Functional Spinal Unit Research Articles

Biomechanical influence of T1 tilt alteration on adjacent segments after anterior cervical fusion

Background: Anterior cervical fusion (ACF) has become a standard treatment approach to effectively alleviate symptoms in patients with cervical spondylotic myelopathy and radiculopathy. However, alteration of cervical sagittal alignment may accelerate degeneration at segments adjacent to the fusion and thereby compromise the surgical outcome. It remains unknown whether changes in T1 tilt, an important parameter of cervical sagittal alignment, may cause redistribution of biomechanical loading on adjacent segments after ACF surgery. Objective: The objective was to examine the effects of T1 tilt angles on biomechanical responses (i.e.range of motion (ROM) and intradiscal VonMises stress) of the cervical spine before and after ACF. Methods: C2-T1 FE models for pre- and postoperative C4-C6 fusion were constructed on the basis of our previous work. Varying T1 tilts of -10°, -5°, 0°, 5°, and 10° were modeled with an imposed flexion-extension rotation at the T1 inferior endplate for the C2-T1 models. The flexion-extension ROM and intradiscal VonMises stress of functional spinal units were compared between the pre- and postoperative C2-T1 FE models of different T1 tilts. Results: The spinal segments adjacent to ACF demonstrated higher ROM ratios after the operation regardless of T1 tilt. The segmental ROM ratio distribution was influenced as T1 tilt varied and loading conditions, which were more obvious during displacement-control loading of extension. Regardless of T1 tilt, intradiscal VonMises stress was greatly increased at the adjacent segments after the operation. As T1 tilt increased, intradiscal stress at C3-C4 decreased under 30° flexion and increased under 15° extension. The contrary trend was observed at the C6-C7 segment, where the intradiscal stress increased with the increasing T1 tilt under 30° flexion and decreased under 15° extension. Conclusion: T1 tilt change may change biomechanical loadings of cervical spine segments, especially of the adjacent segments after ACF. Extension may be more susceptible to T1 tilt change.

Contribution of Shape Features to Intradiscal Pressure and Facets Contact Pressure in L4/L5 FSUs: An In-Silico Study

Finite element models (FEMs) of the spine commonly use a limited number of simplified geometries. Nevertheless, the geometric features of the spine are important in determining its FEM outcomes. The link between a spinal segment’s shape and its biomechanical response has been studied, but the co-variances of the shape features have been omitted. We used a principal component (PCA)-based statistical shape modelling (SSM) approach to investigate the contribution of shape features to the intradiscal pressure (IDP) and the facets contact pressure (FCP) in a cohort of synthetic L4/L5 functional spinal units under axial compression. We quantified the uncertainty in the FEM results, and the contribution of individual shape modes to these results. This parameterisation approach is able to capture the variability in the correlated anatomical features in a real population and sample plausible synthetic geometries. The first shape mode (phi _1) explained 22.6% of the shape variation in the subject-specific cohort used to train the SSM, and had the largest correlation with, and contribution to IDP (17%) and FCP (11%). The largest geometric variation in (phi _1) was in the annulus-nucleus ratio.

An ex-vivo model for the biomechanical assessment of cement discoplasty.

Percutaneous Cement Discoplasty (PCD) is a surgical technique developed to relieve pain in patients with advanced degenerative disc disease characterized by a vacuum phenomenon. It has been hypothesized that injecting bone cement into the disc improves the overall stability of the spinal segment. However, there is limited knowledge on the biomechanics of the spine postoperatively and a lack of models to assess the effect of PCD ex-vivo. This study aimed to develop a biomechanical model to study PCD in a repeatable and clinically relevant manner. Eleven ovine functional spinal units were dissected and tested under compression in three conditions: healthy, injured and treated. Injury was induced by a papain buffer and the treatment was conducted using PMMA cement. Each sample was scanned with micro-computed tomography (CT) and segmented for the three conditions. Similar cement volumes (in %) were injected in the ovine samples compared to volumes measured on clinical PCD CT images. Anterior and posterior disc heights decreased on average by 22.5% and 23.9% after injury. After treatment, the anterior and posterior disc height was restored on average to 98.5% and 83.6%, respectively, of their original healthy height. Compression testing showed a similar stiffness behavior between samples in the same group. A decrease of 51.5% in segment stiffness was found after injury, as expected. The following PCD treatment was found to result in a restoration of stiffness—showing only a difference of 5% in comparison to the uninjured state. The developed ex-vivo model gave an adequate representation of the clinical vacuum phenomena in terms of volume, and a repeatable mechanical response between samples. Discoplasty treatment was found to give a restoration in stiffness after injury. The data presented confirm the effectiveness of the PCD procedure in terms of restoration of axial stiffness in the spinal segment. The model can be used in the future to test more complex loading scenarios, novel materials, and different surgical techniques.

