Cessation of strain facilitates bone formation in the micromotion chamber implanted in the rabbit tibia

Short, daily periods of externally-applied strain have been shown previously to affect the differentiation of mesenchymal tissue. In this study, we examine the effects of discontinuing a strain protocol known to produce primarily fibrous tissue rather than bone in the micromotion chamber (MC). Five MCs were inserted into the proximal tibial metaphysis of mature male New Zealand white rabbits. The MC has a 1 x 1 x 5 mm pore for tissue ingrowth. After osseointegration of the fixed outer cylinder of the chamber, the inner movable core was manipulated for 40 cycles per day delivered at a rate of 1 Hertz ('40'). This provided motion at the interface between the cylinder and the core. The tissue in the pore was harvested after 3 wks. The MCs were then manipulated at 40 cycles per day for 3 wks and then the manipulations were discontinued for 3 additional wks ('40 + 0'); the contents of the chamber were harvested after 6 wks. Finally, the chambers were left without manipulation ('0') and harvested after 3 wks. Histological sections from unmoved chambers ('0') contained extensive trabecular bone, embedded in a fibrovascular stroma. The '40' specimens were composed primarily of longitudinally orientated fibrous tissue. The '40 + 0' specimens were similar histologically to the '0' specimens. The amount of bone ingrowth expressed as a percentage of the area of the section averaged 37 +/- 6 (mean +/- standard error of the mean) for the '0' specimens, 20 +/- 2 for the '40' specimens and 37 +/- 7 for the '40 + 0' specimens.(ABSTRACT TRUNCATED AT 250 WORDS)