, extension, left-leaning, right-leaning, rotate left, and rotate right, were simulated in the FE models with different types of fixations. The biomechanical stability of the spinal segment after different fixations was investigated. Results: Regarding the range of motion (ROM) of the fused segment, OLIF + Bilateral Pedicle Screws (BPS) has a maximum ROM of 1.82° during backward bending and the smallest ROM in all directions of motion compared with other models. In terms of the von Mises stress distribution on the cage, the average stress on every motion direction of OLIF + BPS is about 17.08MPa, and of OLIF + Unilateral Vertebral Screw - Pedicle Screw (UVS-PS) is about 19.29 MPa. As for the von Mises stress distribution on the internal fixation, OLIF + BPS has the maximum internal fixator stress in left rotation (31.85 MPa) and OLIF + Unilateral Pedicle Screw (UPS) has the maximum internal fixator stress in posterior extension (76.59 MPa). The data of these two models were smaller than those of other models. Conclusion: OLIF + BPS provides the greatest biomechanical stability, OLIF + UPS has adequate biomechanical stability, OLIF + UVS-PS is inferior to OLIF + UPS synthetically, and OLIF + Double row vertical screw (DRVS) and Individual OLIF (IO) do not present significant obvious advantages.

Computational comparison of anterior lumbar interbody fusion and oblique lumbar interbody fusion with various supplementary fixation systems: a finite element analysis

BACKGROUND AND OBJECTIVE

Anterior lumbar interbody fusion (ALIF) and oblique lumbar interbody fusion (OLIF) have shown a great surgical potential, while it has always been controversial which surgical approach and which type of fixation system should be selected. This study investigated the biomechanical response of ALIF and OLIF with various supplementary fixation systems using the finite element method.

MATERIALS AND METHODS

Lumbar L4-L5 ALIF and OLIF models stabilized by different supplementary fixation systems (stand-alone cage, integrated stand-alone cage, anterior plate, and bilateral pedicle screw) were developed to assess the segmental range of motion (ROM), endplate stress (EPS), and screw-bone interface stress (SBIS).

EXPERIMENTAL RESULTS

ALIF showed lower ROM and EPS than OLIF in all motion planes and less SBIS in the most of motion planes compared with OLIF when the anterior plate or pedicle screw was used. ALIF induced higher ROM, while lower EPS and SBIS than OLIF in the majority of motion planes when integrated stand-alone cage was utilized. Using a stand-alone cage in ALIF and OLIF led to cage migration. Integrated stand-alone cage prevented the cage migration, whereas caused significantly larger ROM, EPS, and SBIS than other fixation systems except for the rotation plane. In the most of motion planes, the pedicle screw had the lowest ROM, EPS, and SBIS. The anterior plate induced a slightly larger ROM, EPS, and SBIS than the pedicle screw, while the differences were not significant.

CONCLUSION

ALIF exhibited a better performance in postoperative segmental stability, endplate stress, and screw-bone interface stress than OLIF when the anterior plate or the pedicle screw was used. The pedicle screw could provide the greatest postoperative segmental stability, less cage subsidence incidence, and lower risk of fixation system loosening in ALIF and OLIF. The anterior plate could also contribute to the stability required and fewer complications, while not as effectively as the pedicle screw. Extreme caution should be regarded when the stand-alone cage is used due to the risk of cage migration. The integrated stand-alone cage may be an alternative method; however, further optimization is needed to reduce complications and improve postoperative segmental stability.

Biomechanical effects of an oblique lumbar interbody fusion combined with posterior augmentation: a finite element analysis

Background

Oblique lateral interbody fusion (OLIF) is widely used to treat lumbar degenerative disc disease. This study aimed to evaluate the biomechanical stability of OLIF, OLIF including posterior pedicle screw and rod (PSR), and OLIF including cortical screw and rod (CSR) instrumentation through finite element analysis.

Methods

A complete L2-L5 finite element model of the lumbar spine was constructed. Surgical models of OLIF, such as stand-alone, OLIF combined with PSR, and OLIF combined with CSR were created in the L3-L4 surgical segments. Range of motion (ROM), end plate stress, and internal fixation peak stress were compared between different models under the same loading conditions.

Results

Compared to the intact model, ROM was reduced in the OLIF model under all loading conditions. The surgical models in order of increasing ROM were PSR, CSR, and stand-alone; however, the difference in ROM between BPS and CSR was less than 0.4° and was not significant under any loading conditions. The stand-alone model had the highest stress on the superior L4 vertebral body endplate under all loading conditions, whereas the end plate stress was relatively low in the BPS and CSR models. The CSR model had the highest internal fixation stress, concentrated primarily at the end of the screw.

Conclusions

OLIF alone significantly reduces ROM but does not provide sufficient stability. Addition of posterior PSR or CSR internal fixation instrumentation to OLIF surgery can significantly improve biomechanical stability of the segment undergoing surgery.

Assessment of L5-S1 anterior lumbar interbody fusion stability in the setting of lengthening posterior instrumentation constructs: a cadaveric biomechanical study

OBJECTIVE

Excessive stress and motion at the L5-S1 level can lead to degenerative changes, especially in patients with posterior instrumentation suprajacent to L5. Attention has turned to utilization of L5-S1 anterior lumbar interbody fusion (ALIF) to stabilize the lumbosacral junction. However, questions remain regarding the effectiveness of stand-alone ALIF in the setting of prior posterior instrumented fusions terminating at L5. The purpose of this study was to assess the biomechanical stability of an L5-S1 ALIF with increasing lengths of posterior thoracolumbar constructs.

METHODS

Seven human cadaveric spines (T9-sacrum) were instrumented with pedicle screws from T10 to L5 and mounted to a 6 degrees-of-freedom robot. Posterior fusion construct lengths (T10-L5, T12-L5, L2-5, and L4-5) were instrumented to each specimen, and torque-fusion level relationships were determined for each construct in flexion-extension, axial rotation, and lateral bending. A stand-alone L5-S1 ALIF was then instrumented, and L5-S1 motion was measured as increasing pure moments (2 to 12 Nm) were applied. Motion reduction was calculated by comparing L5-S1 motion across the ALIF and non-ALIF states.

RESULTS

The average motion at L5-S1 in axial rotation, flexion-extension, and lateral bending was assessed for each fusion construct with and without ALIF. After adding ALIF to a posterior fusion, L5-S1 motion was significantly reduced relative to the non-ALIF state in all but one fused surgical condition (p < 0.05). Longer fusions with ALIF produced larger L5-S1 motions, and in some cases resulted in motions higher than native state motion.

