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 prev iously to affect the differentiation of mesenchymal tissue. In this st udy, we examine the effects of discontinuing a strain protocol known t o produce primarily fibrous tissue rather than bone in the micromotion chamber (MC). Five MCs were inserted into the proximal tibial metaphy sis of mature male New Zealand white rabbits. The MC has a 1 x 1 x 5 m m 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 m otion at the interface between the cylinder and the core. The tissue i n 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 discontin ued for 3 additional wks ('40 + 0'); the contents of the chamber were harvested after 6 wks. Finally, the chambers were left without manipul ation ('0') and harvested after 3 wks. Histological sections from unmo ved chambers ('0') contained extensive trabecular bone, embedded in a fibrovascular stroma. The '40' specimens were composed primarily of lo ngitudinally orientated fibrous tissue. The '40 + 0' specimens were si milar 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. Bone i ngrowth was less in the '40' specimens, compared with either the '0' o r the '40 + 0' specimens. As in fracture healing, cessation of strain within the MC creates a new functional biomechanical environment which facilitates maturation of tissue to bone.