Influence of local complications on capsule formation around model implants in a rat model

We studied the capsule formation around various filled breast implants and other related changes in distant organs (e.g., liver, spleen, lymph nodes) in a rat model 3 and 6 months after implantation. Model implants, one per rat, (filled with saline, n = 19; silicone, n = 14; cohesive silicone, n = 17; and hydrogel, n = 19) were implanted subcutaneous in the lower back of rats. The animals were sacrificed regularly after 3 and 6 months of implantation or when wound defects occurred. The capsules and organs were examined histologically, and immunohistology of the capsules was performed. A monoclonal antibody specific for AIF-1 (allograft inflammatory factor 1) was used to detect activated macrophages. Wound defects occurred most frequently after implantation of saline and hydrogel implants. Increased capsule thickness was associated with increased grade of inflammation, fibrosis, and the type of implant filling. There was a significant positive correlation between capsule thickness, presence of chronic inflammation, and AIF-1-positive macrophages (p < 0.0001), indicating that inflammation plays an important role in capsule formation. Remarkably, saline and silicone implants (in absence of local complications) cause only a blande slight fibrosis in capsules after 6 months of implantation, whereas capsules around cohesive silicone implants exhibited a more severe fibrosis with an increased capsule thickness. Most importantly, hydrogel seems to be most potent to induce an inflammatory infiltrate with AIF-1 expressing macrophages at the implantation site, independent of implantation time, and capsules also produced a significant increase in thickness after 6 months.