CONCLUSIONS

Posterior fusion constructs up to L4-5 could be appropriately stabilized by a stand-alone L5-S1 ALIF when using a nominal threshold of 80% reduction in native motion as a potential positive indicator of fusion. The results of this study allow conclusions to be drawn from a biomechanical standpoint; however, the clinical implications of these data are not well defined. These findings, when taken in appropriate clinical context, can be used to better guide clinicians seeking to treat L5-S1 pathology in patients with prior posterior thoracolumbar constructs.

Biomechanical specimen assessment by low dose biplanar X-ray study of fusion constructions using a posterior lumbar cage with integrated anchors and posterior adjunctive fixators

The objective was to compare L4/5 range of motions of fusion constructs using anchored cages. Twelve human cadaveric spine were tested in intact condition, and divided into TLIF and PLIF groups. Testing consisted in applying pure moments in flexion-extension, lateral bending and axial rotation. The computation of intersegmental motion was assessed using 3 D biplanar radiographs. In TLIF group, the addition of contralateral transfacet decreased flexion-extension motion (39%; p = 0.036) but without difference with the ipsilateral pedicle screw construction (53%; p = 0.2). In PLIF group, the addition of interspinous anchor reduced flexion-extension motion (12%; p = 0.036) but without difference with the bilateral pedicle screw construction (17%; p = 0.8).

Feasibility of anterior pedicle screw fixation in lumbosacral spine: a radiographic and cadaveric study

Background

The anterior pedicle screw (APS) technique for L5 and S1 is crucial for proper anterior lumbar interbody fusion (ALIF). This study aimed to determine the projection, screw trajectory angle, and bone screw passageway length (BSPL), as well as the screw insertion regularity and the operating area within which it is safe to perform insertion.

Methods

Forty patients with low back pain, all of whom had lumbar computed tomography scans available, was included in this retrospective analysis. Radiographic parameters were measured, including: the distances from the projection to the upper endplate, lower endplate, and midline; the transverse and sagittal screw angles; and the BSPL. In addition, 10 fresh adult cadaveric lumbosacral spine segments were selected to determine the safe anatomic area in which to operate. Finally, APSs were inserted in L5 and S1 to determine the regularity of APS insertion.

Results

We measured the anterior projection parameters, including: the distances to the upper endplate (L5: 12.5±1.3 mm; S1: 4.54±0.87 mm), lower endplate (L5: 17.3±1.6 mm), and midline (L5: 6.6±0.7 mm; S1: 6.6±0.6 mm); the screw trajectory angle, including the transverse screw angle (L5: 25.3±2.8°; S1: 25.7±2.6°), sagittal screw angle (L5: 17.1±1.7°; S1: 22.4±1.1°); and the BSPL (L5: 48.6±3.5 mm; S1: 48.0±3.5 mm). The regularity of APS insertion in L5 and S1 was determined. Upon the needle reaching a point in the lateral view, it reached the corresponding point in the anteroposterior (AP) view. The anatomic parameters of the safe operating area were as follows: the distance from the abdominal aortic bifurcation to the L5 lower edge (40.50±9.40 mm); the distance from the common iliac vein confluence to the L5 lower edge (27.80±8.60 mm); and the horizontal distance from the inner edge of the common iliac vein to the L5 lower edge (37.50±1.30 mm). We also determined the distance between S1 holes (29.30±1.30 mm), the L5/S1 intervertebral height (17.20±1.50 mm), and the safe operating area (2,058.20±84.30 mm²).

Conclusions

This study has determined the projection, screw trajectory angle, and BSPL of APSs in L5 and S1, their insertion regularity, and the area in which the operation can be safely performed.

Lumbar Intervertebral Spacer With Cement Augmentation of Endplates and Integrated Screws as a Fixation Device in an Osteoporotic Model: An In Vitro Kinematic and Load-to-Failure Study

BACKGROUND

Integrated lateral lumbar interbody fusion (LLIF) devices have been shown to successfully stabilize the spine and avoid complications related to posterior fixation. However, LLIF has increased subsidence risk in osteoporotic patients. Cement augmentation through cannulated pedicle screws enhances pedicle fixation and cage-endplate interface yet involves a posterior approach. Lateral application of cement with integrated LLIF fixation has been introduced and requires characterization. The present study set out to evaluate kinematic and load-to-failure properties of a novel cement augmentation technique with an integrated LLIF device, alone and with unilateral pedicle fixation, compared with bilateral pedicle screws and nonintegrated LLIF (BPS + S).

METHODS

Twelve specimens (L3-S1) underwent discectomy at L4-L5. Specimens were separated into 3 groups: (1) BPS + S; (2) polymethyl methacrylate (PMMA) augmentation, integrated LLIF, and unilateral pedicle screws (PMMA + UPS + iS); and (3) PMMA and integrated LLIF (PMMA + iSA) without posterior fixation. Flexion-extension, lateral bending, and axial rotation were applied. A compressive load was applied to L4-L5 segments until failure. An analysis was performed (P < .05).

RESULTS

Operative constructs significantly reduced motion relative to intact specimens in all motion planes (P < .05). BPS + S provided the most stability, reducing motion by 71.6%-86.4%, followed by PMMA + UPS + iS (68.1%-79.4%) and PMMA + iSA (62.9%-81.9%); no significant differences were found (P > .05). PMMA + UPS + iS provided the greatest resistance to failure (2290 N), followed by PMMA + iSA (1970 N) and BPS + S (1390 N); no significant differences were observed (P > .05).

CONCLUSIONS

Cement augmentation of vertebral endplates via the lateral approach with integrated LLIF moderately improved cage-endplate strength compared to BPS + S in an osteoporotic model; unilateral pedicle fixation further improved failure load. Reconstruction before and after application of unilateral pedicle screws and rods was biomechanically equivalent to anteroposterior reconstruction. Overall, initial results suggest that integrated LLIF with cement augmentation may be a viable alternative in the presence of osteoporosis.

