Pedicle Region Research Articles

Effect of the cone-beam CT acquisition trajectory on image quality in spine surgery: experimental cadaver study

BACKGROUNDIntraoperative 3D imaging with cone-beam CT (CBCT) improves assessment of implant position and reduces complications in spine surgery. It is also used for image-guided surgical techniques, resulting in improved quality of care. However, in some cases, metal artifacts can reduce image quality and make it difficult to assess pedicle screw position and reduction. PURPOSEThe objective of this study was to investigate whether a change in CBCT acquisition trajectory in relation to pedicle screw position during dorsal instrumentation can reduce metal artifacts and consequently improve image quality and clinical assessability. STUDY DESIGNExperimental cadaver study. METHODSA human cadaver was instrumented with pedicle screws in the thoracic and lumbar spine region (Th11 to L5). Then, the acquisition trajectory of the CBCT (Cios Spin, Siemens, Germany) to the pedicle screws was systematically changed in 5° steps in angulation (−30° to +30°) and swivel (−25° to +25°). Subsequently, radiological evaluation was performed by 3 blinded, qualified raters on image quality using 9 questions (including anatomical structures, implant position, appearance of artifacts) with a score (1–5 points). For statistical evaluation, the image quality of the different acquisition trajectories was compared to the standard acquisition trajectory and checked for significant differences. RESULTSThe angulated acquisition trajectory significantly increased the score for subjective image quality (p<.001) as well as the clinical assessability of pedicle screw position (p<.001) with particularly strong effects on subjective image quality in the vertebral pedicle region (d=1.61). Swivel of the acquisition trajectory significantly improved all queried domains of subjective image quality (p<.001) as well as clinical assessability of pedicle screw position (p<.001). CONCLUSIONSIn this cadaver study, the angulation as well as the swivel of the acquisition trajectory led to a significantly improved image quality in intraoperative 3D imaging (CBCT) with a constant isocenter. The data show that maximizing the angulation/swivel angle towards 30°/25° provides the best tested subjective image quality and enhances clinical assessability. Therefore, a correct adjustment of the acquisition trajectory can help to make intraoperative revision decisions more reliably. CLINICAL SIGNIFICANCEThe knowledge of enhanced image quality by changing the acquisition trajectory in intraoperative 3D imaging can be used for the assessment of critical screw positions in spine surgery. The implementation of this knowledge requires only a minor change of the current intraoperative imaging workflow without additional technical equipment and could further reduce the need for revision surgery.

MRI-based pedicle bone quality score: correlation to quantitative computed tomography bone mineral density and its role in quantitative assessment of osteoporosis

BACKGROUND CONTEXTBone quality in the pedicle region generally determines screw pullout strength, insertion torque, and vertebral body loading characteristics. Dual-energy X-ray absorptiometry (DEXA), as the gold standard for evaluating bone mineral density (BMD), cannot measure the BMD of specific parts, such as pedicle, and DEXA is limited in many ways. Recent studies have shown a correlation between the magnetic resonance imaging (MRI)-based vertebral bone quality (VBQ) score and BMD measured using DEXA or quantitative computed tomography (QCT). However, no studies have been reported on the MRI-based pedicle bone quality (PBQ) score. Moreover, few studies have investigated the relationship between MRI-based PBQ and osteoporosis. PURPOSETo create a new site-specific MRI-based PBQ assessment method and assess its diagnostic capacity in patients with normal BMD and osteopenia/osteoporosis. STUDY DESIGN/SETTINGA retrospective study. PATIENT SAMPLEA total of 156 patients underwent lumbar fusion surgery for chronic low back pain at our hospital between 2021 and 2022, with lumbar QCT and T1-weighted MRI performed before surgery. OUTCOME MEASURESCorrelation of the PBQ score with QCT BMD, and the association between the PBQ score and presence of osteopenia/osteoporosis. METHODSBMD of the lumbar was calculated as the mean BMD of the L1 and L2 vertebral bodies on the basis of asynchronous QCT measurements. The PBQ score, which is the average of the bone quality values of both pedicles on the basis of site-specific T1-weighted sagittal MRI images, was calculated by dividing the median signal intensity of the L1–L4 pedicles by the signal intensity of the cerebrospinal fluid at the L3 level. The interobserver reliability of the PBQ score was assessed using the intraclass correlation coefficient (ICC). A receiver operating characteristic curve was drawn, and the area under the curve (AUC) was calculated to assess the predictive performance of PBQ for osteoporosis. The PBQ score was compared with QCT BMD, as the gold standard, using Pearson correlation analysis. RESULTSIn total, 156 patients participated in this study, including 51 in the Normal BMD group and 105 in the osteopenia/osteoporosis group. The PBQ score in the osteopenia/osteoporosis group was significantly higher than that in the normal BMD group (3.19±0.55 vs. 2.84±0.51, p<.001). The VBQ and PBQ scores were calculated by 2 authors and were in good agreement (intraclass correlation coefficient=0.949 and 0.929, respectively). Pearson's test showed a significant negative correlation between PBQ and QCT BMD (r=−0.4887, p<.001). The optimal cutoff PBQ score to differentiate patients with osteopenia/osteoporosis from those with normal BMD was 3.160, with a sensitivity of 66.7%, specificity of 72.5%, and AUC of 0.776. The PBQ score correlated more strongly with QCT BMD (r=−0.4887) than VBQ (r=−0.4078). CONCLUSIONSIn this study, we propose a novel, MRI-based pedicle-specific bone quality score. This is the first study to investigate the relationship between the PBQ score and QCT BMD. The PBQ score showed diagnostic utility, differentiating between patients with osteopenia/osteoporosis and those with normal BMD (AUC=0.776), and the PBQ score correlated more strongly with QCT BMD than VBQ.

