Implantation Testing: Scientifically Proving the Tissue Compatibility of Medical Devices

The biocompatibility assessment of medical devices covers not only the device’s contact with blood or skin, but also its long-term interaction with body tissues. In particular, invasive products such as implants, surgical instruments, bone grafts, suture materials, stents, and orthopedic devices can cause local reactions in the tissues where they are placed. These reactions may manifest as inflammation, fibrosis, necrosis, granuloma formation, degeneration, or tissue integration.
Implantation testing enables the scientific evaluation of these local effects. Our laboratory conducts implantation tests in accordance with the ISO 10993-6 standard, under GLP principles, and with a team of experienced veterinary pathologists.

The Scientific Basis and Importance of the Implantation Test

The implantation test involves the surgical placement of a medical device or biomaterial into living tissue, followed by histopathological examination at specific time points. The primary purpose of this test is to evaluate the device’s compatibility with tissue (biocompatibility), potential toxic effects, and tissue integration. The implantation test differs fundamentally from other biocompatibility tests (cytotoxicity, sensitization, irritation) because it directly observes the device’s actual behavior and the tissue response within the body.

The importance of the implantation test stems from the clinical usage scenario of medical devices. An implant can remain in the body for months or years and is in constant interaction with surrounding tissues during this time. The device’s surface properties, chemical composition, degradation products, and mechanical properties directly influence the tissue response. A good implant must be biocompatible, must not cause chronic inflammation, must not release toxic substances, and must be able to maintain its function over the long term. The implantation test ensures that these criteria are scientifically validated.

Implantation test results are directly used in the device’s regulatory approval process, in the preparation of the biocompatibility dossier, and in the safety assessment prior to clinical use. For permanent implants in particular, this test constitutes one of the most critical pieces of evidence for patient safety. Furthermore, for biodegradable devices, the implantation test is indispensable for evaluating the effects of degradation products on tissue and the degradation kinetics.

Scope and Test Types of the ISO 10993-6 Standard

ISO 10993-6 is the internationally recognized fundamental standard for evaluating the local effects of medical devices following implantation.
This standard requires that the test material or device be surgically implanted directly into the target tissue of an appropriate animal model in a manner that best reflects the device’s clinical usage scenario, followed by histopathological examination at specified time points. The evaluation involves the qualitative and semi-quantitative characterization of the tissue response at the implantation site.

Under ISO 10993-6, different test methods are applied depending on the device’s intended clinical use and the implantation site. Subcutaneous (under the skin) implantation is used for surface devices, catheters, wound dressings, and suture materials; the test material is placed in the subcutaneous connective tissue of the back. Intramuscular (intramuscular) implantation is used for devices and drug delivery systems implanted into muscle; the paravertebral region or the thigh muscle is used. Intraosseous (in-bone) implantation is used for orthopedic implants, bone screws, plates, dental implants, and bone grafts; bone tissues such as the femur, tibia, or cranium are preferred. Intracranial/nerve tissue implantation is used for neurostimulators and nerve guide tubes. Intraperitoneal (intra-abdominal) implantation is used for surgical mesh (hernia patches) and adhesion barriers. Intravascular implantation is used for stents, vascular grafts, and catheters.

Test durations are determined based on the device’s clinical usage period. For short-term contact devices, 1–4 weeks are recommended; for long-term contact devices, 4–12 weeks; and for permanent implants, 12–26 weeks or longer. For biodegradable devices, the test must be continued until the device has completely degraded (6, 12, 18 months, or longer). These durations are determined based on scientific rationale, taking into account the device’s residence time in the body and its degradation profile.

Histopathological Evaluation Criteria

The most critical component of the implantation test is the systematic and semi-quantitative evaluation of the tissue response at the implantation site. This evaluation is performed by certified veterinary pathologists using a blinded method and internationally accepted INHAND terminology. Eight key parameters are examined during the evaluation.

Inflammation is scored on a scale of 0–4 based on inflammatory cell infiltration (neutrophils, lymphocytes, macrophages, plasma cells); this serves as a key indicator of the device’s tissue compatibility. Fibrosis / fibrotic capsule formation is assessed by measuring the thickness and density of the connective tissue capsule surrounding the implant; excessive fibrosis can impair implant function, but normal capsule formation is an acceptable response.
Necrosis—areas of tissue death (coagulative, caseous, and fibrinoid necrosis)—are scored on a scale of 0–4, and the percentage of the necrotic area is calculated; this is a direct indicator of tissue damage. Granuloma formation is assessed based on foreign-body giant cells and clusters of epithelioid macrophages; excessive granuloma formation indicates chronic inflammation.

Foreign body reaction refers to the reaction that forms around particles or degradation products of the implant material; it is particularly critical for biodegradable devices and serves as an indicator of particle toxicity. Vascularization/angiogenesis assesses new blood vessel formation at the implant site; it is an indicator of tissue integration and the healing process. Tissue integration measures the degree of integration between the implant and the surrounding tissue; it is particularly critical for evaluating osseointegration in bone implants. Degeneration/calcification evaluates tissue degeneration and dystrophic calcification; it is an indicator of late-stage complications in long-term implants.

The table below summarizes the basic histopathological parameters and scoring approach evaluated under ISO 10993-6.

