Back pain in relation to musculoskeletal disorders in the jaw-face: A matched case–control study

Pain 131 (2007) 311–319 www.elsevier.com/locate/pain

Back pain in relation to musculoskeletal disorders in the jaw-face: A matched case–control study Birgitta Wiesinger a b

a,b

, Hans Malker b, Erling Englund b, Anders Wa¨nman

a,*

Department of Clinical Oral Physiology, Umea˚ University, SE-90187 Umea˚, Sweden Department of Research and Development, Va¨sternorrland County Council, Sweden

Received 2 September 2006; received in revised form 19 February 2007; accepted 12 March 2007

Abstract Back pain and temporomandibular disorders are both common conditions in the population with influence on the human motor system, but a possible co-morbidity between these conditions has not been fully investigated. The aim of this study was to test the hypothesis of an association between long-term back pain and pain and/or dysfunction in the jaw-face region. Back pain was defined as pain in the neck, shoulders and/or low back. The study-population comprised 96 cases with long-term back pain and 192 controls without back pain. We used a screening procedure, a questionnaire and a clinical examination of the jaw function. The questionnaire focused on location, frequency, duration, intensity and impact on daily life of symptoms in the jaw-face and back regions. The analysis was conducted on 16 strata, matched by age and sex for case vs. control, using Mantel–Haenszel estimates of matched odds ratio (OR) and 95% confidence interval (CI) as well as the corrected Mantel–Haenszel v2 test. The overall prevalence of frequent symptoms in the jaw-face region, as reported in the questionnaire, was 47% among cases and 12% among controls. The difference was statistically significant (P < 0.0001) with a sevenfold odds ratio (CI: 3.9–13.7). Moderate to severe signs from the jaw region were clinically registered among 49% of the cases and 17% of the controls (P < 0.0001, OR: 5.2, CI: 2.9–9.2). The results showed statistically significant associations between long-term back pain and musculoskeletal disorders in the jaw-face and indicate co-morbidity between these two conditions. Ó 2007 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved. Keywords: Back pain; Temporomandibular disorders; Matched case–control study; Co-morbidity; Musculoskeletal disorders; Jaw dysfunction

1. Introduction Pain in the spinal region is one of the most common long-term pain conditions with a reported prevalence of 15–30% (Brattberg et al., 1989; Andersson et al., 1993; Elliott et al., 1999; Picavet and Schouten, 2003; Von Korff et al., 2005). It is a major reason for health-care utilization (Hart et al., 1995; Maniadakis and Gray, 2000), for being on sick leave (Linton, 1998; Hansson and Hansson, 1999) and for early retirement (Hansson and Hansson, 1999). The total cost of back pain is estimated at 1.7% of the gross national product in the Neth*

Corresponding author. Tel.: +46 90 7856082; fax: +46 90 132578. E-mail address: [email protected] (A. Wa¨nman).

erlands (van Tulder et al., 1995) and 1.3% in Sweden (Hansson and Hansson, 2005). The jaw is an essential part of the human motor system with a close functional relationship with the head-neck motor system (Eriksson et al., 2000). An interdependence between the cervical and the trigeminal sensory-motor systems has been hypothesized (Browne et al., 1998). A recent study has shown that experimental trapezius muscle pain induced a reduction of jaw opening (Komiyama et al., 2005). Population-based studies have found relationships between the presence of signs and symptoms in the jaw region and in the neck/shoulder area (Wa¨nman, 1995; Ciancaglini et al., 1999). Patients with musculoskeletal pain and dysfunction in the jaw-face (i.e. temporomandibular disorders) are likely to present signs and

0304-3959/$32.00 Ó 2007 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.pain.2007.03.018

