Effect of strain on bone nodule formation by rat osteogenic cells in vitro

The purpose of this study was to assess in vitro bone nodule formation by cells exposed to a range of microstrain, at a sub-optimal oscillation frequency for bone formation. Fetal rat calvarial cells experienced a Flexercell regimen within either FLEX I (deformable) or FLEX II (non-deformable) substrates. Cells in FLEX I plates were exposed to growth medium only; those in FLEX II plates were exposed to either growth medium only, or growth medium + 10(-7) M IGF-1. Cell numbers were assessed from 1 to 6 days. Other cells were exposed to the Flexercell regimen (-2 kPa, 0.05 Hz) for 1-3 (Group 1), 3-6 (Group 2), 1-9 (Group 3) or 10-15 (Group 4) days and were maintained, at other times, under standard conditions. After 21 days, nodules were counted within each well and within the compression, <999, 1000-4900, 5000-9999, 10,000-14,999 and 15,000-25,000 microstrain regions of the FLEX I membrane. Cyclic deformation inhibited cell numbers from 1 to 6 days, compared to control or IGF-1 groups (P<0.001). The number of nodules in Groups 2 and 4 were greater than Groups 1 or 3 (P<0.001), but not different from control or IGF-1 groups. Compression or tensile microstrain significantly affected nodule formation in all groups, with Group 4 producing more nodules than other groups in most microstrain regions. Thus, the number of bone nodules produced by osteogenic cell cultures exposed to cyclic deformation was significantly affected by the timing of initiation and the characteristics and magnitude of the deformation regimen.