Intervertebral Disc Prosthesis Research Articles

Structural modification and biomechanical analysis of lumbar disc prosthesis: A finite element study

BackgroundMost ball-in-socket artificial lumbar disc implanted in the spine result in increased hypermobility of the operative level and overloading of the facet joint. MethodsA finite element model was established and validated for the lumbar spine (L1-L5). The structure of the Mobidisc prosthesis was modified, resulting in the development of two new intervertebral disc prostheses, Movcore and Mcopro. The prostheses were implanted into the L3/L4 level to simulate total disc replacement, and the biomechanical properties of the lumbar spine model were analyzed after the operation. FindingsFollowing the implantation of the prostheses, the mobility of operative level, peak stress of lumbar spine models, and peak stress of facet joint increased. The performance of mobility was found to be more similar between Movcore and Mobidisc. The mobility and facet joint peak stress of the Mcopro model decreased progressively with an increase in the Young's modulus of the artificial annulus during flexion, extension, and lateral bending. Among all the models, the Mcopro50 model had the mobility closest to the intact model. It showed a 3% decrease in flexion, equal range of motion in extension, a 9% increase in left lateral bending, a 7% increase in right lateral bending, and a 3% decrease in axial rotation. InterpretationThe feasibility of the new intervertebral disc prostheses, Movcore and Mcopro, has been established. The Mcopro prosthesis, which features an artificial annular structure, offers significant advantages in terms of reduced mobility of the operative level and peak stress of facet joint.

Rationale for Design Criteria for the Second Generation of Intervertebral Disc Prosthesis

Rationale for Design Criteria for the Second Generation of Intervertebral Disc Prosthesis

Experimental measurements of micromotions of the cementless intervertebral disc prostheses in the cadaver bone

BackgroundSufficient primary stability is mandatory for successful bony prosthetic incorporation. Therefore, defined micromotion values of 150 μm should not be exceeded as higher values might compromise the ingrowth of bone trabeculae to the implant surface. The aim of this study was to evaluate the primary stability of different cementless disc prosthesis in a cadaver model. MethodsFour different implants with different anchoring and bearing concept were tested with a target level of L4/5. 26 specimens were randomly allocated to 1 of the 4 different implants with 6 speciments in each group. Two groups were formed depending on the anchoring (spikes vs. fin) and bearing concept (non−/semi- vs. constrained). Each implant was tested regarding primary stability in a hydraulic simulator allowing simultaneous polyaxial segment movements and axial loading. The measurements were recorded on the lower plate of the prosthesis. FindingsThe majority of the implants showed micromotion values below 200 μm in all planes. Only one prosthesis presented borderline longitudinal amplitudes that were significant higher than the other planes. Furthermore, significant differences were observed in the sagittal plane when comparing spike and keel anchoring. Spike anchoring implants showed superior tresults to keel anchoring implants (40 μm vs. 55 μm; p = .039), while the non−/semi-constrained bearing concept was more advantageous compared to constrained group (40 μm vs. 63 μm; p = .001). InterpretationSpike anchoring and non-constrained implants might provide better primary stability.

Primary stability of the Activ L® intervertebral disc prosthesis in cadaver bone and comparison of the keel and spike anchoring concept

BackgroundHigh primary stability is the key prerequisite for safe osseointegration of cementless intervertebral disc prostheses. The aim of our study was to determine the primary stability of intervertebral disc prostheses with two different anchoring concepts – keel and spike anchoring.MethodsTen ActivL intervertebral disc prostheses (5 x keel anchoring, 5 x spike anchoring) implanted in human cadaver lumbar spine specimens were tested in a spine movement simulator. Axial load flexion, extension, left and right bending and axial rotation motions were applied on the lumbar spine specimens through a defined three-dimensional movement program following ISO 2631 and ISO/CD 18192-1.3 standards. Tri-dimensional micromotions of the implants were measured for both anchor types and compared using Student’s T-test for significance after calculating 95 % confidence intervals.ResultsIn the transverse axis, the keel anchoring concept showed statistically significant (p < 0.05) lower mean values of micromotions compared to the spike anchoring concept. The highest micromotion values for both types were observed in the longitudinal axis. In no case the threshold of 200 micrometers was exceeded.ConclusionsBoth fixation systems fulfill the required criteria of primary stability. Independent of the selected anchorage type an immediate postoperative active mobilization doesn’t compromise the stability of the prostheses.