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symptoms of cervical involvement (Clark et al., 1987; de Wijer et al., 1996b; De Laat et al., 1998; Visscher et al., 2001). In a case–control study, patients with temporomandibular disorders (TMD) had a significantly higher number of painful sites, including low back pain, compared to controls (Hagberg, 1991). Presence of long-term back pain was however not specifically addressed. Whether patients with neck, shoulder and low back pain are more susceptible to TMD than subjects without spinal pain is not clarified. Divergent results have been reported regarding patients with neck/shoulder pain (Carossa et al., 1993; de Wijer et al., 1996a), and studies concerning low back pain have not been conducted. Several studies have shown co-morbidity between long-term spinal pain and pain at other sites (Thomas et al., 1999; Cote et al., 2000; Raspe et al., 2003; Hestbaek et al., 2004; Von Korff et al., 2005), but a possible co-morbidity between longterm back pain and TMD has not been fully investigated. Persistence, amplification and spread of pain in longterm musculoskeletal pain conditions may involve central sensitization (Graven-Nielsen and Arendt-Nielsen, 2002). This pathophysiological mechanism is also indicated in jaw-face pain (Maixner et al., 1998; Svensson et al., 2001; Sarlani and Greenspan, 2003), shoulder pain (Leffler et al., 2002) and low back pain (Giesecke et al., 2004). The fusimotor-muscle spindle system (Johansson et al., 1999) has been suggested to play a role in the spread of muscle pain and tension in the cervical and the jaw-face area (Hellstro¨m et al., 2000). Recent experimental results (Ambalavanar et al., 2006) also raise questions about a possible effect of afference from craniofacial structures on pain perception in spinally innervated regions. The objective of the study was to test the null-hypothesis of no association between long-term pain in the back and pain and/or dysfunction in the jaw-face region. 2. Methods The design was that of an age and sex group-matched case– control study. The cases were 96 patients with back pain (pain location presented in Table 1). The controls were 192 subjects without back pain. Back pain in this context is defined as ongoing pain in the neck, shoulders and/or low back. All subjects gave informed consent to participation in the study, for which the Ethics Committee, Umea˚ University, granted permission. Questionnaire. All participants who were scheduled for an examination answered a questionnaire focusing on symptoms specifically in the jaw-face, auricular, head, neck, shoulder and low back regions. Presence of symptoms was marked according to frequency (never; not now, but previously; once or twice a month; once or twice a week; several times a week; daily), duration (less than 1 month; 1 month–1 year; 1–5 years; more than 5 years) as well as intensity and impact on daily activities (11-point numerical rating scales).

Examination of the jaw function. The jaw function was evaluated during a standardized clinical examination performed by one dentist. The subjects lay in a supine position during the examination, apart from the mandibular movement tests, which were conducted with the subjects in a sitting position, with the head unsupported, allowing for unrestricted motion of the neck. The examination included: 1. Temporomandibular joint (TMJ) sounds. Auscultation, without a stethoscope, was performed to detect the presence of TMJ sounds on opening and closing movements of the jaw. The sounds that occurred were classified as sharp clicking, dull clicking, or crepitating. 2. TMJ pain/tenderness to palpation. The TMJ was palpated laterally and posteriorly through the auditory meatus. Pain/tenderness was registered only if the palpation elicited a palpebral reflex in the eye or a protective reflex. 3. TMJ pain on free movement (opening wide, laterotrusion, protrusion). 4. TMJ loading. The subject was asked to bite hard for 30 s on a double wooden spatula (3 mm) placed in the region of the first molars, each side separately. Elicited pain in the contralateral side of the joint was registered as TMJ load pain. 5. TMJ joint play. The examiner’s 1st finger on the right hand was placed on the subject’s left molar region. While holding the mandible it was pulled forwards and backwards with the operator’s 3rd finger on the left hand placed over the left TMJ. The TMJ movement was registered if restricted, painful or both. The procedure was repeated on the other side. 6. TMJ end-feel. At maximal jaw opening position the examiner assisted further opening with the fingers. Presence of elicited pain was registered. 7. A deviation of >2 mm on opening wide was registered. 8. Mandibular mobility. Unassisted maximal opening, protrusive and laterotrusive capacity was measured to the nearest millimetre with a ruler. 9. Loading of jaw muscles. The subject was asked to clench his/her teeth hard in the intercuspal position for 30 s. Development of fatigue or pain in the head, face or jaws during clenching was registered. 10. Muscle pain/tenderness to palpation. The presence of pain/tenderness to palpation of muscles was registered in the same way as for the TMJ. The following sites were palpated: the region of the lateral pterygoid muscles, medial pterygoid muscles, anterior and posterior parts of the temporal muscles, tendon of the temporal muscles, superficial and deep parts of the masseter muscles, sternocleidomastoid muscles, trapezius muscles, neck muscles in the region of the linea nuccae and the thumb muscles. If the latter elicited a pain response, the muscles on the underside of the forearms and the calf muscles were also palpated to measure signs of a generalized pain response to palpation. Some of the variables collected in the questionnaire and some of the variables registered clinically were used to calculate the anamnestic dysfunction index (Ai) and the clinical

