Apparent absence of local response to bone screws in guinea pigs with contact sensitivity.

Guinea pigs sensitized to nickel, cobalt, or chromium were used to study if impaired fixation occurred as a result of allergy against metal implants in bone. Screws made from stainless-steel (ASTM F55, F138-139) or cobalt chromium (ASTM F75-76) were inserted in the right and left proximal tibiae, and the mechanical strength of the fixation to the bone was evaluated after 4 months. In addition, we studied the histology on decalcified sections of the implant bone interface from the distal femur where the same type of screws had been inserted. To see if changes in bone density occurred in the proximal tibia as a result of allergy, the amount of ash was determined. Although the animals maintained their contact sensitivity throughout the experimental period, there were no differences between allergic and control animals for any of the parameters studied. All screws were well fixed in the tibia at the end of the experiment, and histologically we did not find any significant differences between allergic and control animals by qualitative examination. The experiment supports our earlier results from a 6-week experiment and indicates that contact allergy may be unimportant for the fate of orthopedic implants.