Do Temporomandibular Disorder Patients with Joint Pain Exhibit Forward Head Posture? A Cephalometric Study

Purpose To evaluate head and cervical posture in individuals with or without temporomandibular disorders (TMDs) and to assess the correlations between pain, severity of symptoms, and posture. Methods A total of 384 patients (129 males and 255 females) was included. The Fonseca Anamnestic Index (FAI) was used to assess the severity and prevalence of TMD and the presence of temporomandibular joint (TMJ) pain. Patients were divided into three groups: the TMD-free group, TMD without TMJ pain group, and TMD with TMJ pain group. Subsequently, the patients with TMJ pain were further divided into mild TMD and moderate/severe TMD groups. Nine parameters were traced on cephalograms to characterize the head and cervical posture. Results TMD patients with TMJ pain showed increased forward head posture (FHP) than patients without TMJ pain and TMD-free subjects. No significant difference was observed between the TMD patients without TMJ pain and TMD-free subjects. In the TMD patients with the TMJ pain group, the moderate/severe TMD patients demonstrated increased FHP compared to mild TMD patients. TMD patients with joint pain had greater CVT/RL (B = 3.099), OPT/RL (B = 2.117), and NSL/C2' (B = 4.646) than the patients without joint pain after adjusting for confounding variables (P < 0.05). Conclusion TMD patients with TMJ pain showed increased FHP compared to other groups, and FHP became more significant as TMD severity increased in male patients, indicating the FHP might play an important role in the development of TMJ pain. In the clinical assessment of TMD, the patients' abnormal head and cervical posture might be considered.


Introduction
Temporomandibular disorders (TMDs), with a reporting prevalence between 28% and 88%, are common in young people aged 20 to 40 years and in female [1]. TMD is a comprehensive disease involving joints, muscles, and nerves, which is mainly characterized by the restriction, deviation, and defection of mandibular movement, clicking sound of temporomandibular joint (TMJ), and pain in masticatory muscles, anterior ear, and the TMJ region [2][3][4]. TMD might be caused by a variety of factors, including trauma, mental pressure, mandibular dysfunction, and malocclusion [5]. Among those, abnormal head and cervical posture has been reported to be associated with TMDs. Studies found that TMD patients had a higher cervical anteversion and an increased craniocervical angle [6]. Signifcant increases in craniovertebral, odontoid plane, and individual vertebral angles were also noted in TMD patients [7], while others did not identify association between head and cervical posture and TMD [8,9]. Terefore, the relationship between the head and cervical posture and TMD remains controversial and needs further investigation.
Based on the biopsychosocial model, diagnostic criteria for temporomandibular disorders (DC/TMD) enable researchers to understand the relationship between physical and psychological factors and the development of TMD cases, which is a commonly used method for the diagnosis of TMD at present [10]. However, the use of DC/TMD is timeconsuming and impractical in TMD fast screening. Currently, a number of simplifed screening questionnaires for TMD have been proposed, and one of them is the Fonseca Anamnestic Index (FAI). FAI composed of only 10 questions has been widely used in many studies for its convenience and ease of use, which can quickly screen TMD patients in a large population [11,12]. Te FAI scale has been shown to be consistent with other tools for diagnosing TMD like Helkimo Index and the American Academy of Orofacial Pain questionnaire [13]. Zhang et al. [14] used DC/TMD as a criterion measure, and the Chinese version of the FAI has demonstrated good sensitivity (0.959) and specifcity (0.719) and can be used to screen for TMD in Chinese population. In addition, FAI could compensate for the inability of DC/ TMD to quantify TMD severity, allowing the researchers to evaluate patients based on the severity of TMD symptoms [11,14].
Although TMD-related pain is not a life-threatening problem, it can afect oral health-related quality of life, and symptoms might transform into a chronic state and be difcult to control [15]. Like many other chronic pain syndromes, the biological mechanism of pain caused by TMD needs to be further studied. Using the FAI scale to diagnose TMD and identify pain in the TMJ area, our previous study has found that patients with TMJ pain had specifc craniofacial features [16]. Nevertheless, the investigation about the relationship between head and cervical posture and TMJ pain is limited. Te purpose of this study is to evaluate the head and cervical posture in patients with or without TMD, and furtherly explore the relationships between TMJ pain, severity of TMD, and the head and cervical posture. Te null hypothesis is that there is no signifcant diference in the head and cervical posture between the TMD patients with and without TMJ pain.

