2023 International Academy of Toxicologic Pathology (IATP) Satellite Symposium: "Medical Device Safety Assessment: Pathology and Toxicology Perspective"

Medical devices represent a complex category of medicinal products with varying definitions depending on the regional jurisdiction of regulatory agencies. A common aspect of these definitions is that a medical device is intended to be used for specific medicinal purpose where the primary intended action of the device is not achieved through pharmacologic (or other chemical) means. While regional regulatory frameworks for medical devices are different than for pharmaceutical or biological products, medical device manufacturers are required to evaluate the safety and performance of these products in the context of their intended use. In biological safety evaluation, histopathology plays a relevant role in assessing medical device biocompatibility. This manuscript provides a broad overview of biocompatibility assessment with a deeper look at the role of the toxicologic pathologist in assessing innovative and emerging bone therapies. The content of this manuscript is based on individual presentations delivered at the 2023 International Academy of Toxicologic Pathology (IATP) Satellite Symposium held in conjunction with the Annual Congress of the European Society of Toxicologic Pathology (ESTP) on 26 September, in Basel, Switzerland.

Histomorphometric Analysis of Osseointegrated Intraosseous Dental Implants Using Undecalcified Specimens: A Scoping Review

Bone histology and histomorphometry are reliable diagnostic tools for the assessment of the bone-implant interface, material safety and biocompatibility, and tissue response. They allow for the qualitative and quantitative analysis of undecalcified bone specimens. This scoping review aims to identify the most common staining techniques, study models for in vivo experiments, and histomorphometric parameters used for quantitative bone evaluation of osseointegrated dental implants in the last decade. The Web of Science, PubMed, and Scopus databases were searched on 1 July 2024 for relevant articles in English, published in the last ten years, and the data were exported to an MS Excel spreadsheet. A total of 115 studies met the eligibility criteria and were included in the present review. The results indicate that the most common study models are dogs, rabbits, and pigs. Some of the most frequently used methods for the assessment of the bone-implant interface are the Toluidine blue, Stevenel's blue with Van Gieson, and Levai-Laczko stainings. The results from this study demonstrate that the most commonly used histomorphometric parameters in implant dentistry are the bone-to-implant contact (BIC), bone area fraction occupancy (BAFO), bone area (BA), and bone density (BD). This review presents the recent trends in histomorphometric analysis of dental implants and identifies some research gaps that necessitate further research.

Tissue changes and tissue reactivity following osteotomy by a conventional rotary device, an ultrasonic unit, and an Er: YAG laser - A comparative study in humans

INTRODUCTION

Innovative technologies from other fields of science are constantly being introduced in medicine. Research works on animals strongly confirm the belief in better results following ultrasound and laser surgery. The main conclusions are based on observations from different animal species, whose biological characteristics differ from those of humans. This requires scientific experiments to be performed and confirmed in humans.

MATERIAL & METHODS

Human mandible specimens harvested in vivo by a conventional low-speed rotary device, an ultrasonic unit, or an Er: YAG laser were studied and analyzed to compare their effects on bone morphology and tissue response following surgery.

RESULTS

The cutting surface of eighty biopsies was studied, as well the facial edema, pain levels, and jaw dysfunction were followed up in the same eighty patients from whom the slices were obtained. In the piezosurgery and drill groups, the borders exhibited irregular edges full of bone fragments and debris. Thermal alterations within the superficial surface in all bone samples were generally minimal. In all specimens, intact osteocytes were detectable away from the area of direct action of the bone-cutting device. It was established, that the reactive facial edema, pain levels, and jaw disfunction were with milder values following in vivo osteotomy by an ultrasonic unit and Er: YAG laser compared to the conventional drilling. Moreover, the recovery of the patients in these two groups was more pronounced. The observed tissue changes proved to affect the follow-up tissue reactions in the postoperative period (p ≤ 0.05).

DISCUSSION

Based on the established effects on human bone by its in vivo cutting, it can be concluded that laser and ultrasonic bone surgery was a superior alternative osteotomy method in humans to conventional drilling.