Load-sharing biomechanics of lumbar fixation and fusion with pedicle subtraction osteotomy

Pedicle subtraction osteotomy (PSO) is an invasive surgical technique allowing the restoration of a well-balanced sagittal profile, however, the risks of pseudarthrosis and instrumentation breakage are still high. Literature studied primary stability and posterior instrumentation loads, neglecting the load shared by the anterior column, which is fundamental to promote fusion early after surgery. The study aimed at quantifying the load-sharing occurring after PSO procedure across the ventral spinal structures and the posterior instrumentation, as affected by simple bilateral fixation alone, with interbody cages adjacent to PSO level and supplementary accessory rods. Lumbar spine segments were loaded in vitro under flexion–extension, lateral bending, and torsion using an established spine tester. Digital image correlation (DIC) and strain-gauge (SG) analyses measured, respectively, the full-field strain distribution on the ventral surface of the spine and the local strain on posterior primary rods. Ventral strains considerably decreased following PSO and instrumentation, confirming the effectiveness of posterior load-sharing. Supplemental accessory rods considerably reduced the posterior rod strains only with interbody cages, but the ventral strains were unaffected: this indicates that the load transfer across the osteotomy could be promoted, thus explaining the higher fusion rate with decreased rod fracture risk reported in clinical literature.

Biomechanical in vitro comparison between anterior column realignment and pedicle subtraction osteotomy for severe sagittal imbalance correction

PURPOSE

To investigate the biomechanical effects of anterior column realignment (ACR) and pedicle subtraction osteotomy (PSO) on local lordosis correction, primary stability and rod strains.

METHODS

Seven cadaveric spine segments (T12-S1) underwent ACR at L1-L2. A stand-alone hyperlordotic cage was initially tested and then supplemented with posterior bilateral fixation. The same specimens already underwent a PSO at L4 stabilized by two rods, a supplemental central rod (three rods) and accessory rods (four rods) with and without adjacent interbody cages (La Barbera in Eur Spine J 27(9):2357-2366, 2018). In vitro flexibility tests were performed under pure moments in flexion/extension (FE), lateral bending (LB) and axial rotation (AR) to determine the range of motion (RoM), while measuring the rod strains with strain gauge rosettes.

RESULTS

Local lordosis correction with ACR (24.7° ± 3.7°) and PSO (25.1° ± 3.9°) was similar. Bilateral fixation significantly reduced the RoM (FE: 31%, LB: 2%, AR: 18%), providing a stability consistent with PSO constructs (p > 0.05); however, it demonstrates significantly higher rod strains compared to PSO constructs with lateral accessory rods and interbody cages in FE and AR (p < 0.05), while being comparable in FE or slightly higher in AR compared to PSO constructs with two and three rods.

CONCLUSION

Bilateral posterior fixation is highly recommended following ACR to provide adequate primary stability. However, primary rod strains in ACR were found comparable or higher than weak PSO construct associated with frequent rod failure; therefore, caution is recommended. These slides can be retrieved under Electronic Supplementary Material.

The rate of fusion for stand-alone anterior lumbar interbody fusion: a systematic review

BACKGROUND

Anterior lumbar interbody fusion (ALIF) has been used for treatment of a variety of spinal conditions including degenerative disc disorders and low-grade spondylolisthesis. Expected fusion rate of stand-alone ALIF constructs is currently unclear. The aim of this study was to examine the fusion rate for ALIF without supplemental posterior fusion or instrumentation (stand-alone ALIF).

METHODS

We queried the MEDLINE, COCHRANE, and EMBASE databases for all literature related to spine fusion rates using a stand-alone ALIF procedure with a publication cutoff date of July 19, 2018. Supplementary combinations of search terms included spine, fusion, fixation, rate(s), and arthrodesis. ALIF surgery was considered stand-alone when not paired with supplemental posterior fusion or posterior spinal instrumentation. Nonhuman and non-English publications were excluded. Cohort fusion rate differences were calculated using Student t test with significance assigned if p value was less than .05.

RESULTS

Title and abstract level review required assessing 840 unique publications. Across the 55 studies that met the inclusion criteria of this systematic review, 5,517 patients and 6,303 vertebral levels were fused. The overall weighted average patient fusion rate following stand-alone ALIF was 88.2% (range: 16.6%-100%). In the 31 studies with at least 50 subjects, the weighted average fusion rate following stand-alone ALIF was 88.6% (range: 57.5%-99.0%). Use of anterior fixation plate devices yielded a fusion rate of 94.2%. Newer zero-profile interbody implants had a fusion rate of 89.2%. Fusion rates were lower in studies with 50% or more subjects having positive smoking and worker's compensation status, however these results were found to be statistically insignificant (p>.05). Fusion rate for subjects in the eight rhBMP-2 study groups was 94.4% (n=889) compared with 84.8% (n=3,102) in 38 study groups without rhBMP-2 used.

CONCLUSIONS

Based on the available data, stand-alone ALIF procedures yield high fusion rates overall. Fusion failure and pseudoarthrosis rates are higher in study populations involving a high percentage of smokers or positive workers compensation status. Allograft utilization does not significantly improve fusion rate when compared with autograft in stand-alone ALIF constructs.

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.

Stand-alone anterior interbody fusion for substitution of iliac fixation in long spinal fixation constructs

INTRODUCTION

The use of distal sacral anchorage solely, in long spinal fusions, may lead to substantial complications. Extending the fixation down to the ilium and the addition of anterior column support are both used to facilitate construct stability and improve fusion rates. In the current study, we aimed to determine whether supplementation of long thoracolumbar fixation constructs with stand-alone anterior interbody fusion (ALIF) cage with embedded screws can eliminate the biomechanical need for iliac screws fixation biomechanically.

METHODS

Seven lumbopelvic human cadavers (L1-full pelvis) were used. All specimens were tested with the following fixation constructs: bilateral L1-S1, bilateral L1-S1 with unilateral iliac screw, and bilateral L1-S1 with bilateral iliac screw. The three constructs were tested with and without the addition of stand-alone ALIF cage. We evaluated the multidirectional rigidity and the axial S1 screw strain.

RESULTS

The addition of an ALIF cage solely did not affect rigidity and resulted in mixed S1 screw strain results. One iliac screw was superior to ALIF in rigidity and inferior in S1 screws strain. Bilateral iliac fixation produced similar rigidity and lower S1 screws strain than unilateral iliac fixation. When ALIF was combined with bilateral iliac screws, it resulted in equal rigidity and lower S1 screws strain.

CONCLUSION

Our results do not support stand-alone ALIF cage as a substitute for iliac fixation in in long posterior lumbosacral fusion. They do support the use of stand-alone ALIF for the supplementation of bilateral iliac fixation in long lumbosacral fusions.