Subjects.
Tis study was conducted in accordance with the Helsinki Declaration and approved by the Ethics Committee of the West China Hospital of Stomatology in Sichuan University (Approval no. WCHSIRB-D-2021-431). After obtaining oral informed consent before the procedure, the participants provided written informed consent when flling the questionnaires. Parents or legal guardians of juvenile patients provided oral informed consent and written informed consent as well.
Patients visiting the Department of Orthodontics in the West China Hospital of Stomatology in Sichuan University from August to December in 2021 were consecutively included in this study. Te patients were requested to fll out a questionnaire including demographic information and the FAI scale and were asked orally if they had a clinical history related to the exclusion criteria. Ten, the lateral cephalograms of the patients performed at the Department of Medical Imaging in our hospital were collected. Te inclusion criteria were as follows: (1) patients participating in this study voluntarily; (2) patients visiting our department for the frst time; (3) patients flling out the questionnaire clearly; (4) patients aged 12 years or above; and (5) patients with clear cephalogram and in natural head position. Te exclusion criteria were as follows: (1) patients with history of orthodontic or orthognathic treatment; (2) patients with head and neck trauma or tumour; (3) patients with congenital deformities of the head and the neck, such as cleft lip; (4) patients with systemic diseases, such as rheumatoid arthritis; (5) patients with severe dental, periodontal, and oral mucosal diseases; (6) patients with psychological disorders; (7) patients with the history of TMD treatment; (8) patients with the history of treatment for cervical; (9) patients with primary headaches; and (10) patients with severe malocclusion and craniofacial abnormalities.

TMD and TMJ Pain Assessment.
Te FAI scale was used to assess the presence and severity of TMD in each patient [14,17]. As described in our previous study [16], the total score of 10 questions refected the presence and severity of TMD. Patients with a score of 0-15 were considered TMD-free patients, while patients with a score of 20 or higher were considered TMD patients. TMD patients could be further categorized based on TMD severity: a score of 20-40 � mild TMD, a score of 45-65 � moderate TMD, and a score of 70-100 � severe TMD [18].
Te TMJ pain assessment was performed according to our previously reported method [16]. For TMD patients, if the answer of item 6 in FAI "Do you have ear pain or pain in the TMJ area?" was "sometimes" or "yes," the patient was considered to be a TMD patient with TMJ pain. If the question was answered with a "no," the patient was considered to be a TMD patient without TMJ pain.

Cephalometric Analysis.
Te lateral cephalograms of the patients were performed at the Department of Medical Imaging. Te patients were required to maintain the natural head position with the mandible in the maximum intercuspal position, remain still, and not to swallow [19]. Te cephalograms were collected from the database in the Department of Medical Imaging. Uceph software (version780, Yacent, Chengdu, Sichuan, China) was used for cephalometric analysis. Figure 1 and Supplement Table 1 show the cephalometric reference points and lines used in this study. Table 1 shows the 9 head and cervical posture parameters, including 2 linear measurements and 7 angular measurements [9,20,21].
Two blinded researchers performed the cephalometric analysis, and the intra-and interobserver reliability on cephalogram tracing was tested as previous described [16,22]. No statistical diferences between the two measurements of each researcher and between the measurements of two researchers were observed, and all intraclass correlation coefcients were >0.80 [23,24].

