Anterior Column Load Research Articles

Improving outcomes in adult spinal deformity surgery

Proximal junctional kyphosis and failure are important complications in reconstructive spine surgery. The impact of proximal junctional pathology on health status is variable, and junctional failure may involve compromise of neural and physical function, and may require revision surgery. The risk factors for proximal junctional kyphosis include multilevel fusion to the sacrum, malalignment of the spine, choice of upper instrumented vertebra, magnitude of realignment, bone and muscle quality, tension band integrity and characteristics of the unfused spine. The pathophysiology of proximal junctional pathology involve fracture and bony failure, ligamentous and soft tissue failure, and combined bone and soft tissue failure. Prevention of proximal junctional pathology is an important goal, and has a significant impact on improving the outcomes and durability of spinal reconstructive surgery. Prevention strategies are derived from an understanding of the underlying causes of junctional pathology. Preoperative planning to determine optimal post-operative alignment, and intraoperative strategies to achieve that alignment are important for prevention of junctional pathology. Preoperative preparation of the patient with optimization of bone quality, extensor muscle strength and body mass index may prevent junctional complications. Surgical strategies including anterior column load sharing and posterior augmentation of fixation are useful for prevention of junctional pathology. Cement augmentation at the upper instrumented vertebra, with possible inclusion of the next cephalad vertebra may be protective of bone failure. Ligamentoplasty and preservation of the tension band may be protective of soft tissue failure. Proximal junctional pathology remains an important complication in spinal reconstructive surgery. Understanding the factors associated with proximal junctional pathology including the mechanisms underlying junctional kyphosis, and awareness of the strategies for avoidance of junctional pathology will empower the surgeon to reduce the risk of post-operative junctional kyphosis and lead to improved outcomes in adult spinal deformity surgery.

173 Hydrogel Matrix Human Stem Cell Based Nucleus Pulposus Intervertebral Disc Regeneration

INTRODUCTION: Degenerative disc disease (DDD) is one of the major causes of chronic debilitating low back pain. DDD is characterized by changes in cell populations that result in loss of extracellular matrix (ECM) water holding capacity of the nucleus pulposus. DDD can lead to translation of physiologic loads from the anterior 2/3 to the posterior 1/3 of the spinal column resulting in stress placed on the facets and ligamentum flavum resulting in hypertrophy and canal stenosis. Novel stem cell regenerative therapies could restore the anterior column load via nucleus pulposus regeneration. METHODS: A hydrogel scaffold was created composed of self-assembling polyethylene glycol (PEG) functionalized with acrylate and thiol end groups. The scaffold was impregnated with nucleus pulposus (NP) cells differentiated from human umbilical cord mesenchymal stem cells (MSCs). Impregnated NP-scaffolds were then implanted into an ex vivo rabbit model of DDD and the viability and function of NP cells analyzed. RESULTS: The NP-scaffolds limited leakage and retained the cells in the NP region of the degenerated disc. Both the NP-scaffold and disc environment promoted differentiation of the MSCs into cells types capable of producing the ECM including sulfated glycoaminoglycans at levels higher than MSCs injected into the intervertebral disc. NP-scaffold cells also expressed chrondrogenic markers like SOX9, COL2 and ACAN at higher levels as well as NP markers FOXF1, K19, and Vimentin determined by real-time polymerase chain reaction (PCR) and immunostaining. CONCLUSION: This study demonstrated that a hydrogen scaffold impregnated with NP cells was much more effective at disc regeneration than NP cells alone and can lead to novel therapies for treating DDD.

External and internal responses of cervical disc arthroplasty and anterior cervical discectomy and fusion: A finite element modeling study

