Delayed maturation of chondrocytes enhances condylar growth

Abstract

AIM: Delayed maturation of chondrocytes allows the mesenchymal cells to reach their full replication capacity of (38 ± 4 cycles). SOX9 and PTHrP genes are known to regulate chondrocyte differentiation and delay maturation respectively. The aim of this study was to investigate the temporal pattern of PTHrP and SOX9 gene expression in the mandibular condyle during functional appliance therapy by two different treatment regimens, one- and two-step advancement. MATERIALS AND METHOD: Gene expression was determined using real-time RT-PCR on a rat model with three randomly allocated groups (control, 1-step and 2-step). Fixed functional appliances were fitted to position the mandible 4 mm forward in the 1-step group. For the 2-step group 2 mm mandibular advancement was achieved at the beginning followed by another 2 mm advancement on day 30. The animals were sacrificed on experimental day 3, 7, 14, 21, 30, 33, 37, 44, 51 and 60. Total RNA was extracted from the mandibular condyle for PTHrP and SOX9 genes quantification. RESULTS: Both PTHrP and SOX9 expression was fairly constant in the control group throughout the whole experimental period. The PTHrP and SOX9 genes expression was increased and reached the peak level on the seventh day after functional appliance insertion, then it gradually returned to the baseline level in the 1-step group. In the 2-step group, a similar expression pattern as in the 1-step group for both genes was noted but with a smaller magnitude, moreover, the second advancement on day 30 triggered a significant increase leading to another peak increment on day 37. CONCLUSION: Functional appliance therapy increases the expression of regulatory genes that delay maturation of chondrocytes thus allowing the mesenchymal cells to reach their full replication capacity and subsequently enhance condylar growth. Stepwise advancement re-activates condylar growth through re-triggering the same cascade of events.