Statistical Analysis.
Te sample size was computed by using G * power (version 3.1.9, Germany). Kang [25] reported that the mean value for OPT/CVT in patients with painful TMD was −12.8°± 1.3°compared with −11.7°± 1.2°i n the control group. Based on the previous cross-sectional studies, the prevalence of TMD was reported to be about 50% among the orthodontic patients [16], and we assumed that half of these patients have painful TMD. Keeping the power of the study as 90% and α as 0.05, the estimated minimum sample size was 108 subjects (54 in TMD-free group, 27 in TMD without TMJ pain group, and 27 in TMD with TMJ pain group) for this study. Considering that it may be benefcial to analyse the genders separately, a total of 384 subjects were included in this study.
All statistical analysis was conducted using IBM SPSS Statistics (version 20.0, IBM Corp, Armonk, NY, USA). P < 0.05 was considered to have a statistically signifcant diference. Te quantitative data were expressed as mean and standard deviation, and the qualitative data were expressed by quantity and frequency. Te Shapiro-Wilk test was used to judge the normality of data distribution. In order to compare the quantitative data of the patients in each group, independent sample t-test and one-way analysis of variance (ANOVA) were used when the data showed a normal distribution, and the Student-Newman-Keuls hoc test was used after ANOVA. Te Mann-Whitney U-test or the Kruskal-Wallis H-test was used when the data did not show a normal distribution. In order to compare the qualitative data of the patients in each group, a chi-squared test was used. Spearman correlation analysis was used to correlate the FAI score and head and cervical posture parameters. Correlations were interpreted as follows: weak correlation, r < 0. 30 and moderate or strong correlation, r >0.30. Also, the results of correlation analysis were visualized by scatterplots.
Multivariate linear regression was used to explore the correlation between TMJ pain in TMD patients and head and cervical posture parameters. Te independent variable was TMJ pain, with "TMD patients without TMJ pain" as the reference group. Te dependent variables were all head and cervical posture parameters. Te other covariables including gender, age, educational level, residence, and family per capita monthly income were adjusted in the regression model.

Results
A total of 384 patients were included in this study, including 169 TMD-free patients (44.01%), 147 TMD patients without TMJ pain (38.28%), and 68 TMD patients with TMJ pain (17.71%). In terms of demographic characteristics, no signifcant diference in gender distribution was noted among the three groups (P � 0.265). TMD patients without TMJ pain were signifcantly older than TMD-free patients, with no diference in the age between the TMD patients with TMJ pain and the other groups (P < 0.001). Concerning the educational level, the proportion of senior high school or lower in the TMD patients with TMJ pain was signifcantly higher than that in the other two groups (P < 0.001). No signifcant diference in residence and family per capita monthly income was observed among the three groups (P > 0.05). In the FAI survey, the score of FAI in the TMD group with TMJ pain was signifcantly higher than that in the other two groups (P < 0.001), and the proportion of moderate/severe TMD patients was signifcantly higher than that in the TMD group without TMJ pain (P < 0.001) ( Table 2).
In all the male patients, the CVT/FH of TMD patients with TMJ pain was signifcantly larger than those of the TMD patients without TMJ pain and TMD-free patients (P � 0.033). In all the female patients, the CVT/RL and NSL/ C2' of TMD patients with TMJ pain were signifcantly larger than those of the other two groups (P < 0.05) ( Table 3).
In adult population, no signifcant diference in the gender distribution among the three groups was noticed (P � 0.276). In adult male patients, the CVT/RL, OPT/RL, and NSL/C2' of the TMD patients with TMJ pain were signifcantly larger than those of the other two groups (P < 0.05). In adult female patients, the CVT/RL and NSL/ C2' of the TMD patients with TMJ pain were signifcantly larger than those of the other two groups (P < 0.05) ( Table 4). Meanwhile, no signifcant diferences in head and neck posture parameters were observed in the minor population (P > 0.05) (Supplement Table 2).
In the TMD patients with TMJ pain, stratifed analysis based on the severity of TMD was performed (Table 5). No signifcant diference in gender distribution between mild TMD patients and moderate/severe TMD patients with TMJ pain existed (P � 0.209). Among male, the craniocervical angle of moderate/severe TMD patients with TMJ pain was signifcantly smaller than that of the mild TMD patients, and the CVT/NSL, CVT/RL, OPT/NSL, and OPT/RL were signifcantly larger than those of the mild TMD patients (P < 0.05). However, no signifcant diferences in head and neck posture parameters were observed between the two groups in female (P > 0.05).
In the overall study sample, the FAI score was weakly and positively correlated with CVT/NSL (r � 0.162, P � 0.018), CVT/RL (r � 0.208, P � 0.002), and NSL/C2' (r � 0.233, P � 0.001). For male and female TMD patients, the FAI score was also weakly and positively correlated with several head and cervical posture parameters (P < 0.05) ( Table 6). Te correlations between the FAI score and CVT/ NSL, CVT/RL, and NSL/C2' in overall patients were demonstrated using scatter plots ( Figure 2).

