Quantitative three-dimensional analysis of chondrocytic kinetic responses to short-term stapling of the rat proximal tibial growth plate.

Although it has been demonstrated clinically that controlled compression across a growth plate will slow the rate of endochondral ossification and thus can be used to correct angular limb deformities, the cellular-based mechanism by which altered growth is achieved is poorly understood. This study used short-term uniaxial stapling of the rat proximal tibial growth plate as an experimental system to study chondrocytic responses in the growth plate that account quantitatively for the decreased rate of growth. Growth plates were labeled with oxytetracycline to measure bone growth, and with bromodeoxyuridine to analyze proliferative cell kinetics. Multiple indicators of chondrocytic activity, measured by stereological parameters, were analyzed using growth rate as the primary dependent variable. The unique feature of this analysis was the creation of three-dimensional reconstructions that allowed analysis of data in all directions with distance from the staple. A significant observation was that for the entire operated limb after both 3 and 6 days, all chondrocytic kinetic parameters were affected, indicating that proliferative and hypertrophic responses both act to decrease growth rate in response to stapling. This contradicted our hypothesis that proliferative and hypertrophic responses could occur independently, and that small changes in rate would be attributed primarily to the former and large changes to the latter. The data from this study also demonstrate that volume regulation during hypertrophy can be affected by a primarily mechanical perturbation. Because changes in hypertrophic cell number and volume throughout the growth plate that occur by day 3 remain similar at day 6, the initial modulation of chondrocytic volume and shape may represent the limit of the response while maintaining a growth plate capable of continued growth.