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RESEARCH REPORTS |
1 Department of Odontology, Clinical Oral Physiology, Umeå University, S-901 87 Umeå, Sweden; and
2 Centre for Musculoskeletal Research, Gävle University, Sweden;
* corresponding author, per-olof.eriksson{at}odont.umu.se
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: chewing • endurance • head • neck • whiplash injury
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Jaw movements are the result of coordinated activation of jaw and neck muscles, leading to simultaneous movements in the temporomandibular, atlanto-occipital, and cervical spine joints (Eriksson et al., 2000). Thus, jaw function involves integrative jaw and neck motor control. Given that natural jaw function requires a healthy state of both mandibular and neck motor systems, injury to any of the joints involved might derange jaw function. In fact, we have recently shown an association between neck injury and deranged jaw function, as reflected by reduced amplitude, speed, and disturbed coordination of head and mandibular movements (Häggman-Henrikson et al., 2002; Eriksson et al., 2004).
No previous study has evaluated if neck injury can impair endurance during chewing. The aim of the present study was to test the hypothesis of a relationship between neck injury and impaired endurance during chewing. A chewing test was designed to evaluate the endurance of the jaw system in individuals with whiplash-associated disorders (WAD) and, for comparison, in patients with pain and dysfunction and musculoskeletal disorders (i.e., temporomandibular disorders [TMD]) in the jaw-face, and in healthy subjects.
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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For the TMD group, the inclusion criterion was jaw-face pain and dysfunction of muscular origin, Group 1, Axis I, according to Research Diagnostic Criteria (Dworkin and LeResche, 1992). Exclusion criteria were temporomandibular joint disorders and/or history of trauma to the head-neck regions.
The WAD patients were primarily diagnosed by a physician. Typically, after a neck injury, they had developed neck pain, impaired and painful head-neck movements, and tenderness to palpation in neck muscles. The traumas—consisting of motor vehicle accidents (29 females, 11 males) and falls (eight females, two males)—had resulted in a WAD grade II-III according to the Quebec Task Force classification (Spitzer et al., 1995). In this classification, grades 0 and I denote no neck pain, and neck pain but no signs, respectively. Grades II and III denote neck pain with musculoskeletal signs, and neurological signs, respectively, and grade IV denotes neck pain and fracture. For the WAD group, the inclusion criterion was pain and dysfunction in the jaw-face that had developed following the neck injury. The duration between trauma and examination of jaw function was from 1 to 9 yrs (median, 4 yrs) for females, and from 2 to 4 yrs (median, 3 yrs) for males. The exclusion criterion was reported jaw-face pain and dysfunction prior to the head-neck injury (Table 1
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Test Procedure
The subjects were comfortably seated in an upright position, with back support up to the mid-scapular level, but without a headrest. They performed unilateral chewing of a standardized bolus (3 x 1 g) of chewing gum (V6®, Stimorol AB, Malmö, Sweden) for 5 min on their preferred chewing side. Subjects were free to discontinue the task at any time if fatigue/pain was intolerable, and this information was incorporated into the data analysis. The subjects were instructed not to talk during the chewing test and to point to the words "fatigue", "stiffness", "weakness", "exhaustion", "ache", and "pain", on a clipboard, if any of these symptoms was experienced. The time point during chewing at which a subject chose a symptom was at the discretion of the individual and was duly noted. The words "fatigue", "exhaustion", "stiffness", and "weakness" were pooled as "fatigue", and "pain" and "ache" were pooled as "pain".
Statistics
For continuous data, we used mean, range, and standard deviation for descriptive statistics. The hypothesis of no differences between groups in (i) fatigue, pain, and endurance and (ii) time for onset of symptoms and failure points was tested by means of (i) the 2x2 chi-square test, and (ii) the Mann-Whitney U test, with a probability level of 0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Fatigue and Pain
More subjects in the WAD group reported fatigue and pain compared with both the healthy and TMD groups. Furthermore, these WAD subjects reported an earlier onset of fatigue compared with both healthy (p < 0.001) and TMD subjects (p < 0.001), and an earlier onset of pain compared with TMD subjects (p = 0.034) (Fig.).
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). In addition to discontinuing the task more often than the TMD and healthy groups, the WAD subjects who failed did so at an earlier point in the task than did the TMD subjects who discontinued (p = 0.014) (Fig.
