INTRODUCTION

Recently there has been an increase in demand for orthodontic treatment. The number of patients is skyrocketing, and females constitute the majority. However, in contrast to males, females are more vulnerable to orthodontic pain, a negative experience that more than 90% of the patients complain of during orthodontic treatment.1,2 According to previous research, the intensity of pain that females perceived was stronger than that perceived by males, which was seen from days 3 to 7 after administration of orthodontic forces. In addition, the pain persisted longer in women. By the day 7, only approximately 25% of male patients reported pain, but, in contrast, >50% of the female patients were still reporting pain.2–4 Therefore, it is of great necessity to predict the possible pain of female patients so that orthodontists can take measures to control the pain in clinical settings.

Pain is an individualized perception that differs among subjects. Prediction of pain is difficult in clinical settings because of these individual variations. Although a few previous studies reported the predictive power of a series of psychological factors5 and personality traits5,6 for orthodontic pain, the prediction was made based on a plethora of psychological scales that can hardly be navigated without the professional assistance of a psychologist. Furthermore, it was time consuming to collect and analyze the data for each patient, which seemed impractical in orthodontic clinics, considering the large number of patients an individual doctor treats on a daily basis.

Primary dysmenorrhea (PD) is a common symptom among adolescent girls and adult women, the prevalence rate of which is as high as 90%.7 Defined as a cramping pain or discomfort in the lower abdomen at the onset of menstruation in the absence of any identifiable pelvic disease, PD is a relatively stable perception of pain that most female orthodontic patients are already acquainted with. Furthermore, PD is reported to be caused by an increased level of pain mediators such as prostaglandin,7–9 which is the same one that induces orthodontic pain.10–12

Thus, we hypothesized that PD could be a reference to predict the orthodontic pain of female patients. The aim of the study was to investigate the relationship between the severity of PD and the intensity of orthodontic pain in females. We also attempted to analyze whether the severity of PD could indicate duration of orthodontic pain.

MATERIALS AND METHODS

RESULTS

Of the total 124 females enrolled, four subjects (one in Mi and three in Mo) were excluded because they failed to record the VAS score at every required time point. The dropout percentage was 3.2%, which was considered within the acceptable limit. Descriptive analysis of malocclusion and baseline parameters is given in Table 1. NRS for PD differed significantly between groups because of the way we grouped the subjects. Except for that, no significant difference was seen in terms of age, ALD, PAR scores, pain threshold and tolerance, anxiety level, personality trait, and motivation between the three groups, indicating that baseline levels were generally balanced.

Table 1.  Comparison of Malocclusion and Baseline Parameters Among the Groups^a

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The general linear model for repeated measures showed that VAS scores demonstrated intergroup differences during the overall period. As Figure 1 shows, pain intensities exhibited a downward tendency along the time, but the changes in pain intensity were significantly different between the three groups. The intergroup comparison at each time point showed that Mi had the lowest VAS scores during the first 7 days. In contrast, S possessed the highest VAS scores, and those of Mo stayed in between. On day 14, the VAS score of Mo declined to a similar level as that of Mi, while that of S remained higher. By day 28, the three groups shared a similar level of VAS scores.

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The results of correlation between NRS scores and reports of pain intensity (VAS scores) are presented in Table 2. Significantly positive correlations were found between NRS for PD and VAS scores reported on days 1, 2, 4, 7, and 14, indicating that the more severe the PD was, the more intense the orthodontic pain would be at the first five time points.

Table 2.  Correlation Between the Severity of PD (NRS) and the Intensity of Orthodontic Pain (VAS) at Different Time Points^a

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Percentages of patients who reported no pain (VAS  =  0) for the last three time points are shown in Table 3. The chi-square tests demonstrated a significantly higher proportion of people reporting no pain on day 7 in Mi than in Mo and S. This proportion of pain-free subjects in Mo approached that of Mi at the end of the second week, whereas this proportion in S did not approach that of Mi until day 28. Thus, though the pain was not perceived by a majority of patients in Mi on day 7, it was still perceived by almost all patients in Mo and S. When only a tiny minority of patients reported pain in Mi and Mo on day 14, a large majority of females in S still suffered from orthodontic pain. The result indicated that orthodontic pain tended to persist among subjects with a higher intensity of PD.