Osseointegration of minimally invasive sacroiliac joint fixation implants-A human retrieval study

Minimally invasive sacroiliac joint fusion has become increasingly prevalent and is described to reduce pain and improve function. In some patients, pain can recur several months after primary surgery. Lack of early implant osseointegration might be a cause of pain and hence an indication for revision surgery. Triangular titanium implants are the most documented implant for minimally invasive sacroiliac joint fusion. There is, however, no knowledge of how triangular titanium implants osseointegrate in humans and whether fusion is induced over the sacroiliac joint. During planned revision surgery due to recurrent pain, six triangular titanium implants were retrieved from six different patients at median 9 months from primary surgery. All six implants were scanned using microcomputed tomography. The presence or absence of bone in-growth, on-growth, and through-growth of the implants was evaluated as an indication of implant osseointegration. Three of six implants showed no or minor signs of osseointegration. Of the three remaining implants, one showed partial osseointegration and two implants showed high degrees of osseointegration. This study showed that triangular titanium implants can osseointegrate into host bone in humans. When osseointegration occurs, triangular titanium implants can give fusion across the sacroiliac joint.

Cortico-cancellous osseointegration into additively manufactured titanium implants using a load-bearing femoral ovine model

Introduction: Titanium-based implants can be used to fill voids in bone reconstruction surgery. Through additive manufacturing (AM), it is possible to produce titanium implants with osteoconductive properties such as high porosity and low stiffness. AM facilitates a level of design flexibility and personalization that is not feasible with traditional techniques. Methods: In this study, osseointegration into titanium alloy (Ti-6Al-4V) lattices was investigated for 12 weeks post-implantation using a novel bicortical load-bearing ovine model. The objective was to assess the safety and efficacy of AM-fabricated implants using two lattice structures of contrasting stiffness spanning the full width of the femoral condyle. Results: This was achieved by evaluating implant osseointegration and bone-implant contact properties by histomorphometry, scoring local implant tissue responses via histopathology, and micro-computed tomography reconstruction. Discussion: We found that Ti-6Al-4V implants facilitated widespread and extensive osseointegration, with bone maturation ongoing at the conclusion of the trial period. Following the implantation period, no adverse clinical indications that could be directly ascribed to the presence of the implanted device were identified, as determined by macroscopic and microscopic observation.

In Vivo Biocompatibility Study on Functional Nanostructures Containing Bioactive Glass and Plant Extracts for Implantology

In this paper, the in vivo behavior of orthopedic implants covered with thin films obtained by matrix-assisted pulsed laser evaporation and containing bioactive glass, a polymer, and natural plant extract was evaluated. In vivo testing was performed by carrying out a study on guinea pigs who had coated metallic screws inserted in them and also controls, following the regulations of European laws regarding the use of animals in scientific studies. After 26 weeks from implantation, the guinea pigs were subjected to X-ray analyses to observe the evolution of osteointegration over time; the guinea pigs' blood was collected for the detection of enzymatic activity and to measure values for urea, creatinine, blood glucose, alkaline phosphatase, pancreatic amylase, total protein, and glutamate pyruvate transaminase to see the extent to which the body was affected by the introduction of the implant. Moreover, a histopathological assessment of the following vital organs was carried out: heart, brain, liver, and spleen. We also assessed implanted bone with adjacent tissue. Our studies did not find significant variations in biochemical and histological results compared to the control group or significant adverse effects caused by the implant coating in terms of tissue compatibility, inflammatory reactions, and systemic effects.

Comparison between digital superimposition and microcomputed tomography methods of fit assessment of removable partial denture frameworks

STATEMENT OF PROBLEM

The fit of removable partial denture frameworks should be assessed to optimize clinical adaptation. Potential discrepancies between framework and supporting structures are typically precisely measured with negative subtracts and high-resolution equipment. The growth of computer-aided engineering technology allows the development of new methods for the direct evaluation of discrepancies. However, how the methods compare is unclear.

PURPOSE

The purpose of this in vitro study was to compare 2 digital methods of fit assessment based on direct digital superimposition and microcomputed tomography indirect analysis.

