Does the degree of advancement during functional appliance therapy matter?

The aim of this study was to assess the effect of varied degrees of mandibular advancement on condylar growth. Three hundred and thirty five 35-day-old female Sprague–Dawley rats were randomly divided into 10 experimental groups (n = 10) and five control groups (n = 5) for analysis of new bone formation and 10 experimental groups (n = 14) and five control groups (n = 14) for molecular analysis. The experimental animals were fitted with bite-jumping appliance to advance the mandible 2 and 4 mm. The rats were sacrificed on days 3, 7, 14, 21, and 30. A computer-assisted image analysing system was used to assess the quantity of new condylar bone formation. Molecular analysis utilizing real-time reverse transcription–polymerase chain reaction was used to assess the different levels of mRNA expression of different growth markers in the condyle. One-way analysis of variance (ANOVA), with a Bonferroni multiple comparison test, showed significantly more newly formed bone in the 4 mm group compared with the 2 mm and control groups on days 21 and 30 (P < 0.05). Most of the examined growth markers demonstrated a significant increase during the 4 mm advancement (P < 0.05). Indian hedgehog (Ihh) mRNA showed a 7- and 5-fold change, parathyroid hormone-related peptide (PTHrP) a 5.2- and 3-fold change and type II collagen a 9.6- and 3.7-fold change in the 4 and 2 mm advancement groups, respectively. Varied degrees of mandibular advancement result in different quantities of new bone formation and levels of expression of growth members: Ihh, PTHrP, and type II collagen.