P23. Inline axial forces associated with implantation of ALIF interbody devices with integrated supplemental fixation

<h3>BACKGROUND CONTEXT</h3> Anterior lumbar interbody fusion (ALIF) allows for an optimal sized interboy device while providing correction for lumbar lordosis, restoration of disc height and indirect decompression. Spinal interbody devices with integrated fixation through blades, screws or other supplemental anchoring components mitigate the need for supplemental instrumentation, thus reducing OR time and preserving the stability of the functional spinal unit. Unfortunately, interbody devices have failed at the bone implant interface resulting in complications including implant migration and expulsion. Gaining a better understanding of the biomechanics associated with interbody placement may help to prevent these complications in the future. The aim of present study is to determine the magnitude and effect of axial forces during the deployment of an interbody device with supplemental integrated fixation. <h3>PURPOSE</h3> To understand the axial loading imparted to the functional spinal unit during lumbar interbody fusion surgery. <h3>STUDY DESIGN/SETTING</h3> Biomechanical axial load mesaurement in cadaveric functional spinal units during lumbar disc preparation and spinal interbody device implant procedures. <h3>METHODS</h3> Cadaveric functional spinal units, levels L2-L3, L3-L4, L4-L5, and L5-S1, were decorticated to the osteoligamentous structures. Posterior elements were removed to allow for MicroCT imaging of each specimen at a voxel size of 27 micron before and after instrumentation. A custom test fixture was used to apply 50N compressive force to stabilize the specimen and also record inline axial forces associated with the deployment of anchor plates during instrumentation procedures. A Chatillon DFS II force gauge was used to record axial forces during implantation of ENZA 1.0 and ENZA 2.0 interbody devices. Axial forces resulting from implant trialing, device insertion and anchor deployment were recorded. MicroCT scans were repeated to allow for quantification of damage to bony structures as a result of implantation procedures. <h3>RESULTS</h3> L2-L3 and L3-L4 force outputs are similar for both insertion, 218.82 N, 219.89 N, and deployment forces, 34.58 N, 44.88 N respectively. Insertion forces throughout all spinal levels were larger than both deployment and removal forces. Insertion forces were larger in the proximal levels, 218.82 N, 219.89 N, when compared to the distal levels, 178.96 N, 113.76 N. However, deployment force was larger in the distal levels, 99.03 N, 77.76 N, when compared to proximal 34.58 N, 44.88 N. <h3>CONCLUSIONS</h3> As evidence mounts for the cost-effectiveness of standalone cages with integrated supplemental fixation, it is imperative that design requirements of these devices continue to improve. Previous studies have shown that the vertebral endplate is susceptible to fracture under compressive loading, with the weakest area of the vertebrae being the center region. In 2001, Hasegawa et al showed a cylindrical-shaped implant caused endplate fracture between 510N and 1335N and that implant size is inversely related to the force required to cause fracture. Endplate fracture is difficult to detect with standard imaging modalities and leaves the patient vulnerable to implant subsidence and reduces segmental stability as a result of changes in load transfer. Furthermore, endplate failure may be a contributor to low back pain, further reducing the chance of a successful outcome. Our study showed interbody devices with integrated fixation plates could safely be inserted and deployed in the disc space without causing endplate fracture. The implant-endplate interface should be investigated in greater detail to allow for design of standalone interbody devices which do not cause unintended distraction or worse, damage to the vertebrae. <h3>FDA DEVICE/DRUG STATUS</h3> ENZA Lumbar Interbody Fusion Device (Approved for this indication).

P4. Independent expansion of the anterior and posterior height of a lateral expandable cage can improve the contact area and load distribution at the cage-endplate interface

