Polyethylene Disks Research Articles

Spatial variation of wear on Charité lumbar discs

Total disc replacement (TDR) is a modern technique employed to treat degenerative disc disease that has the benefit of preserving motion compared with the clinically established spinal fusion. The wear performance of implants based on articulating designs is a key factor that determines their longevity and it is hypothesized that this will be the case for TDR devices. A detailed analysis of the surface of Charité lumbar disc replacements during simulated wear for five million cycles (MC), with inputs defined by the ISO18192-1 standard, is presented. After each million cycles the disc asperity heights, asperity curvature radii and their distributions on the surface of the core of the implant were determined at different locations. Two distinct areas on the surface of Charité polyethylene disc were identified based on the surface topography change during the wear simulation process. Within the area corresponding to the dome the initial roughness decreased, but after 2 MC the surface appeared to roughen with material build-up. More peripherally on the dome the surface roughness decreased after the first MC and remained constant. No effect was noticed on the rim. Furthermore, no statistical difference was noticed between the inferior and superior sides of the core of the disc. The study demonstrated that the wear on the two surfaces of the disc was uneven. This spatial variation is important in modelling the wear processes and providing strategies for reducing wear through enhanced design and modifications to the biotribological properties of the device.

TU‐E‐BRB‐07: Feasibility of a Dedicated Breast CT Platform for Orthovoltage Rotational External Beam Radiation Therapy: A Simulation Study

Purpose: To evaluate the feasibility of a dedicated breast computed tomography (bCT) platform to deliver therapeutic dose distributions, specifically for partial breast irradiation (PBI), whole breast irradiation (WBI), and dose painting, through orthovoltage rotational external beam radiation therapy (EBRT).Methods: Rotational EBRT using the geometry of a prototype bCT platform was evaluated by means of MCNPX Monte Carlo simulator. A 178 keV monoenergetic photon source was used to approximate 320 kVp photons filtered by 4 mm of copper, as validated by depth‐dose characteristics in polyethylene. The source was rotated around a 14 cm voxelized polyethylene disk (0.1 cm tall) or cylinder (9 cm tall) to simulate primary and primary + scattered photon interactions, respectively. Simulations were also performed using bCT patient images. Beam collimation was varied in the x‐y plane of rotation (1–14 cm) and in the z‐direction (0.1–10 cm).Results: As x‐y collimation narrowed, 2D dose profiles shifted from a cupped profile with high edge dose (14 cm beam) to an increasingly peaked central dose. A 1 cm beam had a center‐to‐cylinder edge dose ratio of 14.8. Similar distributions were observed experimentally. Dose painting for multiple foci, a line distribution, and a ring distribution was demonstrated using multiple rotations with varying collimation. A homogeneous dose distribution (<5% fluctuation) with skin sparing was demonstrated by weighted summation of four dose profiles. Using 2 cm z‐collimation, scatter tails decreased exponentially towards the cylinder top/bottom to 3% of maximum dose. Simulations using patient bCT images demonstrated the potential for treatment planning and image‐guided therapy. Conclusions: A bCT platform can feasibly deliver orthovoltage rotational EBRT for the treatment of breast cancer. A variety of dose distributions can be generated allowing for PBI to a single focus, dose painting, and WBI with skin sparing. Dedicated bCT is a potential platform for image‐guided radiation therapy.One author is a consultant to Varian Imaging Systems (Palo Alto, Calif) and Artemis (Erlangen, Germany) and receives funding from Varian Imaging Systems, Fuji Medical Systems (Stamford, Conn), and Hologic (Bedford, Mass).

Photochemical Immobilization of Cucurbita Fruit Ascorbate Oxidase onto Polyethylene Disc for Determination of Ascorbate in Serum and Foodstuffs

Abstract: An ascorbate oxidase purified from the green fruit of zucchini squash (Cucurbita pepo medullosa) was immobilized photochemically on a polyethylene disc with 85% retention of initial activity of free enzyme. The optimum pH (5.5) was unchanged, while K(m) was decreased. The polyethylene discs were employed for determination of ascorbic acid in serum and foodstuffs. The working linear range was 2.8 μM to 16 μM. The mean value of ascorbic acid in serum as measured by the method was 0.16 mg/dl in males and 0.209 mg/dl in females. The immobilized enzyme was used 100 times over 4 months, when stored at 4°C.