Long-term clinical and radiological outcomes after total lumbar disc replacement at the lumbosacral junction

The study objective is to analyze long-term clinical and radiological outcomes in patients after performing total lumbar disc replacement at the level of the lumbosacral junction.Materials and methods. A multicenter retrospective observational cohort study was performed. The study included medical records of patients who underwent total lumbar disc replacement at the level of the lumbosacral junction. Both clinical and instrumental parameters were assessed. According to the eligibility criteria, 93 patients were included in the study (38 men, 55 women, mean age 39.9 ± 7.5 years). The average postoperative observation period for the respondents was 33.4 ± 9.5 months.Results. The severity of pain in the lumbar spine on a visual analogue scale significantly decreased (from 7.3 ± 2.5 to 1.9 ± 2.2 cm, p &lt;0.001). The indicators of the quality of life according to the Oswestry Disability Index and according to the Short Form-36 scale (including the mental and physical components) of the studied group of respondents significantly improved (from 43.4 ± 17.6 to 18.9 ± 16.0 %, р &lt;0.001 and from 30.8 ± 8.5 to 52.3 ± 12.4 %, p &lt;0.001, respectively). From the 12th month of observation of the respondents, a significant decrease in the values of the range of motions of the operated segment was noted from 5.3 ± 3.2° to 4.1 ± 3.6° (p &lt;0.009). Also, from the 12th month of postoperative observation, a slight decrease in the values of the range of motions of the adjacent segment from 8.6 ± 4.1° to 7.8 ± 3.8° (p &gt;0.05) was verified. The value of global lumbar lordosis after the installation of a lumbar intervertebral disc prosthesis at the level of the lumbosacral junction significantly increased from 49.1 ± 4.8° to 55.6 ± 5.5° (p &lt;0.001). Intervertebral disc prostheses with a lordotic angle of 10° reliably positively correlate with an increase in the values of segmental lordosis, in contrast to implants with an angle of 6° (rS = 0.86, p = 0.04). The severity of pain in the lumbar spine according to visual analogue scale and the range of motion of the operated segment has a significant negative correlation in the late periods of postoperative follow-up (24 and 36 months, rS = -0.74, p = 0.015).Conclusion. The total lumbar disc replacement technique is an effective method of surgical treatment of patients with degenerative diseases of the intervertebral discs of the lumbosacral spine, including at the level of the lumbosacral junction.

Postoperative Spinal CT: What the Radiologist Needs to Know.

During the past 2 decades, the number of spinal surgeries performed annually has been steadily increasing, and these procedures are being accompanied by a growing number of postoperative imaging studies to interpret. CT is accurate for identifying the location and integrity of implants, assessing the success of decompression and intervertebral arthrodesis procedures, and detecting and characterizing related complications. Although postoperative spinal CT is often limited owing to artifacts caused by metallic implants, parameter optimization and advanced metal artifact reduction techniques, including iterative reconstruction and monoenergetic extrapolation methods, can be used to reduce metal artifact severity and improve image quality substantially. Commonly used and recently available spinal implants and prostheses include screws and wires, static and extendable rods, bone grafts and biologic materials, interbody cages, and intervertebral disk prostheses. CT assessment and the spectrum of complications that can occur after spinal surgery and intervertebral arthroplasty include those related to the position and integrity of implants and prostheses, adjacent segment degeneration, collections, fistulas, pseudomeningoceles, cerebrospinal fluid leaks, and surgical site infections. Knowledge of the numerous spinal surgery techniques and devices aids in differentiating expected postoperative findings from complications. The various types of spinal surgery instrumentation and commonly used spinal implants are reviewed. The authors also describe and illustrate normal postoperative spine findings, signs of successful surgery, and the broad spectrum of postoperative complications that can aid radiologists in generating reports that address issues that the surgeon needs to know for optimal patient management.©RSNA, 2019.

