INTRODUCTION

Although pain is a common experience reported by about 90% of people undergoing orthodontic treatment,1,2 there is a paucity of information about the nature, quality, and intensity of the pain experienced.3,4 Previous investigations5–8 have primarily used visual analogue scales (VASs) to measure only pain intensity during orthodontic therapy. Other types of pain measures, such as verbal rating scales9 and pressure algometry,10 have seen limited employment. There is evidence that pain experiences should be avoided or minimized to prevent sensitization of the central nervous system,11 particularly in individuals who may be genetically predisposed to pain.12 However, without more comprehensive information, appropriate pain management during tooth movement is compromised.

The McGill Pain Questionnaire (MPQ) was developed, tested, and refined to a short form (MPQ-SF) to measure the intensity of sensory and affective aspects of pain in adults.13 This questionnaire has been applied to evaluate pain efficiently in adult dental patients14 and in adolescent,15 preadolescent, and adult orthodontic patients16; however, in its original form it has questionable applicability to young individuals undergoing elective therapy. Although some researchers have recommended the MPQ to investigate orthodontic pain,15,16 to date, its psychometric utility has not been established.

An efficient, valid instrument to improve the qualification and quantification of the pain experienced during orthodontic treatment is a key prerequisite to collecting foundational data and addressing this relatively neglected facet of care.17 Hence, the objectives of this project were to modify and validate the MPQ-SF for assessment of the nature, quality, and intensity of pain experienced by adolescent orthodontic patients.

MATERIALS AND METHODS

The methods in this study involved a two-step process. The first step was to develop a modified MPQ-SF (MMPQ-SF) for the target group: adolescents undergoing active orthodontic treatment. This was accomplished by an internal expert panel and then reviewed for face validity by an external expert panel. The second step was to test and validate the MMPQ-SF on a sample of the target population. The associated methods will be described in more detail in the following sections.

Testing of the MMPQ-SF

The resulting MMPQ-SF15 (Figure 1) was then tested in a sample of adolescents to establish construct validity. The protocol was approved by the Children's Mercy Hospital Institutional Review Board and subjects' rights were accordingly protected. Patients aged 11 to 17 years attending the same clinic were invited to participate if they met the following inclusion criteria: full-arch maxillary and mandibular fixed orthodontic appliances were planned or in place; they were in the initial, middle, or end stage of treatment; and they spoke English, were literate, and were in good general health. Subjects with chronic pain, or those taking pain medications prior to participating or during the validation study, were excluded. The three stages of treatment were defined as follows:

• Initial treatment (IT): Subjects with the first archwires placed for aligning and leveling. These archwires were made of nickel-titanium or stainless steel and were round in cross section.

• Middle of treatment (MT): Subjects with the first archwires, rectangular in cross section, that were not the initial archwires. These archwires were made of nickel-titanium, stainless steel, or titanium-molybdenum alloy.

• End of treatment (ET): Subjects with any archwires of rectangular cross section that had been in place for ≥4 months.

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The clinic schedule was reviewed to identify equal numbers of female and male patients in each of the three stages of treatment as potential subjects. Informed consent to participate in the study was obtained from the parents (or legal guardians) of 75 subjects who fit the inclusion and exclusion criteria. All subjects also agreed to participate.

Each subject was given a single-page pain evaluation form (Figure 1) and instructed to complete its three parts on the day following the IT, MT, or ET adjustment appointment. A stamped, addressed envelope was also given to each subject to complete and return by mail. Part 1 of the form was the MMPQ-SF15, for which the subject was asked to consider the 15 descriptors and rate each on a 4-point Likert severity response scale (0  =  no pain, 1  =  mild pain, 2  =  moderate pain, and 3  =  severe pain). Part 2 of the form was the VAS, on which the subject was asked to indicate the severity of their overall pain by placing a mark along a line ranging from “no pain” to “worst pain possible.” Part 3 of the form was the PPI, on which the subject was asked to select a ranking for their current level of pain on a scale of 0 to 5 (0  =  no pain, 1  =  little pain, 2  =  moderate pain, 3  =  bad pain, 4  =  horrible pain, and 5  =  extreme pain). If the questionnaire was not returned by mail within 10 days after expected completion, the subject was excused from the study.

Responses were initially grouped according to gender and treatment stage, and pain perception was quantified by the total score of the 15 descriptors in the MMPQ-SF15. Analysis of variance was used to determine whether or not there were differential effects of gender on pain perception as measured by MMPQ-SF15. In addition, effect size estimates were obtained using η² values. Divergent validity of the MMPQ-SF15 was explored by comparing VAS scores from individuals in the highest and lowest MMPQ-SF15 quartiles using the Mann-Whitney U-test. Additionally, scores for each of the descriptors in the MMPQ-SF15 were compared in a similar manner to determine whether or not each was sufficiently discriminating. The level of significance for these statistical comparisons was set at P < .05. Subsequently, the underlying constructs of the MMPQ-SF15 were explored for formulation of subscales using principal components factor analyses with varimax rotation. Finally, criterion-related validity was assessed by examining the correlation between the total and subscale scores for MMPQ-SF15 and the other accepted pain measures (VAS and PPI) using Spearman correlation coefficients.