Finite element mechanical analysis of ipsilateral approach and contralateral approach in unilateral bilateral endoscopic spine surgery

BackgroundUnilateral bilateral endoscopic spine surgery (UBE) is often performed to treat lumbar spinal stenosis and disc herniation. It has become a prominent method in endoscopic spine surgery because of its very low learning curve and broader operative field of vision. Currently, the ipsilateral approach and contralateral approach have been established for disc herniation in the foraminal area, intervertebral foramen region, or pedicle region. The contralateral method offers many benefits over the ipsilateral approach, including less bone labour during microsurgical decompression and the preservation of facet joints. However, because it uses the interlaminar window approach, it inevitably involves osteotomy of the patient’s superior and inferior articular processes, which may result in corresponding deterioration in the spine’s biomechanical stability and subsequent adjacent facet joint diseases caused by facet joint degeneration postoperatively.ObjectiveAs a result, the purpose of this work is to use a finite element model to evaluate how the ipsilateral approach and contralateral approach in unilateral bilateral endoscopic spine surgery affect spinal stability while treating identical intervertebral disc herniation.Study designIn this study, a three-dimensional lumbar-sacral spine model was built and verified. Osteotomies were conducted for armpit-type lumbar disc herniation (LDH), periradicular-type LDH, and shoulder-type LDH. Postoperative lumbar spine models of the ipsilateral approach and contralateral approach in unilateral bilateral endoscopic spine surgery were developed. The von Mises stress on the endplate, shear force on the annulus fibrosus, pressure inside the intervertebral disc, and range of motion (ROM) of the L3 segment were all determined. The results of our well-validated model showed that osteotomy done in the ipsilateral approach deteriorated most biomechanical metrics.ResultsIn the majority of loading conditions, the contralateral approach caused the intervertebral disc’s biomechanical properties to increase, and the ipsilateral approach caused the intervertebral disc’s biomechanical properties to increase sharply more than the contralateral approach.ConclusionThe contralateral approach, which is now extensively employed in unilateral bilateral endoscopic spine surgery, may be regarded as an ideal surgical alternative for treating lumbar disc herniation without producing iatrogenic instability. This approach has a low facet joint reduction rate, minimum soft tissue injury, and precisely identifies the midline of the central spinal canal during the retraction of the thecal sac and nerve roots.

Effect of changing the acquisition trajectory of the 3D C-arm (CBCT) on image quality in spine surgery: experimental study using an artificial bone model

BackgroundIntraoperative 3D imaging using cone-beam CT (CBCT) provides improved assessment of implant position and reduction in spine surgery, is used for navigated surgical techniques, and therefore leads to improved quality of care. However, in some cases the image quality is not sufficient to correctly assess pedicle screw position and reduction, especially due to metal artifacts. The aim of this study was to investigate whether changing the acquisition trajectory of the CBCT in relation to the pedicle screw position during dorsal instrumentation of the spine can reduce metal artifacts and consequently improve image quality as well as clinical assessability on the artificial bone model.MethodsAn artificial bone model was instrumented with pedicle screws in the thoracic and lumbar spine region (Th10 to L5). Then, the acquisition trajectory of the CBCT (Cios Spin, Siemens, Germany) to the pedicle screws was systematically changed in 5° steps in angulation (− 30° to + 30°) and swivel (− 30° to + 30°). Subsequently, radiological evaluation was performed by three blinded, qualified raters on image quality using 9 questions (including anatomical structures, implant position, appearance of artifacts) with a score (1–5 points). For statistical evaluation, the image quality of the different acquisition trajectories was compared to the standard acquisition trajectory and checked for significant differences.ResultsThe angulated acquisition trajectory increased the score for subjective image quality (p < 0.001) as well as the clinical assessability of pedicle screw position (p < 0.001) highly significant with particularly strong effects on subjective image quality in the vertebral pedicle region (d = 1.06). Swivel of the acquisition trajectory significantly improved all queried domains of subjective image quality (p < 0.001) as well as clinical assessability of pedicle screw position (p < 0.001). The data show that maximizing the angulation or swivel angle toward 30° provides the best tested subjective image quality.SummaryAngulation and swivel of the acquisition trajectory result in a clinically relevant improvement in image quality in intraoperative 3D imaging (CBCT) during dorsal instrumentation of the spine.