Discussion
Te main fnding of this study was that there were signifcant diferences in head and cervical posture between the TMD patients with and without TMJ pain, and the patients with TMJ pain have a signifcant trend of forward head posture (FHP). In the multivariate linear regression analysis, after adjustment for potential confounders, the increased in CVT/ RL, OPT/RL, and NSL/C2' were independently associated with the TMJ pain risk. Tus, the null hypothesis was rejected.
Te CVT/FH of male patients with TMJ pain and the CVT/RL and NSL/C2' of female patients with TMJ pain were signifcantly larger than those of the patients in other groups. Cephalometric results showed that the TMD patients with TMJ pain of both genders had a forward inclination of the upper segment of the cervical column and cervical hyperfexion. We then analysed head and cervical posture in adult patients. Adult TMD patients with TMJ pain for both genders also showed increased FHP when compared to TMD-free patients and TMD patients without TMJ pain.    Notes: Te chi-squared test, one-way analysis of variance, and the Kruskal-Wallis H-test were used. In the post hoc test column, numbers "1", "2," and "3" represent TMD-free, TMD without TMJ pain, and TMD with TMJ pain groups of the subjects, respectively, * P < 0.05. TMD, temporomandibular disorders; TMJ, temporomandibular joint. Notes: Te chi-squared test, one-way analysis of variance, and the Kruskal-Wallis H-test were used. In the post hoc test column, numbers "1", "2," and "3" represent TMD-free, TMD without TMJ pain and TMD with TMJ pain groups of the subjects, respectively, * P < 0.05. TMD, temporomandibular disorders; TMJ, temporomandibular joint.

