This was a cross-sectional study where sixty-three consecutive patients with diagnosis of IMPS from our outpatient pain clinic in the pediatric rheumatology division were evaluated and forty were included in the final sample. IMPS was defined by the presence of generalized musculoskeletal pain in 3 or more areas of the body for at least 3 months, and these symptoms are not explained by other causes or diseases . FM was characterized by widespread musculoskeletal pain associated with fatigue, sleep, memory and mood issues  and pain amplification syndrome was defined as by the presence of musculoskeletal pain without a well-defined organic basis.
Twenty healthy subjects were considered as control group.
The inclusion criteria were patients with IMPS (FM and pain amplification syndrome) as defined in the literature [3, 5]. Healthy subjects who were referred for control orthodontic evaluation were included as control group and were gender and age matched to the patients. Other inclusion criteria for both groups were the presence of the permanent central incisors completed erupted, first permanent molars in occlusion, and the absence of dental related pain. Twenty-three patients were excluded because they presented problems with their teeth in the areas where the BF was measured.
After informed consent was obtained, demographic and clinical data were collected from the patients’ medical records.
All patients underwent a rheumatologic examination performed by a single pediatric rheumatologist, and an orofacial examination performed by a single dentist. These examinations were scheduled for the same week.
Assessment of subjective symptoms of the TMJ
The subjects were interviewed according to a standardized questionnaire concerning the presence of pain and functional impairment. Patients and their parents answered questions regarding the presence of headaches, abdominal pain, TMJ or masticatory muscle pain at rest or during functional mandibular movements, impaired maximal mouth opening and impaired chewing ability. The questions were answered by categorical (yes/no) responses.
Only one blind assessor (dentist) performed the clinical evaluation following a structured protocol. The following registrations were made:
Maximum mouth opening between the incisal edges of the front teeth (in mm) was measured using a millimeter ruler and adjusted for overbite and open bite as necessary. The subjects were asked to open his or her mouth as widely as possible. The patients performed the movements without any help from the examiner. Limitation of opening was defined as a range of movement in the central incisor region of less than 40 mm from the fully occluded to maximal open position [16, 17].
Presence of pain on palpation on facial sites (6 sites):
Presence of pain on function: during active mandibular movements (open, laterotrusion and protrusion).
Bite force measurement
The BF registration procedure was carefully explained to all participants. Subjects sat in an upright position without head support. The BF was measured unilaterally in the area of the first permanent molars (right and left sides) and central incisors region, using a calibrated dynamometer (Crown DBC, Oswaldo Filizola, São Paulo, SP, Brazil). The stainless steel rods were 40 mm long X 12.7 mm wide X 13.5 mm thick. A piece of simple disposable foam tape (15 mm long X 13 mm wide X 4 mm tall) covered the BF application site. The peak force measurements were displayed digitally on its screen and recorded for further analysis. The dynamometer was cleaned with alcohol, a piece of sterile latex encased the device and it was replaced after each subject evaluation. Subjects were instructed to bite as hard as possible. The measurements were repeated three times, with one-minute rest between them. The highest value from the three recordings was considered as the maximal BF. The same operator performed all measurements. BF was not taken if the anterior teeth or the first permanent molars were not completely erupted or if they had had extensive restorations. The measured BF was calculated in Newton (N) and displayed digitally.
Written informed consent was obtained from all participants. This study was reviewed and approved by the local Medical Ethics Committee.
In order to verify the association between the variables presence of headaches, abdominal pain, and TMD symptoms and signs in the two patient groups, Pearson’s chi-square test and Fisher’s exact test were used. These tests were also used to evaluate the association between the TMD symptoms and signs in each patient group and the control group. Student’s t-test was used to verify the variation of age, maximal mouth opening capacity, and the BF measurements between each patient group and the control group. The STATISTICA 12.7 (Dell®) program was used for the statistical analysis. The level of significance was set to 5% (p < 0.05).