Biointegrative, mineral fiber-reinforced bone fixation implants recently introduced in orthopedic surgery have expanded available treatment options for fractures and bone deformities. This new technology aims to address the disadvantages of permanent metallic implants while overcoming inherent concerns of adverse inflammatory reactions when using polymer-based orthopedic implants. The purpose of this double-arm preclinical study was to evaluate the safety, biocompatibility, and biointegration of fiber-reinforced plates, following implantation on the tibias of eight sheep. Left tibias underwent periosteal elevation, allowing for implant attachment directly onto the cortical surface; right tibia plates were implanted over intact periosteum. Microcomputed tomography and histopathology were performed at 13, 26, 52, 78, 104, and 134 weeks postimplantation. All animals were evaluated clinically at each time point, with no evidence of local adverse reactions. Histopathology demonstrated anti-inflammatory M2-like macrophages and multinucleated giant cells corresponding to implant bioabsorption, similar for both groups at each time point, and indicating expected implant biocompatibility. Inflammatory cells (i.e., eosinophils, lymphophyctes, plasma cells, and M1-like macrophages) were absent throughout the study. The bioabsorption process had started at 13 W, with the highest rate at 52-78 W. At 104 W, only residual polymer material was left (& SIM;5% of implant area). Low amounts of mineral fibers were evident at 78 W and were absent (fully remodeled) by 104 W. At 134 W, implants at both sites were fully bioabsorbed. In conclusion, these new fiber-reinforced implants demonstrated bone remodeling and complete biointegration, with no adverse tissue response. Clinical significance: In this double-arm, 2.5-year study, a biointegrative, fiber-reinforced plate implanted on the tibias of sheep was fully absorbed within 134 weeks, with no adverse tissue reaction. Bioabsorption was similar, with or without periosteal elevation, mimicking conditions like those observed in traumatic injuries disrupting the periosteum, open reduction and internal fixation, or minimally invasive surgeries. These results demonstrate the feasibility, versatility, and safety of this new class of biointegrative bone implants. This newly developed technology avoids the complications of the removal of metal implants.

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.

Critical-sized midfacial bone defects present a unique clinical challenge due to their complex three-dimensional shapes and intimate associations with sensory organs. To address this challenge, a point-of-care treatment strategy for functional, long-term regeneration of 2 cm full-thickness segmental defects in the zygomatic arches of Yucatan minipigs is evaluated. A digital workflow is used to 3D-print anatomically precise, porous, biodegradable scaffolds from clinical-grade poly-ε-caprolactone and decellularized bone composites. The autologous stromal vascular fraction of cells (SVF) is isolated from adipose tissue extracts and infused into the scaffolds that are implanted into the zygomatic ostectomies. Bone regeneration is assessed up to 52 weeks post-operatively in acellular (AC) and SVF groups (BV/DV = 0.64 ± 0.10 and 0.65 ± 0.10 respectively). In both treated groups, bone grows from the adjacent tissues and restores the native anatomy. Significantly higher torque is required to fracture the bone-scaffold interface in the SVF (7.11 ± 2.31 N m) compared to AC groups (2.83 ± 0.23 N m). Three-dimensional microcomputed tomography analysis reveals two distinct regenerative patterns: osteoconduction along the periphery of scaffolds to form dense lamellar bone and small islands of woven bone deposits growing along the struts in the scaffold interior. Overall, this study validates the efficacy of using 3D-printed bioactive scaffolds with autologous SVF to restore geometrically complex midfacial bone defects of clinically relevant sizes while also highlighting remaining challenges to be addressed prior to clinical translation.

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.

Orthopedic medical devices are continuously evolving for the latest clinical indications in craniomaxillofacial, spine, trauma, joint arthroplasty, sports medicine, and soft tissue regeneration fields, with a variety of materials from new metallic alloys and ceramics to composite polymers, bioresorbables, or surface-treated implants. There is great need for qualified medical device pathologists to evaluate these next generation biomaterials, with improved biocompatibility and bioactivity for orthopedic applications, and a broad range of knowledge is required to stay abreast of this ever-changing field. Orthopedic implants require specialized imaging and processing techniques to fully evaluate the bone-implant interface, and the pathologist plays an important role in determining the proper combination of histologic processing and staining for quality slide production based on research and development trials and validation. Additionally, histomorphometry is an essential part of the analysis to quantify tissue integration and residual biomaterials. In this article, an overview of orthopedic implants and animal models, as well as pertinent insights for tissue collection, imaging, processing, and slide generation will be provided with a special focus on histopathology and histomorphometry evaluation.