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).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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The fact that all healthy subjects completed the task, and that very few reported fatigue or pain, is in line with previous reports of no symptoms (Farella et al., 2001), or low frequency of fatigue and pain (Bakke et al., 1996), during the chewing of soft gum. These observations—that the jaw system generally seems to be resistant to fatigue during dynamic loading—are in accord with the fact that jaw-closing muscles are composed of a high proportion of fatigue-resistant (Type I) fibers (Eriksson and Thornell, 1983) and a high density of capillaries (Stål et al., 1996). Moreover, our finding that the TMD patients reported fatigue and pain more often than did healthy subjects corroborates previous reports that fatigue and pain are common symptoms in TMD, and that chewing can increase these symptoms (Dao et al., 1994; Gavish et al., 2002).
It has been reported that almost 25% of TMD patients have a history of trauma to the head-neck, mainly whiplash trauma (De Boever and Keersmaekers, 1996). Compared with TMD patients without a history of trauma, patients with post-traumatic TMD seem to present with more severe jaw-facial pain and dysfunction (De Boever and Keersmaekers, 1996; Goldberg et al., 1996; Kolbinson et al., 1997a), and it has been suggested that the prognosis for recovery from jaw-face pain and dysfunction is lower in this group (Kolbinson et al., 1997b). Furthermore, it has been reported that post-traumatic TMD patients have more symptoms associated with affective disorders, e.g., sleep disturbances (Kolbinson et al., 1997a), respond more poorly in reaction time tests, and overall tire more easily (Goldberg et al., 1996) than do non-traumatic TMD patients. The fact that many of these symptoms are also associated with closed-head injuries indicates that the etiology of post-traumatic TMD differs from that of non-traumatic TMD. Our results corroborate the suggestion that patients with jaw-face pain and dysfunction and with a history of neck injury are unique and more complex with regard to the spread and severity of pain and dysfunction (Goldberg et al., 1996).
It is well-known that females outnumber males in pain clinics. The patients in this study were examined consecutively, and the predominance of females represents the sex difference for patients referred to our department during the study period. All healthy subjects completed the chewing task without age or sex differences for reported symptoms and endurance. However, in the TMD group, women reported fatigue more often than men, and women in the WAD group reported an earlier onset of both fatigue and pain. These results are consistent with indications that, compared with males, females seem to respond earlier to nociceptive input and have a lower pain threshold (Plesh et al., 1998; Wise et al., 2002) and lower pain tolerance (Bragdon et al., 2002; Wise et al., 2002). They also seem to report more pain after tooth-clenching (Plesh et al., 1998) and chewing (Karibe et al., 2003). Our present finding of a reduced endurance indicates a lower functional capacity of the jaw motor system in females, which might contribute to a higher susceptibility for developing more and more severe pain, exhaustion, and dysfunction in the jaw motor system. No conclusive explanation for these sex differences has as yet been reported, either for TMD or for musculoskeletal disorders in other body regions. Thus, many persistent pain conditions have a higher prevalence in females (Unruh, 1996), but the neurobiological mechanisms underlying sex differences are unknown.
Recent studies have proposed that the fusimotor muscle spindle system plays an important role in the development of musculoskeletal pain conditions (Johansson et al., 1999). It has also been shown that reflex connections between chemosensitive muscle afferents and the fusimotor system exist intersegmentally, i.e., between the trigeminal (masseter muscle) and the cervical regions (Hellström et al., 2000). Furthermore, there is support for intersegmental nociceptive connections between the cervical spine and the trigeminal regions (Hu et al., 1993). Analysis of these data, taken together, suggests a tight coupling between the jaw and the neck sensory-motor systems in the onset and spread of pain and dysfunction in the jaw and head-neck regions. Our present results are consistent with these experimental data.
In conclusion, the present finding of a severely reduced endurance during chewing in WAD individuals suggests an association between neck injury and impaired functional capacity of the human jaw motor system. Based on present and previous results, examination of jaw function seems recommendable as part of the routine evaluation of WAD patients, and for this, the endurance test described in this study could be a useful tool. Finally, from data suggesting that jaw function involves simultaneous neuro-muscular activation of movements in the temporomandibular, atlanto-occipital, and cervical spine joints (Eriksson et al., 2000), and that neck injury can disturb natural jaw function (Häggman-Henrikson et al., 2002; Eriksson et al., 2004), we propose that a suitable term for the condition involving both neck and jaw disorders could be "Cervico-Cranio Mandibular Disorders" (CCMD).
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ACKNOWLEDGMENTS |
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Received May 13, 2003; Last revision September 16, 2004; Accepted September 23, 2004
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  H. Sale and A. Isberg Delayed temporomandibular joint pain and dysfunction induced by whiplash trauma: A controlled prospective study J Am Dent Assoc, August 1, 2007; 138(8): 1084 - 1091. [Abstract] [Full Text] [PDF]
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