Resin/plastic histology is typically used to examine hard tissue, such as bone, and to study how implanted medical devices interact with surrounding tissues. There are two basic methods for resin histology: grinding, also called cutting and grinding (or ‘saw and grind’) using a precision diamond saw and a grinder–polisher, and sectioning, which is conducted using a rotary microtome. For both methods, samples are first dehydrated, and then infiltrated with a plastic monomer, which is subsequently polymerized into a plastic tissue block. With cut and grind, a diamond edge saw is used to slice a thin ‘wafer’ out of the resin-embedded tissue block. This wafer is then glued onto the slide and ground to the thickness required. The cut and grind technique is used for devices comprised of larger metal or those containing ceramic components. Cut and grind is commonly used for large devices, such as orthopedic hip replacements, which allows the researcher to see the interface between bone and metal under the microscope.
Microtomy vs. Cut & Grind (Pros and Cons)
When it comes to microtomy, sections can typically be cut to 5 microns or less. Conversely, the cut and grind method can produce sections to a thickness of between 20 and 40 microns. Microtomy characteristically yields a higher resolution and staining quality than cut and grind. With microtomy, there are more staining options to choose from, which can be limited with the cut and grind method. When working with softer implants, microtomy tends to produce better results than cut and grind. When sectioning metals and/or ceramics, microtomy is not possible; therefore, cut and grind is the method of choice. When working with hard devices/tissue-interfaces, microtomy may compromise quality, while cut and grind will generally keep these tissue interactions intact. Immunohistochemistry is possible with both cut and grind and microtomy.
Resin Histology Technique by Device Type
There are a variety of hard and soft implant materials used for a number of different surgical applications. Some common materials used for implants include: metals, plastics, fabrics, tissue-engineered absorbable scaffolds, and hydrogels. With some devices, only one method of resin histology is possible; with others, there are clear advantages and drawbacks of choosing one method over the other. See below for a summary of the advantages and disadvantages of microtomy vs. cut and grind by device type.
1. Hydrogels: Hydrogel-coated materials are commonly used in biomedical applications, especially in blood-contacting devices. With hydrogel-type devices, the method of choice would be microtomy, as the type of resin used softens with water while using the cut and grind technique.
2. Fabrics: Implantable medical fabrics are created by traditional textile methods using biomaterials appropriate for long-term implant in the body. Weaving, knitting, braiding and/or non-woven forming technologies are typically used to create implantable fabrics. These types of materials are made using a wide-ranging spectrum of polymeric and metallic biomaterials. For fabrics, microtomy leads to optimal results, as deplastination of the type of resin used in this case provides the best possible resolution, and there are a wider range of stains to choose from.
3. Stents: The most common use for stents is in coronary arteries, into which a bare-metal stent, a drug-eluting stent, a bio-absorbable stent, or a covered stent is inserted. Ideally, histological and immunohistochemical sections should be 5 microns, while keeping the tissue-stent interface in-tact. For stents, both microtomy and cut and grind are acceptable methods. With cut and grind, there is a better tissue-stent interface present, while microtomy leads to better resolution and staining.
4. Dental Implants: A dental implant is a device, usually comprised of titanium, used in dentistry to support restorations. For dental implants, the device is typically too hard to be cut using a microtome; therefore, cut and grind would be the ideal method to use.
5. Orthopedics: An orthopedic device is any device that is placed within the body’s skeletal system to provide stability or correct medical conditions (such as knee and hip replacements). Much like dental implants, orthopedic devices tend to be larger and quite hard; therefore, cut and grind is the best method for resin histology to ensure optimal results.
Each material type has its own unique requirements for optimal processing and embedding. Some materials are sensitive to chemicals and others to light and heat. Each resin histology method (cut and grind vs. microtomy) offers its own unique advantages and disadvantages, specifically by device type. To achieve the best possible results, medical device researchers should be aware of these differences.
To discuss your medical device study requiring histopathology, contact Wax-it Histology Services via email at science@waxitinc.com or by phone at 604.822.1595 for a complementary scientific consultation.
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About Wax-it Histology Services, Inc.
Incorporated in 2003, Wax-it Histology Services, Inc. brings a wealth of knowledge to our internationally recognized pharmaceutical, biotechnology, and medical device partners. Wax-it Histology Services, Inc. is a GLP-recognized CRO that offers general histology services (paraffin and frozen), immunohistochemistry services (general IHC, cross-reactivity studies, antibody and biomarker testing/validation), GLP and non-GLP histopathology for preclinical toxicology studies, and resin/plastic sectioning for implantable medical devices and bone.