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However, substantial concerns have been raised over the safety of some plasticizers. Several ortho-phthalates, for example, have been classified as potential endocrine disruptors that may cause developmental toxicity. Other concerns have been raised about possible carcinogenicity and the effects of plasticizers on the environment.

In 2000, the orthopedic community received a wake up call when one manufacturer, Sulzer, began to receive notices from surgeons that one of their acetabular shells, the InterOp, was failing to show osseointegration in a number of patients after a few months. The InterOp was designed with a titanium porous back to allow fixation by bony ingrowth. A thorough investigation ensued to determine why osseointegration was occurring for some patients. A number of consultants and laboratories, including Cambridge Polymer Group, were enlisted in this investigation.

Following a few months of analysis, it was determined that two key manufacturing step changes resulted in the poor outcomes. Firstly, Sulzer introduced an additional lathe-turning step following the porous titanium coating sintering process. The lathe-turning step introduced a lubricating oil into the porous backing that was insufficiently removed during the cleaning cycle. Any lubricants introduced prior to the titanium sintering process would be burned cleanly away. The second manufacturing step change was the removal of a nitric acid passivation process. Passivation is normally included in metallic devices to clean away any iron-based fragments introduced by machining tools.

Cambridge Polymer Group quantified oil content on hundreds of devices, comparing the manufacturing lots where clinical failures occurred. Interestingly, the bulk of the clinical failures occurred only in lots lacking the passivation step, despite the fact that other lots with passivation had higher levels of oil as well.

In the end, it was postulated that an endotoxin residing in the oil was responsible for the lack of osseointegration. Such an endotoxin would be readily removable with nitric acid passivation.

In the end, patients with failed InterOps received replacement devices, and a new ASTM sub-committee was formed to develop standards for determining cleanliness of medical devices. Device cleanliness has become a standardized test for medical device manufacturers.

More information on medical device cleanliness can be found here.

LabView, made by National Instruments, is a versatile programming language that has good application for laboratory equipment automation, motion control, image collection, and data analysis. Engineers and scientists at Cambridge Polymer Group routinely use LabView in their design of custom analytical instruments to characterize materials. The LabView code allows the users to set up the experimental conditions, control the equipment, collect and save the data, and analyze the data. The final data set is easily viewed in Excel, Word, or other formats.

Recently, two CPG scientists completed their coursework allowing them to become Certified LabView developers. CPG has designed custom software for clients for existing instruments and for automated data analysis algorithms.

A link to the video from a CPG-generated LabView program that collects crack propagation lengths automatically in a Fatigue Crack Propagation test can be found here.

 

Polymers can be classified as linear or branched. Branched polymers contain chains hanging off the backbone of the polymer, which could include a single side chain or multiple side chains. Linear polymers do not have branches. Branching can strongly influence the processing behavior and ultimate properties of a polymer. In the melt state, branching will increase the melt viscosity of a polymer compared to its linear analog. Branching will also reduce crystallinity in semi-crystalline polymers, as the branches partially inhibits the packing of the polymer chains. Branching can occur during polymerization due to a variety of reasons, including the use of divalent monomers, mixed monomers with different side groups, backbiting during polymerization, radiation grafting, or other reasons. Branching can also occur during processing, particularly if the material undergoes scissioning or further reaction. As a consequence, measurement of branching is important to assess process conditions and ultimate properties in the polymer in question.

Branching can be assessed rheologically, with nuclear magnetic resonance spectroscopy (NMR), and inferred with techniques that monitor end properties such as crystallinity. Detailed information can be determined by triple detection gel permeation chromatography (GPC). In this technique, a polymer is run through a standard GPC column that is equipped with a refractive index detector, a light scattering detector, and a viscometer. The refractive index detector provides information about the concentration of the polymer chains at a given elution time, light scattering provides absolute measurement of the weight-average molecular weight at each elution time, and the viscometer provides the effective density of the polymer chains at each elution time. With these measurements, a Mark-Houwink plot can be generated, as shown below. By comparing the test sample in question to a sample that is known to be a linear polymer, it is possible to assess if the material is branched. For a given molecular weight, a branched polymer will have a smaller volume and hence a reduced viscosity compared to a linear polymer. The amount of deviation from the linear analog is an indication of how much branching there is in the test sample. In this matter, the degree of branching can be calculated for each molecular weight.

The Spring ASTM meeting of F04 (Biomedical Materials and Devices) met in Indianapolis, IN this week. Some key highlights of the meeting are as follows:

Cleanliness of Biomedical Devices

The standard for shipping of potentially infectious tissues and devices was approved, and is now available as ASTM F2995-13.

ASTM F561 (Device retrieval) will be amended to contain a new method for isolating polyethylene particles from tissue.

Two draft standards were discussed. The first, Validating Cleanlines for Biomedical Devices and Instruments, is making good progress, and should be ready for another sub-committee vote prior to the next meeting. The second, Test Soils, is awaiting more information on formulas from some committee members. Two additional draft standards (Designing devices for cleaning, and Establishing threshold limits for residue levels) are expected to be ready for discussion by the November meeting.

Polymers
The committee on UHMWPE discussed several topics, including the need for 3 new standards:
1. Electron spin resonance spectroscopy of free radical content in UHMWPE.
2. Fatigue crack propagation analysis of UHMWPE
3. Oxidation induction time quantification of antioxidant concentration in UHMWPE

Cambridge Polymer Group is working on these three draft standards.

Additional discussion included a small punch round robin study on UHMWPE, which is expected to take place in the next few months. The PEEK standard was also discussed, with debate on the need for different classifications of PEEK, along with additional test methods for extractables and metals levels in PEEK.

A workshop on polymer additives is being considered for a meeting 1 year from now. Interested parties should contact Cambridge Polymer Group for more details.

Bone Cement
CPG researchers suggested a modification to the existing benzoyl peroxide titration method in F451. A small round robin will take place in the next 6 months trying this new protocol.

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 in steady decline due to concerns about metal ion release and its 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 material of choice, has greater compliance, and thus can realign in vivo if there is misalignment.



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