Difference in bone ingrowth after one versus two daily episodes of micromotion: experiments with titanium chambers in rabbits.

Mechanical stimulation has been shown to affect the differentiation and development of mesenchymal tissue. In the present study, we compared the histological and histomorphometric results of tissue ingrowth into micromotion chambers that were moved at 0 cycles per day, 20 cycles once per day, and 20 cycles twice per day over 20-30 sec, for 3 weeks. In each case, a chamber having a 1 x 1 x 5 mm square-holed groove for tissue ingrowth was used. The total amplitude of motion was 0.75 mm. Histological sections from nonmoved chambers contained extensive trabecular bone, embedded in a fibrovascular stroma. Histomorphometric analysis disclosed that bone comprised a mean of 31 +/- 2% (mean +/- SEM) of the ingrown tissue. Twenty movements per day appeared to further stimulate bone ingrowth (46 +/- 5%). Extensive ingrowth of more immature woven and trabecular bone was noted in a more cellular stroma. In general, increasing the degree of micromotion to 20 movements twice per day resulted in a decreased amount of bone formation (19 +/- 7%). In several of these specimens, little or no bone could be found. These experiments have demonstrated that, for the parameters chosen in this study, a short daily period of low frequency, micromotion may facilitate bone ingrowth; however, when the same motion is delivered twice daily, bone ingrowth is depressed. Thus a "window" of externally applied strain appears to exist, which may facilitate or discourage tissue differentiation to bone.