Impact of extracellular matrix and collagen network properties on the cervical intervertebral disc response to physiological loads: A parametric study.

Current intervertebral disc finite element models are hard to validate since they describe multi-physical phenomena and contain a huge number of material properties. This work aims to simplify numerical validation/identification studies by prioritizing the sensitivity of intervertebral disc behavior to mechanical properties. A 3D fiber-reinforced hyperelastic model of a C6-C7 intervertebral disc is used to carry out the parametric study. 10 parameters describing the extracellular matrix and the collagen network behaviors are included in the parametric study. The influence of varying these parameters on the disc response is estimated during physiological movements of the head, including compression, lateral bending, flexion, and axial rotation. The obtained results highlight the high sensitivity of the disc behavior to the stiffness of the annulus fibrosus extracellular matrix for all the studied loads with a relative increase in the disc apparent stiffness by 67% for compression and by 57% for axial rotation when the annulus stiffness increases from 0.4 to 2 MPa. It is also shown that varying collagen network orientation, stiffness, and stiffening in the studied configuration range have a noticeable effect on rotational motions with a relative apparent stiffness difference reaching 6.8%, 10%, and 22%, respectively, in lateral bending. However, the collagen orientation does not affect disc response to axial load.

Flip-flop method: A new T1-weighted flow-MRI for plants studies.

The climate warming implies an increase of stress of plants (drought and torrential rainfall). The understanding of plant behavior, in this context, takes a major importance and sap flow measurement in plants remains a key issue for plant understanding. Magnetic Resonance Imaging (MRI) which is well known to be a powerful tool to access water quantity can be used to measure moving water. We describe a novel flow-MRI method which takes advantage of inflow slice sensitivity. The method involves the slice selectivity in the context of multi slice spin echo sequence. Two sequences such as a given slice is consecutively inflow and outflow sensitive are performed, offering the possiblility to perform slow flow sensitive imaging in a quite straigthforward way. The method potential is demonstrated by imaging both a slow flow measurement on a test bench (as low as 10 µm.s-1) and the Poiseuille's profile of xylemian sap flow velocity in the xylematic tissues of a tomato plant stem.

Experimental evaluation of stent retrievers' mechanical properties and effectiveness.

BACKGROUND: Five randomized controlled trials recently appeared in the literature demonstrating that early mechanical thrombectomy in patients with acute ischemic stroke is significantly related to an improved outcome. Stent retrievers are accepted as the most effective devices for intracranial thrombectomy. OBJECTIVE: To analyze the mechanical properties of stent retrievers, their behavior during retrieval, and interaction with different clots and to identify device features that might correlate with the effectiveness of thrombus removal. MATERIALS AND METHODS: All stent retrievers available in France up to June 2015 were evaluated by mechanical and functional tests aimed at investigating the variation of their radial force and their behavior during retrieval. Devices were also tested during in vitro thrombectomies using white and red experimental thrombi produced with human blood. Functional tests and in vitro thrombectomies were conducted using a rigid 3D printed vascular model. RESULTS: Mechanical tests showed a variation in radial force during retrieval for each stent. A constant radial force during retrieval was related to continuous cohesion over the vessel wall and a higher rate of clot removal efficacy. All stent retrievers failed when interacting with white large thrombi (diameter ≥6 mm). CONCLUSIONS: None of the tested devices were effective in removing white clots of large diameter (≥6 mm). Constant radial force during retrieval allows constant cohesion to the vessel wall and pressure over the clot; such features allow for a higher rate of clot removal.

The pressure reduction coefficient: A new parameter to assess aneurysmal blood stasis induced by flow diverters/disruptors.