B. Wiesinger et al. / Pain 131 (2007) 311–319 Table 1 Distribution of spinal pain location among the cases (pain at least once a week) Back pain location

Number of women

Number of men

Total

Neck Neck and shoulder Neck, shoulder and low back Neck and low back Shoulder Shoulder and low back Low back

0 10 13 4 3 7 10

2 9 17 3 0 6 12

2 19 30 7 3 13 22

dysfunction index (Di), respectively, as devised by Helkimo (Helkimo, 1974). This numerical classification grades the severity of symptoms (Ai 0, I or II) and signs (Di 0, I, II or III) of TMD. In the calculation of Ai, only symptoms reported at least once a week were used and were referred to as frequent symptoms. The Research Diagnostic Criteria for RDC/TMD criteria Axis I (RDC/TMD) was used for diagnosis of myofascial pain (Dworkin and LeResche, 1992).

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back or neck trouble during the previous year?’’ ‘‘Have you been on sick leave due to back or neck trouble during the previous year?’’. Three hundred and forty-one subjects were examined, 289 who reported no back trouble in the screening and 52 who stated that they had back trouble in the screening. People who reported ‘‘never back pain’’ or ‘‘not now, but previously back pain’’ in the questionnaire and fitted the matching criteria were included as controls. Matching was done by sex and age group (20–24, 25–29, 30–34, 35–39, 40–44, 45–49, 50–54, 55–59, 60–64 years), resulting in nine strata for women, and seven strata for men, thus standardizing for sex and age on a group level. Two controls were included for each case. In strata with more controls available than needed, the following exclusion criteria were added: reported previous back pain, health-care utilization due to previous back pain. When strata were still over-represented, the controls were randomly chosen. The original control group included 200 subjects. Eight controls were excluded to adjust for matching, after the withdrawal of cases with generalized pain (i.e. fibromyalgia). The study population (controls) thus consisted of 192 subjects with no ongoing back pain. 2.2. Subsample test

2.1. Subjects Table 2 shows the sex and age characteristics of the study groups. Cases. The cases were drawn from patients referred to the Institute for Vocational Rehabilitation (Rygginstitutet), which deals with the rehabilitation of patients with long-term back problems. The rational for choosing these people as cases for the study, apart from accessibility, was that all had long-term back problems documented by a physician. One hundred and twelve consecutive back pain patients were invited to participate in the study, of whom 102 accepted. Subjects that reported back pain at a frequency of at least once a week were included as cases. Two people did not meet this inclusion criterion. The original patient group thus consisted of 100 subjects. Four patients with a generalized pain pattern (i.e. fibromyalgia) were excluded from this sample. The study population (cases) consequently consisted of 96 subjects with ongoing back pain. Controls. In total approximately 2000 people were invited through advertisements to participate in the study (see flowchart, Fig. 1). The majority were employees at four companies, and the others were students or members of a sport center. Eight-hundred and eighty-four subjects volunteered and answered two questions in a screening form: ‘‘Have you had

Table 2 Number, sex and age characteristics of subjects with back pain (cases) and subjects without back pain (controls) Number

Age range (years)

Mean age (years)