Pedicle Screw Placement Accuracy in Robot-Assisted Spinal Fusion in a Multicenter Study.

Pedicle screw fixation is a spinal fusion technique that involves the implantation of screws into vertebral pedicles to restrict movement between those vertebrae. The objective of this research is to measure pedicle screw placement accuracy using a novel automated measurement system that directly compares the implanted screw location to the planned target in all three anatomical views. Preoperative CT scans were used to plan the screw trajectories in 122 patients across four surgical centers. Postoperative scans were fused to the preoperative plan to quantify placement accuracy using an automated measurement algorithm. The mean medial-lateral and superior-inferior deviations in the pedicle region for 500 screws were 1.75 ± 1.36mm and 1.52 ± 1.26mm, respectively. These deviations were measured using an automated system and were statistically different from manually determined values. The uncertainty associated with the fusion of preoperative to postoperative images was also quantified to better understand the screw-to-plan accuracy results. This study uses a novel automated measurement system to quantify screw placement accuracy as it relates directly to the planned target location, instead of analyzing for breaches of the pedicle, to quantify the validity of using of a robotic-guidance system for accurate pedicle screw placement.

Biomechanical limitations of partial pediculectomy in endoscopic spine surgery

BACKGROUND CONTEXTTransforaminal endoscopic decompression is an emerging minimally invasive surgical technique in spine surgery. The biomechanical effects and limitations of resections associated with this technique are scarce. PURPOSEThe objective of this study was to analyze the effects of three different extents of reduction at the craniomedial pedicle (10%, 25%, and 50%) and to compare them with the intact native side. In addition, the influence of bone quality on the resistance of the pedicle after reduction was investigated. STUDY DESIGNBiomechanical cadaveric study. METHODSThirty lumbar vertebrae originating from six fresh frozen cadavers were tested under uniaxial compression load in a ramp-to-failure test: (1) the reduced pedicle on one side, and (2) the native pedicle on the other side. Of the 30 lumbar vertebrae, ten were assigned to each reduction group (10%, 25%, and 50%). RESULTSOn the intact side, the median axial compression force to failure was 593 N (442.4–785.8). A reduction of the pedicle by 10% of the cross-sectional area resulted in a decrease of the axial load resistance by 4% to 66% compared to the intact opposite side (p=.046). The median compression force to failure was 381.89 N (range: 336–662.1). A reduction by 25% resulted in a decrease of 7% to 71% (p=.001). The median compression force to failure was 333 N (265.1–397.3). A reduction by 50% resulted in a decrease of 39% to 90% (p<.001). The median compression force to failure was 200.9 N (192.3–283.9). At 10% pedicle reduction, the Hounsfield units (HU) value and the absolute force required to generate a pedicle fracture showed significant correlations (ρ=.872; p=.001). At 25%, a positive correlation between the two variables could still be identified (ρ=.603; p=.065). At 50%, no correlation was found (ρ=-.122; p=.738). CONCLUSIONResection of the inner, upper part of the pedicle significantly reduces the axial resistance force of the pedicle until a fracture occurs. CLINICAL SIGNIFICANCEThe extent of pedicle reduction itself plays only a limited role: once the cortical bone in the pedicle region is compromised, significant loss of resistance to loading must be anticipated.

Reversed windshield-wiper effect leads to failure of cement-augmented pedicle screw: Biomechanical mechanism analysis by finite element experiment

The failure mode of cement-augmented pedicle screw (CAPS) was different from common pedicle screw. No biomechanical study of this failure mode named as “reversed windshield-wiper effect” was reported. To investigate the mechanisms underlying this failure mode, a series of finite element models of CAPS and PS were modified on L4 osseous model. Nine models were created according to the cement volume at 0.5 mL interval (range: 1–5 mL). Pullout load and cranio-caudal loads were applied on the screws. Stress and instantaneous rotation center (IRC) of the vertebra were observed. Under cranio-caudal load, the stress concentrated on the screw tip and pedicle region. The maximal stress (MS) at the screw tip region was +2.143 MPa higher than pedicle region. With cement volume increasing, the maximal stress (MS) at the screw tip region decreased dramatically, while MS at pedicle region was not obviously affected. As dose increased to 1.5 mL, the MS at pedicle region became higher than screw tip region and the maximal stress difference was observed at 3.5 mL. IRC of the vertebra located at the facet joint region in PS model. While IRC in CAPS models shifted anteriorly closer to the vertebral body with the increasing of cement volume. Under axial pull-out load, the maximal stress (MS) of cancellous bone in CAPS models was 29.53–50.04% lower than that 2.228 MPa in PS model. MS in the screw-bone interface did not change significantly with cement volume increasing. Therefore, the possible mechanism is that anterior shift of IRC and the negative difference value of MS between screw tip and pedicle region due to cement augmentation, leading to the screw rotate around the cement-screw complex as the fulcrum point.