Table 3.  Comparison of the Percentage of Subjects That Reported No Pain Among the Groups on Days 7, 14, and 28^a

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DISCUSSION

An important finding of our study was the good correlation between NRS scores of menstrual pain and VAS scores of orthodontic pain within the initial 2 weeks after force initiation. The females who had experienced heavier PD were prone to report more intense pain during the initial 2 weeks of orthodontic treatment. They also tended to have prolonged pain experiences in contrast to those who reported milder PD.

Prediction of females' orthodontic pain is necessary for both medical and social reasons. However, the existing methods of prediction are not convenient enough for widespread application in clinical settings. A simple method to predict orthodontic pain is thus required. In the present study, we evaluated PD, a pain experience common to most female patients. We confined our subjects to college students so that the ages and educational backgrounds of the subjects were similar. We assigned the subjects to one of three groups based on the established classifications for NRS in terms of PD. Orthodontic treatment was initiated within 1 week after the latest menstrual period ended to prevent concurrence of PD that might have interfered with assessment of orthodontic pain. The orthodontic force was balanced among groups by confining the extent of malocclusion and the size of initial archwire, because the magnitude of force was previously reported to be associated with orthodontic pain.17 In addition, the baseline parameters that were considered to influence orthodontic pain were balanced among the groups, primarily due to a rigorous selection of subjects.

Our results exhibited differences in reported pain between the three groups, which were further confirmed by correlation analysis. The heavier the PD was, the higher the level of orthodontic pain a patient might perceive. This tendency was especially of clinical significance for the initial few days after archwire placement because it was the period when the pain was most intense for the majority of patients. Our results also revealed differences in pain duration according to severity of PD. In our study, the percentage of patients whose pain disappeared was calculated and compared from day 7 on, because the pain was reported to begin disappearing on day 7.18 Our results showed that the proportion of subjects reporting no pain in S was significantly lower compared to that of the other groups until day 14, at which point the proportions of Mi and Mo reached a similarly high level, approximating 100%. This suggests that females with severe PD tend to suffer from a persistent orthodontic pain. Taken together, our data indicated that PD could potentially serve as an intuitive and simple reference to preliminarily predict orthodontic pain in females.

The relationship between the two types of pain could possibly be explained by the following aspects. From a molecular level, both PD and orthodontic pain share some vital pain-mediating substances that activate nociceptors to induce pain perception. Calcitonin gene-related peptide (CGRP) is one of those mediators. It in one way enhances pain perception by promoting release of substance P (SP), a common neurotransmitter released in response to nociceptive stimulus, and in another way prolongs pain perception by postponing degradation of SP.19 The role of CGRP in inducing orthodontic pain has been widely acknowledged by researchers.20,21 Recently it was found to be highly expressed in the dorsal root ganglia cultured with peritoneal fluids from patients with dysmenorrhea.22 Taken altogether, CGRP is a possible pathway that to some extent explains why PD victims tend to react more intensely and more persistently to orthodontic pain. From a macroscopic perspective, both types of pain bear some resemblance in terms of their temporal features. To be specific, both PD and orthodontic pain generally occur on a monthly basis and peak during period onset. As severity and duration of pain are closely related with psychogenic processes, the negative pain memory and related anxiety associated with PD might induce heavier and more persistent nociception in response to a temporally similar type of pain. These assumptions about joint pathways of pain generation need to be further investigated step by step.

One limitation to this preliminary investigation is that only college students were studied, which restricted the extension of our findings to a broad majority of female orthodontic patients. Moreover, patients with minor crowding could be merely representative for a minority of the patient population. Additional studies with a larger panel of subjects should be carried out to validate these findings and provide more detailed insights into prediction of orthodontic pain using PD.

To summarize, PD is potentially predictive for orthodontic pain in female patients, after it has been confirmed on an independent panel of female patients. The present study renders orthodontists some implications to roughly predict the intensity and duration of orthodontic pain by referring to patients' experiences with menstrual pain, a possible method that might help in addressing a significant side-effect during orthodontic treatment.

CONCLUSIONS

• Females with a higher level of menstrual pain tended to perceive orthodontic pain with higher intensity and prolonged duration.

• PD could potentially serve as a reference to predict orthodontic pain in clinical settings.