Absorbable metallic implants have been under investigation for more than a century. Animal and human studies have shown that magnesium (Mg) alloys can be safely used in bioresorbable scaffolds. Several cardiovascular and orthopedic biodegradable metallic devices have recently been approved for use in humans. Bioresorbable Mg implants present many advantages when compared to bioabsorbable polymer or nonabsorbable metallic implants, including similar strength and mechanical properties as existing implant-grade metals without the drawbacks of permanence or need for implant removal. Imaging visibility is also improved compared to polymeric devices. Additionally, with Mg-based cardiovascular stents, the risk of late stent thrombosis and need for long-term anti-platelet therapy may be reduced as the host tissue absorbs the Mg degradation products and the morphology of the vessel returns to a near-normal state. Absorbable Mg implants present challenges in the conduct of preclinical animal studies and interpretation of pathology data due to their particular degradation process associated with gas production and release of by-products. This article will review the different uses of Mg implants, the Mg alloys, the distinctive degradation features of Mg, and the challenges confronting pathologists at tissue collection, fixation, imaging, slide preparation, evaluation, and interpretation of Mg implants.

Bioabsorbable implants can be advantageous for certain surgical tissue bioengineering applications and implant-assisted tissue repair. They offer the obvious benefits of nonpermanence and eventual restoration of the native tissue's biomechanical and immunological properties, while providing a structural scaffold for healing and a route for additional therapies (i.e., drug elution). They present unique developmental, imaging, and histopathological challenges in the conduct of preclinical animal studies and in interpretation of pathology data. The bioabsorption process is typically associated with a gradual decline (over months to years) in structural strength and integrity and may also be associated with cellular responses such as phagocytosis that may confound interpretation of efficacy and safety end points. Additionally, as these implants bioabsorb, they become increasingly difficult to isolate histologically and thus imaging modalities such as microCT become very valuable to determine the original location of the implants and to assess the remodeling response in tandem with histopathology. In this article, we will review different types of bioabsorbable implants and commonly used bioabsorbable materials; additionally, we will address some of the most common challenges and pitfalls confronting histologists and pathologists in collecting, handling, imaging, preparing tissues through histology, evaluating, and interpreting study data associated with bioabsorbable implants.

Emerging orthopedic biomaterials must first be evaluated for preclinical safety and efficacy before they are approved by regulatory agencies. This review summarizes the biocompatibility framework to demonstrate an appropriate biological response, as well as implant innocuity and inertness. When the biocompatibility is determined to be favorable, then surgical implantation studies can proceed for efficacy, functionality and performance testing. State-of-the-art preclinical models are presented for study designs that simulate the clinical setting in the field of orthopedics. Histology, histomorphometry, and histopathology analyses, as well as imaging modalities and mechanical testing are also described to evaluate orthopedic biomaterials for safety and performance.

In order to validate imaging modalities (angiography, IVUS, OCT) and to increase their accuracy in identifying vulnerable plaques, a reliable and reproducible animal model that mimics human atherosclerotic disease is necessary to correlate the in vivo findings with the histological features of

BACKGROUND:Renal denervation (RDN) emerged as a therapeutic option for resistant hypertension. Nerve regrowth after RDN has been questioned. We aimed to characterize the nerve response after RDN.METHODS AND RESULTS:Swine underwent bilateral RDN and were followed up for 7, 30, and 90 days and evaluated with S100 (Schwann cell), tyrosine hydroxylase (TH; efferent nerves), and growth-associated protein 43 (neurite regeneration) markers. At 7 days, nerve changes consisted of necrosis associated with perineurial fibrosis and distal atrophy with inflammation. At 30 days changes were substituted by healing changes (ie, fibrosis). This response progressed through 90 days resulting in prominent neuroma formation. Immunohistochemistry at 7 days: TH staining was strongly decreased in treated nerves. Early regenerative attempts were observed with strongly TH and growth-associated protein 43 positive and weak S100 disorganized nerve sprouts within the thickened perineurium. Distal atrophic nerves show weak staining for all 3 markers. At 30 days, affected nerves show a weak TH and S100 staining. Evident growth-associated protein 43+ disorganized neuromatous tangles in the thickened perineurium of severed nerves were observed. At 90 days, some TH expression was observed together with prominent S100+ and growth-associated protein 43+ neuromatous tangles with disorganized architecture. The potential for regenerative activity is unlikely based on the disrupted architecture of these neuromatous tangles at the radiofrequency lesion sites.CONCLUSIONS:This study is the first documentation that a progressive regenerative response occurs as early as 7 days after RDN, resulting in a poorly organized neuromatous regeneration. This finding is of paramount importance to further establish the potential functional significance of a regeneration after RDN.