Biomechanical Analysis of Lateral Lumbar Interbody Fusion Constructs with Various Fixation Options: Based on a Validated Finite Element Model

BACKGROUND

Lateral lumbar interbody fusion using cage supplemented with fixation has been used widely in the treatment of lumbar disease. A combined fixation (CF) of lateral plate and spinous process plate may provide multiplanar stability similar to that of bilateral pedicle screws (BPS) and may reduce morbidity. The biomechanical influence of the CF on cage subsidence and facet joint stress has not been well described. The aim of this study was to compare biomechanics of various fixation options and to verify biomechanical effects of the CF.

METHODS

The surgical finite element models with various fixation options were constructed based on computed tomography images. The lateral plate and posterior spinous process plate were applied (CF). The 6 motion modes were simulated. Range of motion (ROM), cage stress, endplate stress, and facet joint stress were compared.

RESULTS

For the CF model, ROM, cage stress, and endplate stress were the minimum in almost all motion modes. Compared with BPS, the CF reduced ROM, cage stress, and endplate stress in all motion modes. The ROM was reduced by more than 10% in all motion modes except for flexion; cage stress and endplate stress were reduced more than 10% in all motion modes except for rotation-left. After interbody fusion, facet joint stress was reduced substantially compared with the intact conditions in all motion modes except for flexion.

CONCLUSIONS

The combined plate fixation may offer an alternative to BPS fixation in lateral lumbar interbody fusion.

Impact of bone quality on the performance of integrated fixation cage screws

BACKGROUND CONTEXT

Commercially available lumbar integrated fixation cages (IFCs) have variable designs. For example, screw-based designs have up to four screws inserted at different locations across the vertebral end plate as well as at different angles in the sagittal and transverse planes. This is important as end plate and trabecular bone quality may vary across the vertebra and may affect the screw's fixation ability, particularly if bone purchase at the bone-screw interface is poor.

PURPOSE

This study aimed to evaluate whether variations in local bone quality surrounding IFC screws inserted at different locations in the vertebrae would affect their mechanical performance.

STUDY DESIGN

This study is an in vitro human cadaveric biomechanical analysis.

MATERIALS AND METHODS

Fourteen lumbar (L3 and L4) vertebrae from 10 cadavers (age: 76±10 years, bone mineral density: 0.89±0.17 g/cm²) were used for this study. Pilot holes (3.5-mm diameter×15-mm length) representing three different IFC screw orientations (lateral to medial [LM], midsagittal [MS], and medial to lateral [ML]) were created in vertebrae using an IFC guide and bone awl. The screw locations and trajectories chosen are representative of commercially available IFC designs. These pilot holes were then imaged with high-resolution microcomputed tomography to obtain a three-dimensional structure of the bone surrounding the pilot hole. Local bone morphology was then quantified by evaluating a 3-mm-thick circumferential volume surrounding the pilot hole. Integrated fixation screws were implanted into pilot holes while recording maximum screw insertional torques. Screws were toggled in the cranial direction from 10 to 50 N for first 10,000 cycles, and the maximum load was increased by 25 N for every 5,000 cycles for a total of 25,000 cycles.

RESULTS

Total bone volume (BV) and trabecular bone volume fraction surrounding ML screws were significantly greater (p<.03) compared with those around MS screws and LM screws. The maximum insertional torque for ML screws were greater (p=.06) than LM and significantly greater (p<.02) than MS screws. The number of cycles to failure for the ML screw was significantly greater (p<.04) than that for the LM and the MS screws. Total BV (R²≤46.2%, p<.03) and the maximum insertional torque (R²≤59.6%, p<.03) provided better correlations to screw loosening compared with all the other bone quality parameters.

CONCLUSIONS

Our findings indicate that bone quality in the vertebral body varied spatially depending on the orientation and the insertion location of the IFC screw. These alterations in local bone quality significantly affected the screw's ability to fixate to bone. These variations in bone quality may be assessed intraoperatively using screw insertional torque measurements. By understanding available bone purchase at the bone-implant interface, the appropriate implant design can be selected to maximize the fixation strength.

Previously unreported complications associated with integrated cage screws following anterior lumbar interbody fusion: report of 2 cases

Anterior lumbar interbody fusion (ALIF) is a widely performed surgical treatment for various lumbar spine pathologies. The authors present the first reports of virtually identical cases of complications with integrated screws in stand-alone interbody cages. Two patients presented with the onset of S-1 radiculopathy due to screw misplacements following an ALIF procedure. In both cases, an integrated screw from the cage penetrated the dorsal aspect of the S-1 cortical margin of the vertebra, extended into the neural foramen, and injured the traversing left S-1 nerve roots. Advanced neuroimaging findings indicated nerve root impingement by the protruding screw tip. After substantial delays, radiculopathic symptoms were treated with removal of the offending instrumentation, aggressive posterior decompression of the bony and ligamentous structures, and posterolateral fusion surgery with pedicle screw fixation. Postoperative radiographic findings demonstrated decompression of the symptomatic nerve roots via removal of the extruded screw tips from the neural foramina.

Evaluation of Two Novel Integrated Stand-Alone Spacer Designs Compared with Anterior and Anterior-Posterior Single-Level Lumbar Fusion Techniques: An In Vitro Biomechanical Investigation

Study Design

In vitro biomechanical investigation.

Purpose

To compare the biomechanics of integrated three-screw and four-screw anterior interbody spacer devices and traditional techniques for treatment of degenerative disc disease.

Overview of Literature

Biomechanical literature describes investigations of operative techniques and integrated devices with four dual-stacked, diverging interbody screws; four alternating, converging screws through a polyether-ether-ketone (PEEK) spacer; and four converging screws threaded within the PEEK spacer. Conflicting reports on the stability of stand-alone devices and the influence of device design on biomechanics warrant investigation.

Methods

Fourteen cadaveric lumbar spines were divided randomly into two equal groups (n=7). Each spine was tested intact, after discectomy (injured), and with PEEK interbody spacer alone (S), anterior lumbar plate and spacer (AP+S), bilateral pedicle screws and spacer (BPS+S), circumferential fixation with spacer and anterior lumbar plate supplemented with BPS, and three-screw (SA3s) or four-screw (SA4s) integrated spacers. Constructs were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Researchers performed one-way analysis of variance and independent t-testing (p≤0.05).

Results

Instrumented constructs showed significantly decreased motion compared with intact except the spacer-alone construct in FE and AR (p≤0.05). SA3s showed significantly decreased range of motion (ROM) compared with AP+S in LB (p≤0.05) and comparable ROM in FE and AR. The three-screw design increased stability in FE and LB with no significant differences between integrated spacers or between integrated spacers and BPS+S in all loading modes.