Location of pedicle screw hold in relation to bone quality and loads.

Introduction: Sufficient screw hold is an indispensable requirement for successful spinal fusion, but pedicle screw loosening is a highly prevalent burden. The aim of this study was to quantify the contribution of the pedicle and corpus region in relation to bone quality and loading amplitude of pedicle screws with traditional trajectories. Methods: After CT examination to classify bone quality, 14 pedicle screws were inserted into seven L5. Subsequently, Micro-CT images were acquired to analyze the screw’s location and the vertebrae were split in the midsagittal plane and horizontally along the screw’s axis to allow imprint tests with 6 mm long sections of the pedicle screws in a caudal direction perpendicular to the screw’s surface. Force-displacement curves in combination with the micro-CT data were used to reconstruct the resistance of the pedicle and corpus region at different loading amplitudes. Results: Bone quality was classified as normal in three specimens, as moderate in two and as bad in two specimens, resulting in six, four, and four pedicle screws per group. The screw length in the pedicle region in relation to the inserted screw length was measured at an average of 63%, 62%, and 52% for the three groups, respectively. At a calculated 100 N axial load acting on the whole pedicle screw, the pedicle region contributed an average of 55%, 58%, and 58% resistance for the normal, moderate, and bad bone quality specimens, respectively. With 500 N load, these values were measured at 59%, 63%, and 73% and with 1000 N load, they were quantified at 71%, 75%, and 81%. Conclusion: At lower loading amplitudes, the contribution of the pedicle and corpus region on pedicle screw hold are largely balanced and independent of bone quality. With increasing loading amplitudes, the contribution of the pedicle region increases disproportionally, and this increase is even more pronounced in situations with reduced bone quality. These results demonstrate the importance of the pedicle region for screw hold, especially for reduced bone quality.

Automatically recognize and segment morphological features of the 3D vertebra based on topological data analysis

Background:For spinal surgery, exact knowledge about the shape of individual vertebra is of great importance. However, due to the complex morphological features of human vertebrae and spine, it is challenging to locate, segment automatically, and recognize the morphological features in vertebral images. Significantly, pedicle recognition is more challenging because of the particular structure. Methods:Topological structures such as the Reeb graph could facilitate effective visualization and interactive exploration of feature-rich data. In this paper, we conducted topological data analysis on the 3D vertebra, whereby some principal morphological features of the 3D vertebra are recognized and segmented. First, a scalar field of the 3D vertebra is created in a vertebra coordinate system (VCS). Then, the Reeb graph is adopted for topological data analysis on the scalar field. Morphological features of the 3D vertebra are separated using a cycle-detect-based algorithm in the Reeb graph, and the valid pedicle region is finally generated. Pedicle morphometry is measured for surgical references. Results:Experiments on the dataset from the CSI 2014 Workshop with our method show that the spinous process and vertebral body are 100% (255/255) recognized, the pedicle is 99.8% (509/510) recognized, the transverse process is 94.1% (240/255) recognized. The parameters incl. chord length and diameter of pedicle morphometry are measured and verify the efficiency of the valid pedicle region deduced from the recognized pedicle. Conclusion:Topological data analysis is an effective and promising automatic tool for segmenting and recognizing morphological features on the 3D vertebra. The final extracted valid pedicle region and its pedicle morphometry can provide good references for pedicle screw placement.

An Automatic Path Planning Method of Pedicle Screw Placement Based on Preoperative CT Images

Due to good fixation and obvious clinical advantages in the treatment of spinal diseases, pedicle screw technology is widely employed in spinal surgery. However, most screw placement paths are manually selected, which is a complicated and time-consuming process. Therefore, various kinds of spine surgery robots have been invented to assist during surgeries. An automatic path planning method of pedicle screw placement is proposed in this article. First, based on preoperative CT scans, the spine of the patient can be segmented and each individual vertebra is classified by using a trained deep learning network model. Additionally, a local coordinate system in every single vertebra is established according to its anatomical characteristics. Second, after analyzing the images to recognize the feature points, the pedicle region and route of screw placement are identified, which renders completion of the surgical path planning automatic. The experimental results show that 93&#x0025; of the planning results can be directly applied in surgeries after surgeon evaluation. In addition to the improvement in accuracy, the whole algorithm does not rely on any human assistance. This fully automatic process greatly reduces the time required for surgical planning.

MRI to C-arm spine registration through Pseudo-3D CycleGANs with differentiable histograms.

