Biomechanical assessment of vertebrae with lytic metastases with subject-specific finite element models

The assessment of risk of vertebral fracture in patients with lytic metastases is challenging, due to the complexity in modelling the mechanical properties of this heterogeneous material. Currently clinical assessment of patients at high risk of fracture is based on the Spinal Instability Neoplastic Score (SINS), which however in many cases does not provide clear guidelines. The goal of this study was to develop a computational approach to provide a comparative biomechanical assessment of vertebrae with lytic lesions with respect to the adjacent controls and highlight the critical vertebrae. The computed tomography images of the thoracolumbar spine of eight patients suffering of vertebral lytic metastases with SINS between 7 and 12 (indeterminate unstable) were analysed. For each patient one or two vertebrae with lytic lesions were modelled and the closest vertebrae without lesions were considered as control. Metastatic vertebrae (N = 12) and controls (N = 18) were converted to subject-specific, heterogeneous, isotropic, nonlinear finite element models for simulating uniaxial compression. Densitometric and mechanical properties were computed for each vertebra. In average, similar mechanical properties were found for vertebrae with lytic lesions and controls (e.g. ultimate force equal to 6.2 ± 2.7 kN for vertebrae with lytic lesions and to 6.2 ± 3.0 kN for control vertebrae). Only in three patients the vertebrae with lytic lesions were found to be mechanically weaker (−19% to −75% difference for ultimate stress) than the controls. In conclusion, in this study we presented an approach to estimate the mechanical competence of vertebrae with lytic metastases. It remains to be investigated in a clinical study if this method, together with the SINS, can better classify patients with vertebrae with lytic lesions at high risk of fracture

Development of a computer-aided design and finite element analysis combined method for affordable spine surgical navigation with 3D-printed customized template

Introduction: Revision surgery of a previous lumbosacral non-union is highly challenging, especially in case of complications, such as a broken screw at the first sacral level (S1). Here, we propose the implementation of a new method based on the CT scan of a clinical case using 3D reconstruction, combined with finite element analysis (FEA), computer-assisted design (CAD), and 3D-printing technology to provide accurate surgical navigation to aid the surgeon in performing the optimal surgical technique by inserting a pedicle screw at the S1 level. Materials and Methods: A step-by-step approach was developed and performed as follows: (1) Quantitative CT based patient-specific FE model of the sacrum was created. (2) The CAD model of the pedicle screw was inserted into the sacrum model in a bicortical convergent and a monocortical divergent position, by overcoming the geometrical difficulty caused by the broken screw. (3) Static FEAs (Abaqus, Dassault Systemes) were performed using 500 N tensile load applied to the screw head. (4) A template with two screw guiding structures for the sacrum was designed and manufactured using CAD design and 3D-printing technologies, and investment casting. (5) The proposed surgical technique was performed on the patient-specific physical model created with the FDM printing technology. The patient-specific model was CT scanned and a comparison with the virtual plan was performed to evaluate the template accuracy Results: FEA results proved that the modified bicortical convergent insertion is stiffer (6,617.23 N/mm) compared to monocortical divergent placement (2,989.07 N/mm). The final template was created via investment casting from cobalt-chrome. The template design concept was shown to be accurate (grade A, Gertzbein-Robbins scale) based on the comparison of the simulated surgery using the patient-specific physical model and the 3D virtual surgical plan. Conclusion: Compared to the conventional surgical navigation techniques, the presented method allows the consideration of the patient-specific biomechanical parameters; is more affordable, and the intraoperative X-ray exposure can be reduced. This new patient- and condition-specific approach may be widely used in revision spine surgeries or in challenging primary cases after its further clinical validation

A novel three-dimensional computational method to assess rod contour deformation and to map bony fusion in a lumbopelvic reconstruction after en-bloc sacrectomy.

Introduction: En-bloc resection of a primary malignant sacral tumor with wide oncological margins impacts the biomechanics of the spinopelvic complex, deteriorating postoperative function. The closed-loop technique (CLT) for spinopelvic fixation (SPF) uses a single U-shaped rod to restore the spinopelvic biomechanical integrity. The CLT method was designed to provide a non-rigid fixation, however this hypothesis has not been previously tested. Here, we establish a computational method to measure the deformation of the implant and characterize the bony fusion process based on the 6-year follow-up (FU) data. Materials and Methods: Post-operative CT scans were collected of a male patient who underwent total sacrectomy at the age of 42 due to a chordoma. CLT was used to reconstruct the spinopelvic junction. We defined the 3D geometry of the implant construct. Using rigid registration algorithms, a common coordinate system was created for the CLT to measure and visualize the deformation of the construct during the FU. In order to demonstrate the cyclical loading of the construct, the patient underwent gait analysis at the 6th year FU. First, a region of interest (ROI) was selected at the proximal level of the construct, then the deformation was determined during the follow-up period. In order to investigate the fusion process, a single axial slice-based voxel finite element (FE) mesh was created. The Hounsfield values (HU) were determined, then using an empirical linear equation, bone mineral density (BMD) values were assigned for every mesh element, out of 10 color-coded categories (1st category = 0 g/cm3, 10th category 1.12 g/cm3). Results: Significant correlation was found between the number of days postoperatively and deformation in the sagittal plane, resulting in a forward bending tendency of the construct. Volume distributions were determined and visualized over time for the different BMD categories and it was found that the total volume of the elements in the highest BMD category in the first postoperative CT was 0.04 cm3, at the 2nd year, FU was 0.98 cm3, and after 6 years, it was 2.30 cm3. Conclusion: The CLT provides a non-rigid fixation. The quantification of implant deformation and bony fusion may help understate the complex lumbopelvic biomechanics after sacrectomy

