Histopathologic observations after short-term implantation of two porous elastomers in dogs

This report describes the effects of pore size and material on soft tissue ingrowth of two medical-grade elastomers. Using the replamineform process, silicone rubber (SR) and bioelectric polyurethane (BEP) were rendered microporous with essentially the same microstructural pore configuration. Implants were prepared in each material having five pore size ranges: 18-25 microns, 30-45 microns, 75-95 microns, 60-120 microns, and 120-180 microns. Implants 1 cm X 1 cm X 1 mm were harvested at 1, 2, 4 and 12 weeks following subcutaneous implantation in mongrel dogs. Ingrowth of the 18-25 microns and 30-45 microns implants in both polymers consisted of histiocytes and dispersed fibrocytic proliferation during the first two weeks. By 12 weeks, the fibrocytic component had increased, but histiocytes remained the principal component of ingrown tissue. In contrast, initial ingrowth of the 75-95 microns, 60-120 microns and 120-180 microns implants showed increased fibrocytic proliferation and minimal histiocytic reaction. By 12 weeks, ingrowth into the larger-pore implants had progressed to broad bands of well organized collagenous stroma. Differences in the rate of tissue ingrowth were found to be related to both material and pore size. Less than 15% of the void spaces were infiltrated by 4 weeks in 18-25 microns and 30-45 microns SR implants, although this increased to approximately 50% by 12 weeks. In contrast, the 3 larger-pore SR implants and all pore sizes in the BEP implants were almost completely ingrown by 4 weeks.