Ultra high molecular weight polyethylene (UHMWPE) is the most commonly used bearing surface in hip and knee arthroplasties. Due to its high molecular weight, UHMWPE cannot be injection molded or extruded with a screw extruder. Compression molding or ram extrusion are the two consolidation processes used for UHMWPE, whereby the combination of temperature and pressure sinter the flakes of UHMWPE together. These processes do not result in co-mingling of the UHMWPE powder, however, so that the original flakes can be readily seen in a cryo-fractured surface (see above). Regulators are interested in verify that consolidation defects, or voids, do not exist in the consolidated UHMWPE. These defects can result in crack formation and failure of the device if the defects are in sufficient quantity and size. SEM analysis of cryo-fractured surfaces is a commonly used technique to look for consolidation defects, along with optical microscopy of microtomed films of the material. Regulatory submissions usually require these analyses, comparing a new formulation of UHMWPE with a cleared and marketed formulation.
The increasing prevalence of Vitamin E, a naturally-occuring antioxidant, in medical grade plastics has resulted in a need for analysis methods that can track the effect of processing on this compound. Vitamin E is effective as an antioxidant due to the hydroxyl group sitting on the aromatic ring at one end of the molecule. This hydroxyl group can readily lose a hydrogen and capture a free radical, a culprit in oxidation reactions. The free radical is thereafter stabilized by the Vitamin E molecule.
When processing medical grade plastics, the plastics are exposed to high heats and pressures during molding. Additionally, cleaning agents and ionizing radiation are often used to clean and sterilize the finished components. All these steps can potentially modify some of the Vitamin E molecules.
As a result, medical device manufacturers are usually required to identify and quantify these modified molecules to ensure both that they are safe for in vivo use, and that the material remains adequately oxidatively stabilized. Researchers at Cambridge Polymer Group have developed a series of assays to analyze potential transformation products of Vitamin E, using a combination of chromatography and spectroscopy. Additionally, we have a series of analyses to test the oxidation resistance of stabilized plastics. All these methods have been successfully used for regulatory submissions.
The ASTM Committee on Medical Devices (F04) met last week in Atlanta, GA. Cambridge Polymer Group staff attended several of the task groups, and reported back the following activities.
Medical Device Cleanliness
Several draft standards are in development, including guidance on cleanline validation, synthetic test soils to verify cleaning efficacy, guidance on how to design for cleaning, and methods of establishing allowable cleanliness levels. The first 3 items will be submitted for a sub-committee ballot in January, and more input is required for the last item. Additionally, a new wear particle isolation method was introduced to ASTM F561 a few years ago, and was ballotted last year. Discussions were held on negative ballots received. The votes on these negatives will occur in January.
Polyether ether Ketone
Discussions of ASTM F2026 suggested that specifications for different grades of PEEK based on molecular weight be introduced into this standard. The three manufacturers of medical grade PEEK (Solvay, Invibio, and Evonik) were invited to submit their data for evaluation.
UHMWPE
It was suggested to remove a few test methods from ASTM F648, including net ash on consolidated resin and Charpy Impact. The committee is considering these items. A round robin study on small punch testing (ASTM F2183) is being developed to establish a precision and bias statement for this method. Lastly, a micro-tensile dogbone standard is being developed for UHMWPE to allow characterization of explanted acetabular cups.
Bone Cement
A new benzoyl peroxide assay was introduced by CPG scientists as a replacement for the existing test method in ASTM F451. The committee is arranging to prepare several formulations of bone cement with varying benzoyl peroxide concentrations to evaluate the new method.
Measuring the volume of a standard shape (e.g. cube, cylinder, sphere) is straightforward, as one only needs to measure the relevant dimensions (length, height, diameter, etc.) and calculate the volume using known geometric equations. Measuring a non-standard shape is also straightforward if you have an analytical balance. Using Archimedes’ principle of buoyancy, the weight of the object, when immersed in a liquid, will decrease by the volume of liquid the object displaces, which is its volume. Using the density attachment for an analytical balance, the mass of the object is first measured. A beaker of a suitable liquid (e.g. water) is then placed on the density attachment, and the object is re-weighed, this time while immersed in the water. By subtracting the difference in the two masses, one calculates the mass of the liquid that was displaced by the object. Knowing the density of the liquid at the test temperature, one can calculate the volume of the test sample by dividing the buoyancy mass by the fluid density.