First measurement of Ωc0 production in pp collisions at s=13 TeV

The inclusive production of the charm–strange baryon 0 c is measured for the first time via its hadronic √ decay into −π+ at midrapidity (|y| <0.5) in proton–proton (pp) collisions at the centre-of-mass energy s =13 TeV with the ALICE detector at the LHC. The transverse momentum (pT) differential cross section multiplied by the branching ratio is presented in the interval 2 < pT < 12 GeV/c. The pT dependence of the 0 c-baryon production relative to the prompt D0-meson and to the prompt 0 c-baryon production is compared to various models that take different hadronisation mechanisms into consideration. In the measured pT interval, the ratio of the pT-integrated cross sections of 0 c and prompt + c baryons multiplied by the −π+ branching ratio is found to be larger by a factor of about 20 with a significance of about 4σ when compared to e+e− collisions

Elliptic flow of charged particles at midrapidity relative to the spectator plane in Pb–Pb and Xe–Xe collisions

Measurements of the elliptic flow coefficient relative to the collision plane defined by the spectator neutrons v2{ SP} in collisions of Pb ions at center-of-mass energy per nucleon–nucleon pair √ 2.76 TeV and Xe ions at √ sNN = sNN =5.44 TeV are reported. The results are presented for charged particles produced at midrapidity as a function of centrality and transverse momentum for the 5–70% and 0.2–6 GeV/c ranges, respectively. The ratio between v2{ SP} and the elliptic flow coefficient relative to the participant plane v2{4}, estimated using four-particle correlations, deviates by up to 20% from unity depending on centrality. This observation differs strongly from the magnitude of the corresponding eccentricity ratios predicted by the TRENTo and the elliptic power models of initial state fluctuations that are tuned to describe the participant plane anisotropies. The differences can be interpreted as a decorrelation of the neutron spectator plane and the reaction plane because of fragmentation of the remnants from the colliding nuclei, which points to an incompleteness of current models describing the initial state fluctuations. A significant transverse momentum dependence of the ratio v2{ SP}/v2{4} is observed in all but the most central collisions, which may help to understand whether momentum anisotropies at low and intermediate transverse momentum have a common origin in initial state f luctuations. The ratios of v2{ SP} and v2{4} to the corresponding initial state eccentricities for Xe–Xe and Pb–Pb collisions at similar initial entropy density show a difference of (7.0 ±0.9)%with an additional variation of +1.8% when including RHIC data in the TRENTo parameter extraction. These observations provide new experimental constraints for viscous effects in the hydrodynamic modeling of the expanding quark–gluon plasma produced in heavy-ion collisions at the LHC

Longevity of elastin in human intervertebral disc as probed by the racemization of aspartic acid

Background Aging and degeneration of human intervertebral disc (IVD) are associated with biochemical changes, including racemization and glycation. These changes can only be counteracted by protein turnover. Little is known about the longevity of IVD elastin in health or disease. Yet, such knowledge is important for a quantitative understanding of tissue synthesis and degradation. Methods We have measured the accumulation of d-Asp and pentosidine in IVD elastin. Samples representing a broad range of ages (28–82 years) and degeneration grades (1–5) were analyzed. Results d/l-Asp for elastin increased linearly with age from 3.2% (early 30s) to 14.8% (early 80s) for normal tissue (grades 1–2) and from 1.7% (late 20s) to 6.0% (until the mid 50s) for degenerate tissue (grades 3–5) with accumulation rates of 16.2 ± 3.1 × 10− 4 and 11.7 ± 3.8 × 10− 4 year− 1, respectively; no significant difference was found between these values (p < 0.05). Above the mid 50s, a decrease in d-Asp accumulation was recorded in the degenerate tissue. d-Asp accumulation correlated well with pentosidine content for elastin from healthy and degenerate tissues combined. We conclude that IVD elastin is metabolically‐stable and long‐lived in both healthy and degenerate human IVDs, with signs of new synthesis in the latter. The correlation of d‐Asp with pentosidine content suggests that both these agents may be used as markers in the overall aging process of IVD. General significance Accumulation of modified IVD elastin argues for its longevity and may have a negative impact on its role in disc function. Weak signs of newly synthesized molecules may act to counteract this effect in degenerate tissue