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The micromotion chamber consists of a titanium outer cylinder and a central core, which are pierced by a transverse 1-mm canal for tissue ingrowth. Six weeks after implantation in the proximal tibia in mature rabbits, the outer cylinder is osseointegrated; the central core can then be moved in relation to the fixed outer cylinder. Thus, the tissue growing through the pore, from the cylinder into the core, can be subjected to motion of a predetermined amplitude and frequency. In this study we investigate the influence of pore cross-sectional shape on tissue ingrowth in the canal. In six animals, the outer cylinder was pierced by a square 1-mm hole that was congruent with the square hole in the core; in five animals, the hole in the cylinder was round. The cross-sectional area of the square hole in the cylinder was about 21% greater than in a round hole. In all cases, the channel in the inner core was 1 x 1 x 5-mm quadrate. All chambers underwent 20 cycles/day of micromotion for a 3-week period. The amplitude of the micromotion was 0.5 mm. Chambers containing cylinders with a round hole demonstrated less bone ingrowth as compared to cylinders with a square hole. This observation may be due to several factors including the greater cross-sectional area of the square versus the round hole in the cylinder and the enhanced congruity provided by the square outer and inner holes, versus a round outer and square inner hole.
Goodman et al. (Mon,) studied this question.