Customised multiphasic nucleus/annulus scaffold for intervertebral disc repair/regeneration

ABSTRACTBackground: In the case of a degenerated intervertebral disc (IVD), even though spinal fusion has provided good short-term clinical results, an alteration of the spine stability has been demonstrated by long-term studies. In this context, different designs of IVD prostheses have been proposed as alternative to spinal fusion. However, over the past few years, much of the recent research has been devoted to IVD tissue engineering, even if several limitations related to the complex structure of IVD are still presented.Purpose/Aim: Accordingly, the aim of the current paper was to develop a strategy in designing customised multiphasic nucleus/annulus scaffolds for IVD tissue engineering, benefiting from the great potential of reverse engineering, additive manufacturing and gels technology.Materials and Methods: The device consisted of a customised additive-manufactured poly(ε-caprolactone) scaffold with tailored architectural features as annulus and a cell-laden collagen-low molecular weight hyaluronic acid-based material as nucleus with specific rheological and functional properties. To this aim, injectability and viscoelastic properties of the hydrogel were analyzed. Furthermore, a mechanical and biological characterization of cell-laden multiphasic nucleus/annulus scaffold was performed.Results and Conclusions: Analyses on the developed devices demonstrated appropriate viscoelastic and mechanical properties. As evidenced by rheological tests, the hydrogel showed a shear-thinning behaviour, supporting the possibility to inject the material. The mechanical characterization highlighted a compressive modulus which falls in the range of lumbar discs, with the typical initial J-shaped stress–strain curve of natural IVDs. Furthermore, preliminary biological tests showed that human mesenchymal stem cells were viable over the culture period.

Ex vivo computed tomography evaluation of loading position on morphometry of the caudal cervical intervertebral disk spaces of dogs.

OBJECTIVE To provide an objective, quantitative morphometric description of the caudal cervical intervertebral disk (IVD) spaces of dogs. SAMPLE Vertebral specimens consisting of C4 through C7 from 5 medium-sized dogs. PROCEDURES CT images were obtained with the specimens positioned in neutral, flexion, extension, and lateral bending positions. Size and shape of the cranial and caudal end plates, angle between the end plates (IVD wedge angle), and craniocaudal distance (IVD width) between end plates for the 4 loading positions were measured and compared for the 3 segments (C4-5, C5-6, and C6-7). RESULTS End plate size and shape, IVD wedge angle, and IVD width were not significantly different among the 3 segments. Caudal cervical end plates were consistently larger than cranial cervical end plates. The IVD wedge angle ranged from -4.8° to 15.2°. Flexion induced a reduction in IVD width in the ventral portion of the IVD, whereas extension induced a decrease in width in the dorsal portion of the IVD. Central IVD width remained unchanged among the loading positions. CONCLUSIONS AND CLINICAL RELEVANCE Unique morphometric and dynamic characteristics of the caudal cervical IVD space of dogs were detected. These findings may help investigators when designing IVD prostheses for dogs with cervical spondylomyelopathy.

Biomechanical response of a novel intervertebral disc prosthesis using functionally graded polymers: A finite element study

With their gradual and continuous properties, functionally graded polymers (FGP) have high potentials to reproduce the regional variation in microstructure/property of the natural intervertebral disc and, therefore, the functional anatomy and biomechanics of the soft tissue. This paper evaluates by finite element analysis the biomechanical response and stress distribution of a novel disc prosthesis using FGP. The kinetics of the FGP parameters is designed using experimental data issued from linear ethylene copolymers over a wide crystallinity range. The radial variation in crystallinity index within the disc prosthesis varies gradually and continuously following a special function in the aim to tailor and optimize the FGP parameters. The experimental data of a healthy human cervical spine segment are used to predict the optimal model of the FGP disc prosthesis loaded under different physiological loading conditions, i.e. rotation, lateral bending and flexion/extension. The results suggest that the FGP parameters can be tailored to control the stiffening, the non-linear behavior, the inelastic effects and the stress distribution in the aim to propose the optimal prosthesis model giving the great opportunity of patient-specific FGP prostheses via 3D printing technologies.

