Acceleration of Secondary Ossification Center Formation Upon Axial Cyclic Microcompressive Stresses in Neonatal Rabbit Femur

Although textbooks recognize that appearance of secondary ossification centers is a complex and genetically determined event, the role of biomechanical stresses played in this process has received little attention. A finite element model predicts deceleration of the formation of secondary ossification centers by intermittent hydrostatic pressure (Stevens et al.: J Orthop Res 17:646-653, 1999). Objective: The objective of the present work was to determine how cyclic compression of the epiphysis would modulate the formation of secondary ossification centers. Methods: Under general anesthesia and aseptic conditions, the femoral head of each of two 7-day-old New Zealand White rabbits was dissected. The femur explants were immediately placed in an organ culture chamber with Dubelco's Modified Eagle Medium (DMEM) supplemented with 10% heat inactivated fetal bovine serum, and 1% Penicillin-streptomycin-L-glutamine. Micromechanical compressive forces were applied to the experimental explants at 200 mN and 1 Hz for 12 hrs, yielding a total dose of 4,320 cycles. Upon completion of loading, both loaded and control explants were transferred to a bioreactor with a slow turning speed of 20 rpm, and incubated for 3 days at 37C degrees. After incubation, all explants were prepared for histological sections and stained using hematoxylin and eosin stain, and safranin O/fast green counterstain. Results: Secondary ossification centers appeared only in the center of the loaded epiphysis (mean area: 1.8 mm²),but not in the control epiphysis. Computer-assisted histomorphometric analysis revealed significantly greater chondrocyte cell count(23± 6.8/300 µm²) than the control(29±9.8/300 µm²) (P < 0.01).The loaded epiphyseal cartilage area was 21.5 mm²,in comparison with 16.5 mm²for the control epiphyseal cartilage. Conclusions: These data provide experimental evidence that appearance of secondary ossification centers can be accelerated by cyclic microcompressive stresses. Supported by Whitaker Biomedical Engineering Research Grant and USPHS Research Grants DE 13088 and DE 13964 from NIH/NIDCR