Surgical treatment for spinal disorders, such as cervical disc herniation and spondylosis, includes the removal of the intervertebral disc and replacement of biological or artificial materials. In the former case, bone graft is used to fill the space, and this conventional procedure is termed anterior cervical discectomy and fusion (ACDF). The latter surgery is termed as artificial disc replacement ADR) or cervical disc arthroplasty (CDA). Surgeries are most commonly performed at one or two levels. The present study was designed to determine the external (range of motion, ROM) and internal (anterior and posterior load sharing) responses of the spines with one-level and two-level surgeries in both models (ACDF and CDA) using a previously validated finite element model (FEM) of the subaxial cervical spinal column. The FEM simulated the vertebra (cancellous core and cortical shell of the body, posterior elements – laminae, pedicles and spinous processes), discs (anulus fibers, ground substance, and nucleus pulposus), anterior and posterior ligaments of the disc and facet joints, and interspinous and supraspinous ligaments. Appropriate material properties were assigned to the spinal components. The United States Food Drug Administration-approved Mobi-C was used for the CDA option. The FEM was exercised under pure flexion and extension moment loading of 2 Nm in the intact state. The overall ROM of the column was obtained. The hybrid loading protocol applied moments that matched the ROM in the intact spine for both one-level (C5–C6) and two-level (C5–C7) ACDF and CDA surgeries. ROM at the level(s) of surgery, termed the index level was obtained. These data along with anterior column load (ACL) and posterior column load (PCL) sharing were obtained for all surgical options at superior and inferior segments (termed adjacent segment outputs). Results for both one-level and two-level surgeries showed that ACDFs decreases ROM at the index level, while CDAs increase motions compared to the intact normal spine. The ROM, ACL, and PCL increased at both adjacent levels for the ACDF while CDA showed a decrease. Although two-level surgeries resulted in increased these biomechanical variables, greater changes to adjacent segment biomechanics in ACDF may accelerate adjacent segment disease. Decreased ROM and lower load sharing in CDAs may limit adjacent segment effects such as accelerated degeneration. Their increased posterior load sharing, however, may need additional attention for patients with suspected facet joint disease.

Biomechanics of Posterior Dynamic Fusion Systems in the Lumbar Spine

A comparative biomechanical human cadaveric spine study of a dynamic fusion rod and a traditional titanium rod. The purpose of this study was to measure and compare the biomechanical metrics associated with a dynamic fusion device, Isobar TTL Evolution, and a rigid rod. Dynamic fusion rods may enhance arthrodesis compared with a rigid rod. Wolff's law implies that bone remodeling and growth may be enhanced through anterior column loading (AL). This is important for dynamic fusion rods because their purpose is to increase AL. Six fresh-frozen lumbar cadaveric specimens were used. Each untreated specimen (Intact) underwent biomechanical testing. Next, each specimen had a unilateral transforaminal lumbar interbody fusion performed at L3-L4 using a cage with an integrated load cell. Pedicle screws were also placed at this time. Subsequently, the Isobar was implanted and tested, and finally, a rigid rod replaced the Isobar in the same pedicle screw arrangement. In terms of range of motion, the Isobar performed comparably to the rigid rod and there was no statistical difference found between Isobar and rigid rod. There was a significant difference between the intact and rigid rod and also between intact and Isobar conditions in flexion extension. For interpedicular displacement, there was a significant increase in flexion extension (P=0.017) for the Isobar compared with the rigid rod. Isobar showed increased AL under axial compression compared with the rigid rod (P=0.024). Isobar provided comparable stabilization to a rigid rod when using range of motion as the metric, however, AL was increased because of the greater interpedicular displacement of dynamic rod compared with a rigid rod. By increasing interpedicular displacement and AL, it potentially brings clinical benefit to procedures relying on arthrodesis.

Stability and Load Sharing Characteristics of a Posterior Dynamic Stabilization Device.

BackgroundLumbar interbody fusion is a common treatment for a variety of spinal pathologies. It has been hypothesized that insufficient mechanical loading of the interbody graft can prevent proper fusion of the joint. The purpose of this study was to evaluate the mechanical stability and anterior column loading sharing characteristics of a posterior dynamic system compared to titanium rods in an anterior lumbar interbody fusion (ALIF) model.MethodsRange of motion, interpedicular kinematics and interbody graft loading were measured in human cadaveric lumbar segments tested under a pure moment flexibility testing protocol.ResultsBoth systems provided significant fixation compared to the intact condition and to an interbody spacer alone in flexion extension and lateral bending. No significant differences in fixation were detected between the devices. A significant decrease in graft loading was detected in flexion for the titanium rod treatment compared to spacer alone. No significant differences in graft loading were detected between the spacer alone and posterior dynamic system or between the posterior dynamic system and the titanium rod.ConclusionsThe results of this study indicate that the posterior dynamic system provides similar fixation compared to that of a titanium rod, however, studies designed to evaluate the efficacy of fixation in a cadaver model may not be sufficiently powered to establish differences in load sharing using the techniques described here.

The vertebral biomechanic previous and after kyphoplasty

The biomechanical understanding of increasing anterior column load with progressing kyphosis leading to subsequent vertebral compression fracture (VCF) established the basic rationale for kyphoplasty. The lumbar spine can support an effort of 500 kg in the axis of the vertebral body, and a bending moment of 20 Nm in flexion. Consequently, if this effort is forward deviated of only 10 cm, the acceptable effort will be reduced to 20 kg so it is important to restore the vertebral anterior wall after a VCF: the authors describe the biomechanical modifications in the spine after kyphoplasty.