Design of a new intervertebral disc prosthesis

In the degenerative disc disease, the traditional treatment consists in a removal of the intervertebral disc followed by the fusion of the two adjacent vertebral bodies (arthrodesis). An alternative treatment is the use of an artificial intervertebral disc. The advantage of an artificial intervertebral disc is that the mobility of the vertebral column will be restored.Aim of this paper consists in the definition of a prosthetic device for insertion in the intervertebral zone.Different types of artificial bone prostheses have been proposed, seldom based on similarity with physiological discs, then made with a central part (nucleus) consisting of hydrogel and an outer containment belt (annulus) consisting of a plastic material of greater stiffness. Upper and lower containment plates allow realizing a closed structure to be inserted in place of the original disc. Then, in our solution, the external parts (annulus and plates) were modeled by HDPE and the inner part (nucleus) by hydrogel. Both the materials are highly biocompatible. Both materials were modeled by the Ogden parameters related to elastomers. In fact, under the action of near-bending and twisting forces, the disc is subject to large deformations that the materials are able to withstand in elastic conditions. In order to assign the correct Ogden parameters to the materials, a procedure was used to fit the curves acquired by literature. The discs were subjected to compressive load as well as to forward and back bending (flexion and extension), lateral bending, torsion and combined loading.

Interrelation of tropism and angulation parameters of facet joints and results of stabilization surgeries for degenerative diseases of the lumbar spine

Objective. To clarify indications for dynamic and rigid stabilization based on the analysis of correlation between neuroimaging parameters of facet joints (FJ) and clinical outcomes of surgical treatment of patients with degenerative diseases of the lumbar spine. Material and Methods. A total of 141 patients with degenerative diseases of the lumbar spine were surgically treated. Patients were divided into three groups: patients of Group I (n = 48) underwent surgical intervention with artificial intervertebral disc prosthesis; those of Group II (n = 42) – with interbody fusion and combined transpedicular and transfacetal stabilization; and those of Group III (n = 51) – with interbody fusion and bilateral transpedicular stabilization. The correlation between long-term clinical outcomes (pain syndrome according to VAS, functional state according to ODI, and satisfaction with surgical result according to MacNab scale) and preoperative neuroimaging parameters of FJ (degenerative changes according to Fujiwara, facet angle magnitudes, and the presence of tropism) was analyzed. Results. A direct significant nonparametric correlation of neuroimaging parameters of facet angles and FJ tropism with long-term clinical outcomes of surgical treatment according to VAS and ODI was revealed. It was established that good clinical outcomes were achieved with the following preoperative parameters: in Group I, the facet angle was less than 60°, while the presence of tropism had no correlation dependence; in Group II, the facet angle – more than 60°, in the absence of FJ tropism; and in Group III, the facet angle – more than 60°, in the presence of FJ tropism. Conclusion. Objective neuroimaging parameters of the facet angle magnitude of less than 60°, regardless of the presence of tropism, allow performing total arthroplasty. If the facet angle is more than 60°, the rigid stabilization of the operated segment is indicated; in the absence of tropism, a contralateral transfacetal fixation is possible, and in its presence – a bilateral transpedicular stabilization is reasonable.

INTERRELATION OF SPONDYLOMETRIC PARAMETERS WITH THE OUTCOME OF SURGICAL TREATMENT OF PATIENTS WITH DEGENERATIVE DISEASES OF THE LUMBOSACRAL JUNCTION

Objective. To evaluate the relationship between the radiological and neuroimaging parameters of the spinal motion segment and the clinical outcome of surgical treatment of patients with degenerative diseases of the lumbosacral junction to clarify the indications for dynamic and rigid stabilization.Material and Methods. The study included 267 patients with degenerative diseases of the lumbosacral spine. Depending on the stabilization method, patients were divided into two groups: Group I (n = 83) with dynamic intervertebral disc (IVD) prosthesis; and Group II (n = 184) with interbody fusion and transpedicular fixation. Long-term clinical parameters and biomechanical characteristics before and after surgery were analyzed.Results. A significant nonparametric correlation of the long-term result of surgical treatment assessed by VAS and Oswestry Disability Index with radiological parameters and results of neuroimaging was revealed. It was determined that the use of artificial IVD allows achieving a minimum level of pain syndrome and good functional recovery with effective preservation of the volume of physiological movements in the operated segment and restoration of the total angle of lumbar lordosis.Conclusion. Objective neuroimaging data (grade II-IV of degeneration according to the measured diffusion coefficient) and radiological parameters (linear displacement of vertebrae not more than 4 mm, sagittal volume of movements in the spinal motion segment less than 6°, decrease in the height of intervertebral disc space no more than 2/3 of the superjacent one) make possible using total arthroplasty. It is advisable to perform interbody fusion and rigid stabilization in grade IV-V of degeneration, linear displacement of vertebrae more than 4 mm, sagittal volume of movements of at least 6°, and decrease in the interbody space height over 2/3 of the superjacent one.

