Cell necrosis, capsule formation, and soft tissue response to implants

The tissue response to implanted polyethylene discs was evaluated. Cells with increased membrane permeability could be identified using fluorescence microscopy by injection of propidium iodide prior to the killing of the rats. Cell death was verified by detection of DNA fragmentation. Early, a considerable amount of the cells close to the implant surface were stained with propidium iodide and especially at the edges of the discs. The numbers of positive interfacial cells decreased markedly over time. Cells with DNA fragmentation displayed initially a scattered distribution, whereas at later time points they mainly appeared in the outer portion of the enveloping fibrous capsule. There was a positive correlation between capsule thickness and propidium iodide positive cells at earlier implantation periods. The results suggest that the thickness of the reactive capsule is related to the extent of cell necrosis, that the major initiator for this cell necrosis is mechanical shear, since cell necrosis was mainly found in areas where mechanical shear could be expected.