Neutronic design and simulated performance of Peking University Neutron Imaging Facility (PKUNIFTY)

Abstract The Peking University Neutron Imaging Facility (PKUNIFTY) is a Radio Frequency Quadruple (RFQ) accelerator based system. The fast neutrons are produced by 2 MeV deuterons bombarding beryllium target. The moderator, reflector, shielding and collimator have been optimized with Monte–Carlo simulation to improve the neutron beam quality. The neutrons are thermalized in water cylinder of Φ 26×26 cm 2 with a polyethylene disk in front of Be target. The size of deuteron beam spot is optimized considering both the thermal neutron distribution and the demand of target cooling. The shielding is a combination of 8 cm thick lead and 42 cm thick boron doped polyethylene. The thermal neutrons are extracted through a rectangular inner collimator and a divergent outer collimator. The thermal neutron beam axis is perpendicular to the D + beam line in order to reduce the fast neutron and the γ ray components in the imaging beam. When the neutron yield is 3×10 12 n/s and the L / D is 50, the thermal neutron flux is 5×10 5 n/cm 2 /s at the imaging plane, the Cd ratio is 1.63 and the n/γ ratio is 1.6×10 10 n/cm 2 /Sv.

Reducing Metal Artifacts in Computed Tomography Caused by Hip Endoprostheses Using a Physics-Based Approach

Metal-induced artifacts may cause severe problems in clinical computed tomography (CT) imaging and may impair diagnosis as well as overall image quality. Many approaches for reducing these artifacts tackle the problem by simply ignoring or interpolating the metal traces in the raw data, which results in a general information loss and additional artifacts in the corrected image. It was the objective of this study to develop an approach aiming at correcting several physical artifact sources. We have also tried to minimize the impact on spatial resolution and attempted to avoid new artifacts resulting from the correction. The algorithm works with a first volumetric reconstruction followed by threshold-based metal prostheses segmentation. The segmented metal implants are then forward projected and the resulting sinogram entries are squared and combined, followed by a second reconstruction to yield correction volumes. The resulting volumes are then combined linearly with a combination weight determined to minimize the flatness of the initial image. A directional filtering algorithm following the beam hardening correction applies a nonlinear convolution in the metal traces of the sinogram which reduces existing metal-induced noise artifacts. Phantom measurements on a polyethylene (PE) disc with different inserts and a semi-anthropomorphic hip phantom with optional bone and titanium inserts were used for evaluating the algorithm. Patient datasets containing uni- and bilateral hip endoprostheses verified the applicability and efficiency on realistic clinical cases. Deviations in CT values were reduced to below 3 HU on average. Image noise reduction of up to 70% was achieved (average noise reduction of 37%) with a more homogeneous CT value distribution in soft-tissue areas. A comparison to standard interpolation methods showed superior artifact suppression without producing artifacts caused by interpolation and without the general information loss in the close vicinity to the implants. The impact on spatial resolution was minimized as compared with interpolation algorithms. Metal artifacts caused by hip-endoprostheses were strongly reduced. Soft tissue areas and skeletal structures surrounding the implants were well restored. The correction works by postprocessing CT datasets and it is applicable to any reconstructed image without a priori knowledge.

Tribological performance of DLC coatings on UHMWPE

Diamond like carbon (DLC) coatings were deposited by several methods (ion beam assisted evaporation, magnetron sputtering, filter cathodic arc, and plasma enhanced chemical vapor deposition) onto medical grade ultra-high-molecular weight polyethylene (UHMWPE) discs. The chemical characteristics and mechanical properties of the deposited DLC coatings were studied by Raman spectroscopy and nanoindentation, respectively. In addition, a set of tribological tests was conducted at human body temperature and under bovine serum lubrication against alumina balls. After testing, wear tracks were both visually inspected and documented using confocal microscopy. Visual inspection of the wear tracks confirmed that the DLC coatings were completely removed in all cases, the only exception being the DLC coating prepared by magnetron sputtering with thickness about 0.5 microns. Although this type of DLC coating exhibited the highest friction coefficient, and therefore it suggested a somewhat lower resistance to abrasive/adhesive wear conditions, no evidence of cracking or delamination was observed after the high contact pressure wear testing. This fact points out a good substrate-coating adhesion, and confirms magnetron sputtered DLC as a potential coating for orthopaedic applications.