MATERIAL AND METHODS

Twelve cobalt chromium removable partial denture frameworks were fabricated by conventional lost-wax casting or additive manufacturing techniques. The thickness of the gap between occlusal rests and respective definitive cast rest seats (n=34) was evaluated by using 2 different digital methods. Silicone elastomer impressions of the gaps were obtained, and microcomputed tomography measurements were used as controls for validation purposes. Digitization of the framework, the respective definitive cast, and the combination was followed by digital superimposition and direct measurements with the Geomagic Control X software program. Because normality and homogeneity of variance were not verified (Shapiro-Wilk and Levene tests, P<.05), the data were analyzed with Wilcoxon signed rank and Spearman correlation tests (α=.05).

RESULTS

The thicknesses measured by microcomputed tomography (median=242 μm) and digital superimposition (median=236 μm) did not reveal statistically significant differences (P=.180). A positive correlation (ρ=0.612) was detected between the 2 methods of assessing fit.

CONCLUSIONS

The frameworks presented median gap thicknesses under the limit of clinical acceptability without differences between the proposed methods. The digital superimposition method was determined to be as acceptable as the high-resolution microcomputed tomography method for assessing removable partial denture framework fit.

Geometric Mismatch Promotes Anatomic Repair in Periorbital Bony Defects in Skeletally Mature Yucatan Minipigs

Porous tissue-engineered 3D-printed scaffolds are a compelling alternative to autografts for the treatment of large periorbital bone defects. Matching the defect-specific geometry has long been considered an optimal strategy to restore pre-injury anatomy. However, studies in large animal models have revealed that biomaterial-induced bone formation largely occurs around the scaffold periphery. Such ectopic bone formation in the periorbital region can affect vision and cause disfigurement. To enhance anatomic reconstruction, geometric mismatches are introduced in the scaffolds used to treat full thickness zygomatic defects created bilaterally in adult Yucatan minipigs. 3D-printed, anatomically-mirrored scaffolds are used in combination with autologous stromal vascular fraction of cells (SVF) for treatment. An advanced image-registration workflow is developed to quantify the post-surgical geometric mismatch and correlate it with the spatial pattern of the regenerating bone. Osteoconductive bone growth on the dorsal and ventral aspect of the defect enhances scaffold integration with the native bone while medio-lateral bone growth leads to failure of the scaffolds to integrate. A strong positive correlation is found between geometric mismatch and orthotopic bone deposition at the defect site. The data suggest that strategic mismatch >20% could improve bone scaffold design to promote enhanced regeneration, osseointegration, and long-term scaffold survivability.

Hydroxyapatite 3D-printed scaffolds with Gyroid-Triply periodic minimal surface porous structure: Fabrication and an in vivo pilot study in sheep

Bone repair is a major challenge in regenerative medicine, e.g. for large defects. There is a need for bioactive, highly percolating bone substitutes favoring bone ingrowth and tissue healing. Here, a modern 3D printing approach (VAT photopolymerization) was exploited to fabricate hydroxyapatite (HA) scaffolds with a Gyroid-"Triply periodic minimal surface" (TPMS) porous structure (65% porosity, 90.5% HA densification) inspired from trabecular bone. Percolation and absorption capacities were analyzed in gaseous and liquid conditions. Mechanical properties relevant to guided bone regeneration in non-load bearing sites, as for maxillofacial contour reconstruction, were evidenced from 3-point bending tests and macrospherical indentation. Scaffolds were implanted in a clinically-relevant large animal model (sheep femur), over 6 months, enabling thorough analyses at short (4 weeks) and long (26 weeks) time points. In vivo performances were systematically compared to the bovine bone-derived Bio-OssⓇ standard. The local tissue response was examined thoroughly by semi-quantitative histopathology. Results demonstrated the absence of toxicity. Bone healing was assessed by bone dynamics analysis through epifluorescence using various fluorochromes and quantitative histomorphometry. Performant bone regeneration was evidenced with similar overall performances to the control, although the Gyroid biomaterial slightly outperformed Bio-OssⓇ at early healing time in terms of osteointegration and appositional mineralization. This work is considered a pilot study on the in vivo evaluation of TPMS-based 3D porous scaffolds in a large animal model, for an extended period of time, and in comparison to a clinical standard. Our results confirm the relevance of such scaffolds for bone regeneration in view of clinical practice. STATEMENT OF SIGNIFICANCE: Bone repair, e.g. for large bone defects or patients with defective vascularization is still a major challenge. Highly percolating TPMS porous structures have recently emerged, but no in vivo data were reported on a large animal model of clinical relevance and comparing to an international standard. Here, we fabricated TPMS scaffolds of HA, determined their chemical, percolation and mechanical features, and ran an in-depth pilot study in the sheep with a systematic comparison to the Bio-OssⓇ reference. Our results clearly show the high bone-forming capability of such scaffolds, with outcomes even better than Bio-OssⓇ at short implantation time. This preclinical work provides quantitative data validating the relevance of such TMPS porous scaffolds for bone regeneration in view of clinical evaluation.

