Influence of callus deformation time. Bone chamber study in rabbits.

Short periods of strain have effects on tissue differentiation in a skeletal defect. Little is known about the importance of the duration of such periods. The authors compared 2 short daily periods of strain pulses that differed only by their duration. This was done by using the micromotion chamber, which is a titanium implant with a transverse intraosseous canal. Fibrous tissue forms in the canal and then is replaced by bone through membranous (metaplastic) ossification. The tissue in the canal can be exposed to cyclic deformation. The chamber allows harvest of the tissue within the canal without disturbing the outer parts of the implant or the surrounding bone, thus enabling repeated experiments in the same animal. Chambers were inserted in 6 rabbits and repeatedly harvested at 3-week intervals. Between harvests, the chambers were subjected to either no motion, 20 cycles once daily during 20 seconds (1 Hz), or 20 cycles once daily during 120 seconds (0.17 Hz). Altogether 39 harvested specimens were studied. The 20-second treatment tended to increase the amount of ingrown bone as compared with no motion, whereas the 120-second treatment caused a marked decrease in bone formation and increase in fibrous tissue. Because the acute tissue trauma appears similar with both deformation treatments, it would appear that the increased fibrous tissue formation with the longer deformation time is caused by the parameters of tissue deformation and not by increased tissue damage.