<h3>BACKGROUND CONTEXT</h3> Compared to their traditional static counterparts, expandable cages offer clinical advantages by allowing for the use of smaller surgical access corridors to achieve similar lordosis correction without endplate damage from large impaction forces. However, as with static cages, the risk of implant subsidence due to stresses on the vertebral endplate remains an area of concern. Although early clinical outcomes have been reported for individual expandable cage designs, how the endplate contact area and load distribution are affected by specific cage expansion trajectories remains unclear. <h3>PURPOSE</h3> To assess and quantify differences in contact area and load distribution from different lateral expandable interbody cage designs. <h3>STUDY DESIGN/SETTING</h3> Comparative biomechanics analysis using established synthetic spine units. <h3>OUTCOME MEASURES</h3> Cage-endplate contact area, anterior-posterior load distribution, segmental lordosis angle. <h3>METHODS</h3> Discectomies and cage placement were performed on synthetic functional lumbar spine units (n = 3). Three different expandable cage designs were evaluated, each with their own unique expansion trajectory: cranio-caudal, arc-based, and independent anterior and posterior height expansion. For the third cage, a differential expander was used to increase the two heights, anterior and posterior, according to the path of least resistance. A Tekscan calibrated pressure mapping sensor was placed between the cage and the synthetic endplate. The functional spine units were mounted on a servo-hydraulic testing frame and a constant 250N compressive axial preload was applied with unconstrained rotations and translations. The cages were then fully expanded and segmental lordosis angle along with pressure measurements were collected. To allow for a consistent comparison among cages, the surface contact area was normalized by the footprint of each respective cage. To determine the uniformity in the contact load profile, the local load distribution was demarcated in half along the anterior-posterior (A-P) aspect of the cage. The symmetry of the loading profile was characterized by dividing the cumulative anterior force magnitude by the overall force (50% indicates perfectly balanced A-P loading) quantified using the pressure mapping sensor. Friedman's test was performed (α = 0.05) to compare the effect of expansion trajectory on the outcome measures (values reported as median [range]). <h3>RESULTS</h3> Expansion trajectory had a significant effect on cage-endplate contact area and the resultant A-P loading distribution (p = 0.050), but not segmental lordosis angle (p = 0.097). Independent expansion resulted in the highest cage-endplate contact area (93% [92 to 98%]) compared to cranio-caudal (84% [81 to 90%]) and arc-based (62% [56 to 63%]) expansion. Furthermore, independent expansion resulted in a near-even A-P loading distribution (48% [42 to 51%]), compared to cranio-caudal (34% [22 to 36%]), and arc-based (97% [94 to 99%]) expansion. <h3>CONCLUSIONS</h3> Of the cage designs evaluated, the expandable cage with independent anterior and posterior height expansion resulted in the highest cage-endplate contact area and most uniform load distribution, both important considerations fundamental for a successful fusion procedure and the mitigation of implant subsidence risk. With a larger sample size, our continuing studies will investigate the interaction of various cage placement options and expansion mechanisms. <h3>FDA DEVICE/DRUG STATUS</h3> MOD-EX XLIF Interbody System (Approved for this indication), RISE-L Spacer (Approved for this indication)

A comparison of intervertebral ligament properties utilized in a thoracic spine functional unit through kinematic evaluation

Ligament properties in the literature are variable, yet scarce, but needed to calibrate computational models for spine clinical research applications. A comparison of ligament stiffness properties and their effect on the kinematic behavior of a thoracic functional spinal unit (FSU) is examined in this paper. Six unique ligament property sets were utilized within a volumetric T7–T8 finite element (FE) model developed using computer-aided design (CAD) spinal geometry. A 7.5 Nm moment was applied along three anatomical planes both with and without costovertebral (CV) joints present. Range of Motion (RoM) was assessed for each property set and compared to published experimental data. Intact and serial ligament removal procedures were implemented in accordance with experimental protocol. The variance in both kinematic behavior and comparability with experimental data among property sets emphasizes the role nonlinear characterization plays in determining proper kinematic behavior in spinal FE models. Additionally, a decrease in RoM variation among property sets was exhibited when the model setup incorporated the CV joint. With proper assessment of the source and size of each ligament, the material properties considered here could be expanded and justified for implementation into thoracic spine clinical studies.