This method is useful for measuring the change in dimensions of samples due to polymerization, crosslinking, crystallization, or other chemical processes.
DJO Global announced that it will be launching its Motivation(tm) total knee system at the American Academy of Orthopedic Surgeons meeting in 2013. The Motivation System incorporates e+(tm), an UHMWPE blended with Vitamin E.
Dr. Michael Kasser, from Center for Devices and Radiological Health at the FDA, has published an article in the Journal of Biomedical Material Research detailing the history of regulation on ultra high molecular weight polyethylene (UHMWPE) and its use in medical devices. This article is available as a early view download for registered users. Dr. Kasser offers a flow chart for the 510K decision making process to decide if a new productis substantially equivalent to a product already on the market, and therefore, if the 510K route can be used, as opposed to the more costly and time-consuming PMA route.
The paper outlines some of the testing used for establishing equivalency, based on ASTM test methods and tests developed within the orthopedic industry. In particular, custom tests designed to test anti-oxidants used in UHMWPE are required for 510K submissions.
Cambridge Polymer Group has assisted several clients in getting regulatory clearance on their UHMWPE materials, and is very familiar witht the testing requirements.
Iconacy Orthopedic Implants, a privately held medical device company in Warsaw, IN, received FDA clearance to market their highly crosslinked UHMWPE prepared using the CIMA process developed at Cambridge Polymer Group and the Massachusetts General Hospital. CIMA is a patented highly crosslinked, low wear and oxidation-resistant ultra high molecular weight polyethylene that is non-exclusively licensed to Iconacy for use in their I-Hip total hip replacement technology.
The Mooney-Rivlin model is a hyperelastic model that can be used to predict the deformation behavior of elastomers to uniaxial, planar, and biaxial extension. This model requires knowledge of two or more Mooney-Rivlin parameters that are specific to the material in question, and usually to the deformation mode in question as well. Some experimental work has shown that the Mooney-Rivlin parameters obtained in uniaxial extension do not lead to good predictions of the same material’s behavior in biaxial extension.
The scientists at Cambridge Polymer Group have developed a system for easily measuring the biaxial deformation behavior of elastomers using a bubble inflation system. Custom software allows generation of biaxial stress-strain data, which can then be fit to obtain the Mooney-Rivlin parameters.
The BBC reports that a team from Glasgow University has developed a technique to created a pitted nanopatterned surface using poly ether ether ketone (PEEK). Tissue growth studies conducted by this group has shown that the nanopattern helps to elicit bone growth through the manipulation of stem cells. These researchers feel that it they can put this pattern onto the hip stems used in total hip replacement surgeries, loosening of the hip stem due to soft tissue growth around the stem could be mitigated. The Glasgow team feels that if successful, it could greatly increase the in vivo duration of the hip stem.
The current standard of care in orthopedic joint replacement relies on the use of hard bearing surfaces comprised of polyethylene, ceramics, and metals. The natural tissues these synthetic materials replace are usually softer, viscoelastic materials that are best described as hydrogels, or hydrophilic network structures of cross linked macromolecules. In this audio conference presentation, our speaker discusses the increased use of hydrogels in biomedical applications, outlining what they are, their properties, and why they may have value in several biomedical applications, including orthopedics and spine. The presentation discusses potential applications, and looks at tissue models based on hydrogels for testing and training. Finally, attendees learn what issues have to be addressed in designing and using these materials, including concerns about how to test these soft, viscoelastic materials reliably in regimes relevant for their application.
This audio conference covers:
What makes hydrogels different from other materials
Challenges with using and designing for hydrogels
Potential applications in biomedical devices and tissue models
Testing issues that must be addressed
This web conference is on August 7th at 11:30 am EST.