Scientific and Regulatory Policy Committee Best Practices: Recommended ("Best") Practices for Informed (Non-blinded) Versus Masked (Blinded) Microscopic Evaluation in Animal Toxicity Studies

This article describes the Society of Toxicologic Pathology's (STP) five recommended ("best") practices for appropriate use of informed (non-blinded) versus masked (blinded) microscopic evaluation in animal toxicity studies intended for regulatory review. (1) Informed microscopic evaluation is the default approach for animal toxicity studies. (2) Masked microscopic evaluation has merit for confirming preliminary diagnoses for target organs and/or defining thresholds ("no observed adverse effect level" and similar values) identified during an initial informed evaluation, addressing focused hypotheses, or satisfying guidance or requests from regulatory agencies. (3) If used as the approach for an animal toxicity study to investigate a specific research question, masking of the initial microscopic evaluation should be limited to withholding only information about the group (control or test article-treated) and dose equivalents. (4) The decision regarding whether or not to perform a masked microscopic evaluation is best made by a toxicologic pathologist with relevant experience. (5) Pathology peer review, performed to verify the microscopic diagnoses and interpretations by the study pathologist, should use an informed evaluation approach. The STP maintains that implementing these five best practices has and will continue to consistently deliver robust microscopic data with high sensitivity for animal toxicity studies intended for regulatory review. Consequently, when conducting animal toxicity studies, the advantages of informed microscopic evaluation for maximizing sensitivity outweigh the perceived advantages of minimizing bias through masked microscopic examination.

Biocompatibility and degradation comparisons of four biodegradable copolymeric osteosynthesis systems used in maxillofacial surgery: A goat model with four years follow-up

Applying biodegradable osteosyntheses avoids the disadvantages of titanium osteosyntheses. However, foreign-body reactions remain a major concern and evidence of complete resorption is lacking. This study compared the physico-chemical properties, histological response and radiographs of four copolymeric biodegradable osteosynthesis systems in a goat model with 48-months follow-up. The systems were implanted subperiosteally in both tibia and radius of 12 Dutch White goats. The BioSorb FX [poly(70LLA-co-30DLLA)], Inion CPS [poly([70–78.5]LLA-co-[16–24]DLLA-co-4TMC)], SonicWeld Rx [poly(DLLA)], LactoSorb [poly(82LLA-co-18GA)] systems and a negative control were randomly implanted in each extremity. Samples were assessed at 6-, 12-, 18-, 24-, 36-, and 48-month follow-up. Surface topography was performed using scanning electron microscopy (SEM). Differential scanning calorimetry and gel permeation chromatography were performed on initial and explanted samples. Histological sections were systematically assessed by two blinded researchers using (polarized) light microscopy, SEM and energy-dispersive X-ray analysis. The SonicWeld Rx system was amorphous while the others were semi-crystalline. Foreign-body reactions were not observed during the complete follow-up. The SonicWeld Rx and LactoSorb systems reached bone percentages of negative controls after 18 months while the BioSorb Fx and Inion CPS systems reached these levels after 36 months. The SonicWeld Rx system showed the most predictable degradation profile. All the biodegradable systems were safe to use and well-tolerated (i.e., complete implant replacement by bone, no clinical or histological foreign body reactions, no [sterile] abscess formation, no re-interventions needed), but nanoscale residual polymeric fragments were observed at every system's assessment.