Image-guided spine surgery increasingly relies on diagnostic MRI for device navigation, as it allows to visualize the nerves and soft tissues during screw insertion in the pedicle region, which is not possible with preoperative CT or cone beam CT. However, registration of MRI to C-arm images remains difficult due to differences in visible tissue. In this paper, we introduce a three-dimensional/two-dimensional (3D/2D) registration method of preoperative T2-weighted MRI of the lumbar spine to C-arm X-ray using synthetic CT images. The registration work is based on a pseudo-3D CycleGAN integrating a new cyclic loss function to ensure consistency in MRI and CT synthesis using differentiable histograms to match the multimodal distributions. The unified framework allows to improve bony tissue inference as opposed to regular 2D CycleGAN for image synthesis. A multiplanar digitally reconstructed radiograph (DRR) registration approach aligns the 3D and 2D images. Experiments performed on a public dataset of 18 pathological spines yielded a mean dice coefficient of 0.84±0.015 on synthetic CTs. The DRR registration experiments, on the other hand, presented a target localization error of 2.1±0.2mm. Intensity distributions and voxel-wise errors in Hounsfield units show encouraging results, illustrating the network's flexibility of producing qualitatively and quantitatively reasonable synthetic CT scans that can be used in a surgical 3D/2D registration framework. These promising results demonstrate the potential of the synthesis tool prior to integration in an image-guidance system.

Dual pitch titanium-coated pedicle screws improve initial and early fixation in a polyetheretherketone rod semi-rigid fixation system in sheep.

Background:Reports on the efficacy of modifications to the thread design of pedicle screws are scarce. The aim of the study was to investigate initial and early fixation of pedicle screws with a plasma-sprayed titanium coating and dual pitch in the pedicle region (dual pitch titanium-coated pedicle screw [DPTCPS]) in a polyetheretherketone (PEEK) rod semi-rigid fixation system.Methods:Fifty-four sheep spine specimens and 64 sheep were used to investigate initial (“0-week” controls) and early (post-operative 6 months) fixation, respectively. Sheep were divided into dual pitch pedicle screw (DPPS), standard pitch pedicle screw (SPPS), DPTCPS, and standard pitch titanium-coated pedicle screw (SPTCPS) groups. Specimens/sheep were instrumented with four screws and two rods. Biomechanical evaluations were performed, and histology at the implant-bone interface was investigated.Results:At 0-week, mean axial pull-out strength was significantly higher for the DPTCPS and SPTCPS than the SPPS (557.0 ± 25.2 vs. 459.1 ± 19.1 N, t = 3.61, P < 0.05; 622.6 ± 25.2 vs. 459.1 ± 19.1 N, t = 3.43, P < 0.05). On toggle-testing, the DPTCPS was significantly more resistant than the SPPS and SPTCPS (343.4 ± 16.5 vs. 237.5 ± 12.9 N, t = 3.52, P < 0.05; 343.4 ± 16.5 vs. 289.9 ± 12.8 N, t = 3.12, P < 0.05; 124.7 ± 13.5 vs. 41.9 ± 4.3 cycles, t = 2.18, P < 0.05; 124.7 ± 13.5 vs.79.5 ± 11.8 cycles, t = 2.76, P < 0.05). On cyclic loading, maximum displacement was significantly lower for the DPTCPS than the SPPS and SPTCPS (1.8 ± 0.13 vs. 3.76 ± 0.19 mm, t = 2.29, P < 0.05; 1.8 ± 0.13 vs. 2.46 ± 10.20 mm, t = 2.69, P < 0.05). At post-operative 6 months, mean axial pull-out strength was significantly higher for the DPTCPS and SPTCPS than the SPPS (908.4 ± 33.6 vs. 646.5 ± 59.4 N, t = 3.34, P < 0.05; 925.9 ± 53.9 vs. 646.5 ± 59.4 N, t = 3.37, P < 0.05). On toggle-testing, the DPTCPS was significantly more resistant than the SPPS and SPTCPS (496.9 ± 17.9 vs. 370.3 ± 16.4 N, t = 2.86, P < 0.05; 496.9 ± 17.9 vs. 414.1 ± 12.8 N, t = 2.74, P < 0.05; 249.1 ± 11.0 vs.149.9 ± 11.1 cycles, t = 2.54, P < 0.05; 249.1 ± 11.0 vs.199.8 ± 7.2 cycles, t = 2.61, P < 0.05). On cyclic loading, maximum displacement was significantly lower for the DPTCPS than the SPPS and SPTCPS (0.96 ± 0.11 vs. 2.39 ± 0.14 mm, t = 2.57, P < 0.05; 0.96 ± 0.11 vs. 1.82 ± 0.12 mm, t = 2.73, P < 0.05). Resistance to toggle testing (370.3 ± 16.4 vs. 414.1 ± 12.8 N, t = 3.29, P < 0.05; 149.9 ± 11.1 vs.199.8 ± 7.2 cycles, t = 2.97, P < 0.05) was significantly lower and maximum displacement in cyclic loading (2.39 ± 0.14 vs.1.82 ± 0.12 mm; t = 3.06, P < 0.05) was significantly higher for the SPTCPS than the DPTCPS. Bone-to-implant contact was significantly increased for the DPTCPS compared to the SPPS (58.3% ± 7.0% vs. 36.5% ± 4.4%, t = 2.74, P < 0.05); there was no inflammatory reaction or degradation of coated particles.Conclusion:DPTCPSs might have stronger initial and early fixation in a PEEK rod semi-rigid fixation system.