Finite Element Modeling of Kinematic and Load Transmission Alterations due to Cervical Intervertebral Disc Replacement

A parametric finite element investigation of the cervical spine. To determine what effect, if any, cervical disc replacement has on kinematics, facet contact parameters, and anterior column loading. Anterior cervical discectomy and fusion has been a standard treatment for certain spinal degenerative disorders, but evidence suggests that fusion contributes to adjacent-segment degeneration. Motion-sparing disc replacement implants are believed to reduce adjacent-segment degeneration by preserving kinematics at the treated level. Such implants have been shown to maintain the mobility of the intact spine, but the effects on load transfer between the anterior and posterior elements remain poorly understood. To investigate the effects of disc replacement on load transfer in the lower cervical spine, a finite element model was generated using cadaver-based computed tomography imagery. Mesh resolution was varied to establish model convergence, and cadaveric testing was undertaken to validate model predictions. The validated model was altered to include disc replacement prosthesis at the C4/C5 level. The effect of disc-replacement on range of motion, anteroposterior load distribution, contact forces in the facets, as well as the distribution of contact pressure on the facets were examined. Three sizes of implants were examined. Model predictions indicate that the properly sized implant retains the mobility, load sharing, and contact force magnitude and distribution of the intact case. Mobility, load sharing, nuclear pressures, and contact pressures at the adjacent motion segments were not strongly affected by the presence of the properly sized implant, indicating that disc replacement may not be a significant cause of postoperative adjacent-level degeneration. Implant size affected certain mechanical parameters, such as anteroposterior load sharing, and did not affect compliance or range of motion. The results of this work support the continued use of motion sparing implants in the lower cervical spine. Load sharing data indicate that implant size may be an important factor that merits further study; although, the deleterious effects of improper size selection may be less significant than those of fusion.

Biomechanical Assessment of a PEEK Rod System for Semi-Rigid Fixation of Lumbar Fusion Constructs

The concept of semi-rigid fixation (SRF) has driven the development of spinal implants that utilize nonmetallic materials and novel rod geometries in an effort to promote fusion via a balance of stability, intra- and inter-level load sharing, and durability. The purpose of this study was to characterize the mechanical and biomechanical properties of a pedicle screw-based polyetheretherketone (PEEK) SRF system for the lumbar spine to compare its kinematic, structural, and durability performance profile against that of traditional lumbar fusion systems. Performance of the SRF system was characterized using a validated spectrum of experimental, computational, and in vitro testing. Finite element models were first used to optimize the size and shape of the polymeric rods and bound their performance parameters. Subsequently, benchtop tests determined the static and dynamic performance threshold of PEEK rods in relevant loading modes (flexion-extension (F/E), axial rotation (AR), and lateral bending (LB)). Numerical analyses evaluated the amount of anteroposterior column load sharing provided by both metallic and PEEK rods. Finally, a cadaveric spine simulator was used to determine the level of stability that PEEK rods provide. Under physiological loading conditions, a 6.35 mm nominal diameter oval PEEK rod construct unloads the bone-screw interface and increases anterior column load (approx. 75% anterior, 25% posterior) when compared to titanium (Ti) rod constructs. The PEEK construct's stiffness demonstrated a value lower than that of all the metallic rod systems, regardless of diameter or metallic composition (78% < 5.5 mm Ti; 66% < 4.5 mm Ti; 38% < 3.6 mm Ti). The endurance limit of the PEEK construct was comparable to that of clinically successful metallic rod systems (135N at 5 × 10(6) cycles). Compared to the intact state, cadaveric spines implanted with PEEK constructs demonstrated a significant reduction of range of motion in all three loading directions (> 80% reduction in F/E, p < 0.001; > 70% reduction in LB, p < 0.001; > 54% reduction in AR, p < 0.001). There was no statistically significant difference in the stability provided by the PEEK rods and titanium rods in any mode (p = 0.769 for F/E; p = 0.085 for LB; p = 0.633 for AR). The CD HORIZON(®) LEGACY(™) PEEK Rod System provided intervertebral stability comparable to currently marketed titanium lumbar fusion constructs. PEEK rods also more closely approximated the physiologic anteroposterior column load sharing compared to results with titanium rods. The durability, stability, strength, and biomechanical profile of PEEK rods were demonstrated and the potential advantages of SRF were highlighted.

