Promotion of bony ingrowth by frequency-specific, low-amplitude mechanical strain

The ability of extremely low-amplitude mechanical strains to promote bony ingrowth was evaluated in an in vivo animal model, the functionally isolated turkey ulna. A cylindrical, porous-coated titanium implant was placed across the dorsal and ventral cortices of the left ulna diaphysis of 12 animals. Back scatter electron microscopy was used to quantify the relative bony ingrowth after eight weeks of: (1) disuse alone, (2) disuse plus 100 seconds per day of a 1-Hz, 150-microstrain (mu epsilon) mechanical stimulus, or (3) disuse plus 100 seconds per day of a 20-Hz stimulus of similar strain magnitude. Disuse alone caused a mean 8.3% (+/- 5.5%) less of bone away from the implant, with the area between implant and bone actively filling with a fibrous membrane. A daily 100-second regimen of low-magnitude, 1-Hz mechanical stimulation caused 28% (+/- 6.2%) of the implant area available for ingrowth to be filled with bone. At 20 Hz, the amount of bony ingrowth increased to 69% (+/- 3.0%). These data demonstrate that brief exposure to extremely low-amplitude mechanical strains can enhance the biologic fixation of cementless implants. Moreover, the degree of ingrowth is dependent on the frequency of the applied strain.