Testing Pullout Strength of Pedicle Screw Using Synthetic Bone Models: Is a Bilayer Foam Model a Better Representation of Vertebra?

Study DesignA biomechanical study.PurposeA new biomechanical model of the vertebra has been developed that accounts for the inhomogeneity of bone and the contribution of the pedicle toward the holding strength of a pedicle screw.Overview of LiteraturePullout strength studies are typically carried out on rigid polyurethane foams that represent the homogeneous vertebral framework of the spine. However, the contribution of the pedicle region, which contributes to the inhomogeneity in this framework, has not been considered in previous investigations. Therefore, we propose a new biomechanical model that can account for the vertebral inhomogeneity, especially the contribution of the pedicles toward the pullout strength of the pedicle screw.MethodsA bilayer foam model was developed by joining two foams representing the pedicle and the vertebra. The results of the pullout strength tests performed on the foam models were compared with those from the tests performed on the cadaver lumbar vertebra.ResultsSignificant differences (p <0.05) were observed between the pullout strength of the pedicle screw in extremely osteoporotic (0.18±0.11 kN), osteoporotic (0.37±0.14 kN), and normal (0.97±0.4 kN) cadaver vertebra. In the monolayer model, significant differences (p <0.05) were observed in pullout strength between extremely osteoporotic (0.3±0.02 kN), osteoporotic (0.65±0.12 kN), and normal (0.99±0.04 kN) bone model. However, the bilayer foam model exhibited no significant differences (p >0.05) in the pullout strength of pedicle screws between osteoporotic (0.85±0.08 kN) and extremely osteoporotic bone models (0.94±0.08 kN), but there was a significant difference (p <0.05) between osteoporotic (0.94±0.08 kN) and normal bone models (1.19±0.05 kN). There were no significant differences (p >0.05) in pullout strength between cadaver and bilayer foam model in normal bones.Conclusions The new synthetic bone model that reflects the contribution of the pedicles to the pullout strength of the pedicle screws could provide a more efficacious means of testing pedicle-screw pullout strength. The bilayer model can match the pullout strength value of normal lumbar vertebra bone whereas the monolayer foam model was able to match that of the extremely osteoporotic lumbar vertebra.

Pedicle screw placement in patients with variant atlas pedicle.

ObjectiveTo investigate how the anatomy of variant atlas vertebra impacts on the strategy used to place pedicle screws used to treat atlantoaxial instability.MethodsThe study enrolled patients with cervical instability who had a posterior arch pedicle height <3.5 mm at the anchor point, a vertebral artery groove height <3.5 mm, or both. Pedicle screws were fitted according to the anatomy of the variant atlas vertebra. Patients were followed-up to evaluate accuracy of the screw placement and maintenance of cervical stability.ResultsA total of 28 patients were enrolled. The mean height of the atlas pedicle proximal section was >5.0 mm. For the vertebral artery groove, the height of the lateral region was significantly greater than that of the medial region. Approximately 60% of atlas vertebrae had lateral heights >3.5 mm (34 of 56). The majority of the posterior arch heights were <3.0 mm. There were no perioperative or postoperative complications observed.ConclusionsPedicle screw placement in the lateral pedicle region is the safest and most reliable strategy to treat variant atlas pedicles.

Static Test of Flexo Compression in Posterior Thoracolumbar Spinal Fixation with Conical Cylindrical Screws