34. The effect of screw length on single-level anterior cervical fusion using finite element

<h3>BACKGROUND CONTEXT</h3> Anterior cervical discectomy fusion and plating (ACDFP) is one of the most common neurosurgical procedures performed. Early in the use of plates to minimize postoperative orthosis duration and to increase healing rates, there was an attempt to use a bicortical screw purchase. Over time, we have typically converted to a lesser screw length to decrease any additional risks from the bicortical technique. Our study looks at the biomechanical effect of screw length at a single-level ACDFP and the effect on adjacent levels in flexion and extension using finite element modeling. <h3>PURPOSE</h3> To determine the effect of screw length on single-level anterior cervical discectomy, fusion and plating, and its clinical application. <h3>STUDY DESIGN/SETTING</h3> Finite element model. <h3>METHODS</h3> A previously validated three-dimensional osteoligamentous finite element model (FEM) of the cervical spine column (C2-T1) was used. The FEM simulated the vertebra of the body, posterior elements, intervertebral disc and ligaments. Each vertebral body consisted of a thin cortical shell, a softer cancellous bone, endplate, and a posterior bony structure. The cortical bone was modeled as a linear isotropic material of 0.5mm thick shell surrounding the cancellous bone and a 0.2mm thick endplate was placed on the superior and inferior surface of the intervertebral disc. Each single motion segment of the FEM consists of vertebral bodies, intervertebral disc and five major vertebral ligaments. The intervertebral disc was modeled and composed of the nucleus pulposus, annulus ground substance and annulus fibrosus. Material property parameters from literature were used as definitions in the model. The intact spine was modified at the C5-C6 functional spine unit level to simulate the biomechanics of ACDF with different screw lengths (12mm, 14mm, 16mm, 18mm). The standard surgical procedure was used to simulate ACDFP with different length screws at the C5-C6 level. The ALL was removed. The disc properties were altered at the surgical level to simulate the trabecular bone, and this represented complete fusion. The plate was placed at the C5-C6 level, and 4 titanium screws were simulated. All FEM were fixed at the first thoracic vertebra in all degrees of freedom, and load was applied at the C2 superior endplate vertebra. Bending moment (1.5 Nm) combined with a follower load (75N) magnitude under flexion and extension was applied to the intact model. The ROM was measured at each level and across the C3-C7 levels. The variable moment loading protocol (otherwise known as hybrid loading) was next applied. The bending moment was varied until the ACDF with different length screw models displayed the same total range of motion across the C3-C7 levels (although individual segments could have different range of motion). The ROM was obtained at the segmental levels under the same loading mode. <h3>RESULTS</h3> The C5-6 area of fusion demonstrated the least flexion (0.17°) and extension (0.36°) with the 18mm screw and the most flexion (0.75°) and extension (1.06°) with the 12 mm screws. The length of the screw effects the adjacent levels of the spine throughout with the largest increase in ROM in flexion (11.51° -11.98° with 12mm and 18mm screw length respectively) and extension (6.71°- 7.71° with 12mm and 18mm screw length respectively occurring at the cephalad C4-5 level. The caudal C6-7 adjacent level was more pronounced with an increase in flexion ROM with 12mm screws with 12.04° vs 11.88° with 18mm screws and increased but less pronounced ROM in extension with 6.34° with 18mm screws and 5.72° with 12mm screws. <h3>CONCLUSIONS</h3> Increased screw length results in higher flexion ROM at the C4-5 level. An inverse relationship is noted with 12mm screws at the C6-7 adjacent levels demonstrating more ROM than the 18mm screws in flexion. In extension both C4-5 and C6-7 demonstrate that increasing screw length results in increased ROM as compared to 12mm screws. This increased ROM would be expected to increase adjacent segment degeneration more prominently at the C4-5 level as compared to C6-7. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.

Fatty infiltration in cervical extensor muscle: is there a relationship with cervical sagittal alignment after anterior cervical discectomy and fusion?

PurposeTo investigate the relationship between the preoperative paraspinal Goutalier grade of fatty infiltration and postoperative cervical sagittal alignment in patients undergoing anterior cervical discectomy and fusion (ACDF).MethodsA total of 101 patients who underwent single-level ACDF with the Zero-profile implant system between March 2011 and April 2020 were included in this study. Cervical sagittal alignment parameters, including the C2-C7 Cobb angle, functional spinal unit (FSU) angle, cervical sagittal vertical axis (SVA), and T1 slope (T1S), were assessed. Preoperative magnetic resonance images were used to classify patients according to Goutalier grade. Clinical outcomes including Neck Disability Index (NDI) scores, Japanese Orthepaedic Association (JOA) scores and Visual Analogue Scale (VAS) scores were collected and analyzed.ResultsAccording to the Goutalier grade, 33 patients were classified as Goutalier 0–1 (Group A), 44 were classified as Goutalier 1.5–2 (Group B), and 24 were classified as Goutalier 2.5–4.0 (Group C). The mean age among the three groups showed significant differences (P = 0.007). At the last follow-up, the C2-C7 Cobb angle, FSU angle, and T1S improved after the surgery among the groups. Although there were varying degrees of loss of curvature among the different groups during the follow-up period, the postoperative cervical sagittal alignment parameters demonstrated no statistical differences among the three groups (P > 0.05). In addition, patients in all groups experienced significant relief of their symptoms, and the clinical scores were comparable among the groups (P > 0.05).ConclusionThe complex nature of anterior cervical surgery requires surgical attention both in decompression and sagittal alignment. Our study demonstrates satisfactory postoperative cervical sagittal alignment of patients despite different grades of fatty infiltration of the multifidus muscle following single-level ACDF. Based on our results, the improvement and maintenance of cervical sagittal alignment after ACDF remains a complex problem that spine surgeons should consider before surgery.

Expandable pedicle screw may have better fixation than normal pedicle screw: preclinical investigation on instrumented L4-L5 vertebrae based on various physiological movements.