SD

Cases

Female Male

47 49

24–61 25–56

39.5 41.4

9.4 9.9

Controls

Female Male

94 98

22–62 25–59

39.6 41.1

9.4 9.8

Jaw pain may be considered a confounder to associations between back pain and jaw-face dysfunction, therefore separate analyses regarding signs and symptoms of dysfunction in the jaw-face region were conducted on participants without frequent jaw pain. Seventy-eight cases and 190 controls fulfilled this inclusion criterion and were accordingly included in these analyses. 2.3. Internal validity test In order to control for bias in the examination, a blind test was incorporated. Employees at two of the participating companies were included in this test. The personnel managers at these companies sorted the screening forms into respondents with and without back trouble. From the former group the personnel managers randomly drew totally 52 screening forms and scheduled the sampled people for examination, together with 76 subjects without back trouble. To control for internal validity the examiner was blinded to the presence or absence of back trouble for these 128 subjects. In the questionnaire 28 people reported back pain once or twice a month and were not included in the internal validity test. Thirtyseven people reported frequent back pain and 63 reported no back pain. These 100 subjects were used for the internal validity test. 2.4. Statistical methods Data were analysed using Epi Infoä version 3.3 and SPSS version 14.0. The analysis was conducted on the 16 strata, matched for cases vs. controls by age and sex. The Mantel– Haenszel estimates of matched odds ratio (OR) and 95% confidence interval (CI) were used. Statistical significance was assessed by the Corrected Mantel–Haenszel v2 test. The Benjamini and Hochberg correction was used to control for multiple tests (Benjamini and Hochberg, 1995). Due to the small

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About 2000 invited

884 answered the screening procedure 533 not scheduled for examination: back trouble at screening, or strata full, or refused examination

351 scheduled for examination: 296 no back trouble at screening 55 back trouble at screenin

10 dropouts: 3 back trouble 7 no back trouble 341 examined

289 no back trouble at screening

235 no back pain in # questionnaire 8

192 used as controls

54 back pain in questionnaire *6

52 back trouble at screening

9 no back pain in questionnaire #9

43 back pain in questionnaire *31

8 used as controls

#

46

200 controls original control group

192 controls (adjustment due to exclusion of cases with fibromyalgia) Fig. 1. Flowchart showing how controls were included in the study, and the affiliation of subjects included in the internal validity test. *Number of subjects with back pain used for the internal validity test. #Number of subjects without back pain used for the internal validity test.

number of subjects with different back pain locations and participants in the internal validity test and sub sample test, estimates of OR and CI were not conducted on strata for these analysis. Differences in jaw movement capacity were tested with independent-samples T test, back pain intensities and impact of pain on daily activities were tested with Mann–Whitney U test, not using strata. Statistical tests were considered statistically significant if the P-value < 0.05.

3. Results 3.1. Symptoms in the jaw-face region Patients with back pain reported pain in the jawface–head region significantly more often (P < 0.0001) than controls. Frequent headaches were reported by

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55% of the cases and by 8% of the controls (P < 0.0001). Affirmative answers regarding the presence of frequent pain in the jaw during rest, chewing or when opening the mouth wide were given by 19% of the cases and by 1% of the controls (P < 0.0001). Patients with back pain reported symptoms of dysfunction in the jaws significantly more often (P < 0.0001) than controls. Frequent temporomandibular joint (TMJ) sounds were reported by 30% of back pain patients and by 8% of controls (P < 0.0001), 12% of the cases experienced an impaired jaw opening capacity compared to 1% of the controls (P < 0.0001). Frequent tinnitus was reported among 25% of the cases and 10% of the controls (OR: 3.0, 95% CI: 1.5–5.8, P < 0.005). The overall prevalence of frequent symptoms (Helkimo’s Ai I and Ai II) in the jaw-face region (pain in the jaw-face, fatigue in the jaws, TMJ sounds, difficulties in opening the mouth wide or TMJ locking) was 47% among cases and 12% among controls. The difference was statistically significant (OR: 7.3, 95% CI: 3.9–13.7, P < 0.0001). Frequent and severe symptoms from the jaws (Ai II) were reported by 26% of the cases and by 3% of the controls (OR: 13.8, 95% CI: 4.9–39.0, P < 0.0001). Odds ratios and 95% confidence intervals for symptoms in the jaw-face–head region are presented in Fig. 2.

