The materials to be examined include 1) Polypropylene (PP: Prime Polypro; Prime Polymer Co., Ltd., Tokyo), 2) Polyethylene (PE: HI-ZEX, Prime Polymer Co., Ltd., Tokyo) and 3) Polytetrafluoroethylene (FE: TeflonRTFE; Du Pont-Mitsui Fluorochemicals Co., Ltd., Tokyo), in discoids 6 mm in diameter. Materials were prepared in the following thickness: PP, 0.54 mm; PE, 0.79 mm; and FE, 0.94 mm. The materials were then disinfected by autoclave treatment.

A total of 18 male 9-week-old ddY mice weighing about 35 g (30-40 g) each were purchased from Japan SLC Inc. (Hamamatsu, Japan). The animals were kept in plastic cages with a floor mat (Paper Clean: Peparlet Co., Ltd., Shizuoka, Japan) under controlled air-conditioned room with water and solid diet (Picolab Rodent Diet 20: Japan SLC Inc., Hamamatsu, Japan) during the experimental periods.

The Matsumoto Dental University Committee for Animal Experimentation approved the study.

Prior to the examination, inhalation anesthesia was used with isoflurane (Isoflu: Dainippon Sumitomo Pharma Co., Osaka, Japan) and gas-air mixture (4.0% concentration). Immediately after the dorsal areas of the mice were disinfected with 70% ethanol, the incisions were made, and the materials were injected subcutaneously into the connective tissues. The skin surface at the incision sites was sutured using suture string The site was marked every 2 weeks until the final examination period at 12 weeks.

At 1 week and 12 weeks after the injection, 3 mice from each group were anaesthetized with isoflurane and gas-air mixture, and the tissues surrounding the injection sites were excised. The excised tissues were immediately fixed in 4% paraformaldehyde/0.5 M phosphate buffered solution, embedded in paraffin, and sections prepared.

The tissue sections were stained with hematoxylin and eosin and examined by light microscopy for histopathological changes. We followed the International Organization for Standardization (ISO) guidelines for evaluating the local effects of injection materials [²].

Histopathological examination of the Polypropylene group, 1-week-specimens, showed granulation tissue proliferation around the embedded material (Figure 1, PP-1). The material attached to the surface area of the granulation tissue was stained well by eosin, showing necrotic changes. Just under the region, inflammatory cell-rich granulation tissue was observed. The enlarged view showed there were some inflammatory cells and a few fibroblasts in the surface eosin-stained region.

After 12 weeks, a thin fibrous tissue capsule tissue was formed (Figure 1, PP-12). There was almost no inflammatory cell infiltration in the tissue; thus, the tissue began to change to fibrous tissue.

In 1-week specimens, proliferation of scattered granulation tissue was observed around the embedded material (Figure 1, PE-1). In the scattered granulation tissues, there was some lymphocyte-rich inflammatory cell infiltration as well as capillaries showing hyperemia. The enlarged view showed that there were some eosin uniformly-stained region, which suggested necrotic change.

In 12-week specimen observations, the inflammatory cell infiltration had almost disappeared in the proliferated granulation tissues. The granulation tissues changed to fibrous tissues (Figure 1, PE-12). Attached the surface of the determination edge of the embedded material was a thin fibrous tissue layer, which covered the scattered connective tissues. Some capillaries showing hyperemia were observed in the tissues.

In histopathological observations of 1-week specimens, we note that a thin layered fibrous capsule tissue formed around the embedded material (Figure 1, FE-1). The fibrous capsule was slightly thick at the determination edge of the material. The thin fibrous capsule tissue had almost no inflammatory cell infiltrations.

In 12-week histopathological specimens, the fibrous tissue gradually changed to a thiner fibrous layered tissue (Figure 1, FE-12). The enlarged view showed more clearly the state of the fibrous tissue.