Computer Modeling of Intervertebral Disk Endoprosthesis

The article describes the model of the intervertebral disc endoprosthesis. The algorithm for constructing the geometric model of the intervertebral disc prosthesis is developed in the ANSYS programming language APDL, which allows to automatically rebuilding the shape and size of the prosthesis depending on the size of the vertebrae and the required intervertebral spaces of different parts of the spine of particular individual. The geometric model of the intervertebral disc prosthesis is based on the experimental sample of a ceramic endoprosthesis developed at the Institute of Strength Physics and Materials Science SB RAS.

Artificial Intervertebral Disc Replacement to Provide Dynamic Stability to the Lumbar Spine: A Finite Element Study.

Currently, intervertebral disc prostheses that are designed to restore mobility to a vertebral segment are possible for the lumbar spine. The ball-and-socket joint is a constrained design, wherein the rotational axis of the intervertebral joint is forced to pass through the center of the spherical surfaces that form the joint. One advantage of ball-and-socket joints versus unconstrained designs includes better shear stability, which results in sufficient flexibility. In this study, finite element analyses were performed in preimplanted and implanted (intervertebral disc replacement [IDR]) lumbar spine (L1-S) models to examine range of motion (ROM) and the resulting mechanical responses in the implant and the adjacent bones. Four physiological loading conditions including flexion, extension, and left and right lateral bending were analyzed to observe the effect on ROM under a 10-Newton meter moment. In terms of mechanical response, this study shows that disc replacement is an viable alternative to fusion surgery. The added advantage of IDR over fusion for degenerative discs is the reduced chance of disc degeneration at the adjacent segment of spinal vertebral column; with fusion surgery, chances of degeneration are increased.

Computer modelling of the mechanical behaviour of the cervical spine segment and intervertebral disk prosthesis

The article describes the model of intervertebral disc prosthesis and the results of study of mechanical behavior of the cervical spine segment. The results show that degenerative changes in the intervertebral disc lead to a change in the location of the regions of maximum Mises stresses in cervical spine segment. The presented geometric model of the intervertebral disc prosthesis is based on the experimental sample of a ceramic endoprosthesis developed at the Institute of Strength Physics and Materials Science SB RAS.

P.097 Tensile properties of Polyvinyl Alcohol Cryogel (PVA-C) formulations and generation of a tissue mimicking artificial lumbar intervertebral disc prototype

Background: Current lumbar intervertebral disc prostheses provide suboptimal symptom relief with little natural load-cushioning. PVA-C is a promising biocompatible material, however previous studies from our lab show that it does not have adequate elastic modulus to mimic the annulus fibrosus. Here we present a prototype of an artificial lumbar intervertebral disc. Methods: The tensile properties of pure (5-35% PVA-C) and particle-reinforced (15% PVA-C with 5% of either Sephadex or hydroxyapatite) composite PVA-C formulations were evaluated. Simple tension and tensile stress relaxation tests were performed. Woven Teflon mesh was embedded in PVA-C and tested under compression. Endplate pull-out tests were performed. Results: Tensile testing showed that all PVA-C formulations behaved linearly for physiologic levels of strain (&lt;20%). Tensile elastic modulus is an order of magnitude lower than the annulus fibrosus. Teflon has similar elastic modulus as collagen and compression of the hybrid Teflon-PVA-C construct revealed good biomechanical mimicry with elastic modulus of 20-25MPa at 20% deformation, similar to human data. Bonding between PVA-C and porous titanium endplate is excellent. Conclusions: A fiber-reinforced PVA-C impregnated composite adequately mimics the annulus fibrosus. Our prototype of a tissue mimicking artificial intervertebral disc utilizes a woven Teflon fiber with 20% PVA-C (+Hydroxyapatite) annulus and 5% pure PVA-C nucleus bonded to porous titanium foam endplates.