Total Spondylectomy of C2

A report on 3 patients undergoing total spondylectomy of the C2 vertebra for tumor and the technique for C1-3 reconstruction. To illustrate the feasibility of complete resection of the C2 vertebra with preservation of the vertebral arteries and cervical nerve roots. Total spondylectomy provides improved progression free survival in many patients with locally aggressive spinal tumors. However, the perceived technical demands of effectively preserving both vertebral arteries, maintaining cervical nerve roots, and biomechanical reconstruction of the cranial-cervical junction often dissuades surgeons from carrying out total spondylectomy of the C2 vertebra. A review of 3 patients undergoing total C2 spondylectomy for tumor (thyroid adenocarcinoma, chordoma, and solitary plasmocytoma) was done. The surgical procedure that was undertaken and the technique used are described. Postoperatively, all 3 patients had uneventful postoperative recovery with gradual improvement in their neurologic functions. Preservation of bilateral vertebral arteries and all cervical nerve roots is feasible when carrying out intralesional total spondylectomy in patients with C2 vertebral body tumors and should be considered in patients thought to benefit from total C2 vertebra excision. In an attempt to augment construct stability and provide anterior column load sharing, we have used mesh cage and iliac crest graft between C1 and C3 held in place with a short cervical plate without complications.

Biomechanical Comparison of a Novel Percutaneous Transfacet Device and a Traditional Posterior System for Single Level Fusion

Posterior spinal fusions are indicated for a variety of spinal disorders. Transfacet fixation minimizes soft tissue disruption and preserves the adjacent facet joint. This technique is uncommon due to concerns with biomechanical stability and proper implant placement. For these reasons, a length adjustable implant may obviate the clinical concerns but necessitates biomechanical study. This study evaluated the in vitro biomechanical stability between a novel transfacet fixation device compared with standard pedicle screws during cyclic physiologic loading in a human cadaveric model. Cadaveric L4-L5 lumbar motion segments from 16 human spines were tested in cyclic flexion/extension, lateral bending, and torsion after insertion of either transfacet fixation devices or 5.5 mm pedicle screw instrumentation. A load cell was used to measure the compressive forces on the anterior column during testing. Motion segment stiffness and anterior column compression were analyzed with a 1-way analysis of variance (P<0.05). The transfacet device demonstrated a statistically similar stiffness when compared with the pedicle screw system for each test direction. For anterior column loading during physiologic testing, there were no biomechanical differences between stabilization systems. Percutaneous transfacet fixation is an attractive surgical option for single-level spinal fusions. A biomechanical evaluation of a novel device for this application demonstrated similar stability to a pedicle screw system. The length adjustability of the device may alleviate concerns for precise device placement and the biomechanical stability may produce similar rates and quality of posterior spinal fusions.

Anterior transvertebral interbody cage with posterior transdiscal pedicle screw instrumentation for high-grade spondylolisthesis

Adult high-grade degenerative spondylolisthesis represents the extreme end of the spectrum for spondylolisthesis and is consequently rarely encountered. Surgical management of high-grade spondylolisthesis requires constructs capable of resisting the shear forces at the slipped L5-S1 interspace. The severity of the slip, sacral inclination, and slip angle may make conventional approaches to 360 degrees fusion difficult or hazardous. Transdiscal pedicle screw fixation, transvertebral fibular graft fusion, and transvertebral cage fixation are techniques that have been developed to establish anterior column load sharing and to resist shear forces at the L5-S1 interspace, given the anatomical constraints accompanying high-grade spondylolisthesis. In this technical note the authors describe the procedure for implanting an in situ anterior L5-S1 transvertebral cage and performing L4-5 anterior lumbar interbody fusion, followed by placement of posterior S1-L5 vertebral body transdiscal pedicle screws for management of high-grade spondylolisthesis.

Letter to the Editor

In their paper “The effect on anterior column loading due to different vertebral augmentation techniques”, Ananthakrishnan et al. ( Ananthakrishnan et al., 2005 Ananthakrishnan D. Berven S. Deviren V. Cheng K. Lotz J.C. Xu Z. Puttlitz C.M. The effect on anterior column loading due to different vertebral augmentation techniques. Clinical Biomechanics. 2005; 20: 25-31 Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar ) experimentally investigated the effects of vertebroplasty and kyphoplasty. Cadaveric three-vertebrae models were fractured and subsequently augmented with cement. Non-destructive loads were applied and the pressure in the nucleus pulposus measured. This pressure was substantially reduced after fracturing of the spine. After treatment a slightly increased pressure was reported, but it was not restored to the intact condition.

