Perioral structures play important roles in the development of either a normal occlusion or a malocclusion. Soft tissues like the lips, cheeks, and tongue affect hard tissues and orthodontic treatment results by perioral pressures, muscle forces, and periodontal attachments. Therefore, soft tissue limitations should be assessed more precisely by orthodontists, and they should consider not only the genetics but also the environmental factors.
According to the balance theory defined by Weinstein et al. , the teeth are balanced by the tongue from the inside and by the lips and cheeks from the outside. At the same time, even though the magnitude of force is low, it may cause a movement in the teeth when applied for a sufficient amount of time [17,18,19]. Graber  stated that the changes observed in the muscle functions may change the normal morphology or may compound the current malocclusion where he examined the muscle morphologies of the Class I, Class II, and Class III malocclusions. Evaluations regarding the soft tissues are very important for determining the malocclusion etiology and the stability of the orthodontic treatment. Thus, in this study, we aimed for clarifying the relationship between the upper incisor protrusion and perioral pressures.
In the evaluation of the soft tissues, it is possible to investigate thickness or volume measurements as well as the electromyographic or electrodynamic measurements. Applying electrodynamic measurement techniques with strain gauges is a reliable method for evaluating soft tissue forces and pressures [21,22,23,24,25,26,27,28,29,30,31]. Lindeman and Moore , comparing three different methods of evaluating the perioral pressure and the force, maintained that the lips cause fluid-like pressure, and thus, they should be evaluated using devices that are sensitive to pressure instead of the force. IOPI is a reliable tool that measures the perioral pressures from the lips, cheeks, and tongue [33,34,35]. Additionally, there is no need for additional laboratory processes and no risk of contamination because of its single use measurement bulbs.
It was mentioned in previous studies that perioral pressures were affected by the alterations in head posture [6, 36,37,38]. According to Thüer et al. , Ingervall and Thüer , and Hellsing and L’Estrange , the perioral pressures were increased with the head extension than the natural head position. Therefore, in our study, we paid attention during the pressure records to the maintenance of natural head position.
Consensus about the relationship between the types of malocclusion and perioral pressures has not been achieved yet. Lapatki et al.  indicated that the upper incisors were exposed to more resting lip pressure in CII/2 malocclusion compared to Class I occlusion. Oppositely in another study, it has been shown that maximum lip pressure was the lowest in individuals with CII/2 malocclusion . Nevertheless, in both studies, the individuals were older and also the methodology of these studies was different from our study. Thüer and Ingervall  found that the ULP on the upper incisors was at the lowest in CII/2 malocclusion and at the highest in Class II division 1 malocclusion. They also mentioned that the LLP did not show significant difference between malocclusions. In a study that included both young and adult individuals, ULP change between Class I and Class II malocclusions did not show significant difference . Additionally, Lambrechts et al.  noted that among the types of malocclusion, no significant differences could be found in terms of tongue pressure. In our study, there are no significant pressure differences except LBP between the study and control groups at the beginning of the treatment. This difference disappeared with the decrease of LBP after the protrusion period. However, this decrease has not been thought to result from orthodontic treatment. Though a utility arch has buccal segments, tooth movement apparently occurs at the anterior region. Additionally, since the utility arch is a symmetric appliance, we would wait to see the same effect on the right side. But, a significant change in RBP has not been determined. It was thought that these differences might be due to transitions in dentition, and although attention is shown, bending and inserting the utility arch may be different from one side to the other.
Mitchell and Williamson  and Posen [42, 43] stated that the perioral forces increase with age. In the present study, it was seen that ULP increased during the retention period in both groups. The retention period covered a longer phase than the protrusion period. Di Fazio et al.  found an increase in the upper lip pressure with age, and he suggested that this increase might be described by the maturation of the orbicularis oris muscle due to growth. Additionally, in another study, it was indicated that the ULP tended to be high in children with a large overjet . These observations might be acceptable for our study.
In the present study, LLP increased and VLP decreased after upper incisor protrusion but these changes did not remain stable in the study group. Some authors [7, 10] stated that the upper incisor position was determined by the increased LLP in CII/2 malocclusion. In one of the studies that investigated the effect of increased overjet in perioral pressures, it was observed that LLP increased , while in another study, VLP decreased because tightening of the lips became harder . These findings are compatible with the findings of our study. However, these studies had a cross-sectional study design, and individuals with an increased overjet were included. Meanwhile, in the control group, VLP did not increase significantly after protrusion and then significantly after the protrusion period. Due to these changes, significant difference occurred at the end of protrusion between the two groups. In fact, although not significant, LLP also increased in the control group. All these lip pressure increases observed in the control group might be described as mentioned above in ULP.
The only correlation between the incisor inclination and the lip pressures was found between the increase of the U1-NA distance and the decrease of VLP. This correlation, even though statistically significant, was moderate. However, a significant correlation between the increase of VLP and the decrease of upper incisor inclination was not observed after the retention. Therefore, VLP which did not remain stable after retention was not associated with the mild relapse of upper incisor protrusion. Such as soft tissue morphology changes, other factors that may be effective on the perioral pressures need to be investigated in further studies. Thüer and Ingervall  determined that ULP was correlated with the morphology of the lips, while Di Fazio et al.  found a significant correlation between ULP and age.
This is the first study to evaluate the perioral pressure changes in terms of lips by IOPI. Furthermore, IOPI can be considered as a diagnostic tool for malocclusions and the progression of myofunctional exercises. Hence, orthodontists can use IOPI for the malocclusions arising from bad oral habits in a practical way. The only limitation of this prospective study was that transitions in mixed dentition might affect the perioral pressure measurements.
It has been known that soft tissues affect skeletal and dentoalveolar hard tissues during growth. Our findings showed that changes that occurred in the anterior teeth had a temporary effect on the soft tissues. Therefore, it should be noted that permanent changes could not be achieved in soft tissue pressures after upper incisor protrusion and relapse resulting from soft tissue pressures should always be kept in mind. In further studies, perioral pressure changes can be examined by including different malocclusions.