Electronic Thesis and Dissertation Repository

Thesis Format



Master of Clinical Dentistry




Tassi, Ali


Introduction: Orthodontic elastics are frequently used in orthodontic treatment but there is little research into how to modify their prescription to improve their performance. The aim of this study was to evaluate potential wear modifications to a standard Class II intermaxillary elastic prescription on force levels and force degradation over time.

Methods: For this in vitro study, Ormco (OR) and American Orthodontic (AO) latex elastics had five modifications made to a standard 1/4”/4.5oz control elastic (Group 1). These included an increase of force level by one unit to 6 or 6.5oz (Group 2); a decrease in lumen size to 3/16” (Group 3); a doubling of the control elastic (Group 4); a 720° twist added between the two ends (Group 5); and an increase in initial stretch length by one tooth (Group 6). Elastics were tested on an elastic stretching apparatus with parameters imitating the use of intermaxillary elastics for the treatment of Class II malocclusions. Intraoral conditions were simulated with a distilled water bath maintained at 37°C and cyclic stretching of 1 stretch/min to mimic oral functioning. Force was assessed at time points 1, 5, 30, 60, 180, 360 and 720 minutes. A Two-way Mixed ANOVA was used to identify statistically significant differences between groups and time points (p<.05).

Results:Mean force level values were significantly greater at all time points for Groups 2, 4, and 6 as compared to control Group 1 in both OR (p

Conclusions: Increasing elastic force by one unit, doubling elastics, and increasing elastic stretch by one tooth showed significant increases in force levels at all time points as compared to the standard control elastic.

Summary for Lay Audience

Introduction: It is known that orthodontic elastics (or rubber bands) wear out and forces decline over time based on many different variables. However, recommendations on how orthodontists can apply this information when deciding which elastics to prescribe patients is incomplete. The aim of this study was to evaluate five potential changes to a commonly used elastic prescription to correct Class II malocclusions and see how force levels change over time.

Methods: A custom built testing machine was used to stretch the common elastic (Group 1) and the elastics with the five changes. The changes included: the same size elastic but increased force (Group 2); a smaller size elastic with the same force (Group 3); two of the common elastics placed at the same time (Group 4); the common elastic with two full twists (720°) placed between the two ends (Group 5); and stretching the common elastic to include one more tooth (Group 6). Elastics were stretched on the testing machine to the average length patients need to wear elastics with their braces. The machine helped imitate the conditions found in the mouth by having the elastics in water at body temperature and stretching the elastic periodically to mimic mouth movements. The elastics were tested for 12 hours on the machine with comparisons made at intervals of 1, 5, 30, 60, 180, 360, and 720 minutes. Elastics tested were from two different companies to see if differences were consistent between manufacturers. Statistics were used to analyze the differences between groups and between the different time intervals

Results: The changes made to Groups 2, 4, and 6 increased the force levels at all the time intervals and in both companies’ elastics. Group 5 had decreased force levels in one of the companies. The forces in groups 3 and 6 decreased the most over time. The biggest changes in elastic decay were seen early on in all groups with elastics wearing out less as time passed.

Conclusions: Increasing the force level of the elastic, doubling the elastic, and increasing the elastic stretch by one tooth increased force values compared to the common elastic, while twisting the elastic decreased force values. Decreasing the size of the elastic or increasing the distance it is stretched caused an increased rate of elastic degradation over time.