The effect on anterior column loading due to different vertebral augmentation techniques

Background. Vertebral compression fractures are a significant cause of morbidity amongst the elderly, resulting in pain, kyphosis, decreased mobility, and often depression. The two most popular operative treatments of compression wedge fractures are vertebroplasty and kyphoplasty, however, which method leads to restoration of normal spinal loading is unknown. Methods. A cadaveric thoracolumbar spine model was fractured and then treated using one of two techniques: vertebroplasty or kyphoplasty. Three non-destructive loading regimes were applied to the intact, fractured and treated cases. Sub-adjacent nuclear pressure, used as a measure of anterior column loading, was determined for each experimental variant and loading case. Findings. The data indicate that nuclear pressure was substantially reduced after the spine was fractured. These pressures were minimally increased after treatment, however, not to the level of the pre-fractured condition. In all loading cases, there were no significant differences between the two treatment types. Interpretation. While our data show that both vertebroplasty and kyphoplasty allow the disc to generate higher nuclear pressures, neither treatment increased nucleus pulposus pressure above the level of the intact state. This directly relates to the clinical condition in that adjacent segment fractures sustained subsequent to either treatment is likely the result of overall disease progression, and not a direct result of the intervention.

Static and Cyclical Biomechanical Analysis of Pedicle Screw Spinal Constructs

Biomechanical evaluation of twelve different spinal devices in vitro employing pedicle screws was performed using static (n = 5) and cyclical testing (n = 3) parameters. In general, the rank order of implant failures was similar between static and cyclical tests, performed at 600 N compressive load, 5 Hz, and 1 million cycles. The mean number of cycles to failure was higher for spinal instrumentation systems employing longitudinal rods than those using plates (ANOVA F = 16.94, P <.001). At 600 N, the compact Cotrel-Dubousset, TSRH, and ISOLA rod systems demonstrated mean cycles to failure ranging from 200,000 to 800,000 cycles. The remaining devices including Dyna-lok, Kirschner plate, and VSP devices had failures ranging from 50,000 to 210,000 cycles. Polyethylene cylinders representing vertebral bodies were used to eliminate the problems of biologic deterioration encountered with cadaveric spines (a full cyclical test to 1 million cycles required 56 hours), and thus to provide analysis of the weak portion of each spinal system. The failure of eleven of the twelve spinal systems occurred by fracture of a pedicle screw, most commonly at the junction of the upper screw thread and the collar (Kirschner, AO fixator, standard CD, ISOLA, and TSRH). However, in Dynalok and VSP systems, fracture of the threaded portion of the screw just posterior to the integral nuts was the most common screw fracture location. The compact CD system was the only spinal implant that consistently failed by fracture of the longitudinal spinal member (rod). The fatigue life of rod based systems was longer than plate based systems. These studies confirm the importance of anterior column load sharing (vertebral body, corpectomy bone graft) as the mean bending strength demonstrated by these implant systems was not inordinately high using this “worst case scenario” model.

Static and cyclical biomechanical analysis of pedicle screw spinal constructs.

Biomechanical evaluation of twelve different spinal devices in vitro employing pedicle screws was performed using static (n = 5) and cyclical testing (n = 3) parameters. In general, the rank order of implant failures was similar between static and cyclical tests, performed at 600 N compressive load, 5 Hz, and 1 million cycles. The mean number of cycles to failure was higher for spinal instrumentation systems employing longitudinal rods than those using plates (ANOVA F = 16.94, P < .001). At 600 N, the compact Cotrel-Dubousset, TSRH, and ISOLA rod systems demonstrated mean cycles to failure ranging from 200,000 to 800,000 cycles. The remaining devices including Dyna-lok, Kirschner plate, and VSP devices had failures ranging from 50,000 to 210,000 cycles. Polyethylene cylinders representing vertebral bodies were used to eliminate the problems of biologic deterioration encountered with cadaveric spines (a full cyclical test to 1 million cycles required 56 hours), and thus to provide analysis of the weak portion of each spinal system. The failure of eleven of the twelve spinal systems occurred by fracture of a pedicle screw, most commonly at the junction of the upper screw thread and the collar (Kirschner, AO fixator, standard CD, ISOLA, and TSRH). However, in Dynalok and VSP systems, fracture of the threaded portion of the screw just posterior to the integral nuts was the most common screw fracture location. The compact CD system was the only spinal implant that consistently failed by fracture of the longitudinal spinal member (rod). The fatigue life of rod based systems was longer than plate based systems. These studies confirm the importance of anterior column load sharing (vertebral body, corpectomy bone graft) as the mean bending strength demonstrated by these implant systems was not inordinately high using this "worst case scenario" model.