THE SUSCEPTIBILITY OF SMOOTH IMPLANT SURFACES TO PERIIMPLANT FIBROSISAND MIGRATION OF POLYETHYLENE WEAR DEBRIS

The purpose of this investigation was to establish whether the tissue response and migration of polyethylene debris differed at noncemented smooth and porous implant surfaces, This was accomplished through 3 se parate but closely related studies: (1) a canine cylindrical implant m odel with smooth and porous surfaces exposed to polyethylene debris; ( 2) a canine total hip arthroplasty model analyzing the interface betwe en bone and femoral implants with various porous-coating configuration s; and (3) a histologic analysis of autopsy-retrieved, human, noncemen ted hip prostheses with noncircumferential porous coating, The cylindr ical implant model involved the placement of split cylinders, 1/2 poro us and 1/2 smooth, into the distal femur and proximal tibia of 4 dogs. Four control implants and 10 test implants (chronically exposed to si mulated polyethylene debris with a mean size of 4.7 microns) were exam ined histologically as long as 30 weeks after surgery, The canine hip study involved the study of 54 noncemented hip prostheses at periods o f 1, 6, and 24 months, The prostheses possessed 4 different porous sur face configurations: 1 with circumferential porous coating, 2 with non circumferential coating, and 1 without porous coating, The human retri eval analysis involved the study of 7 cadaveric femora (age, 6 months- 5 years) implanted with a straight titanium-alloy prosthesis possessin g proximal pads of titanium fiber metal on the anterior, posterior, an d medial aspects, With all implants in all 3 studies, there was the co mmon finding of bone ingrowth at the porous implant surface and a fibr ous interface or periprosthetic cavity around the portion of the impla nt that was smooth surfaced, The periprosthetic cavity typically was e ncapsulated by a thin continuous shell of trabecular bone, In addition , polyethylene debris was found to have preferentially migrated along the smooth implant surfaces, In the longer-term canine and human hip r etrievals, polyethylene particles in the micron size range were presen t within histiocytes, whereas larger particles as much as 100 microns were found within foreign-body giant cells, Of importance for the impl ants from all 3 studies, with the exception of some pronounced cavitie s on the lateral aspect of the human hip prostheses, the periimplant c avities around the smooth surfaces were not detectable radiographicall y. This study clearly established a fundamental principle of relative barriers to particulate debris migration, Smooth implant surfaces are more susceptible than porous surfaces to the development of a fibrous tissue filled periimplant cavity and the subsequent migration of polye thylene wear debris, The relative protective effect afforded by porous coatings against debris migration and polyethylene granuloma formatio n is a crucial design criterion influencing the longevity of noncement ed prostheses.