In part 1, we discussed why bone implantation studies are important. But once you’ve decided you need one, how do you go about performing it?
There are several basic components of an implantation study: what test model to use, where to implant, how long the in-life component should be (in other words, what intervals at which to terminate), how the sites will be processed afterward, and what they’ll be evaluated for. Let’s look at these in more detail:
Test Model. There are several sizes of models that can be used in a bone implant study, depending on your needs. Smaller test models are inexpensive, easy to get, and commonly used in R&D for quick data; especially for startups and in academia. Mid-sized models are also relatively inexpensive. They have been extensively used in implantation studies of all kinds, so using mid-sized models means a large potential database of historical data. Large models are more laborious and expensive to keep, but can be useful for devices that need to be tested full-size rather than in coupons.
Generally speaking, the test model to choose is the smallest that will provide the data you need. If you want a relatively quick feasibility study for a material, small test models are your best choice. If you have a heavy device that needs to be tested for efficacy, large models will be best.
Implantation site. There are two basic types of bone to choose from: cortical and cancellous. Cortical bone is compact and dense, forming most of the human skeleton, especially along the length of long bones. The midshaft of the femur is a common site for cortical-bone implantation. Cancellous (or trabecular) bone is more spongy and flexible than cortical bone, and is found at the end of bones and near joints.
Often, choosing cortical or cancellous bone is sufficient for defining which implantation site to use. However, the site of clinical use can sometimes be more important. If the indication is unique for a critical location (such as the spine or skull), an implantation study at that site may be required. “A bone void filler or cranioplasty material used in the skull would require a calvarial model to address the interaction with neural tissue,” explains Dr. Joseph Carraway, NAMSA’s scientific director for biocompatibility. “If material is intended for spinal fusion, they would need a spinal fusion model to demonstrate efficacy in this location.”
Termination intervals. ISO 10993-6 states that multiple intervals should be evaluated, especially when the implant will dissolve or resorb. For non-absorbable materials, its recommendation is a short (1-4 weeks) and long (12 weeks or more) interval for muscle and connective tissue, with potentially longer periods for bone implants. Absorbable materials should be evaluated at several points along their expected absorption time. The most common intervals we recommend at NAMSA are 4, 12, and 26 weeks.
Processing. Once removed from the test model, bone implantation sites will normally be preserved in 10% neutral buffered formalin and further processed for histology. Bone can be decalcified in order to be embedded in paraffin (as is commonly done with tissue implants). However, decalcification can be a lengthy process, and for implant materials that cannot be embedded and cut in paraffin anyway, it does not add any benefit. For those materials, bone implant sites can be processed with either hard or soft plastic. Once fixed and processed, the sites—now “blocks”—are sliced thinly, placed on slides, and stained in various ways to assist in microscopic evaluation. There are several common stains for bone:
- Hematoxylin & eosin (H&E) is by far the most common stain used in histology. It stains different tissue structures differently and provides an overall view of cellular structure around the implantation site.
- Masson’s Trichrome is a common stain for new bone growth and osteoids, used in paraffin embedding.
- Masson’s Trichrome Goldner is used with plastic-processed slides.
- Toluidine Blue is used to define osteons to track new bone growth.
- Von Kossa is used to show areas of mineralization.
Evaluation. This is the payoff of any implantation study: determining what effect your product had. There are several qualitative and quantitative methods for evaluation, which we will address in the last post in this series.