Lumbar spine fracture is typically treated by means of screw fixation, the primary aim of which is to reduce fracture by achieving bony union such that the spinal anatomy is restored. Pedicle screw fixation has certain advantages over conventional vertebral screws, e.g. 3-column fixation and improved surgical alignment. However, expandable pedicle screws have been reported to impart better anchorage as compared to conventional pedicle screws, both in case of healthy and osteoporotic bone. The clinical studies notwithstanding, there is a paucity of preclinical investigations on expandable pedicle screws used on lumbar vertebrae. By employing anatomically viable FE models, the present study intended to estimate stress-strain fields of a functional spinal unit (FSU) of intact L4-L5 vertebra and to further compare the same with FSUs instrumented with normal and expandable pedicle screws under different physiological loading condition. The various physiological loading regimes appeared to have significant influence on the overall load transfer in the L4-L5 vertebrae. The expandable pedicle screw predicted marginally improved anchorage as compared to the normal pedicle screws, with more contact area with the bone resulting in higher stresses (~ 1.6MPa) and high strain at the contact sites. This is indicative of improved stability albeit having marginally greater risk of screw pullout. Greater area (15-80%) with peak stresses at the bone-screw interfaces also indicates lesser degree of stress shielding. Thus, stability aside, one may expect to have lower loosening issues too with the use of expandable pedicle screws.

Even mild intervertebral disc degeneration reduces the flexibility of the thoracic spine: an experimental study on 95 human specimens

BACKGROUND CONTEXTIntervertebral disc degeneration represents one of multiple potential trigger factors for reduced passive spinal mobility and back pain. The effects of age-related degenerative intervertebral disc changes on spinal flexibility were however mainly investigated for the lumbar spine in the past, while intervertebral disc degeneration is also highly prevalent in the thoracic spine. PURPOSETo evaluate the effect of the degeneration grade on the range of motion and neutral zone of the thoracic spine. STUDY DESIGNExperimental study including combined radiological grading of intervertebral disc degeneration and biomechanical testing of 95 human thoracic functional spinal units (min. n=4 per level from T1–T2 to T11–T12) from 33 donors (15 female / 18 male, mean age 56 years, age range 37–80 years). METHODSDegeneration grades of the intervertebral discs were assessed using the validated x-ray grading scheme of Liebsch et al. (0=no, 1=mild, 2=moderate, 3=severe degeneration). Motion segments were loaded with pure moments in flexion/extension, lateral bending, and axial rotation to determine range of motion and neutral zone at 5 Nm. RESULTSAll tested specimens exhibited degeneration grades between zero and two. Range of motion significantly decreased for grades one and two compared with grade zero in any motion direction (p<.05), showing the strongest decrease in extension comparing grade two with grade zero (-42%), while no significant differences were detected between grades one and two. Similar trends were found for the neutral zone with the strongest decrease in extension also comparing grade two with grade zero (-47%). Donor age did not significantly affect the range of motion, whereas the range of motion was significantly reduced in specimens from male donors due to the significantly higher degeneration grade in this study. CONCLUSIONSEven mild intervertebral disc degeneration reduces the range of motion and neutral zone of the thoracic spine in any motion plane, whereas progressing degeneration does not further affect its flexibility. This is in contrast to the lumbar spine, where a more gradual decrease of flexibility was found in prior studies, which might be explained by differences between thoracic and lumbar intervertebral disc morphologies. CLINICAL SIGNIFICANCEThoracic intervertebral disc degeneration should be considered as one of multiple potential causal factors in patients showing reduced passive mobility and middle back pain.

In Vitro Biomechanics of the Cervical Spine: A Systematic Review.

In vitro testing has been conducted to provide a comprehensive understanding of the biomechanics of the cervical spine. This has allowed a characterization of the stability of the spine as influenced by the intrinsic properties of its tissue constituents and the severity of degeneration or injury. This also enables the preclinical estimation of spinal implant functionality and the success of operative procedures. The purpose of this review paper was to compile methodologies and results from various studies addressing spinal kinematics in pre- and postoperative conditions so that they could be compared. The reviewed literature was evaluated to provide suggestions for a better approach for future studies, to reduce the uncertainties and facilitate comparisons among various results. The overview is presented in a way to inform various disciplines, such as experimental testing, design development, and clinical treatment. The biomechanical characteristics of the cervical spine, mainly the segmental range of motion (ROM), intradiscal pressure (IDP), and facet joint load (FJL), have been assessed by testing functional spinal units (FSUs). The relative effects of pathologies including disc degeneration, muscle dysfunction, and ligamentous transection have been studied by imposing on the specimen complex load scenarios imitating physiological conditions. The biomechanical response is strongly influenced by specimen type, test condition, and the different types of implants utilized in the different experimental groups.