•

Foreign-body reactions are a major concern of biodegradable osteosyntheses.

•

Amorphous poly(DLLA) showed the most predictable degradation profile.

•

Nanoscale residual polymeric fragments could still be observed after 4 years.

Scientific and Regulatory Policy Committee Points to Consider for Medical Device Implant Site Evaluation in Nonclinical Studies

Nonclinical implantation studies are a common and often critical step for medical device safety assessment in the bench-to-market pathway. Nonclinical implanted medical devices or drug-device combination products require complex macroscopic and microscopic pathology evaluations due to the physical presence of the device itself and unique tissue responses to device materials. The Medical Device Implant Site Evaluation working group of the Society of Toxicologic Pathology's (STP) Scientific and Regulatory Policy Committee (SRPC) was tasked with reviewing scientific, technical, and regulatory considerations for these studies. Implant site evaluations require highly specialized methods and analytical schemes that should be designed on a case-by-case basis to address specific study objectives. Existing STP best practice recommendations can serve as a framework when performing nonclinical studies under Good Laboratory Practices and help mitigate limitations in standards and guidances for implant evaluations (e.g., those from the International Organization for Standardization [ISO], ASTM International). This article integrates standards referenced by sponsors and regulatory bodies with practical pathology evaluation methods for implantable medical devices and combination products. The goal is to ensure the maximum accuracy and scientific relevance of pathology data acquired during a medical device or combination drug-device implantation study.

Novel resin tissue array system reduces sample preparation time, labour and reagent costs in bone tissue histology

Resin histology plays an essential role in the analysis of hard tissues, such as bone and teeth, as well as in the context of metallic implant analysis. However, the techniques of resin embedding, followed by ground sectioning, are very costly due to significantly increased reagent cost and labour time when compared to the conventional paraffin histology approach. In the present study, a novel resin array system was developed to increase the affordability of a project analysing rat femur tissues containing metallic or polymeric implants. The resin array system enabled the simultaneous embedding of the femur samples in groups of eight samples compared to the conventional resin method where samples are processed individually. The ground sections produced with the resin array system allowed uniform ROI selection, ground section thickness, staining consistency, and histological analysis with Goldner's trichrome stain, offering a substantial opportunity for reproducible immunohistochemistry which is unable to be achieved when processing samples embedded individually. The application of this novel resin array system significantly reduced resource usage when compared to doing the same analysis on individual samples. A reduction of approximately 40% was achieved for both total labour time and total reagent cost through the use of the array system compared with individual embedding. This novel resin array system has widespread applicability to many bone, hard tissue, and metallic implant studies, offering substantial conservation of research funds and increased accessibility to advanced techniques for commercial partners due to more cost-effective sample preparation and more accurate, reproducible data.

Neutron microtomography to investigate the bone-implant interface-comparison with histological analysis

Bone properties and especially its microstructure around implants are crucial to evaluate the osseointegration of prostheses in orthopaedic, maxillofacial and dental surgeries. Given the intrinsic heterogeneous nature of the bone microstructure, an ideal probing tool to understand and quantify bone formation must be spatially resolved. X-ray imaging has often been employed, but is limited in the presence of metallic implants, where severe artifacts generally arise from the high attenuation of metals to x-rays. Neutron tomography has recently been proposed as a promising technique to study bone-implant interfaces, thanks to its lower interaction with metals. The aim of this study is to assess the potential of neutron tomography for the characterisation of bone tissue in the vicinity of a metallic implant. A standardised implant with a bone chamber was implanted in rabbit bone. Four specimens were imaged with neutron tomography and subsequently compared to non-decalcified histology to stain soft and mineralised bone tissues, used here as a ground-truth reference. An intensity-based image registration procedure was performed to place the 12 histological slices within the corresponding 3D neutron volume. Significant correlations (p < 0.01) were obtained between the two modalities for the bone-implant contact (BIC) ratio (R = 0.77) and the bone content inside the chamber (R = 0.89). The results indicate that mineralised bone tissue can be reliably detected by neutron tomography. However, theBICratio and bone content were found to be overestimated with neutron imaging, which may be explained by its sensitivity to non-mineralised soft tissues, as revealed by histological staining. This study highlights the suitability of neutron tomography for the analysis of the bone-implant interface. Future work will focus on further distinguishing soft tissues from bone tissue, which could be aided by the adoption of contrast agents.

