Development of an animal model for testing human breast implantation materials

Although breast implant materials have been tested in laboratory species since the early 1950s, a standardized evaluation system does not currently exist in which human-made polymers are exposed directly to the mammary milieu of female animals. The present study evaluated such a model as the basis for future experiments on long-term tissue effects. Polyesterurethane disks, 6 mm in diameter x 3 mm thick, were inserted bilaterally beneath the axillobrachial and inguinal mammary/fat pads of 50 9-wk-old female B6D2F1 mice (4 implants each). Implant sites were examined histologically at time points 24 hr to 47 wk after surgery. An acute inflammatory reaction at the implant edges began within 24 hr, and macrophages were found lining the smooth polyurethane fiber surfaces at the periphery by day 2. Multinucleated foreign body giant cells formed by day 4, and by week 4 giant cells contained polyurethane fragments within the cytoplasm, implying degradation of the material. Implant sites showed declining subacute inflammatory responses and increasing fibrosis by week 5. By 13 wk, the polyurethane disks appeared to be integrated into the growing adipose and mammary tissues. Although not apparent on gross inspection, microscopic examination showed that polyurethane fibers moved progressively into adjacent tissues and were always associated with chronic granulomatous inflammation. Histologic findings in the present study are strikingly similar to the human response to polyurethane-coated breast implants. These results suggest the applicability of this model to appropriately test mammaplasty materials in mammary tissues.