Vortex counter-current chromatography

A novel counter-current chromatographic system is developed by mounting a vortex column on a type-I coil planet centrifuge. The column is fabricated from a high-density polyethylene disk (16cm diameter and 5cm thick) by making multiple holes of various diameters (3–12.5mm) each arranged in a circle and connected with narrow transfer ducts. The performance of this vortex column is tested with three different two-phase solvent systems with a broad range in hydrophobicity. The results indicated that the smallest diameter column (3mm diameter, 120 units with 42.8ml capacity) yielded the best separation with the height equivalent to a theoretical plate of 2cm compared with 20cm required by the conventional multilayer coil column of high-speed CCC. By avoiding the use of an Archimedean Screw Force, the system shows a low column pressure which would permit safe operation of a large preparative column without a risk of leakage of solvent and column damage.

Surface Roughness of CoCr and ZrO2 Femoral Heads with Metal Transfer: A Retrieval and Wear Simulator Study

Metal transfer to femoral heads may result from impingement against the metallic acetabular shell following subluxation/dislocation, or when metallic debris enters the articulation zone. Such transfers roughen the head surface, increasing polyethylene wear in total hip replacements. Presently, we examined the surface roughness of retrieved femoral heads with metallic transfer. Profilometry revealed roughness averages in regions of metal transfer averaging 0.380 μm for CoCr and 0.294 μm for ZrO2 which were one order of magnitude higher than those from non-implanted controls. Scanning electron microscopy (SEM) revealed adherent transfers on these retrievals, with titanium presence confirmed by electron dispersive spectroscopy. Due to the concern for increased wear, metal transfer was induced on non-implanted heads, which were then articulated against flat polyethylene discs in multidirectional sliding wear tests. Increased polyethylene wear was associated with these specimens as compared to unaltered controls. SEM imaging provided visual evidence that the transfers remained adherent following the wear tests. Pre- and post-test roughness averages exceeded 1 μm for both the CoCr and ZrO2 heads. Overall, these results suggest that metal transfer increases the surface roughness of CoCr and ZrO2 femoral heads and that the transfers may remain adherent following articulation against polyethylene, leading to increased polyethylene wear.

Coupled whispering gallery mode resonators in the Terahertz frequency range

We report on coupling of two whispering gallery mode resonators in the Terahertz frequency range. Due to the long wavelength in the millimeter to submillimeter range, the resonators can be macroscopic allowing for accurate size and shape control. This is necessary to couple specific modes of two or more resonators. Sets of polyethylene (PE) and quartz disk resonators are demonstrated, with medium (loaded) quality (Q)-factors of 40-800. Both exhibit coinciding resonance frequency spectra over more than ten times the free spectral range. Loading effects of single resonators are investigated which provide strong Q-factor degradation and red-shifts of the resonances in the 0.2% range. By coupling two resonators of the same size, we observe mode splitting, in very good agreement with our numerical calculations.

Process analytical technology (PAT): Effects of instrumental and compositional variables on terahertz spectral data quality to characterize pharmaceutical materials and tablets

The aim of this study was to use terahertz spectroscopy to characterize pharmaceutical materials and tablets, and to understand the effects of measuring conditions and compositional variability on the data quality. Tests were performed on five formulation components (theophylline, lactose, starch, Avicel, magnesium stearate) and a series of tablets composed of various concentrations of theophylline and excipients. Transmission spectra of polyethylene (PE) disks derived from each of the samples were analyzed. Three factors (component loading, component chemistry, and disk drying time) were screened as critical factors associated with the magnitude and location of THz absorbance peaks. Applying the standard sample spectra divided by PE reference spectra ratio method revealed that, to a large extent, PE was responsible for the disk drying time dependence. Direct spectral feature analysis along with mass-transfer analysis of the disk drying process revealed THz absorption peak maxima of lactose (255 cm −1) and water (54 and 210 cm −1) which is also supported by literature values for the peak maxima assignment for water. Particle scattering due to specimen and PE was found to be also partially responsible for the observed spectral intensities. The importance of THz spectroscopy was demonstrated for characterization of pharmaceutical materials and tablet.