For Which Devices Is Implantation Testing Required?

The ISO 10993-1 standard adopts a risk-based approach to the biological evaluation of medical devices. Within this framework, the requirement for implantation testing is determined based on the type and duration of contact.

Surface devices—products that come into contact with the skin (wound dressings, electrodes, transdermal patches)—generally do not require implantation testing; skin irritation and sensitization tests are sufficient. Surface devices—products that come into contact with mucous membranes (contact lenses, dental materials)—may require an implantation test; mucosal reactions should be evaluated, particularly in cases of prolonged contact. Externally connected devices—products that come into contact with tissue or blood (intravenous catheters, dialysis devices, drainage tubes)—require implantation testing; local reactions following subcutaneous or intramuscular placement are evaluated.

Implantable devices – implantation testing is mandatory for products that come into contact with bone or tissue (orthopedic implants, screws, plates, dental implants); osteointegration, fibrotic capsule formation, chronic inflammation, and biodegradation are evaluated. Implantable devices—products in contact with blood (stents, vascular grafts, heart valves)—require an implantation test; endothelialization, thrombosis, neointimal hyperplasia, and the vascular tissue response are evaluated.

Implantation testing is mandatory for biodegradable devices (absorbable sutures, biodegradable stents, tissue scaffolds), and the test must be continued until degradation is complete; degradation kinetics, particle reaction, tissue regeneration, and the presence of any permanent damage are evaluated. Implantation testing is also mandatory for combination devices (drug-eluting stents, antibiotic-impregnated bone cement); the local interaction between the drug and the device, as well as the tissue response, are evaluated.

Conducting Implantation Tests in Our Laboratory

In our GLP-certified laboratory, we conduct implantation tests in accordance with ISO 10993-6, paying the utmost attention to quality control at every stage, from the surgical procedure to the histopathological evaluation. Surgical procedures are performed under aseptic conditions by experienced veterinary surgeons using appropriate anesthesia and analgesia protocols. Postoperative monitoring is standard practice, and animal welfare is maintained at the highest level.

During test material preparation, test and control materials are prepared with the same dimensions, shape, and surface properties; their sterilization status and implant dimensions are documented. As a negative control, reference materials recommended by ISO 10993-12 and ISO 10993-6 (e.g., high-density polyethylene—HDPE) are used. This is critical for verifying the sensitivity of the test system and distinguishing the response resulting from the implantation procedure itself from the effect of the test material.

In the histopathological evaluation, the implantation site and surrounding tissue are completely excised; paraffin or resin embedding is performed. Special sectioning techniques and staining methods are used for hard tissue (bone). The evaluation is performed by a certified veterinary pathologist using a blinded method and semi-quantitative scoring in accordance with INHAND terminology. The report includes macroscopic findings, histopathological scores, representative microphotographs, and an interpretation section; a complete report format compliant with ISO 10993-6 Annex A and Annex B is used.

One of our laboratory’s key strengths is our extensive experience with various animal models and implantation sites. Rats and rabbits are used for small implants; sheep, goats, or mini-pigs for orthopedic devices; and dogs, sheep, or pigs for vascular devices. The selection of animal models is based on scientific rationale according to the device’s target tissue and size and is documented in the study protocol.

Frequently Asked Questions

One of the most common questions we receive regarding implantation testing is which device classes require this test. According to the EU MDR (2017/745), implantation testing under ISO 10993-6 is mandatory for all implantable devices and devices that come into long-term contact with tissue or blood. The device’s duration of contact and degree of invasiveness determine the testing requirement.

We are also frequently asked which animal model should be selected for the test. The animal model depends on the device’s target tissue and size. Rats and rabbits are preferred for small implants; sheep, goats, or mini-pigs for orthopedic devices; and dogs, sheep, or pigs for vascular devices. The selection must be documented with scientific justification.

ISO 10993-6 recommends evaluating the device at multiple time points; to distinguish between short-term acute reactions and long-term chronic effects, evaluation at a minimum of two time points (early and late phases) is recommended. For permanent implants, durations of 12–26 weeks are typical. In response to the question of why a negative control is necessary, the answer is that the negative control (e.g., HDPE) is used to verify the sensitivity of the test system and to distinguish the response resulting from the implantation procedure itself from the effect of the test material. Uncontrolled results are not accepted by regulatory authorities.

There is also interest in how long the test period should be for biodegradable devices. Testing for biodegradable devices must be continued until the device has completely degraded or has fully integrated into the tissue. This period can range from 6 months to 2 years, depending on the device’s degradation profile. As for the question of what format the histopathology report should follow, the answer is that the report must include macroscopic findings, histopathological scoring tables, representative microphotographs, statistical analyses, and the pathologist’s interpretation; the format recommended in the annexes of ISO 10993-6 must be followed.

The implantation test provides scientific evidence of your medical device’s compatibility with body tissues. A rigorous implantation study conducted in accordance with ISO 10993-6 directly impacts your device’s safety, performance, and regulatory acceptability. Our laboratory offers services in a wide range of applications—from subcutaneous applications to orthopedic and vascular implants—in accordance with GLP standards and with a team of experienced veterinary pathologists. Contact us to be your trusted partner on your medical device’s biocompatibility journey.