Background and purpose Pore density (PD), surface metal coverage (SMC) and the number of wires are all different parameters which can influence the efficacy of a flow disruptor/diverter. Nevertheless, the relative importance of a parameter to induce intra-aneurysmal blood stasis is still poorly evaluated. Therefore, comparison between devices based on a unique value is not reliable. The aim of this study was to propose a new bench top parameter (the pressure reduction coefficient (PRC; ξ)) in order to assess the global haemodynamic effect of each flow diverter/disruptor to slow flow. Methods Eight devices were tested in vitro during three different flow conditions. For the eight devices, the PRC was computed at different volumetric flow rates to characterise flow reduction. Comparison was made with SMC, PD and the number of wires. Results The PRC obtained for flow disruptors was on average 1.5 times more efficient in reducing flow compared to flow diverters. PD (mm2) ranged from 24 to 38 for flow diverters and did not independently correlate with the PRC. The SMC of flow diverters ranged from 25% to 70%, and ranged from 20% to 100% for flow disruptors, without independent correlation to the PRC. The number of wires ranged from 48 to 96 for the flow diverters and did not correlate independently to the PRC. Conclusion There were no direct correlations between individual device characteristics and the PRC, suggesting a multifaceted and interrelating association of the overall design of each implant. Hence, the PRC could be used as a simple, reliable parameter to assess the overall capacity of flow disruptors/diverters to induce intra-aneurysmal blood stasis.

Influence of asymmetric tether on the macroscopic permeability of the vertebral end plate.

We implemented an experimental model of asymmetrical compression loading of the vertebral end plate (VEP) in vivo. The macroscopic permeability of the VEP was measured. We hypothesized that static asymmetrical loading on vertebrae altered the macroscopic permeability of the VEP. In scoliosis, solute transport to and from the disc is dramatically decreased especially at the apical intervertebral disc. The decrease in permeability could be induced by mechanical stress. Nine skeletally immature pigs were instrumented with left pedicle screws and compression rod at the T5/T6 and L1/L2 levels. After 3 months, three cylindrical specimens of the VEP were obtained from each of the tethered levels. A previously validated method for measuring permeability, based on the relaxation pressure due to a transient-flow rate was used. A pistoning device generated a fluid flow that fully saturated the cylindrical specimen. The decrease in upstream pressure was measured using a pressure transducer, which allowed the macroscopic permeability to be derived. A microscopic study completed the approach. Overall macroscopic permeability was lower for the tethered VEPs than for the VEPs of the control group, respectively -47% for flow-in (p = 0.0001) and -46% for flow-out (p = 0.0001). In the tethered group, macroscopic permeability of the specimens from the tethered side was lower than macroscopic permeability of those from the non-tethered side, -39% for flow-out (p = 0.024) and -47% for flow-in (p = 0.13). In the control group, the macroscopic permeability was greater in the center of the VEP than in its lateral parts for flow-out (p = 0.004). Macroscopic permeability of the center of the VEPs was greater for flow-out than for flow-in (p = 0.02). There was no significant difference between thoracic and lumbar. This study demonstrated that compression loading applied to a growing spine results in decreased permeability of the VEP. This result could be explained by local remodeling, such as calcification of the cartilage end plate or sclerosis of the underlying bone.

Influence of location, fluid flow direction, and tissue maturity on the macroscopic permeability of vertebral end plates.