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task occurred significantly more frequently (P < 0.0001) among cases (47%) than among controls (17%), (OR: 4.8, 95% CI: 2.7–8.6, P < 0.0001). The maximal jaw opening and the maximal jaw protrusive capacity were significantly lower among cases than among controls (P < 0.0001). One or more clinical signs of dysfunction in the jaw in accordance with Helkimo (Di I, II and III) were registered among 91% of patients with back pain and among 52% of controls. The difference was statistically significant (OR: 9.4, 95% CI: 4.4–20.1, P < 0.0001). Moderate to severe signs from the jaws (Di II–Di III) were registered among 49% of the cases and 17% among the controls (OR: 5.2, 95% CI: 2.9–9.2, P < 0.0001). Odds ratios and 95% CI for clinically registered signs are presented in Fig. 3. 3.3. Oral parafunctions Patients with back pain were significantly more often aware of tooth grinding (OR: 2.7, 95% CI: 1.5–5.1, P < 0.005), tooth clenching (OR: 3.9, 95% CI: 2.2–7.1, P < 0.0001), tongue pressing (OR: 8.6, 95% CI: 2.1–35.2, P < 0.005) and lip/cheek/tongue biting (OR: 2.3, 95% CI: 1.2–4.4, P < 0.05) than controls. There was no statistically significant difference for awareness of nail biting. Odds ratios and 95% confidence intervals for pooled grinding and clenching are presented in Fig. 2.

3.2. Signs of dysfunction in the jaw-face region 3.4. Controlling for multiple tests The presence of pain/tenderness to palpation at both jaw muscle and neck–shoulder muscle sites was significantly more often (P < 0.0001) registered among cases (55%) than among controls (8%). Thirty-four percent of back pain patients met the criteria for myofascial pain, according to RDC/TMD, compared to 1% among the controls (OR: 75.7, 95% CI: 14.6–392.2, P < 0.0001). Symptoms (pain or fatigue) elicited by a tooth clenching

The results between cases and controls were also significant with Benjamini and Hochberg correction. 3.5. Signs of dysfunction in participants without jaw pain The sub sample analyses based on subjects without frequent jaw pain showed a similar pattern of

Fig. 2. Odds ratio and 95% confidence interval for 96 back pain patients compared with 192 matched subjects with no back pain in regard to reported frequent symptoms from the jaw, face and head region and reported oro-facial parafunctions.

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Fig. 3. Odds ratio and 95% confidence interval for 96 back pain patients compared with 192 matched subjects with no back pain in regard to clinically registered signs of TMD. Di denotes the presence of moderate (Di II) to severe signs (Di III) of mandibular dysfunction; TMJ disk displacement denotes clinical signs of a symptomatic temporomandibular joint disk displacement with reduction; clicking or crepitation denotes the presence of these TMJ sounds during jaw opening and closing; TMJ dysfunction denotes temporomandibular joint sounds and/or deviation >2 mm on jaw opening; TMJ pain denotes the presence of TMJ pain/tenderness to palpation and/or pain elicited during a load test and/or pain during jaw opening and closing; myofascial pain denotes the presence of pain in the jaw muscles in accordance with the RDC/TMD criteria Axis I; clench symptoms denotes elicited pain during a 30-s jaw clenching task; jaw muscle tenderness denotes presence of pain/tenderness to palpation at one or more sites over 16 jaw muscle sites; neck and jaw muscle tenderness denotes presence of pain/tenderness to palpation at one or more sites over 16 jaw muscle sites and one or more sites over six neck muscle sites.