Conclusions

Integrated spacers provided fixation statistically equivalent to traditional techniques. Comparison of three-screw and four-screw integrated anterior lumbar interbody fusion spacers revealed no significant differences, but the longer, larger-diameter interbody spacer with three-screw design increased stabilization in FE and LB; the diverging four-screw design showed marginal improvement during AR.

Biomechanical Analysis of Porous Additive Manufactured Cages for Lateral Lumbar Interbody Fusion: A Finite Element Analysis

BACKGROUND

A porous additive manufactured (AM) cage may provide stability similar to that of traditional solid cages and may be beneficial to bone ingrowth. The biomechanical influence of various porous cages on stability, subsidence, stresses in cage, and facet contact force has not been fully described. The purpose of this study was to verify biomechanical effects of porous AM cages.

METHODS

The surgical finite element models with various cages were constructed. The partially porous titanium (PPT) cages and fully porous titanium (FPT) cages were applied. The mechanical parameters of porous materials were obtained by mechanical test. Then the porous AM cages were compared with solid titanium (TI) cage and solid polyetheretherketone (PEEK) cage. The 4 motion modes were simulated. Range of motion (ROM), cage stress, end plate stress, and facet joint force (FJF) were compared.

RESULTS

For all the surgical models, ROM decreased by >90%. Compared with TI and PPT cages, PEEK and FPT cages substantially reduced the maximum stresses in cage and end plate in all motion modes. Compared with PEEK cages, the stresses in cage and end plate for FPT cages decreased, whereas the ROM increased. Comparing FPT cages, the stresses in cage and end plate decreased with increasing porosity, whereas ROM increased with increasing porosity. After interbody fusion, FJF was substantially reduced in all motion modes except for flexion.

CONCLUSIONS

Fully porous cages may offer an alternative to solid PEEK cages in lateral lumbar interbody fusion. However, it may be prudent to further increase the porosity of the cage.

Transforaminal Lumbar Interbody Fusion Versus Mini-open Anterior Lumbar Interbody Fusion With Oblique Self-anchored Stand-alone Cages for the Treatment of Lumbar Disc Herniation: A Retrospective Study With 2-year Follow-up

STUDY DESIGN

A retrospective study.

OBJECTIVE

The aim of this study was to evaluate the clinical and radiological outcomes of mini-open ALIF (MO-ALIF) with self-anchored stand-alone cages for the treatment of lumbar disc herniation in comparison with transforaminal lumbar interbody fusion (TLIF).

SUMMARY OF BACKGROUND DATA

Currently, whether ALIF is superior to TLIF for the treatment of lumbar disc herniation remains controversial.

METHODS

This study retrospectively reviewed 82 patients who underwent MO-ALIF with self-anchored standalone cages (n = 42) or TLIF (n = 40) for the treatment of lumbar disc herniation between April 2013 and October 2014. Patient demographics, intraoperative parameters, and perioperative complications were collated. Clinical outcomes were evaluated using the visual analog scale (VAS) scoring, the Oswestry Disability Index (ODI) for pain in the leg and back, and radiological outcomes, including fusion, lumbar lordosis (LL), disc height (DH), and cage subsidence were evaluated at each follow-up for up to 2 years.

RESULTS

Patients who underwent TLIF had a significantly higher volume of blood loss (295.2 ± 81.4 vs. 57.0 ± 15.2 mL) and longer surgery time (130.7 ± 45.1 vs. 60.4 ± 20.8 min) than those who had MO-ALIF. Compared with baseline, both groups had significant improvements in the VAS and ODI scores and DH and LL postoperatively, though no significant difference was found between the two groups regarding these indexes. All patients reached solid fusion at the final follow-up in both groups. Three patients (3/42) with three levels (3/50) suffered from cage subsidence in the MO-ALIF group; meanwhile, no cage subsidence occurred in the TLIF group.

CONCLUSION

MO-ALIF with self-anchored stand-alone cages is a safe and effective treatment of lumbar disc herniation with less surgical trauma and similar clinical and radiological outcomes compared with TLIF.

LEVEL OF EVIDENCE

3.

Integrated Fixation Cage Loosening Under Fatigue Loading

BACKGROUND

Screw loosening is a well-known adverse event in traditional spinal fusion instrumentation. This phenomenon may hinder segmental stability of the spine leading to bony non-union. In recent years numerous lumbar integrated fixation cages (IFC) have been introduced that offer a low profile alternative to a standard cage with an anterior plate (AP+C). The fixation approach for IFCs is different than a traditional anterior approach; therefore, it is unclear whether IFCs may loosen from the surrounding bone over time. The purpose of this study was to quantify screw loosening of IFC devices compared to AP+C implants under fatigue loading using micro-CT and image processing techniques.

METHODS

L2-3 and L4-5 functional spinal units (FSUs) were obtained from nine human lumbar spines. These FSUs were then reconstructed with either AP+C or IFC implants designed to attach to vertebral bodies using four screws (two top and two bottom for AP+C; two medial and two lateral for IFC). The reconstructed specimens were fatigued in flexion-extension load of ±3 Nm at 1Hz for first 5,000 cycles and it was increased to ±5 Nm until 20,000 cycles. After removing screws to prevent image artifact, micro-CT scans were performed on all FSUs post-fatigue. These images were post-processed to calculate three-dimensional volumes around screw holes created due to damage at the screw-implant interface.

RESULTS

IFC screws had significantly greater (p=0.008) screw hole volumes compared to AP+C screws after fatigue testing. This increased screw hole volume for IFC devices was mainly due to loosening in medial screws. Medial screws had significantly greater (p<0.003) screw hole volumes compared to lateral IFC screws and all AP+C screws. There was no difference (p>0.888) between the screw hole volumes of lateral IFC, top AP+C, and bottom AP+C screws.

CONCLUSIONS

This study elucidated screw-loosening mechanisms in integrated fixation cages under simulated physiological loading. In particular, spatial differences in fixation was observed for IFC screws across the vertebra where medial screws loosened at a greater frequency compared to lateral screws post-fatigue. This novel technique may also be used to quantitatively investigate screw fixation post-fatigue testing in a variety of spinal devices.

Biomechanical evaluation of an integrated fixation cage during fatigue loading: a human cadaver study

OBJECTIVE

Lumbar cages with integrated fixation screws offer a low-profile alternative to a standard cage with anterior supplemental fixation. However, the mechanical stability of integrated fixation cages (IFCs) compared with a cage with anterior plate fixation under fatigue loading has not been investigated. The purpose of this study was to compare the biomechanical stability of a screw-based IFC with a standard cage coupled with that of an anterior plate under fatigue loading.