STUDY DESIGN: We implemented a pilot study in a growing animal model. The macroscopic permeability of the vertebral endplates was measured. The influence of location, tissue maturity, and fluid flow direction was quantified. OBJECTIVE: We hypothesized that the macroscopic permeability of vertebral endplate may decrease with maturity of the vertebral segment. SUMMARY OF BACKGROUND DATA: The alternation of loading induced by the diurnal cycle generates convective flux into the vertebral segment with the dominant flow path through the vertebral endplates. The alteration of mass transport at the disc-vertebrae interface may interrupt the mechanobiologic balance, and have an effect such as degenerative changes or scoliosis. METHODS: A previously validated method for measuring permeability, based on the relaxation pressure caused by a transient-flow rate was used. Three specimens were extracted from each L1 to L5 endplate. Seventy-one specimens were frozen, and 64 were stored fresh in a standard culture media. A microscopic analysis completed the biomechanical analysis. RESULTS: At 2, 4, and 6 months, the mean permeability (10(-14) m/N x s, flow-in/flow-out) of the central zone was respectively: 1.23/1.66, 1.03/1.29, and 0.792/1.00. Laterally, it was 1.03/1.19, 1.094/1.001, and 0.765/0.863. For all groups, cartilage endplate and growth plate were both thinner in the center of the plate. Weak differences of the vascular network were detected, except for a small increase of vascular density in the central zone. CONCLUSION: The results from this animal study showed that the central zone of the vertebral endplate was more permeable than the periphery and the flow-out permeability was up to 35% greater than the flow-in permeability. Increase of permeability with decrease of cartilage thickness was noticed within the same age group. We also found a statistically significant decrease of the macroscopic permeability correlated with the tissue maturity.

A measurement technique to evaluate the macroscopic permeability of the vertebral end-plate.

The remodelling response of vertebral segments in idiopathic scoliosis or disc degeneration, shows a modification of the disc hydration. The investigation of mass transport between the disc and the vertebral body is relevant to understanding the normal and pathological behaviour of the spine. The measurement method we adopted, to derive the macroscopic permeability of the vertebral end-plate, used the relaxation pressure due to a transient-flow rate into the biological structure. Our approach mimicked the in vivo conditions of mass transfer between the disc and the vertebral bodies. It minimized the flow-induced matrix compaction and it allowed the boundary conditions of the specimen to be controlled. We proposed a specific theoretical method and an associated device according to poroelastic theory. A preliminary evaluation with a controlled porous medium, preceded a pilot study in an animal model (lumbar L(4)-L(5) segment of a 4 months old female pig). The macroscopic permeability of multilayered cylindrical specimens including the trabecular bone, subchondral bone and cartilage end-plate was measured, using a 'cartilage-towards-bone' fluid flow direction. A histologic evaluation completed the biomechanical approach. Results showed that the central zone was more permeable than the periphery and this concurred with qualitative studies from the literature.

Use of high-resolution MRI for investigation of fluid flow and global permeability in a material with interconnected porosity.

We present a multi-scale experimental approach designed to improve the investigation of both localized and global fluid flow in biomaterials with randomly interconnected porosity. Coralline hydroxyapatite (ProOsteon 500 from Interpore-Cross), having a relatively well-defined porosity, was used as an in vitro model of typical bone architecture. Axial fluid velocity profiles within the pores of a cylindrical hydroxyapatite sample were characterized using high-resolution MRI in conjunction with the measurement of global flow and associated permeability based on the Darcy-type relationship. Assuming Newtonian fluid behaviour, image analysis permitted computation of local porosity, intra-pore fluid shear, and visualization of flow heterogeneity within the sample. These results may benefit applications in biomaterials for the evaluation of factors influencing bony incorporation in porous scaffolds and on porous implant and bone surfaces. Normal and diseased biological tissues are also clinical relevant applications.

High-resolution magnetic resonance flow imaging in a model of porous bone-implant interface.

