IN VITRO MECHANICAL TESTING OF POROUS-COATED ORTHOPEDIC IMPLANT SUPPORT IN BONE AFTER 1 YEAR: DIFFERENCES BETWEEN FIBROUS TISSUE SUPPORT AND BONE INGROWTH.

If bone ingrowth is to occur with porous-coated orthopedic implants, relative movement between the device and the surrounding bone must be limited. Otherwise, calcification does not occur, and the device is supported (if at all) by soft fibrous tissue. Using a canine segmental replacement model, Pilliar et al. demonstrated that early loading of intramedullary rods can lead to the appearance of a radiopaque line (or halo) surrounding the implant. A similar radiographic feature has been observed in some human cases. They showed that this line corresponds histologically to a region of dense bone surrounding the implant but not contacting it. The gap is filled with fibrous tissue aligned in the direction of stress. We have used this model system to produce intramedullary rods supported by: (1) bone ingrowth, and (2) fibrous tissue. An in vitro testing jig has been developed for direct testing of the viscoelastic properties of the tissue support under physiological conditions.