Metal on Metal Hip Implants

J&J Depuy, maker of the Ultamet Metal-on-Metal acetabular hip system, has announced that they will no longer market this and related metal-on-metal products after August 31st.  Since around 2007, the use of metal-on-metal has been insteady decline due to concerns about metal ion release and their possible association with pseuodo tumour formation in some patients. The main advantage of metal-on-metal components is the ability to use a thin liner, which allows the surgeon to use the largest possible femoral head without the removal of an extensive amount of bone from the acetabulum. A larger head normally will result in less subluxation and dislocation in patients prone to these phenomena. The lack of compliance in metal bearings, however, results higher wear rates and metal release if there is misalignment in the bearing. UHMWPE, the other bearing surface of choice, has greater compliance, and thus can realign in vivo if there is misalignment.



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MIT Polymer Day

MIT held its annual MIT Polymer Day in March. Cambridge Polymer Group was a sponsor, and also supplied two judges for the poster contest, where students and post-doctoral research fellows presented their work on polymer technology.

Link to PPST

Crystallinity in PEEK

Polyether ether ketone (PEEK) is finding its way into more and more medical devices, due to its high strength, resistance to oxidation, bioinertness, and ease of manufacturing. PEEK is a semi-crystalline thermoplastic, and the crystallinity is strongly dependant on the processing conditions used to form a PEEK component, such as molding and cooling temperatures, sample size, shear rate, and post-molding conditions, such as tool speed and cold working. In a paper presented at the International PEEK meeting in Philadelphia in April, 2013, CPG researchers, along with collaborators from Brigham and Woman's Hospital and Stryker Orthopedics, discuss different techniques to measure crystallinity in PEEK, including density, DSC, X-ray, and infrared spectroscopy. Work was performed on 4 different PEEK formulations with differing molecular weights.

Link to presentation

Polymers and Plastics in Medical Devices

Cambridge Polymer Group researcher Gavin Braithwaite will be presenting his work on hydrogels used in medical devices at the upcoming Polymers and Plastics in Medical Devices conference held in San Francisco, CA from June 26-28th, 2013. Dr. Braithwaite's work discusses injectable hydrogels for nucleus pulposus and tissue augmentation, as well as cartilage replacement technologies.

Link to conference information

HFIP Quantification

Hexafluoroisopropanol (HFIP), also known as hexafluoro-2-propanol, is a fluorinated alcohol commonly used in processing of polyethylene terephthalate, polyacrylonitriles, some polyketones, and polyamides. It is a fairly toxic material, causing several eye damage and respiratory problems. Consequently, manufacturers who use HFIP need to measure how much residual HFIP remains in their processed goods, particularly if the goods are being used for biomedical purposes.

HFIP can be quantified with solvent extraction followed by gas chromatography-mass spectroscopy. This method can be used to detect HFIP concentrations down to approximately 100 ppb. The technique requires a reliable extraction methodology, and a GC-MS protocol that will provide separation of HFIP from any other co-eluting species that may be extracted from the test sample.

Contact Cambridge Polymer Group for more information.

A peek at PEEK

On September 25-26, 2013, an international symposium on polyetherether ketone (PEEK) will be held at Drexel University. PEEK is finding increasing use in the biomedical community, particularly in the area of permanent implants. Spinal implants have been composed of PEEK for several years, finding use as stabilization rods, spacers, and articulating surfaces.

Characterization of PEEK has received increasing attention as a consequence. This material has unusual crystallization behavior, and the method of analysis can lead to different results. In a paper written by researchers from Cambridge Polymer Group, Brigham and Women's Hospital, and Stryker "Macromolecular and Morphological Characterization of Medical Grade PEEK", we describe 4 methods to measure crystallinity in PEEK (X-ray, DSC, density, and FTIR), and compare the results of multiple grades of PEEK.


Details on the conference

Fatigue Crack Propagation

 

 

Fatigue crack propagation (FCP) analysis is a method to monitor the resistance of a material to crack inception and propagation under cyclical loading. ASTM E647 describes the methodology for measuring crack propagation in materials. An example of the typical data obtained in FCP analysis is shown above for GUR 1020 UHMWPE subjected to ionizing radiation measured at Cambridge Polymer Group. There are two principle regimes in a crack propagation plot: (1) crack inception, where the minimum load range required to start a crack to grow is determined; (2) Paris regime, where steady crack growth occurs. The x-axis shows DK, which is derived from linear elastic mechanics and is dependent on the cyclical load range (Pmax-Pmin) and the crack length (a). The expression for DK will depend on the shape of the test specimen, which is often a compact tensile geometry. The y-axis shows the crack growth as a function of number of fatigue cycles.  The main reportable items for FCP analysis are the DKincep, or the load conditions for crack growth to reach 1e-6 mm/cycle, and the slope and intercept of the curve in the Paris regime (m and C, respectively). With highly crosslinked UHMWPE, the DKincep tends to decrease, and the material sometimes shows a higher sensitivity to DK in the Paris regime.

Contact Cambridge Polymer Group for more information on E647 testing.

Link to Application Note on fatigue crack testing