Long-term Follow-Up Results of Dynamic Cervical Implant in Patients with Cervical Disk Diseases: Compared with Arthroplasty.

Arthroplasty has been proven to be a safe and effective treatment for patients with cervical degenerative disk disease (DDD). Dynamic Cervical Implant (DCI) has emerged as a novel implantation device for cervical DDD. This study aimed to compare the outcomes of these procedures after 5 years of follow-up in the DCI and arthroplasty groups. This study retrospectively enrolled 79 consecutive cervical DDD patients with 41 DCI and 47 prostheses implanted. Radiographs were analyzed for intervertebral height and range of motion (ROM). Neural function of the patients was assessed using the Neck Disability Index score, visual analog scale, Japanese Orthopaedic Association score and 36-Item Short Form Survey. The DCI group had statistically lesser flexion/extension and bilateral bending ROM than the arthroplasty group at the operated level(s) (p < 0.05). The DCI group but not the orthoplast group showed improved lordotic alignment of C2-C7 and operated functional spinal unit (p < 0.05). No statistical difference was observed in the neural function of the two groups. Heterotopic ossification was found in 7 and 14 patients in the DCI and arthroplasty groups, respectively. The 5-year follow-up results were comparable between the two groups. We believe that DCI implantation is a safe and effective procedure and could possibly become an alternative treatment for cervical DDD.

Examining the protective role of the posterior elements of the spine against endplate fractures in a porcine model.

Previous studies have shown that the posterior elements/facet joints provide strength to the overall functional spine unit (FSU) by taking 3-25% of vertical compressive load off the intervertebral disc (IVD). However, little is known regarding whether this offloading has a protective effect against endplate fracture. Therefore, the purpose of this study was to investigate if the posterior elements provide a protective role to the endplate in porcine cervical spines under fracture-inducing conditions. Twenty-two cervical porcine FSUs (C5/6 level) were randomized into two groups: 1) a control group which had their posterior elements left intact (n = 11); 2) an experimental group which had the posterior elements removed (n = 11). Each FSU underwent a previously reported rapid IVD pressurization protocol in order to create endplate fractures. Briefly, hydraulic fluid was rapidly injected into the IVD via a standard inflation needle inserted through the anterior annulus which was connected to a hydraulic pump and pressure transducer. Post pressurization, each FSU was dissected to determine the presence and size of endplate fracture. Peak pressurization and rate of pressurization were not found to differ between intact and cut specimens (p = 0.313 and 0.101, respectively). In contrast, significantly, more cut FSUs sustained an endplate fracture (11/11) compared to intact FSUs (5/11); p = 0.012. Further, cut FSUs resulted in a fracture area 1.91 times greater in size compared to the fractures seen in the intact FSUs (p = 0.011). Therefore, posterior elements appear to decrease the risk and severity of endplate fracture.

Anterior Cervical Discectomy and Fusion With a No-Profile Integrated Fixation Allograft Device: An In Vitro Biomechanical Analysis and Clinical Case Series.

No-profile anterior cervical discectomy and fusion (ACDF) devices are commonplace in spinal surgery. Contained within the intervertebral margins, these devices diminish risks associated with anterior cervical plating, while also marginalizing cage migration and subsidence. However, these devices have been limited in their scope of implant material. Accordingly, a no-profile ACDF (npACDF) device supporting a machined allograft implant body with a connected load-sharing fixation interface was developed. However, it is not established in the literature whether the device supports early mechanical stability and subsequent boney fusion. The objective of this study was to assess this device in both the clinical and preclinical settings. Biomechanical Analysis: Twenty-four functional spinal units (FSUs) were divided into 4 groups (n = 6). Each group would receive a single construct: (1) npACDF device with connected polyetheretherketone (PEEK) body, (2) npACDF with connected allograft body, (3) npACDF (allograft body) with nonconnected fixation plate, and (4) standard ACDF plate and PEEK interbody cage. FSUs were subjected to pure moment loading (flexion/extension, lateral bending, and axial rotation) via a kinematic test machine in their intact state and then following instrumentation. Vertebral motion was recorded and range-of-motion (ROM) reduction, relative to intact, was calculated.Clinical Case Series: Ten patients receiving single-level ACDF with the npACDF allograft (connected) device for the treatment of persistent cervical radiculopathy or myelopathy were retrospectively reviewed at 12 months. Radiographic and patient reported outcomes were reported. No differences in ROM existed between the npACDF constructs (P ≥ 0.99). Standard ACDF achieved more ROM reduction than the npACDF constructs in all directions (P ≥ 0.04). All subjects achieved fusion at 12 months. The npACDF allograft device supported less motion reduction in comparison to traditional plating; however, case series data suggest the device provides clinically effective stability resulting in quality radiographic fusion and pain improvement. This report provides both clinical and preclinical insight into a device which offers alternative design features to traditional continuous/rigid ACDF device designs.