METHODS

Eighteen functional spinal units were implanted with either a 4-screw IFC or an anterior plate and cage (AP+C) without integrated fixation. Flexibility testing was conducted in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) on intact spines, immediately after device implantation, and post-fatigue up to 20,000 cycles of FE loading. Stability parameters such as range of motion (ROM) and lax zone (LZ) for each loading mode were compared between the 2 constructs at multiple stages of testing. In addition, construct loosening was quantified by subtracting post-instrumentation ROM from post-fatigue ROM.

RESULTS

IFC and AP+C configurations exhibited similar stability (ROM and LZ) at every stage of testing in FE (p ≥ 0.33) and LB (p ≥ 0.23) motions. In AR, however, IFCs had decreased ROM compared with AP+C constructs at pre-fatigue (p = 0.07) and at all post-fatigue time points (p ≤ 0.05). LZ followed a trend similar to that of ROM in AR. ROM increased toward intact motion during fatigue cycling for AP+C and IFC implants. IFC specimens remained significantly (p < 0.01) more rigid than specimens in the intact condition during fatigue for each loading mode, whereas AP+C construct motion did not differ significantly (p ≥ 0.37) in FE and LB and was significantly greater (p < 0.01) in AR motion compared with intact specimens after fatigue. Weak to moderate correlations (R2 ≤ 56%) were observed between T-scores and construct loosening, with lower T-scores leading to decreased stability after fatigue testing.

CONCLUSIONS

These data indicate that a 4-screw IFC design provides fixation similar to that provided by an AP+C construct in FE and LB during fatigue testing and better stability in AR motion.

Anterior Lumbar Interbody Fusion Integrated Screw Cages: Intrinsic Load Generation, Subsidence, and Torsional Stability

OBJECTIVE

To perform a repeatable idealized in vitro model to evaluate the effects of key design features and integrated screw fixation on unloaded surface engagement, subsidence, and torsional stability.

METHODS

We evaluated four different stand-alone anterior lumbar interbody fusion (ALIF) cages with two, three, and four screw designs. Polyurethane (saw-bone) foam blocks were used to simulate the vertebral bone. Fuji Film was used to measure the contact footprint, average pressure, and load generated by fixating the cages with screws. Subsidence was tested by axially loading the constructs at 10 N/s to 400 N and torsional load was applied +/-1 Nm for 10 cycles to assess stability. Outcome measures included total subsidence and maximal torsional angle range.

RESULTS

Cages 1, 2, and 4 were symmetrical and produced similar results in terms of contact footprint, average pressure, and load. The addition of integrated screws into the cage-bone block construct demonstrated a clear trend towards decreased subsidence. Cage 2 with surface titanium angled ridges and a keel produced the greatest subsidence with and without screws, significantly more than all other cages ( P < 0.05). Angular rotation was not significantly affected by the addition of screws ( P < 0.066). A statistically significant correlation existed between subsidence and reduced angular rotation across all cage constructs ( P = 0.018).

CONCLUSION

Each stand-alone cage featured unique surface characteristics, which resulted in differing cage-foam interface engagement, influencing the subsidence and torsional angle. Increased subsidence significantly reduced the torsional angle across all cage constructs.

Stand-alone anterior lumbar interbody fusion: indications, techniques, surgical outcomes and complications

INTRODUCTION

Anterior lumbar interbody fusion (ALIF) is a well-established technique to achieve lumbar spine fusion with various indications including degenerative disk disease, spondylolisthesis, recurrent disk herniation, adjacent level disease, pseudoarthrosis, as well as being used as part of the overall strategy to restore sagittal balance. ALIF can be an extremely useful tool in any spine surgeon's armamentarium. However, like any surgical procedure, proper patient selection is key to success. A solid understanding of the biomechanics, careful surgical planning, along with clear knowledge of the advantages and disadvantages of stand-alone ALIF will ensure optimal clinical outcome. Stand-alone ALIF may be a suitable surgical option in carefully selected patients that can provide good clinical results and adequate fusion rates without the need for posterior instrumentation. Areas covered: A brief overview of the indications, techniques, biomechanics, surgical outcome and complications of stand-alone ALIF is provided in this article with a review of the pertinent literature. Expert commentary: In this review we discuss the clinical evidence of using a stand-alone ALIF compared to other fusion techniques of the lumbar spine. The development of interbody cages with integrated screws has increased the arthrodesis rate and improved clinical outcomes while decreasing morbidity and operative time.

Applying the Mini-Open Anterolateral Lumbar Interbody Fusion with Self-Anchored Stand-Alone Polyetheretherketone Cage in Lumbar Revision Surgery

The author retrospectively studied twenty-two patients who underwent revision lumbar surgeries using ALLIF with a self-anchored stand-alone polyetheretherketone (PEEK) cage. The operation time, blood loss, and perioperative complications were evaluated. Oswestry disability index (ODI) scores and visual analog scale (VAS) scores of leg and back pain were analyzed preoperatively and at each time point of postoperative follow-up. Radiological evaluation including fusion, disc height, foraminal height, and subsidence was assessed. The results showed that the ALLIF with a self-anchored stand-alone PEEK cage is safe and effective in revision lumbar surgery with minor surgical trauma, low access-related complication rates, and satisfactory clinical and radiological results.

Choice of Approach Does Not Affect Clinical and Radiologic Outcomes: A Comparative Cohort of Patients Having Anterior Lumbar Interbody Fusion and Patients Having Lateral Lumbar Interbody Fusion at 24 Months

STUDY DESIGN

Retrospective analysis of prospectively collected registry data.

OBJECTIVE

This study aimed to compare the clinical and radiologic outcomes between comparative cohorts of patients having anterior lumbar interbody fusion (ALIF) and patients having lateral lumbar interbody fusion (LLIF).

METHODS

Ninety consecutive patients were treated by a single surgeon with either ALIF (n = 50) or LLIF (n = 40). Inclusion criteria were patients age 45 to 70 years with degenerative disk disease or grade 1 to 2 spondylolisthesis and single-level pathology from L1 to S1. Patient-reported outcome measures included pain (visual analog scale), disability (Oswestry Disability Index [ODI]), and quality of life (Short Form 36 physical component score [PCS] and mental component scores [MCS]). Assessment of fusion and measurement of lordosis and posterior disk height were performed on computed tomography scans.

