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The Histological Analysis of Implantable Medical Devices
Resin/Plastic Sectioning vs Cutting and Grinding
Resin/plastic histology is typically used to examine hard tissue such as bone and implanted medical device interaction with surrounding tissues. There are two basic methodologies: grinding, also called cutting and grinding, and sectioning, which is done on a rotary microtome. In either method, samples are first dehydrated then infiltrated with a plastic monomer which is subsequently polymerized into a plastic tissue block. Each method offers its own unique advantages and disadvantages. Resin/plastic histology is more labour intensive than paraffin histology and processing is usually conducted manually due to required lengthy resin infiltration times.
Cut and Grind Technique
With the cut and grind method, a diamond edge saw is used to slice a thin wafer out of the resin embedded tissue block. The wafer is then glued onto a slide and ground to the desired thickness. This technique is used for devices comprised of larger metal or those containing ceramic components. Cutting and grinding is commonly used for large devices such as orthopaedic hip replacements. The researcher would be able to see the interface between bone and metal under the microscope.
Benefits and Drawbacks of Cut and Grind
An advantage of this method over sectioning is that one can produce slides from larger and harder medical devices. However, thicker sections (50 to 100 microns) are produced, which reduces the resolution under the microscope. The equipment required is typically expensive. Also, the resins used for cut and grind may not be able to be dissolved or deplastinated; this reduces the choice and quality of stains available.
Resin Sectioning Technique
Resin/plastic sectioning is closer to paraffin microtomy than the cut and grind method. This method requires the use of a rotary microtome and a Tungsten Carbide or glass knife. Resin sectioning is the preferred process when working with tissues or devices that are too hard to cut using paraffin but not hard or large enough to require the cut and grind method. Un-decalcified bone, small stents, and fabric implants would be cut using this method. The most common plastics used are JB4 (glycol-Methacrylate), MMA (Methyl-Methacrylate), and Spurr's.
Benefits & Drawbacks of Sectioning
Both JB4 and Spurr's are the easiest resins to section and collect on a slide, while MMA is more difficult to collect onto a slide but allows for deplastination which can be followed by special stains or IHC staining. The sectioning technique produces thinner sections (3-5 micron) than the cut and grind method. These thinner sections offer better resolution under the microscope. The sectioning technique cannot be used for hard medical devices made of ceramic or larger metal components. However, devices with smaller metal components, such as arterial stents, can be sectioned quite easily using this method. Wax-it Histology Services uses the resin sectioning technique for the histological analysis of medical devices such as stents, sutures, artificial muscle, heart valves, silk, artificial fibres, bio-absorptive materials, and polymers, to name a few.
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