RESULTS

Sixty-three subjects completed the three pain assessment measures and submitted the forms according to instructions. Twice as many females compared to males accepted the invitation to participate (50∶25) and completed the study (42∶21). Results from two subjects were excluded because they reported taking pain medications after their orthodontic visit but before completing the form. Overall, the 61 remaining respondents had a mean age (± standard deviation) of 14.2 (± 1.6) years (Table 1). These respondents were grouped by gender and treatment stage: initial (15 female, 7 male), middle (10 female, 7 male), end (16 female, 6 male). There was a trend toward females reporting slightly higher overall median scores for the three pain measures. However, analyses showed that there were no significant main effects as a function of gender for VAS (P  =  .179, η²  =  0.033), PPI (P  =  .712, η²  =  0.003), or MMPQ-SF15 (P  =  .963, η²  =  0.0001) scores. Similarly, there were no significant interactions between gender and treatment stage for VAS (P  =  .401, η²  =  0.033), PPI (P  =  .172, η²  =  0.062), or MMPQ-SF15 (P  =  .310, η²  =  0.042) scores. Hence, subsequent analyses were collapsed across gender. The MMPQ-SF15 was found to be discriminating for pain, as supported by the finding that median MMPQ-SF15 scores for individuals in the lowest and highest quartiles for VAS scores were significantly different (P  =  .0001, Table 2).

Table 1.  Subject Demographics and Summary Pain Scores for Treatment Groups

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Table 2.  Median Scores (Semi-interquartile Range, SIQ) for Total MMPQ-SF15^a and MMPQ-SF11 for Adolescent Orthodontic Patients and for Each Descriptor for Subjects with Lowest (First Quartile) and Highest (Fourth Quartile) Scores on the VAS

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Eleven of the 15 descriptors were discriminating for pain, as supported by findings that the median scores for these descriptors (pressure, sore, aching, throbbing, tight, pulling, uncomfortable, strange, frustrating, annoying, miserable) from individuals in the lowest and highest quartiles for the VAS were significantly different (P  =  .0001 to .024; Table 2). Four descriptors—cutting, burning, tingling, and dull—were not discriminating for pain (P > .05, Table 2). Because these items did not contribute meaningfully to the total score, they were eliminated from the MMPQ-SF15, and MMPQ-SF11 scores were computed by summing the 11 discriminating descriptors. Subsequent analyses compared total MMPQ-SF11 scores from individuals in the lowest and highest VAS score quartiles, and these were found to be significantly different (P  =  .0001, Table 2).

To explore the underlying constructs in the MMPQ-SF11, the data were subjected to principal components factor analysis with varimax rotation. Results yielded a two-factor solution (Kaiser-Meyer-Olkin  =  0.88) that explained 64% of the variance in responses (Table 3). The two factors of descriptors distinguished by the analysis were categorized as general/emotional pain or localized pain. Internal consistency estimates of reliability (Cronbach's α) for the general/emotional and localized pain subscales were 0.883 and 0.857, respectively.

Table 3.  Two-Factor Model from Principal Components Factor Analysis of MMPQ-SF11^a Showing Descriptor Loadings and Pain Subscales

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Consequently, the severity scores for subscale descriptors were summed and used to determine whether or not measures of generalized/emotional vs localized pain differed across the treatment stages. Comparison of the two subscale scores as a function of treatment stage showed that the localized pain subscale scores (factor 2 descriptors) were not different as a function of treatment stage (P  =  .235, Table 4). However, comparison of the generalized/emotional subscale scores (factor 1 descriptors) revealed that scores from the initial stage (median  =  6.0) and the other two stages (middle, median  =  2.0; end, median  =  3.0) were significantly different (P  =  .011 and .014, respectively; Table 4).

Table 4.  Median Scores (Semi-interquartile Range, SIQ) for the Generalized/Emotional and Localized Subscales of the MMPQ-SF11^a for Different Stages of Treatment

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Criterion-related validity for the MMPQ-SF15, MMPQ-SF11, and the two pain subscales was assessed via correlation with the PPI and VAS scores. The resulting Spearman rank order correlation coefficients between measures were positive and ranged from 0.64 to 0.85 (Table 5). The coefficients of determination (r²_s) of MMPQ-SF15 and MMPQ-SF11 with PPI (r²_s  =  0.71 and 0.72, respectively) were somewhat higher than with VAS (both r²_s  =  0.61). The proportions of variance explained in the localized pain subscale by PPI and VAS (0.72 and 0.59, respectively) were notably larger than those for the generalized/emotional pain subscale (0.49 and 0.41, respectively).