RESULTS

At 24 months, patients having ALIF had significant improvements in back (64%) and leg (65%) pain and ODI (60%), PCS (44%), and MCS (26%; p < 0.05) scores. Patients having LLIF had significant improvements in back (56%) and leg (57%) pain and ODI (52%), PCS (48%), and MCS (12%; p < 0.05) scores. Fourteen complications occurred in the ALIF group, and in the LLIF group, there were 17 complications (p > 0.05). The fusion rate was 100% for ALIF and 95% for LLIF (p = 0.1948). ALIF added ∼6 degrees of lordosis and 3 mm of height, primarily measured at L5-S1, and LLIF added ∼3 degrees of lordosis and 2 mm of height between L1 to L5. Mean follow-up was 34.1 months.

CONCLUSIONS

In comparative cohorts of patients having ALIF and patients having LLIF at 24 months postoperatively, there were no significant differences in clinical outcomes, complication rates, or fusion rates.

Impact of screw location and endplate preparation on pullout strength for anterior plates and integrated fixation cages

BACKGROUND CONTEXT

Numerous integrated fixation cages (IFCs) have recently been introduced to the market with "zero-profile" designs that incorporate screw fixation through the vertebral endplate. It is unclear whether differences in bone quality and quantity in this insertion location may affect fixation compared with screws used in traditional anterior plate (AP) fixation. Moreover, endplate preparation for IFC implantation may affect fixation strength.

PURPOSE

This study aimed to compare pullout strength of screws used in IFCs with screws used for AP implantations.

STUDY DESIGN

A biomechanical cadaveric study.

METHODS

T12 and L1 vertebrae from 13 human cadaver spines were prepared for pullout testing. End plates in T12 vertebrae were scraped according to surgical practice for fusion procedures. Conversely, endplates in L1 vertebrae were kept intact (unscraped). Integrated fixation cage screws were implanted at a 45° angle into the endplate and AP screws were implanted horizontally into the same vertebral body. Pullout testing was performed on all screws, and peak pullout force (PPF) and work were compared between groups to determine fixation strength. In addition, micro-CT imaging was used to assess bone quantity and quality parameters such as trabecular bone volume fraction, endplate and anterior cortex thickness at screw insertion location, endplate mineralization, and anterior cortex mineralization.

RESULTS

Peak pullout force for IFC screws (176±68 N) with scraped endplates was similar (p=.26) to AP screws (192±84 N). However, PPF for IFC screws (231±75 N) with unscraped endplates was significantly greater (p<.01) than AP screws (176±50 N). Peak pullout force for IFC screws with scraped endplates was significantly lower (p=.03) than IFC screws with unscraped endplates. Scraped endplates group (0.17±0.05 mm) were thinner (p=.05) than unscraped endplates (0.21±0.06 mm) by approximately 40 µ on average.

CONCLUSIONS

These data indicate that IFC and AP screws exhibited similar fixation behavior when the endplate is prepared according to common surgical practices. However, endplate scraping reduces endplate thickness by 20% on average, resulting in a decrease in fixation strength when compared with the unscraped endplates and provides bounds for IFC screw fixation strength.

Anterior lumbar interbody fusion with integrated fixation and adjunctive posterior stabilization: A comparative biomechanical analysis

BACKGROUND

Interbody fusion cages with integrated fixation components have become of interest due to their ability to provide enhanced post-operative stability and mitigate device migration. A recently approved anterior lumbar interbody fusion cage with integrated fixation anchors has yet to be compared in vitro to a standard polyetheretherketone cage when used in combination with an interspinous process clamp.

METHODS

Twelve human cadaveric lumbar segments were implanted at L4-L5 with a Solus interbody cage (n=6) or standard polyetheretherketone cage (n=6) following Intact testing and discectomy. Each cage was subsequently evaluated in all primary modes of loading after supplementation with the following posterior constructs: interspinous process clamp, bilateral transfacet screws, unilateral transfacet screw with contralateral pedicle screws, and bilateral pedicle screws. Range of motion results were normalized to Intact, and a two-way mixed analysis of variance was utilized to detect statistical differences.

FINDINGS

The Solus cage in combination with all posterior constructs provided significant fixation compared to Intact in all loading conditions. The polyetheretherketone cage also provided significant fixation when combined with all screw based treatments, however when used with the interspinous process clamp a significant reduction was not observed in lateral bending or axial torsion.

INTERPRETATION

Interbody cages with integrated fixation components enhance post-operative stability within the intervertebral space, thus affording clinicians the potential to utilize less invasive methods of posterior stabilization when seeking circumferential fusion. Interspinous process clamps, in particular, may reduce peri-operative and post-operative comorbidities compared to screw based constructs. Further study is necessary to corroborate their effectiveness in vivo.

Biomechanical characteristics of an integrated lumbar interbody fusion device

Introduction

We hypothesized that an Integrated Lumbar Interbody Fusion Device (PILLAR SA, Orthofix, Lewisville, TX) will function biomechanically similar to a traditional anterior interbody spacer (PILLAR AL, Orthofix, Lewisville, TX) plus posterior instrumentation (FIREBIRD, Orthofix, Lewisville, TX). Purpose of this study was to determine if an Integrated Interbody Fusion Device (PILLAR SA) can stabilize single motion segments as well as an anterior interbody spacer (PILLAR AL) + pedicle screw construct (FIREBIRD).

Methods

Eight cadaveric lumbar spines (age: 43.9±4.3 years) were used. Each specimen's range of motion was tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) under intact condition, after L4-L5 PILLAR SA with intervertebral screws and after L4-L5 360° fusion (PILLAR AL + Pedicle Screws and rods (FIREBIRD). Each specimen was tested in flexion (8Nm) and extension (6Nm) without preload (0 N) and under 400N of preload, in lateral bending (±6 Nm) and axial rotation (±5 Nm) without preload.

Results

Integrated fusion using the PILLAR SA device demonstrated statistically significant reductions in range of motion of the L4-L5 motion segment as compared to the intact condition for each test direction. PILLAR SA reduced ROM from 8.9±1.9 to 2.9±1.1° in FE with 400N follower preload (67.4%), 8.0±1.7 to 2.5±1.1° in LB, and 2.2±1.2 to 0.7±0.3° in AR. A comparison between the PILLAR SA integrated fusion device versus 360° fusion construct with spacer and bilateral pedicle screws was statistically significant in FE and LB. The 360° fusion yielded motion of 1.0±0.5° in FE, 1.0±0.8° in LB (p0.05).

Conclusions

The PILLAR SA resulted in motions of less than 3° in all modes of motion and was not as motion restricting as the traditional 360° using bilateral pedicle screws. The residual segmental motions compare very favorably with published biomechanical studies of other interbody integrated fusion devices.