PURPOSE: To evaluate the application of high-resolution MRI methodology for characterizing the fluid velocity field and evaluate fluid shear field within a simplified in vitro model of a bone-implant interface. MATERIALS AND METHODS: The study used a specific micromotion canine bone implant that has been used for over a decade in the experimental evaluation of anatomical, biomaterial, mechanical and surgical factors influencing the quality of the implant interface. To allow its implementation in an MR coil, a nonmagnetic model of the micromotion implant was fabricated. The model consisted of a cylinder of polymethylmethacrylate (PMMA) representing the implant, located within an annular controlled gap into a block of coralline-derived bulk porous hydroxyapatite (HA; Interpore Cross International, Irvine, CA, USA). The assembly was potted in a polycarbonate shell and connected to a gravity-feed flow system consisting of a water fluid reservoir and peristaltic pump. Cross-sectional fluid velocity images through the principal axis of the implant were generated using a phase-encoding MR imaging technique; axial fluid flow was derived, and fluid shear was evaluated using a Newtonian fluid model. RESULTS: Due to the nonuniform gap of the actual experimental construct, a highly nonuniform flow through the annular gap and a secondary flow through the porous HA block were observed. Axial velocity magnitudes in the range 0.04 to 14 mm/s were measured, and the flow velocities within the annular gap and the surrounding bone differed by nearly two orders of magnitude. Image analysis showed that 95% of total flow passed through the annular gap and 5% was transported through the porous HA block. Fluid shear was computed within the porous structure and the annular gap, and they differed by one order of magnitude. CONCLUSION: We demonstrated that high-resolution MR flow imaging has the resolution to measure fluid transport processes noninvasively through a nonmagnetic model bone implant. Gap fluid flow and fluid flow into the permeable skeleton (HA block) were quantified, and these data allowed the noninvasive determination of fluid shear. These promising results are encouraging for applications in biological tissue, artificial bone substitutes, tissue engineering and clinically relevant studies concerning implant fixation.

Computer assisted planning and orbital surgery: patient-related prediction of osteotomy size in proptosis reduction.

BACKGROUND: Proptosis is characterized by a protrusion of the eyeball due to an increase of the orbital tissue volume. To recover a normal eyeball positioning, the most frequent surgical technique consists in the osteotomy of orbital walls combined with the manual loading on the eyeball. Only a rough clinical rule is currently available for the surgeons but it is useless for this technique. The first biomechanical model dealing with proptosis reduction, validated in one patient, has been previously proposed by the authors. METHODS: This paper proposes a rule improving the pre-operative planning of the osteotomy size in proptosis reduction. Patient-related poroelastic finite element models combined with sensitivity studies were used to propose two clinical rules to improve the pre-operative planning of proptosis reduction. This poroelastic model was run on 12 patients. Sensitivity studies permitted to establish relationships between the osteotomy size, the patient-related orbital volume, the decompressed tissue volume and the eyeball backward displacement. FINDINGS: The eyeball displacement and the osteotomy size were non-linearly related: an exponential rule has been proposed. The patient-related orbital volume showed a significant influence: a bi-quadratic analytical equation liking the osteotomy size, the orbital volume and the targeted eyeball protrusion has been established. INTERPRETATION: Two process rules derived from patient-related biomechanical FE models have been proposed for the proptosis reduction planning. The implementation of the process rules into a clinical setting is easy since only a sagittal radiography is required. The osteotomy size can be monitored using optical guided instruments.

Prediction of tissue decompression in orbital surgery.

OBJECTIVE: A method to predict the relationships between decompressed volume of orbital soft tissues, backward displacement of globe after osteotomy, and force exerted by the surgeon, was proposed to improve surgery planning in exophthalmia reduction. DESIGN: A geometric model and a poroelastic finite element model were developed, based on computed tomography scan data. BACKGROUND: The exophthalmia is characterized by a protrusion of the eyeball. Surgery consists in an osteotomy of the orbit walls to decompress the orbital content. A few clinical observations ruling on an almost linear relationship between globe backward displacement and tissue-decompressed volume are described in the literature. METHODS: Fast prediction of decompressed volume is derived from the geometric model: a sphere in interaction with a cone. Besides, a poroelastic finite element model involving morphology, material properties of orbital components and surgical gesture was implemented. RESULTS: The geometric model provided a better decompression volume estimation than the finite element model. Besides, the finite element model permitted to quantify the backward displacement, the surgical gesture and the stiffness of the orbital content. CONCLUSIONS: The preliminary results obtained for one patient, in accordance with the clinical literature, were relatively satisfying. An efficient aid for location and size of osteotomies was derived and seemed to be able to help in the surgery planning.