Porous polymers for biological fixation.

Porous polymers are recommended as coatings for femoral stems on the basis of their low modulus, which allows uniform distribution of stress to surrounding bone and the fabrication of more flexible stems by reducing the metallic cross section. Application of the porous polymer coating to the metallic substrate reduces metal ion release; the polymer coating does not affect the mechanical properties of the device. Porous polytetrafluoroethylene-carbon fiber composite, (PCFC), porous polyethylene, and porous polysulfone are undergoing investigation for this application. However, the low strength of the PCFC makes its use for this application questionable. Recently reported unfavorable clinical results (i.e., a high incidence of pain) have led to the discontinuation of one trial of porous polyethylene. Clinical investigation of porous polysulfone-coated titanium alloy devices are in progress. Animal studies have shown bone ingrowth into porous polyethylene and porous polysulfone implants in cortical and cancellous bone and in canine femoral stems coated with these materials. Pull-out testing of porous polysulfone implants revealed that the strength of the porous material was higher than the strength of surrounding bone. Less cortical bone loss was found around porous polysulfone-coated canine stems than was reported for fully coated porous metallic prostheses, presumably because of the lower stiffness.