A systematic review of preclinical in vivo testing of 3D printed porous Ti6Al4V for orthopedic applications, part I: Animal models and bone ingrowth outcome measures

For Ti6Al4V orthopedic and spinal implants, osseointegration is often achieved using complex porous geometries created via additive manufacturing (AM). While AM porous titanium (pTi) has shown clinical success, concerns regarding metallic implants have spurred interest in alternative AM biomaterials for osseointegration. Insights regarding the evaluation of these new materials may be supported by better understanding the role of preclinical testing for AM pTi. We therefore asked: (a) What animal models have been most commonly used to evaluate AM porous Ti6Al4V for orthopedic bone ingrowth; (b) What were the primary reported quantitative outcome measures for these models; and (c) What were the bone ingrowth outcomes associated with the most frequently used models? We performed a systematic literature search and identified 58 articles meeting our inclusion criteria. We found that AM pTi was evaluated most often using rabbit and sheep femoral condyle defect (FCD) models. Additional ingrowth models including transcortical and segmental defects, spinal fusions, and calvarial defects were also used with various animals based on the study goals. Quantitative outcome measures determined via histomorphometry including ''bone ingrowth'' (range: 3.92-53.4% for rabbit/sheep FCD) and bone-implant contact (range: 9.9-59.7% for rabbit/sheep FCD) were the most common. Studies also used 3D imaging to report outcomes such as bone volume fraction (BV/TV, range: 4.4-61.1% for rabbit/sheep FCD), and push-out testing for outcomes such as maximum removal force (range: 46.6-3092 N for rabbit/sheep FCD). Though there were many commonalities among the study methods, we also found significant heterogeneity in the outcome terms and definitions. The considerable diversity in testing and reporting may no longer be necessary considering the reported success of AM pTi across all model types and the ample literature supporting the rabbit and sheep as suitable small and large animal models, respectively. Ultimately, more standardized animal models and reporting of bone ingrowth for porous AM materials will be useful for future studies.

Histomorphometric Evaluation of Critical-Sized Bone Defects Using Osteomeasure and Aperio Image Analysis Systems

Most histological evaluations of critical-sized bone defects are limited to the analysis of a few regions of interest at a time. Manual and semiautomated histomorphometric approaches often have intra- and interobserver subjectivity, as well as variability in image analysis methods. Moreover, the production of large image data sets makes histological assessment and histomorphometric analysis labor intensive and time consuming. Herein, we tested and compared two image segmentation methods: thresholding (automated) and region-based (manual) modes, for quantifying complete image sets across entire critical-sized bone defects, using the widely used Osteomeasure system and the freely downloadable Aperio Image Scope software. A comparison of bone histomorphometric data showed strong agreement between the automated segmentation mode of the Osteomeasure software with the manual segmentation mode of Aperio Image Scope analysis (bone formation R² = 0.9615 and fibrous tissue formation R² = 0.8734). These results indicate that Aperio is capable of handling large histological images, with excellent speed performance in producing highly consistent histomorphometric evaluations compared with the Osteomeasure image analysis system. The statistical evaluation of these two major bone parameters demonstrated that Aperio Image Scope is as capable as Osteomeasure. This study developed a protocol to improve the quality of results and reduce analysis time, while also promoting the standardization of image analysis protocols for the histomorphometric analysis of critical-sized bone defect samples. Impact Statement Despite bone tissue engineering innovations increasing over the last decade, histomorphometric analysis of large bone defects used to study such approaches continues to pose a challenge for pathological assessment. This is due to the resulting large image data set, and the lack of a gold standard image analysis protocol to quantify histological outcomes. Herein, we present a standardized protocol for the image analysis of critical-sized bone defect samples stained with Goldner's Trichrome using the Osteomeasure and Aperio Image Scope image analysis systems. The results were critically examined to determine their reproducibility and accuracy for analyzing large bone defects.