P.096 Evaluation of Poly Vinyl Alcohol Cryogel (PVA-C) composites for mimicking biomechanical properties of the lumbar interverterbral disc

Background: Current lumbar intervertebral disc prostheses provide suboptimal symptom relief with little natural load-cushioning. PVA-C is a promising biocompatible material, and our previous study finds that it can closely mimic the properties of nucleus pulposus. However, pure PVA-C does not possess adequate stiffness to mimic the annulus fibrosus. Methods: Composite particle-reinforced PVA-C formulations were tested to identify methods that could increase the elastic modulus. This included: sephadex, hydroxyapatite (stock) and hydroxyapatite (in-solution synthesis). All formulations were tested using 15% PVA-C and 5% reinforcing agent. Indentation and durometer tests were performed as well as simple compression, compressive stress relaxation and creep. Results: Indentation and durometer results did not clearly reveal any specific formulations that significantly improved stiffness. The addition of in-solution synthesized hydroxyapatite resulted in 1.15 to 2 time increase in elastic modulus (0.3-0.9 MPa) and associated decrease in stress relaxation and creep. The addition of stock hydroxyapatite and spehadex (G100f and G50sf) lowered the elastic modulus and increased stress relaxation and creep. Conclusions: In-solution synthesized hydroxyapatite is the only particle-reinforced composite PVA-C formulation that exhibited greater stiffness than pure PVA-C. The elastic modulus will need to be increased by 5-10x to adequately mimic the annulus fibrosus. A fiber-reinforced composite will likely be needed to accomplish this.

Wear of a total intervertebral disc prosthesis

Abstract In this study we characterized the wear behavior of a tribological system with material combinations of the total intervertebral disc prosthesis InDisc (Institute of Medical and Polymer Engineering) – consisting of two laser sintered titanium alloy plates with an elastomer inbetween. For the tests a translatory oscillation test bench was used which simulates the kinematic and the physiological environment of the lumbar spine. In the interface between the elastomer (Silopren LSR 2670) and the laser sintered titanium alloy (Ti6Al4V) abrasion cords out of silicone rubber with a maximal area of 500 × 700 μm2 were generated as well as individual particles with a size of 10 × 20 to 40 × 50 μm2. In the second prosthesis interface of Ti6Al4V and bone, bone particles with an area of 0.5 × 0.5 to 1 × 1 μm2 were detected. The wear particles and the amount of wear volume influence the biological response of a total intervertebral disc prosthesis in the body.

The outcomes of arthroplasty in degenerative lesions in the lumbar spine

A retrospective study. To investigate the safety and effectiveness of total intervertebral disc replacement with a Maverick prosthesis in patients with discogenic pain in the lower lumbar spine. Lumbar disc arthroplasty has been developed as an alternative to rigid interbody fusion for patients with pathology in the lower lumbar discs. According to the developers, motion preservation in the operated segment should prevent secondary pathology of the adjacent segments. From November 2007 to date, 41 patients have been operated on and 42 disc prostheses have been placed. All patients underwent surgery in the same hospital by the same surgeon. The clinical and radiological results of treatment of 35 patients were studied. The patients were examined before surgery, immediately after surgery and then in the intervals of 3-6, 6-12, 12-24, and 36-48 months. The longest follow-up was 61 months. Mean follow-up was 30 months. Patients operated on using total intervertebral disc prosthesis demonstrated a significant improvement compared to their preoperative status. A significant reduction of back and leg pain intensity was observed. The quality of life significantly improved. The intervertebral disc space height, the sagittal balance and the balance in the adjacent levels were restored and preserved after correction. The motion in the implanted segment was preserved. Total lumbar disc replacement with a Maverick prosthesis has proved to be an effective and safe technique. The clinical and radiological methods showed the significant improvement of the patients' status.

Non-linear constitutive model for the oligocarbonate polyurethane material

The polyurethane, which was the subject of the constitutive research presented in the paper, was based on oligocarbonate diols Desmophen C2100 produced by Bayer®. The constitutive modelling was performed with a view to applying the material as the inlay of intervertebral disc prostheses. The polyurethane was assumed to be non-linearly viscohyperelastic, isotropic and incompressible. The constitutive equation was derived from the postulated strain energy function. The elastic and rheological constants were identified on the basis of experimental tests, i.e. relaxation tests and monotonic uniaxial tests at two different strain rates, i.e. $$\dot \lambda = 0.1$$ min−1 and $$\dot \lambda = 1.0$$ min−1. The stiffness tensor was derived and introduced to Abaqus® finite element (FE) software in order to numerically validate the constitutive model. The results of the constants identification and numerical implementation show that the derived constitutive equation is fully adequate to model stress-strain behavior of the polyurethane material.