Pain Research and Management
After adjusting for potential confounding factors, increased in CVT/RL, OPT/RL, and NSL/C2' remained signifcant risk factors for TMJ pain in the TMD patients. It demonstrated that the TMD patients with TMJ pain exhibited more pronounced FHP. Also, the increased in the length of the upper segment of the cervical column (C2ap-C4ip) might also be associated with the possibility of presenting TMJ pain. Te signifcance was not statistical, but it was very close to the P � 0.05 threshold. In this study, the TMD patients with TMJ pain showed a signifcant FHP compared to the TMD-free patients and the TMD patients without TMJ pain. It is worth noting that there was no signifcant diference in head and cervical posture between the latter two groups. Terefore, a hypothesis was developed that a close relationship may exist between the FHP and the specifc type of TMD with TMJ pain.
Previous studies have found that abnormal head and cervical posture was signifcantly associated with head and neck pain [26,27]. Tis might be attributed to the complex anatomical structure and muscle connection between the functional unit formed by the cranium, mandible, and cervical spine, i.e., the craniocervical-mandibular system [28,29]. From an anatomical point of view, when the FHP occurs, the postural misalignment could lengthen the anterior cervical muscles as well as shorten the posterior cervical muscles, and the increased pressure is exerted on the cervical facet joints and neck muscles [30,31]. At the same time, a greater electromyographic activity in the temporal and masseter muscles could be observed in the TMD subjects [32,33]. Increased pain sensitivity in the neck muscles and masticatory muscles in patients with TMD has also been detected [34][35][36]. Researchers reported that changes in cervical curvature could infuence the muscle tension of the neck, subsequently afect the mandible movement and the muscle function in the TMJ area and eventually induce TMD [37,38]. As for TMJ pain and headache related to TMD, they were also found to be associated with the dysfunction of the masticatory muscles [39,40]. In conclusion, there is a tight relationship among head and cervical posture, muscle function, and TMD-related pain. It is very likely that FHP Table 5: Head and cervical posture parameters of mild TMD patients with TMJ pain and moderate/severe TMD patients with TMJ pain.   Head and cervical  posture parameters   Mild TMD with TMJ  pain   Moderate/severe TMD with  TMJ    Similarly, the FHP was found in patients with back pain, migraine, and chronic headache. Researchers also suggested that excessive stretch of muscles, soft tissues, and capsular ligaments beyond biological limitations from FHP may reduce the threshold of pain sensation in nerve endings and irritate proprioceptors in the joint capsules, which may contribute to these pain symptoms [26,[41][42][43][44].
Meanwhile, a severe FHP might not simply be a matter of the presence or absence of TMJ pain. We then further explored the relationship between TMD severity and head and cervical posture in the TMD patients with TMJ pain. Limited by the sample size, we had to combine patients with moderate and severe TMD into one group for statistical analysis, which inevitably overlooked some detailed information. We observed that the FHP appeared to become more signifcant in male patients with moderate/ severe TMD than patients with mild TMD. Similarly, TMD severity in the TMD patients was weakly positively correlated with FHP in correlation analysis for both males and females. Tis fnding suggested a possible "dose effect" could exist between the FHP and the signs and symptoms of TMD, and this requires further investigation based on a large sample size. Few studies have focused on the relationship between the head and cervical posture and TMD severity or the frequency of TMD-related symptoms, and the fndings observed in this study in the TMD patients may provide some useful insights for subsequent research studies.
TMJ region pain and ear pain are common complaints in TMD patients [45]. To the best of our knowledge, there are few studies evaluating head and cervical posture in the TMD patients with TMJ pain. In this study, the fndings of the specifc head and cervical posture in these patients could contribute to our understanding of the underlying mechanisms of the development of TMD-related TMJ pain. So far, noninvasive, minimally invasive, and invasive treatments for TMJ pain usually focus on local pain conditions [46], while less attention has been given to the role of the head and cervical spine as a functional unit on the development of joint pain. In the clinical treatment for TMJ pain in the TMD patients, the efect of abnormal head and cervical posture might be considered. Although the previous studies have found that cognitive behavioral treatment [47,48] and posture training [49,50] can signifcantly relieve TMD symptoms including pain, treatments vary greatly among these studies, and the role of psychological factors and conventional treatments cannot be adequately adjusted [51]. Tis study identifed organic changes in the head and neck posture in the TMD patients with joint pain, which provided the basic evidence of physiotherapy for pain-related TMD. In addition, the examination and intervention of abnormal head and neck postures are rarely included in the routine clinical practice. When treating TMD patients with main complaints of pain, it  is necessary for physicians to pay attention to the relative position of their head and shoulders and to provide necessary explicit reminders and suggestions for posture training.
Tere are some limitations in this study. First, this study was a cross-sectional study and cannot establish a causal relationship between TMJ pain and head and cervical posture, and longitudinal investigations are necessary in the future. Second, this study only stratifed patients based on the presence or absence of TMJ pain and did not quantify pain duration and pain intensity. Tird, this study did not strictly distinguish between adults and adolescents. Adolescents at diferent growth stages may have specifc cervical spine morphologies and head and neck posture characteristics due to their growth potential. Since this study included only a small number of adolescent TMD patients, the analysis of adolescent subjects alone may not meet the sample size requirement. It would be meaningful to increase the sample size of adolescents and analyse diferences in their head and neck postures in further studies. Fourth, this study only used FAI to assess the severity and prevalence of TMD in patients without further verifcation of the diagnosis, which may result in grouping error and mislead the fnal conclusion. Te use of DC/TMD could perform an accurate diagnosis of TMD and more detailed clinical subtypes of TMD patients in the followup studies.

Conclusion
TMD patients with TMJ pain showed abnormal head and cervical posture and increased FHP compared with TMDfree patients and TMD patients without TMJ pain. In male patients experiencing TMJ pain, the FHP appeared to become more signifcant in moderate/severe TMD patients compared to mild TMD patients. Te FHP may play an important role in the development of TMJ pain in TMD. In the clinical treatment for TMD-related TMJ pain, the efect of abnormal head and cervical posture might be considered. Longitudinal investigations are necessary to defne the causal relationship between the FHP and TMJ pain in the TMD patients.

Data Availability
Te data used to support the fndings of this study are available from the corresponding author upon request.

Conflicts of Interest
Te authors declare that there are no conficts of interest.

Supplementary Materials
Supplement Table 1: cephalometric reference points and lines used in this study and their defnition. Supplement Table 2: head and cervical posture parameters of TMD-free  patients, TMD patients without TMJ pain, and TMD