Tribology of ultra-high molecular weight polyethylene disks molded at different temperatures

Tribological characteristics of ultra-high molecular weight polyethylene (UHMW-PE) disks molded at 130–190 °C were studied. The highest crystallinity was obtained for the sheet molded at 130 °C, but crystallinity decreased with increasing molding temperature. Beyond 150 °C, the resultant crystallinity reached a constant level. The dynamic friction coefficients of these UHMW-PE disks were measured using a ball-on-disk friction tester. The friction coefficient decreased with increasing number of rotations in the early stage of the measurement, and achieved at an equilibrium level, independent of the molding temperature. The steady-state friction coefficient was 0.04 for the disk molded at 130 °C and increased with increasing molding temperature. The disks molded at 150–190 °C always had a steady-state friction coefficient of 0.065. The surface deformation of each disk was evaluated from the observation of the resultant wear track. Analyzing the relationship between the above friction coefficient and width of the wear track enabled us to interpret the tribological mechanism generated in this study.

Experimental study of the friction and wear behaviour of a polymer disc/primer coating combination used in ball‐joints by means of large‐scale testing

AbstractThe surfaces of a heavily loaded ball‐joint were initially covered with a sliding spray, and suffer wear. A solution is found by incorporating ultra high molecular weight polyethylene (UHMWPE) discs with a carbon fibre/epoxy reinforced ring as sliding material into the chairs of the structure. The ball side is covered with a zinc phosphate primer coating. For design purposes the local static and dynamic behaviour of the hybrid UHMWPE discs in contact with steel or Zn‐coated counterfaces should be large‐scale tested in terms of their loading capacity, low friction and wear resistance. Also the influence of creep and wear on friction is examined. After the large‐scale verification tests in laboratory, a good correlation is found with a test in the field. Copyright © 2006 John Wiley & Sons, Ltd.

The Effect of Human Bone Marrow Stromal Cells and Dermal Fibroblasts on Angiogenesis

A cell therapy methodology for angiogenesis using fibroblasts has already been developed. Bone marrow stromal cells, which contain mesenchymal stem cells, have a low rate of immunity-assisted rejection and are capable of expanding profoundly in culture. Therefore, these cells offer several advantages for transplantation over mature cells. The aim of this study was to compare the angiogenic activity of bone marrow stromal cells with that of fibroblasts. For in vitro study, cultured human bone marrow stromal cells and dermal fibroblasts were seeded onto 96-well culture plates. After 1, 3, and 5 days, the levels of the basic fibroblast growth factor and vascular endothelial growth factor were compared. For in vivo study, porous polyethylene disks were loaded with bone marrow stromal cells, fibroblasts, or no cells and then implanted in the backs of rats. At three time intervals ranging from 1 to 3 weeks, the microvascular density was measured. In the in vitro study, the basic fibroblast growth factor levels in the bone marrow stromal cell group were 47, 89, and 68 percent higher than in the fibroblast group at each time interval (p < 0.05). The vascular endothelial growth factor levels of the bone marrow stromal cell group were seven, 12, and 12 times higher than those of the fibroblast group (p < 0.05). In the in vivo study, there was little difference in the microvascular density among the three groups by the second week. However, the 3-week specimens showed a significantly greater difference. The microvascular density averaged 52.88, 26.12, and 17.50 for the bone marrow stromal cell, fibroblast, and no-cell groups, respectively. These results suggest that bone marrow stromal cells may possibly be used as a replacement for fibroblasts for angiogenesis.

