Four different modifications of the keratin polymer were made as small rods and inserted into the subcutaneous tissue of the hind limbs of adult female sheep. Material 1 was porous, whereas materials 2, 3, and 4 had little or no internal porosity. The tissue response to each material was determined with regard to the extent of the inflammatory reaction and formation of a fibrous capsule around the implant, and the integrity and morphological appearance of the implant was assessed. An inflammatory cell infiltrate and fibrous capsule formed at an early stage around implants of material 1. Subsequently the inflammation decreased, the fibrous capsule became more mature, and the implants became cavitated and invaded by mono- and multi-nucleated macrophages, fibroblasts and blood vessels, with breakdown of the implant material occurring at its periphery. Similar changes were observed for implants made of materials 2 and 4. Implants of material 3 were remarkable in that, while surrounded by an inflammatory infiltrate and a fibrous capsule, they did not show any disturbance even at 24 weeks. The fibrous capsule around material 3 was thinner than that around material 1 at 6 to 24 weeks (both materials prepared using ammonium thioglycollate). No difference in capsule thickness was found for materials 2 and 4 (both materials prepared using thioglycollic acid).
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