Introduction The aim of this paper is to describe the results obtained of a static test of flexo compression in polyaxial transpedicular conical cylindrical screws; which we have used for more than 20 years; in posteriors thoracolumbar spine fixings based on the ASTM F 1717–04 standard. This trial was made at the National Institute of Industrial Technology at its Center for Research and Development in Mechanical in Buenos Aires, Argentina. Material and Methods Two spinal fixation systems were evaluated, both composed by transpedicular conical soul and cylindrical thread screws, bars and tulips. In Table 1 the information provided by the manufacturer of the screws and identification adopted in the test for each sample is presented. [Table: see text] A universal testing machine Tinius Olsen H50-KT was used for the study. The proposed paragraph 8.1.1 of ASTM F1717–04 procedure was followed. The assembly of the samples was performed using the steps proposed by us, the tulips were set at the distance established by the Standard and the terminal screw of them was set by applying a torque of 10 Nm. The assays were performed at a speed of 10 mm / min and were interrupted when the load drop was observed in the instant graphic verifying in this way the loss of poliaxiality. The test setup used and the location of the devices during the test are shown in Fig. 1 . [Figure: see text] The active length of longitudinal component (L) was revealed (see Fig. 1 ). From this value the corresponding displacement of 2% of the permanent deformation was calculated, Point B =Lx 0, 02 (see Fig. 2 ). From the assay were obtained (see Fig. 2 ): The Load-displacement curve; The fluence load flexo-compression (N) (point D); The fluence displacement 2% (mm) (point A) # the elastic displacement (mm): difference between the value of point A and point B. Elastic displacement = Point A-Point B # the flexo-compression rigidity (N / mm): ratio between the loading of flexo-compression fluence and the elastic displacement. Rigidity = Point D Point A – Point B # the final displacement (mm), and # the final load (N) [Figure: see text] Results Fig. 3 shows the curve Load versus Displacement obtained for samples 018565- (1–2) [Figure: see text] In Table 2 the results are described. [Table: see text] Conclusion Screws and prisoners remained adjusted and there were not apparent permanent deformation in any of the system components. Its cylindrical thread and conical soul provides a high strength profile improving primary fixation and the resistance to the pull out. The major diameter of the soul in the pedicle region increases the bending and the resistance in fixations without arthrodesis preventing the breaking of the screw. All tests were performed within the framework of Management System for Quality Assurance of INTI-Mechanics.

Variation on a Theme: Alternative to Plastic Bag in ALPPS Procedures: Feasibility and Clinical Safety of COVA+™ Membrane in ALPPS Procedures.

Associating liver partition and portal vein ligation for staged hepatectomy (ALPPS) got wide success among hepatic surgeons as an efficient way to shorten to 7 days from the 4 weeks interval of classic 2-staged hepatectomy. The main disadvantage of ALPPS is the onset of inflammatory adhesions, particularly on the hepatic pedicle region, previously dissected. The aim of the study is the evaluation of a resorbable collagen membrane (CM) indicated in the prevention of postoperative adhesions as an alternative to the use of a plastic bag (PB) during ALPPS procedure. All patients undergoing ALPPS procedure in our department were prospectively included in a database. At the end of the first surgery, at least one resorbable CM (COVA+™, Biom'Up, France) was placed instead of a PB. Intraoperative adhesions during the second step and clinical short-term safety were assessed. Ten patients with a mean age of 57.5 years underwent a 2-staged hepatectomy through ALPPS approach. At the second stage, 90 % of the patients experienced either grade-0 (no adhesion) or grade-I adhesions (mild adhesions easily divided). None of the reported complications were related to the use of the CM. To our knowledge, this is the first clinical study evaluating the use of an anti-adhesion resorbable CM as a safe and efficient alternative to PB in ALPPS procedures.

English

The major goal of this study is to determine the stress on vertebrae subjected to hyperextension loading. In addition, probabilistic analysis was adopted in finite element analysis (FEA) to verify the parameters that affected failure. Probabilistic finite element (PFE) analysis plays an important role today in solving engineering problems in many fields of science and industry and has recently been applied in orthopaedic applications. A finite element model of the L2 vertebra was constructed in SolidWorks and imported by ANSYS 11.0 software for the analysis. For simplicity, vertebra components were modelled as isotropic and linear materials. A tetrahedral solid element was chosen as the element type because it is better suited to and more accurate in modelling problems with curved boundaries such as bone. A Monte Carlo simulation (MCS) technique was performed to conduct the probabilistic analysis using a built-in probabilistic module in ANSYS with 100 samples. It was found that the adjacent lower pedicle region depicted the highest stress with 1.21 MPa, and the probability of failure was 3%. The force applied to the facet (FORFCT) variable needs to be emphasized after sensitivity assessment revealed that this variable is very sensitive to the stress and displacement output parameters.

Biomechanical Analysis of Pedicle Screws in Osteoporotic Bone With Bioactive Cement Augmentation Using Simulated In Vivo Multicomponent Loading