Sensitive Detection of Botulinum Neurotoxin Types C and D with an Immunoaffinity Chromatographic Column Test

A sensitive and specific immunoassay for the simultaneous detection of Clostridium botulinum type C (BoNT/C) and type D neurotoxin was developed. Goat anti-mouse immunoglobulin G was bound to polyethylene disks in a small disposable column used for this assay. The sample was preincubated together with monoclonal antibodies specific for the heavy chain of BoNT/C and D and affinity-purified, biotinylated polyclonal antibodies against these neurotoxins. This complex was captured on the assay disk. Streptavidin-poly-horseradish peroxidase was used as a conjugate, and a precipitating substrate allowed the direct semiquantitative readout of the assay, if necessary. For a more accurate quantitative detection, the substrate can be eluted and measured in a photometer. Depending on the preincubation time, a sensitivity of 1 mouse lethal dose ml(-1) was achieved in culture supernatants.

Fast characterization of carbon/epoxy rings for use in the ball-joints of the Maeslant storm surge barrier

In 2004, the rehabilitation works of the Maeslant storm surge barrier in The Netherlands were finished. The two gates of the barrier are attached to a pivotal point on both banks. The pivotal points are designed as two large ball-joints (10 m diameter) that must allow for both horizontal and vertical movements of the gates. Five hundred individual composite material discs provide a low friction sliding contact with the ball. The composite material consists of a ultra-high molecular weight polyethylene disc, reinforced by a filament-wound carbon/epoxy ring. In this paper, the characterization and qualification tests for the carbon/epoxy rings are discussed. The design of the tests and their numerical validation is presented. It is shown that this combined numerical/experimental approach is essential to obtain reliable results.

Acidic demineralization of apatites studied by scanning X-ray microradiography and microtomography

AbstractThe mineral in bones and teeth is an impure form of hydroxylapatite (HAP), the principal impurity being 2—5 wt.% carbonate. This mineral dissolves during remodelling of bone and also in dental caries as a result of the action of acids produced by osteoclasts and by bacteria, respectively. In enamel, demineralization proceeds with preferential loss of carbonate relative to phosphate. Surprisingly, in the early stages, the demineralization is subsurface. In order to facilitate the understanding of physical chemical aspects of these processes, we have undertaken studies of demineralization in model systems. We give three examples here. The first two used scanning microradiography in which the specimen is stepped across a 10—30 μm diameter X-ray beam. Intensity measurements allow calculation of the mineral mass per unit area in the X-ray path through the specimen. In the first experiment, porous HAP sections were separated from a reservoir of acidic buffer by a column initially filled with water (the diffusion length) and scanned with the X-ray beam perpendicular to the axis of the diffusion length. The rate of total loss of mineral along each profile was calculated from the scans. The rate of demineralization fell as the diffusion length increased. We believe the explanation is that the rate-controlling step is the diffusion of dissolved HAP away from the solid to the buffer reservoir. In the second experiment, demineralizing solution and water were pumped alternately, for equal lengths of time, past blocks of porous HAP or enamel. The X-ray beam was perpendicular to the exposed surface. As the rate of switching between solutions decreased, the mean rate of demineralization also fell. We propose that this effect is due to retention of acid in the pores of the HAP during the time when water flows, allowing further demineralization to take place during this time. The third study used X-ray microtomography, a form of 3D microscopy, to study the loss of mineral in compacted carbonate apatite powders. The powders were packed in six 10 mm internal diameter acrylic cylinders to a depth of 4 mm (after pressing). One end was covered with a porous polyethylene disc and each tube placed in acidic buffer for 70 days. Periodic examination by microtomography showed the development of subsurface demineralization. Infrared spectroscopy of the dissected-out surface layers showed preferential loss of carbonate over phosphate by comparison with deeper layers. Rietveld analysis of X-ray powder diffraction data showed changes in the crystallographic structures of the apatites between the initial and dissected-out apatite.

Artificial tumor model suitable for monitoring 31P and 13C NMR spectroscopic changes during chemotherapy‐induced apoptosis in human glioma cells