Table 5.  Spearman Rank Order Correlation Coefficients (r_s) for Total MMPQ-SF15 and MMPQ-SF11 and Scores from the Generalized/Emotional (G/E) and Localized (L) Pain Subscales (PS) Compared with VAS and PPI^a

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DISCUSSION

The results of the current study provide initial evidence to support the validity of the MMPQ-SF for orthodontic pain assessment in adolescents. Findings showed that the MMPQ-SF15 was strongly related to the accepted standard for assessing pain intensity, the VAS (r²_s  =  0.61). Moreover, it was also correlated strongly with a single-item global rating scale for pain, the PPI (r²_s  =  0.71). This indicates consistency among all three pain scales and suggests that the MMPQ-SF total score from 15 descriptors was a valid measure of overall orthodontic pain experienced by adolescent subjects undergoing orthodontic treatment. Furthermore, the results of the MMPQ-SF11 showed similar criterion-related validity. Important features of any questionnaire are parsimony and efficiency in administration time. Analysis of the discriminant capacity of the 15 MMPQ-SF descriptor terms for individuals reporting high vs low pain on the VAS allowed an efficient evaluation while still capturing the intensity and quality of orthodontic pain. These analyses provided empirical evidence for the elimination of four descriptors (cutting, burning, tingling, and dull), resulting in a highly efficient 11-item instrument for this patient group.

The MPQ-SF was developed to capture measures of the intensity of sensory and affective aspects of pain in adults. From a domain perspective, whether or not the results from this preliminary exploration could differentiate between sensory and affective aspects of pain in a younger population was of interest. Results from the factor analysis suggest that the 11 descriptors fell into either of the domains. The results for sensory aspects of pain were not significantly different across treatment stages. However, the exact types and magnitudes of forces experienced between and within treatment stages were not determined in this study. On the other hand, the results for the affective aspects of pain, when compared across the three treatment stages, performed conceptually as expected. It may be anticipated that adolescents would become more comfortable emotionally after initial treatment, and the current results supported this hypothesis. This type of information is not provided by commonly used pain measures such as the VAS, which only quantify the intensity of the pain. Words in the generalized/emotional pain subscale—uncomfortable, strange, frustrating, annoying (factor 1 descriptors)—were scored significantly higher by subjects in the IT stage compared to the MT and ET stages. These words are notable because they tend to describe perceptions of an experience (affective) rather than feelings of pain (sensory). These differences could reflect the emotional aspects of new and different sensations associated with activation of the fixed orthodontic appliances in the IT group patients, as compared to patients in the MT and ET groups, who were likely more accommodated to their appliances. Individuals who are uncertain about what type of sensations to expect are often more apprehensive initially and may experience more pain than those who are adequately prepared and accepting of the treatment.16,18 Therefore, taking these types of descriptors into consideration seems important in educating orthodontic patients about what they can expect to feel during the course of treatment.

The current study sample was of sufficient size to achieve statistically significant and clinically useful results for this preliminary validity study. Furthermore, results from the effect size analysis, which is independent of sample size, showed that only very small amounts of the variance in the pain assessments used could be attributed to gender differences (≤3.3%) and the interaction of gender and treatment stage (≤6.2%). Nevertheless, the current study had limitations that should be addressed in future studies. For example, a next step to further validate the MMPQ-SF11 as an age-appropriate instrument to measure pain would be an assessment of face validity through obtaining evaluations and feedback from adolescent subjects regarding the relevance of the descriptors. In addition, a larger sample that is more balanced for gender is indicated for future investigations to determine whether or not there are differences between adolescent females and males in measures of generalized/emotional and localized pain at different stages of treatment. Future studies are also needed to determine the intrarater reliability of the MMPQ-SF11 and whether or not it is sensitive to changes within individuals and between different genders over time. Also, the definitions for treatment stages in this study may not generalize to comparable treatment in a different population, in that the applied mechanics and types of dental malalignments were variable between and within stages. Although it has been proposed that patients are sensitive to orthodontic force magnitudes and directions, there is scant evidence to support this statement to date.7,19 Hence, studies of the pain experience when the applied forces are carefully calibrated, quantified, and controlled and the tooth positions and movements are measured should be conducted. In addition, there is growing evidence that interindividual variability in inflammatory responses and pain sensitivity to a given stimulus is high and may be influenced by previous experiences.12 This should be further tested in studies employing controlled and quantified force systems by including genetic profiling for inflammation and pain sensitivity. Candidate genes linked to inflammation, tooth movement, and pain sensitivity include those of the interleukin-1 gene cluster,20 catechol-O-methyltransferase,12 prostaglandin-endoperoxide synthase 2,21 and μ-opioid recepter.22 Combining these improvements in a study design with the validated MMPQ-SF11 should provide improved knowledge of the intensity as well as the quality of pain experienced by adolescents during orthodontic tooth movement. Future application of the MMPQ-SF11 to measure orthodontic pain more specifically could lead to appropriate management of this important facet of therapy.

CONCLUSION

• The investigated MMPQ-SF, particularly MMPQ-SF11, demonstrated efficiency and utility in the assessment of pain in adolescent orthodontic patients and correlated well with VAS and PPI ratings.