Detection of collagen degradation products from subcutaneously implanted organic bone matrix

Pathology of Bioabsorbable Implants in Preclinical Studies

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.

Xenogeneic ossicular implants: an experimental study of heterotopic, demineralized, lyophilized, porcine implants in the guinea-pig

This study was done to compare the outcome of porcine ossicular implants in the middle ear and the subcutaneous dorsal region of the guinea-pig to those of allo-implants implanted in parallel in the dorsal region. The implants were heteropic, xenogeneic, demineralized (HCl), lyophilized and sterilized. The evaluation was histological (light microscopy and scanning electron microscopy) and immunological (immunofluorescence staining). Fifty-four guinea-pigs were implanted in the middle ear and 14 of them were also implanted subcutaneously in the dorsal region with xeno-implants and allo-implants. The middle ear implants were found to be constantly reossified and coated with normal mucosa with only a minimal immune reaction. In contrast, the dorsal xeno-implants were found to be the target of mononucleic infiltration, fibrous encapsulation and an influx of immunoglobulins resulting in segregation. The corresponding allo-implants were found to be partially reoccupied and reossified. These findings highlight the value of HCl demineralization in the induction of non-species-specific Bone Morphogenetic Protein and the failure of attempts at immuno-despecification. It appears that the fate of the implant depends less on its antigenic load than on the site of implantation. In this regard the middle ear is apparently very advantageous. The very good short-term tolerance and recovery observed in the middle ear xeno-implant suggest that these implants offer sufficiently good results to warrant clinical testing.

Effect of mineral content of human bone on in vitro resorption

Osteoclasts, mechanically isolated from chick long bones, were grown in vitro on slices of human rib and femur. Evidence of their activity was assessed by secondary electron and backscattered electron (BSE) imaging in the SEM. BSE imaging was also used to study the relative degree of mineralisation of the bone matrix in which resorption had taken place. All bone phases were resorbed, from osteoid through to densely mineralised interstitial bone and reversal (cement) lines. Resorbing osteoclasts crossed reversal lines between osteons of different mineral density and moved both from higher to lower and lower to higher density phases. Where single loci spanned reversal lines, and thus breached bone of two different mineral densities, depth of demineralisation was inversely related to mineral density. The presence of an annular zone around some resorption loci, which may be caused by demineralisation beneath the osteoclast clear zone, was confirmed. Also, BSE imaging of polished substrata showed that significantly more osteoclastic activity had occurred at their surfaces than was apparent from the amount of cavitation present.

Fate of mineralized and demineralized osseous implants in cranial defects

We have evaluated the fate of mineralized osseous implants placed into cranial defects in rats. By 2 weeks, 100% of the defects that had been filled with demineralized bone powder (DBP, 75-250 micrometer) showed bony repair as judged by histomorphometric analysis and incorporation of 45Ca. The DBP was not appreciably resorbed but rather was amalgamated within the new bone. Histomorphometric evaluation of osteo-genesis induced by equal masses of demineralized bone powders of various particle sizes (less than 75, 75-250, 250-450 micrometer) revealed that the smaller particles induced more bone per field than did the larger particles. In contrast, mineralized bone powder (BP) was completely resorbed by 3 weeks, without bony repair of the cranial defect. These specimens contained large multinucleated cells within 7 days and completely resorbed by 3 weeks. It is concluded that (a) demineralized bone powder predictably induces a osteogenic healing of cranial defects, (b) demineralized bone powder is not appreciably resorbed prior to bone induction, (c) the extent of bone induction is a function of the surface area of the demineralized bone implant, and (d) mineralized bone powder undergoes obligatory resorption.