BIOMECHANICAL EFFECTS OF E-PTFE IMPLANT STRUCTURE ON SOFT-TISSUE IMPLANTATION STABILITY - A STUDY IN THE PORCINE MODEL

Successful implantation of biocompatible materials depends on physical aspects of its structure. Meshed implants are stable but cannot be ea sily removed. Nonporous materials are easily removed, but subject to e xtrusion. We hypothesized that the microporous structure of expanded p olytetrafluoroethylene (e-PTFE) would permit limited fibrous ingrowth into the substance of the material, and that tubular implant shape wou ld increase tissue integration while preserving ease of removal. A two -tailed in vivo study was done comparing implant retention, strength o f fixation, and removability between tubular and solid-strip e-PTFE im plants. Differences in implant retention within tissues were assessed by implanting 396 implants subcutaneously in five swine for observatio n periods ranging from 3 weeks to 12 months. Strength of implant attac hment to host soft tissues was measured at 52 sites by extraction with a tensiometer with forces both parallel and perpendicular to the impl ant used. Implant porosity was assessed with scanning electron microgr aphy of tubular and solid-strip e-PTFE implants. Measurements of the f orce and stress tolerances of the implant-tissue interface demonstrate d significantly stronger attachment in tubular than strip-shaped impla nts (P < 0.005). The 11 N (2.75 lb) force sustained by the tubular imp lant exceeded the 3.4 N (<1 lb) force for the e-PTFE strip by a statis tically significant margin on two-tailed Student's t-test (P < 0.005). Even greater forces were tolerated when applied at right angles to th e axis of the tubular implant, emulating tissue suspension (21 N, 5.25 lb). The forces and stresses tolerated by both e-PTFE implants far ex ceeded the fracture stress measured for the implants. Implant extrusio n rates were significantly smaller in tubular (0.85%) than in strip-sh aped (4.4%) e-PTFE implants (P < 0.05). Standard error of the mean (SE M) demonstrated lesser porosity in tubular than strip implants, sugges ting lesser direct tissue attachment. Tubular e-PTFE implant structure facilitates ingrowth of soft tissue through the tube's lumen. This in creases the attachment to surrounding soft tissues, increasing fixatio n strength, decreasing extrusion rate, but still allowing easy removal . These properties may improve clinical applications in facial implant ation.