co-morbidity between back pain and musculoskeletal disorders in the jaw-face as the total study population. However, the Odds ratios in the sub sample analyses were generally lower. Patients with back pain but without jaw pain reported frequent TMJ sounds (OR: 3.8, 95% CI: 1.8–7.7, P < 0.0001), difficulty in jaw opening (OR: 7.8, 95% CI: 1.5–39.7, P < 0.05) and tiredness in the jaws (OR: 7.3, 95% CI: 2.7–19.6, P < 0.0001) significantly more often than their controls. Patients with back pain but not jaw pain had lower maximal jaw opening capacity (P < 0.05) and lower ability to protrude the mandible (P < 0.0001) than their controls. TMJ sounds (OR: 2.2, 95% CI: 1.1–4.2, P < 0.05), pain/tenderness to palpation of the jaw muscles (OR: 4.5, 95% CI: 2.6– 8.0, P < 0.0001) and moderate to severe signs (Di II–Di III) of jaw dysfunction (OR: 3.6, 95% CI: 2.0–6.5, P < 0.0001) were recorded significantly more often among these cases than among their controls. 3.6. Symptoms and signs of dysfunction with regard to location of back pain An analysis based on the reported location of back pain among the cases (Table 1) in relation to presence of signs (Ai I–Ai II) and symptoms (Di II–Di III) of TMD showed similar odds ratio patterns regardless of location (Fig. 4). 3.7. Internal validity test In the internal validity test non-cases with frequent back pain (n = 37) had a higher prevalence of signs

and symptoms of TMD compared to subjects without back pain (n = 63). The odds ratio for any symptoms (Ai I–Ai II) of TMD was 3.6 (95% CI: 1.4–9.3). The OR for moderate to severe signs (Di II–Di III) of TMD was 1.9 (95% CI: 0.8–4.7). Compared to the true cases in this study (n = 96), the non-cases with back pain had a significantly lower degree of neck and low back pain intensity, and back pain had a significantly lower impact on their daily activities. 4. Discussion This study showed that patients with long-term pain in their spinal region were likely to present signs and symptoms of musculoskeletal disorders in the jawface. The null-hypothesis was thus rejected. This was, as far as we know, the first time that the presence of long-term spinal pain has been found to be strongly associated with both pain and dysfunction in the jaw-face region. The study results support previous observations of a higher probability among patients with long-term spinal pain to report pain at other sites. The presence of one pain condition has been shown to be associated with an increased risk of developing a new pain condition (Von Korff et al., 1993). Widespread pain has been a suggested risk factor for the onset of dysfunctional TMD pain among women, but not among men. The probability of reporting dysfunctional TMD pain at baseline was associated with the number of pain sites (other than in the temporomandibular region) in a dose–response relationship (John et al., 2003).

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Fig. 4. Odds ratios (OR) and 95% confidence intervals for Ai I–Ai II and Di II–Di III with regard to back pain location and compared to 192 controls.

The results of the present study mostly show significant associations between pains in different and fairly localized spinal regions and musculoskeletal disorders in the jaw-face. Two of the first researchers to study a possible relationship between upper spinal and temporomandibular joint (TMJ) dysfunction were Alanen and Kirveskari at the beginning of the 1980s (Alanen and Kirveskari, 1984). Based on the results from a sample of females, exposed and non-exposed, respectively, to cervicobrachial stress in their work, the authors hypothesized that TMJ dysfunction may predispose to symptoms in the upper-spine region. More recent case– control studies, based on rather small samples, have shown that patients with TMD, compared to individuals without TMD, have a significantly higher probability of reporting pain (Hagberg, 1991; Visscher et al., 2001), tenderness to palpation (Clark et al., 1987; De Laat et al., 1998; Stiesch-Scholz et al., 2003), and to exhibit dysfunction (De Laat et al., 1998) in the neck/shoulder region. In the studies by Carossa (Carossa et al., 1993) and by de Wijer et al. (1996a) the authors compared their results, based on samples of patients with cervical disorders, to previously reported population surveys. The divergent results between these two studies may be related to differences in examination methods and criteria for defining TMD in both the cases and in the population samples. Our study supports the findings that there is an increased prevalence of pain and dysfunction in the jaw-face region in patients with long-term pain in the upper part of the back, compared to control subjects. Our results also showed a significant relationship between long-term pain localized in the low back region and the presence of musculoskeletal disorders in the jawface. Based on these results it seems logical that patients