An artificial tumor method was developed to study cells inside the sensitive volume of an NMR spectrometer during growth and apoptosis. The tumor was composed of a 50:50 mixture of tightly packed porous-collagen and nonporous-polystyrene microspheres. The porous collagen served as a growth surface for the tumor cells, and the nonporous polystyrene served as a structural support to limit compression of the packed bed during perfusion. The microspheres were held between two porous polyethylene discs that were tightly sealed inside the NMR perfusion chamber. The new method was evaluated with two cell types: a mouse mammary tumor line (EMT6/SF) and a human glioma line (SF188). The results indicate that for both lines, approximately 10(9) metabolically active cells could be sustained for at least 1 week in the 12-cm(3) artificial tumor. Further, cells undergoing chemotherapy-induced apoptosis (which is known to cause detachment of cells from their surroundings) were retained in the artificial tumor. In preliminary 31P NMR studies, glioma cells treated with temozolomide (TMZ) exhibited reduced phosphocholine (PCh) levels relative to glycerophosphocholine (GPC) and diphosphodiester (DPDE) levels. They also exhibited sharply reduced oxygen consumption and TCA cycle 13C labeling, while they retained glycolytic activity. These metabolic changes are consistent with those that would be expected during mitochondrially-mediated apoptosis.

High-frequency investigation on nonradiative single-mode dielectric resonators

High-frequency investigations of the recently proposed nonradiative single-mode dielectric resonators are presented. In particular, the TE011 mode of high-density polyethylene and single-crystal quartz discs has been experimentally characterized, by means of a simple setup, in a frequency interval around 190 GHz. The obtained unloaded merit factors of 1400 for polyethylene and 2480 for single-crystal quartz lead to magnetic field conversion factors of 15 G/W1/2 and 49 G/W1/2, respectively. In the latter case, the obtained value represents the state of the art among the room temperature conversion efficiencies. The close agreement with the theoretical predictions demonstrates that the performances of the proposed resonators are only limited by intrinsic factors, as finite conductivity of metallic mirrors and dielectric losses of the employed materials. Ideal performances are then expected in suitably realized nonradiative resonators, also in the high-frequency regime.

Groningen temporomandibular joint prosthesis. Development and first clinical application

Patients with a severely degenerated temporomandibular joint (TMJ) may benefit from an alloplastic TMJ replacement. The aim of the study was to develop a safe and properly functioning TMJ prosthesis. The design was based on imitation of anterior condylar translation by an inferiorly located centre of rotation, unrestricted mandibular movements by a double articulation, correct fit to the skull by a self-adjusting skull part consisting of two connected parts, and stable fixation by bone screws that are rigidly connected to the prosthesis parts. The prosthesis consists of a titanium skull part with ceramic inlay, a titanium mandibular part with a ceramic spherical head, and an intervening polyethylene disc. Titanium-alloy bone screws are used for fixation. All parts are available in a number of different shapes. In vitro laboratory and in vivo animal tests showed a low wear rate, the possibility of a close fit to the skull, a stable fixation, sufficient mechanical strength, appropriate choice of materials and proper functioning. Thereafter the step to first patient application was made. First patient application was carried out without adverse events. In conclusion, the presented TMJ prosthesis passed the pre-clinical tests and has progressed to clinical application. The fit to the skull, the expected lifetime of the device and the reliability of the implantation procedure require further evaluation in well-designed clinical trials.

A three-axis knee wear simulator with ball-on-flat contact

A three-axis knee wear simulator with ball-on-flat contact was designed and built for basic wear and friction tests of prosthetic knee materials. The three-axis motion consisted of flexion–extension (FE), anterior–posterior translation (APT) and inward–outward rotation (IOR). Preliminary tests were done with non-irradiated, and with gamma-irradiated, artificially aged ultra-high molecular weight polyethylene disks. The counterface was a 54 mm diameter, polished CoCr ball. The load was static 2 kN, and lubricant diluted calf serum. The wear of non-irradiated polyethylene proved to be insensitive to the disk thickness. Gamma-irradiation and aging resulted in higher wear rates, which further increased with decreasing disk thickness. The steady-state wear rates of the disks varied from 10.7 to 47.1 mg per one million cycles, and the average coefficients of friction from 0.043 to 0.063. The wear zone was burnished in all disks, the dominating wear mechanism being adhesive. Severe delamination occurred only in a disk made from a gamma-irradiated tibial component which had been on the shelf for 10 years. In accordance with clinical findings, the majority of the polyethylene wear particles had a diameter between 0.1 and 1 μm, with an average of 0.7 μm. As the tibial components made of polyethylene are often damaged by oxidation, the effects of aging conditions on polyethylene wear are important subjects of further studies.