Biomechanical analysis of bioactive cements augmenting pedicle screw resistance to loosening in osteoporotic synthetic bone. To simulate in vivo loading-loosening of pedicle screws in osteoporotic vertebrae; and to compare biomechanical efficacy of the following bioactive cements: calcium phosphate (CP), calcium sulfate (CS), and proprietary mixture (M). Pedicle screw instrumentation in osteoporotic spines is limited by poor bone-screw interface strength, resulting in screw loosening fixation failure. Previous in vivo studies evaluated augmented pedicle screw resistance to pure pullout, not simulating in vivo loading/failure. A pedicle screw-instrumented osteoporotic thoracic vertebra subjected to combined pullout, transverse, moment loading was simulated. Unconstrained 3-dimensional screw motion relative to vertebra was optically measured during quasi-static, and dynamic loading. Augmented groups (CP, CS, M) produced (P < 8.0E-07) higher quasi-static failure initiation force (61.2,45.6, 40.3 N) than those by the nonaugmented group (21.0 N), with no significant difference in small screw displacement up to these loads. Nonaugmented screw motion after failure initiation was primarily rotation (toggle-migration) with minimal pullout until the screw tip contacted the superior endplate, followed by more prominent screw pullout. Augmented screw motion (with cement remaining intact on screw) was similar, but with eventual bone fracture anterior to the pedicle region. Dynamic loading produced similar failure initiation force and screw motion. We believe our test protocol produced screw loosening failure similar to that observed clinically, and that it has the ability to detect differences in failure initiation force and failure modes to compare short-term efficacy of screw augmentation techniques. All cements improved screw resistance to failure. The CP > CS > M failure initiation force (P < 0.006) was because of differences in cement distribution. Animal studies may be required to characterize the remodeling activity of bioactive cements and their longer term efficacies.

Finite element investigation of the loading rate effect on the spinal load-sharing changes under impact conditions

Sudden deceleration and frontal/rear impact configurations involve rapid movements that can cause spinal injuries. This study aimed to investigate the rotation rate effect on the L2–L3 motion segment load-sharing and to identify which spinal structure is at risk of failure and at what rotation velocity the failure may initiate? Five degrees of sagittal rotations at different rates were applied in a detailed finite-element model to analyze the responses of the soft tissues and the bony structures until possible fractures. The structural response was markedly different under the highest velocity that caused high peaks of stresses in the segment compared to the intermediate and low velocities. Under flexion, the stress was concentrated at the upper pedicle region of L2 and fractures were firstly initiated in this region and then in the lower endplate of L2. Under extension, maximum stress was located in the lower pedicle region of L2 and fractures started in the left facet joint, then they expanded in the lower endplate and in the pedicle region of L2. No rupture has resulted at the lower or intermediate velocities. The intradiscal pressure was higher under flexion and decreased when the endplate was fractured, while the contact forces were greater under extension and decreased when the facet surface was cracked. The highest ligaments stresses were obtained under flexion and did not reach the rupture values. The endplate, pedicle and facet surface represented the potential sites of bone fracture. Results showed that spinal injuries can result at sagittal rotation velocity exceeding 0.5°/ms.

MRI Signal Changes of the Pedicle as an Indicator for Early Diagnosis of Spondylolysis in Children and Adolescents

Clinical review of pediatric patients with lumbar spondylolysis and biomechanical analysis using finite-element lumbar spine model. To evaluate the usefulness of the signal changes observed on MR images of the pedicle for the early diagnosis of spondylolysis, and to investigate the pathomechanism of the signal changes based on the stresses in pedicles, as predicted using finite-element analyses. Furthermore, to evaluate the usefulness of the signal change to predict the bony healing following conservative treatment. Since early-stage spondylolysis can achieve osseous healing conservatively, it is important to diagnose this disorder as early as possible. Presently, there is no well-established, noninvasive, and reliable diagnostic tool for the early diagnosis. Thirty-seven pediatric patients with spondylolysis were included. Sixty-eight defects were examined and their stages as revealed on CT scans were recorded. High signal changes (HSC) of the pedicles on axial T2-weighted MRI were compared with the CT-based stages of the defect. Among them, 16 patients, including 15 boys and 1 girl, were treated conservatively for at least a 3-month period. Bony healing of the fracture site was evaluated on CT, and the results were compared between two groups with or without HSC at the initial consultation. Using a three-dimensional nonlinear finite-element model of the L3-L5 segment, stress distributions in the pars and pedicle regions were analyzed in response to 400 N compression and 10.6 Nm moment. Based on CTs, 68 pars defects were classified as follows: 8 very early, 24 late-early, 16 progressive, and 20 terminal stages. All defects in very early and late-early stages (100%) showed HSC on T2-weighted MRI at the ipsilateral pedicle. Among 16 progressive stages, eight (50%) showed HSC, while no defects of the terminal stage (0%) were found to have HSC. In total, 29 pars defects were treated conservatively out of 16 patients. In 19 of the HSC positive defects, 15 (79%) showed bony healing after the conservative treatment, whereas none of the 10 HSC negative defects (0%) showed any healing. The results were statistically significant at P < 0.05 (chi). Stress results from the finite-element model indicated that pars interarticularis showed the highest value in all loading modes, and the pedicle showed the second highest. The correlation between the high stresses in the pedicle and the corresponding HSC suggest that signal changes in MRI could be used as an indicator for early diagnosis of spondylolysis. The HSC of the pedicle provided useful information to diagnose early stage spondylolysis. Furthermore, the HSC may be a good indicator as to whether a bony union will result from conservative treatment.