Dorsal horn volume loss and pain pathway changes in Cavalier King Charles Spaniels with syringomyelia, signs of pain, and phantom scratching.

Central neuropathic pain is a core clinical sign of syringomyelia in humans and Cavalier King Charles Spaniel (CKCS) dogs. This histopathological study used spinal cords from CKCS dogs with syringomyelia to investigate the following conditions: (1) whether specific structural cervical spinal cord entities involved in nociception were affected by loss of neuroparenchyma or other pathological changes in CKCS dogs with pain-related behaviour and phantom scratching, (2) whether pain-related behaviour or phantom scratching correlated with loss of a specific anatomical entity or upregulation of glia cells, and (3) whether syringomyelia-related lesions affected specific functional spinal cord units of nociception. Spinal cord segments C1-C8 from CKCS dogs with magnetic resonance imaging-confirmed syringomyelia and clinical signs of pain and phantom scratching (n = 8) were compared with those from CKCS dogs without syringomyelia (n = 4). Dogs with unilateral scratching (n = 7) had a volume loss ( P = 0.043) of the dorsal horn laminae I-III in the ipsilateral side compared with the contralateral dorsal horn. A clear pattern of ipsilateral changes in the dorsal root entry zone characterised by deafferentation and reorganization of first-order axons into deeper laminae was found in cases with lateralised scratching. Significant changes in cell number density were not found for astrocytes or microglia, suggesting that the dogs represented cases of end-stage syringomyelia and thus could not reveal astrogliosis and microgliosis, which may be involved in the early phases of syrinx development and phantom scratching. The present relationship between clinical findings and dorsal horn and pain pathway pathology in CKCS dogs suggests that these dogs may be of interest as a supplement to experimental model pain research.

Characterizing the Mechanical and Viscoelastic Response of the Porcine Facet Joint Capsule Ligament in Response to a Simulated Impact.

The facet capsule ligament (FCL) is a structure in the lumbar spine that constrains motions of the vertebrae. Subfailure loads can produce microdamage resulting in increased laxity, decreased stiffness, and altered viscoelastic responses. Therefore, the purpose of this investigation was to determine the mechanical and viscoelastic properties of the FCL under various magnitudes of strain from control samples and samples that had been through an impact protocol. Two hundred FCL tissue samples were tested (20 control and 180 impacted). Impacted FCL tissue samples were obtained from functional spinal units that had been exposed to one of nine subfailure impact conditions. All specimens underwent the following loading protocol: preconditioning with five cycles of 5% strain, followed by a 30 s rest period, five cycles of 10% strain, and 1 cycle of 10% strain with a hold duration at 10% strain for 240 s (4 min). The same protocol was followed for 30% and 50% strain. Measures of stiffness, hysteresis, and force-relaxation were computed. No significant differences in stiffness were observed for impacted specimens in comparison to control. Impacted specimens from the 8 g flexed and 11 g flexed and neutral conditions exhibited greater hysteresis during the cyclic-30% and cyclic-50% portion of the protocol in comparison to controls. In addition, specimens from the 8 g and 11 g flexed conditions resulted in greater stress decay for the 50%-hold conditions. Results from this study demonstrate viscoelastic changes in FCL samples exposed to moderate and highspeed single impacts in a flexed posture.

Are rotational passive stiffness and translational passive stiffness correlated? A porcine in vitro study

BackgroundQualitative clinical assessments of spinal stiffness have been demonstrated to show moderate correlations with one-another. We hypothesized that these correlations would improve in an in vitro model of the functional spinal unit. If the stiffness of spinal units are different across loading regimes (e.g. flexion-extension versus shear), then it may provide one explanation as to the variability in findings from clinical assessments, since these tests tend not to discriminate rotational and translational degrees-of-freedom. Therefore, the purpose of this investigation was to quantify the relationships between rotational and translational stiffness measures in vitro. MethodsForty-eight porcine cervical spine functional units were used in this investigation (20 C3-C4, 28 C5-C6). While under constant 300 N compressive load, range-of-motion tests for both flexion-extension (± 8 Nm, 0.5 deg./s) and anteroposterior shear (± 400 N, 0.2 mm/s) were conducted, to quantify moment-angle and force-deflection curves. Representative stiffness values were then obtained for flexion, extension, anterior shear, and posterior shear using segmented regression. The correlation matrix between these four measures was then used to explore their potential relationships. FindingsOf the six correlations conducted, only the relationship between posterior shear and extension stiffness was statistically significant (p = 0.014), despite featuring a low correlation coefficient (R2 = 0.123). InterpretationThe poor correlations between stiffness metrics in this study supports the disparate findings of tissue stiffness in vivo. Results from this investigation suggest that clinicians should be cognizant of which degrees-of-freedom they are assessing in the spine, as their stiffness values vary independently.

