Objective: To measure the difference in force delivery between aligners fabricated using positive air pressure versus negative air pressure.

Material and methods: A series of clear aligners of similar material composition (PET-G), manufacturer (Scheu Dental), thickness (0.75mm) and activation of 0.2mm uncontrolled palatal tipping on the upper left central incisor were designed and fabricated using two different methods of fabrication. The first set of aligners (n=5) were fabricated via positive air pressure using MiniStarS pressure thermoforming and the second set (n=5) were fabricated via negative air pressure using the PlastVac P7 vacuum forming machine. A force change measurement system was used to measure changes in the forces delivered by the pressure formed and vacuum formed PET-G materials. Statistical analysis was carried independent samples t test to compare between pressure formed and vacuum formed force values using Significance level was considered at P < 0.05.

Results: There was a statistically significant increase in force values delivered by aligners thermoformed using positive air pressure compared to aligners fabricated using negative air pressure. Forces delivered by vacuum formed aligners averaged to 0.40N compared with 1.01N for aligners thermoformed using positive air pressure.

Conclusions: The forces delivered by aligners are influenced by the fabrication process; aligners fabricated using positive pressure showed increased fit and adaptation on the resin model which led to an increase in the delivered forces compared to aligners fabricated using negative pressure.