Method: Six fresh skulls of 1-month old piglets (equivalent to human preadolescent age) were used. Strain gages were implanted at the surface of 5 circumaxillary (MZ: maxillary-zygomatic; MP: maxillary-premaxillary; ML: maxillary-lacrimal; ZT: zygomatic-temporal; IN, internasal) sutures. Through arms anchored on maxillary molars, maxillary protraction was performed by using varied force levels (100, 250 and 500g) generated by latex elastics. Midpalatal suture opening was achieved by inserting metal shims with standardized thickness (1-5mm) into this suture. Circumaxillary suture strains were recorded during maxillary protraction and MSO. Strain changes with protraction force level and MSO amount were compared by repeated measures ANOVA.
Result: When isolated effect was considered, maxillary protraction produced compressive and tensile strains at the MP and MZ sutures, respectively, both of which increased significantly (p<0.05) with protraction force level but decreased significantly (p<0.05) with the amount of pre-protraction MSO (p<0.05). MSO alone caused tensile and compressive strains at the MP and MZ sutures, respectively, both of which increased significantly with the amount of MSO (p<0.05). When strain changes relative to initial baselines (pre-MSO and pre-protraction) were considered, strains at the MP and MZ sutures were dominated by MSO-generated strains (tensile and compressive, respectively) with lessening effects from maxillary protraction. Generally, strains in the other three sutures tested were smaller than the MP and MZ sutures.
Conclusion: Compared to maxillary protraction, opening of the midpalatal suture generates stronger but opposite strains at anterior and posterior circumaxillary sutures, suggesting that the immediate mechanical effect caused by midpalatal suture opening may counteract maxillary protraction mechanics.
Keywords: Animal, Biomechanics, Growth & development, Orthodontics and Strain