Force Distribution Within Spinal Tissues During Posterior to Anterior Spinal Manipulative Therapy: A Secondary Analysis.

BackgroundPrevious studies observed that the intervertebral disc experiences the greatest forces during spinal manipulative therapy (SMT) and that the distribution of forces among spinal tissues changes as a function of the SMT parameters. However, contextualized SMT forces, relative to the ones applied to and experienced by the whole functional spinal unit, is needed to understand SMT’s underlying mechanisms.AimTo describe the percentage force distribution between spinal tissues relative to the applied SMT forces and total force experienced by the functional unit.MethodsThis secondary analysis combined data from 35 fresh porcine cadavers exposed to a simulated 300N SMT to the skin overlying the L3/L4 facet joint via servo-controlled linear motor actuator. Vertebral kinematics were tracked optically using indwelling bone pins. The functional spinal unit was then removed and mounted on a parallel robotic platform equipped with a 6-axis load cell. The kinematics of the spine during SMT were replayed by the robotic platform. By using serial dissection, peak and mean forces induced by the simulated SMT experienced by spinal structures in all three axes of motion were recorded. Forces experienced by spinal structures were analyzed descriptively and the resultant force magnitude was calculated.ResultsDuring SMT, the functional spinal unit experienced a median peak resultant force of 36.4N (IQR: 14.1N) and a mean resultant force of 25.4N (IQR: 11.9N). Peak resultant force experienced by the spinal segment corresponded to 12.1% of the total applied SMT force (300N). When the resultant force experienced by the functional spinal unit was considered to be 100%, the supra and interspinous ligaments experienced 0.3% of the peak forces and 0.5% of the mean forces. Facet joints and ligamentum flavum experienced 0.7% of the peak forces and 3% of the mean forces. Intervertebral disc and longitudinal ligaments experienced 99% of the peak and 96.5% of the mean forces.ConclusionIn this animal model, a small percentage of the forces applied during a posterior-to-anterior SMT reached spinal structures in the lumbar spine. Most SMT forces (over 96%) are experienced by the intervertebral disc. This study provides a novel perspective on SMT force distribution within spinal tissues.

Місце малоінвазивної нейрохірургії на хребетно-рухових сегментах у багатопрофільному реабілітаційному центрі

Надано 100 пацієнтів, прооперованих з приводу поперекових міжхребцевих гриж, зі стабілізацією хребетно-рухового сегмента міжтіловим титановим кейджем. Проаналізовано закономірність залежності локалізації міжхребцевої грижі від рівня хребетного стовпа, віку, статі та латералізації. Показана ефективність включення малоінвазивних нейрохірургічних операцій в умовах реабілітаційного центру.

Reaction Forces and Flexion-Extension Moments Imposed on Functional Spinal Units With Constrained and Unconstrained In Vitro Testing Systems.

A mechanical goal of in vitro testing systems is to minimize differences between applied and actual forces and moments experienced by spinal units. This study quantified the joint reaction forces and reaction flexion-extension moments during dynamic compression loading imposed throughout the physiological flexion-extension range of motion. Constrained (fixed base) and unconstrained (floating base) testing systems were compared. Sixteen porcine spinal units were assigned to both testing groups. Following conditioning tests, specimens were dynamically loaded for 1 cycle with a 1 Hz compression waveform to a peak load of 1 kN and 2 kN while positioned in five different postures (neutral, 100% and 300% of the flexion and extension neutral zone), totaling ten trials per functional spinal unit (FSU). A six degree-of-freedom force and torque sensor was used to measure peak reaction forces and moments for each trial. Shear reaction forces were significantly greater (25.5 N-85.7 N) when the testing system was constrained compared to unconstrained (p < 0.029). The reaction moment was influenced by posture (p = 0.037), particularly in C5C6 spinal units. In 300% extension (C5C6), the reaction moment was, on average, 9.9 N·m greater than the applied moment in both testing systems and differed from all other postures (p < 0.001). The reaction moment error was, on average, 0.45 N·m at all other postures. In conclusion, these findings demonstrate that comparable reaction moments can be achieved with unconstrained systems, but without inducing appreciable shear reaction forces.