with TMD should also have a higher probability of reporting low back pain than people without TMD. This point is supported in a case–control study on TMD and on-going general musculoskeletal complaints (Hagberg, 1991). Since spinal pain (Sternbach, 1986; Ma¨kela et al., 1991; Andersson et al., 1993; Webb et al., 2003) and TMD (De Kanter et al., 1993; Wa¨nman, 1996; Carlsson, 1999) may be related to age and sex, these factors were included in the matching procedure. When collecting case histories with the aid of a questionnaire, the reliability has been shown to increase when symptoms reported as occurring once a week or more often are included (Wahlund, 2003). We, therefore, used this time frame in the present report and referred to it as a frequent symptom. The clinical examination followed a standardized routine procedure, comprising well-known variables. The examiner was always blinded to the answers from the questionnaires during the examinations, but since all cases were examined at Rygginstitutet, and the majority of the controls at their place of work, the examiner was not blinded to each subject’s affiliation. In order to investigate whether there was any examiner bias, a validity test was incorporated. This test showed the same pattern as for the case–control sample, despite the fact that the back pain in non-cases was not as severe as the back pain in cases, indicated by a significantly lower degree of reported pain intensity and impact on daily life activities. Hence, we are confident that the non-blinded procedure did not produce any major bias in the clinical part of the study. It may be questioned if the controls were representatives of an extremely healthy population. When merging cases and controls into one cross-sectional sample, as a simple

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test of the possibility of extreme values, prevalence of back pain (33%), symptoms in the jaw-face (37%) and clinical signs of dysfunction (65%) reached prevalence figures well in line with population based epidemiological studies. The grouping of back pain into different locations in this study was entirely based on the patients’ reporting in the questionnaire. Separate neck and shoulder pain seldom occurred in this sample, reflecting the view that the cervical spine, upper thoracic spine and shoulder girdle are considered to represent a functional unit (Mannheimer and Dunn, 1991). Cases with a recognized, generalized pain pattern, which is known to involve also the jaw-face region (Plesh et al., 1996; Aaron et al., 2000), were excluded from the original study population. We also tested the possibility of frequent pain in the jaw, as a confounder to associations between back pain and jaw-face dysfunction. Even after exclusion of subjects with jaw pain, signs of dysfunction of the jaw-face were significantly more common in subjects with back pain than in controls. The pathophysiology for these indicated sensorymotor disturbances is not clear. Experimentally induced pain outside the jaw can influence the motor function of the jaw system (Komiyama et al., 2005). Ericsson et al. have shown that functional jaw movements are the result of activation of jaw and neck muscles, leading to coordinated movements in the temporomandibular, atlanto-occipital, and cervical spine joints (Eriksson et al., 2000). Intersegmental reflexes from nociceptors in the temporomandibular region to the fusimotor system of neck muscles have been demonstrated in cat (Hellstro¨m et al., 2000). The authors suggest that these reflex connections could be involved in the spread of muscle stiffness and pain as well as motor control disturbances. Non-specific effects of central pain processing and sensitization may contribute to the co-morbidity between back pain and jaw-face pain. Thus, similar results may also be found in relation to other regional pain conditions. However, central sensitization does not explain why jaw dysfunction was more common in subjects with back pain but without jaw pain. In addition to the above-mentioned possible pathophysiological mechanisms, also biomechanical (Armijo Olivo et al., 2006) and psychological factors (Linton, 2000; Vlaeyen and Linton, 2000) may be involved in the registered disturbances. The link between back pain and jaw dysfunction shown in the present study indicates that these musculoskeletal disorders are much more closely related than hitherto recognized. This may have implications for the strategies in rehabilitation of patients with long-term musculoskeletal pain and dysfunction. In conclusion, we have demonstrated that long-term back pain was significantly associated with pain and dysfunction in the jaw-face. The results imply that back pain and musculoskeletal disorders in the jaw-face are not two

separate entities. The co-morbidity shown suggests a mutual influence, or that these conditions are caused by the same contributing factors. The study design does not allow us to draw any conclusions regarding the causation or temporal sequence between back pain and musculoskeletal disorders in the jaw-face. The mechanisms behind the co-morbidity warrant further research.

Acknowledgements The authors would like to thank the employees and management at Va¨sternorrland County Council, Emhart Glass, Metso Paper, Metso Panelboard and the Swedish Social Insurance Agency, Development Division, for participating in the study. This study was supported by grants from Va¨sternorrland County Council Dept. of Research and Development, the Swedish Dental Society and Stiftelsen Goljes Minne.

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