Widening the maxilla, or upper jaw, is a common treatment in orthodontics used to generate additional room in the correction of tooth misalignments. Expansion is achieved through activation of an appliance inserted in the patients’ upper jaw. Currently, appliances utilizing expansion screws, spring, magnets, or shape memory alloys are being used by clinicians. The unfused midpalatal suture, soft connective tissue between maxillary bones, is also widened during this procedure. In the literature concerning this treatment and its impact on patient response, little has been done to consider the suture’s viscoelastic properties. Development of a viscoelastic model would allow for accurate prediction of suture response to the various expansion appliances and aid in guiding future appliance design and treatment protocols.

In the presented thesis research, complete viscoelastic creep-relaxation models are developed for the unfused midpalatal suture. First, nonlinear creep-strain models are established based on experimental data from the rabbit midsagittal suture. Then, interrelation techniques are utilized to generate subsequent stress-relaxation relations based on the previously obtained creep-strain constants. Development of an overall creep-relaxation model allows for prediction of suture response to expansion appliances that exert a constant or decaying force (springs, magnets, shape memory alloys) as well as step-wise increases in displacement (expansion screws).

In using developed creep-relaxation models to simulate the suture’s response to expansion appliances, several key observations were made. In regards to screw-activated appliances, it was found that stresses resulting from a single step-wise activation likely would not generate tissue failure;​ however, as few as two or three rapid activations may certainly do so. Additionally, stresses decayed rapidly to negligible values within minutes of screw activation. When considering appliances that exert a continuous force during expansion, it was determined that it is imperative to maintain as constant a force as possible;​ as the applied force decays over treatment, the amount of suture expansion generated closely follows this trend. Overall, it was found that an appliance able to generate a constant force throughout the entirety of treatment will be most effective in physiologically expanding the midpalatal suture. This in turn will decrease treatment time and improve overall results.