Primary stiffness of a modified transforaminal lumbar interbody fusion cage with integrated screw fixation: cadaveric biomechanical study

STUDY DESIGN

In vitro biomechanical study using human fresh-frozen vertebrae.

OBJECTIVE

To investigate the influence of the additional screw fixation on the stability of a noncommercially available prototype transforaminal lumbar interbody fusion (TLIF) cage, when used as a stand-alone fusion device and in combination with pedicle screws (PSs).

SUMMARY OF BACKGROUND DATA

Generally interbody fusion cages are supplemented by additional fixation devices such as PS. However, such posterior instrumented techniques are associated with additional soft-tissue trauma and potentially increased complication rate. To limit such drawbacks, a conventional posterior TLIF cage was modified to allow supplemental screw fixation to the adjacent vertebral bodies, to increase initial stiffness and possibly allow as a stand-alone posterior interbody cage.

METHODS

Six monosegmental lumbar spine segments were loaded in a spine simulator with pure bending moments of 7.5 Nm in lateral bending, flexion/extension, and axial rotation. The following paradigms were tested: intact spines; a destabilized spine (i.e., after discectomy and unilateral facetectomy); and the modified TLIF cage with (i.e., fixed TLIF cage) and without (i.e., TLIF cage) integrated screw fixation as a stand-alone model and with and without additional posterior fixation with bilateral PS. The range of motion (RoM) was recorded by a 3-dimensional motion analysis system.

RESULTS

The TLIF cage with integrated screw fixation had minimal additional stabilizing effect in all motion planes with or without supplemental PS fixation. Moreover, compared with the intact spines, the stand-alone TLIF cage with and without integrated screw fixation did not reduce the RoM in any of the 3 motion planes. Comparison of the TLIF cage with integrated screw fixation to the TLIF cage supplemented with PS showed a significantly greater RoM in all testing conditions (P < 0.05).

CONCLUSION

In several testing paradigms, the prototype TLIF cage with the integrated screw fixation had limited effect in reducing RoM and providing stability. The PS was the main contributor in reducing RoM in the destabilized spine and remains the current "gold standard" in posterolateral spinal fixation.

LEVEL OF EVIDENCE

N/A.

Biomechanics of lateral lumbar interbody fusion constructs with lateral and posterior plate fixation: laboratory investigation

OBJECT

Lumbar interbody fusion is indicated in the treatment of degenerative conditions. Laterally inserted interbody cages significantly decrease range of motion (ROM) compared with other cages. Supplemental fixation options such as lateral plates or spinous process plates have been shown to provide stability and to reduce morbidity. The authors of the current study investigate the in vitro stability of the interbody cage with a combination of lateral and spinous process plate fixation and compare this method to the established bilateral pedicle screw fixation technique.

METHODS

Ten L1-5 specimens were evaluated using multidirectional nondestructive moments (± 7.5 N · m), with a custom 6 degrees-of-freedom spine simulator. Intervertebral motions (ROM) were measured optoelectronically. Each spine was evaluated under the following conditions at the L3-4 level: intact; interbody cage alone (stand-alone); cage supplemented with lateral plate; cage supplemented with ipsilateral pedicle screws; cage supplemented with bilateral pedicle screws; cage supplemented with spinous process plate; and cage supplemented with a combination of lateral plate and spinous process plate. Intervertebral rotations were calculated, and ROM data were normalized to the intact ROM data.

RESULTS

The stand-alone laterally inserted interbody cage significantly reduced ROM with respect to the intact state in flexion-extension (31.6% intact ROM, p < 0.001), lateral bending (32.5%, p < 0.001), and axial rotation (69.4%, p = 0.002). Compared with the stand-alone condition, addition of a lateral plate to the interbody cage did not significantly alter the ROM in flexion-extension (p = 0.904); however, it was significantly decreased in lateral bending and axial rotation (p < 0.001). The cage supplemented with a lateral plate was not statistically different from bilateral pedicle screws in lateral bending (p = 0.579). Supplemental fixation using a spinous process plate was not significantly different from bilateral pedicle screws in flexion-extension (p = 0.476). The combination of lateral plate and spinous process plate was not statistically different from the cage supplemented with bilateral pedicle screws in all the loading modes (p ≥ 0.365).

CONCLUSIONS

A combination of lateral and spinous process plate fixation to supplement a laterally inserted interbody cage helps achieve rigidity in all motion planes similar to that achieved with bilateral pedicle screws.

Utility of postoperative radiographs after anterior lumbar interbody fusion with or without posterior instrumentation

STUDY DESIGN

Retrospective clinical and radiographical review.

OBJECTIVE

To evaluate the utility of plain radiographical surveillance after anterior lumbar interbody fusion and determine to what extent radiographical findings affect postoperative decision making.

SUMMARY OF BACKGROUND DATA

Postoperative radiographical surveillance is a ubiquitous practice among spine surgeons, which lacks evidence and has received growing attention in an environment of increasing health care cost and receding resources. Lumbar interbody fusions are being performed with increasing frequency from numerous approaches; nonetheless, there are no evidence-based guidelines for postoperative radiographical evaluation of patients after these procedures.

METHODS

One hundred forty-six consecutive patients who underwent anterior lumbar interbody fusion with or without short segment posterior fusion from 2008 to 2011 were reviewed. Exclusion criteria were less than 6 months of follow-up, prior surgery, hybrid constructs with disc arthroplasty, and concurrent posterior fusion of greater than 3 levels. Three hundred fifty-nine radiographical series and 330 clinic notes of the included 67 patients were reviewed. Radiographs were evaluated for abnormalities and clinic notes reviewed for any changes in clinical management by multiple reviewers. Interobserver reliability, sensitivity, specificity, and positive and negative predictive values were calculated.

RESULTS

There was no single instance of a change in treatment course based on radiographical findings alone in any of the 330 clinic visits of the 67 included patients during an average 15.8-month postoperative follow-up period. Thirty-four of the 67 patients (51%) had some change in their management on the basis of their clinical symptoms and/or examination. Interobserver agreement for change in management was 0.96 (κ = 0.918). Sensitivity (6%), specificity (97%), positive predictive value (67%), and negative predictive value (50%) were calculated.

CONCLUSION

Routine postoperative radiographical surveillance has minimal value for asymptomatic patients after anterior lumbar interbody fusion with or without posterior fusion. Obtaining a limited number of postoperative films of these patients in the absence of clinical symptoms or risk factors could significantly reduce health care costs and unnecessary radiation exposure.