Short-term effects of strain produced on a split palatal screw-type hyrax appliance after rapid maxillary expansion: A clinical trial

• Facial biotypes determine the patterns of accumulated stress.
• Greater displacement of the maxilla at the anterior zone and less bone cause greater strain.
• Accumulated strain is not dissipated in the posterior zone of dolichocephalic biotypes.
• A protocol based on each patient's characteristics will improve the quality of treatment.

IntroductionThe aim of this study was to establish an accumulated strain pattern in different parts of rapid maxillary expansion appliances and relate them to different vertical growth patterns. A clinical study was conducted of 40 patients with posterior crossbite who required rapid palatal expansion.MethodsPatients (mean age, 8.48 years) were recruited and treated at the Dental Hospital of Bellvitge, Hospitalet de Llobregat, Barcelona, Spain. Strain gauges were placed on the arms of the RME hyrax screw appliance to record deformation (strain) during the expansion and the retention phases for 75 days. A finite element model was used to place the gauge at the point where the strain was most expressed. The vertical coefficient of variation was used to classify the patients by their vertical growth pattern. P = 0.05 was considered to be statistically significant.ResultsDuring the expansion phase, the highest values of accumulated strain were measured in the posterior part of the appliance for all facial biotypes, but these values passed to the anterior area at the end of the retention phase of the mesocephalic and brachycephalic patients. There was statistically significant difference in the strain of the posterior arms in accordance with the vertical growth pattern (P = 0.05) during the retention phase. At 75 days of retention, 61.25% of the arms had already begun to have strain dissipation.ConclusionsThe accumulated strain pattern in the rapid maxillary expansion appliance can vary depending on the facial biotype. In the future, orthodontists should try to tailor the activation and retention protocol based on each patient's characteristics.