Correlation between bruxism and cervical function

Background Temporomandibular joint has biomechanical and neurological interactions with cervical region in a complex functioning that is addressed as cranio‑cervical mandibular system. Subjects and methods In a prospective cohort study of 67 patients, their mean age 24.3 ± 5.6 years of both genders with temporomandibular disorder “bruxism,” patients had fulfilled a questionnaire regarding bruxism manifestations. Pain pressure algometer was used to assess pain pressure threshold for trigger point in masseter, temporalis, ster‑ nocleidomastoid, and trapezius, and bubble inclinometer was used to assess cervical proprioception for all cervical ranges. Results There was a statistical significant indirect weak correlation between proprioception error of cervical flexion and pain pressure threshold of masseter ( r = − 0.333) ( p = 0.006), between proprioception error of cervical flexion and pain pressure threshold of trapezius ( r = − 0.363) ( p = 0.003), and also between proprioception error of cervi‑ cal left‑side bending and pain pressure threshold of trapezius ( r − 0.298; p 0.014), while there was significant direct weak correlation between proprioception error of cervical left rotation and cervical bending to left ( r 0.315; p 0.009), where the p ‑value was < 0.05. Conclusion The study proved that temporomandibular joint pain was correlated with impaired neck proprioception in whom with bruxism.


Introduction
Temporomandibular disorders (TMDs) include painful temporomandibular joint (TMJ) dysfunction or masticatory musculatures.Masticatory system functional role includes both speaking and chewing, where its muscular hyperactivity "parafunctional" is required for teeth grinding and/or clenching, i.e., bruxism [1,2].Recently, many authors had stated that masticatory system pain has close interaction with cervical disorders and vice versa [3].
Bruxism is recognized as oral unconscious dysfunctional rhythmic habit of teeth clenching, grinding, or pressing either throughout mastication resulting in traumatic occlusion [4] and/or while thrusting or mandibular bracing, and double particular circadian features take place during daylight or nighttime [5].No doubt, bruxism aggravates pathological wear of tooth, periodontal lesions, and even TMD [6].This is multifactorial etiologies with critical value for underlying pathomechanics for genesis, i.e., pathophysiological (central), peripheral (morphological), and psychosocial factors.Current additional focus on various bruxism either wakefulness or nighttime forms those rarely differentiated academically due to almost examining together and also recognition under the same medical term [1,7].
According to the grading system, there are three states of bruxism: "possible bruxism" is the lowest diagnostic category, denoted by pain in the TMJ with maximal opening.The more advanced diagnosis category is "probable bruxism, " which combines contralateral laterotrusion and TMJ pain during maximal mouth opening.When there is silent TMJ arthritis, severe arthritis with disturbance in growth or damage to the structure but without pain that cannot be identified by clinical examination, it is called "definite bruxism, " the highest diagnostic level [11].
These are possible (based only on a self-report), probable (based on a clinical findings with or not a self-report), and definite (based on an instrumental examination, with or not a self-report and also a clinical finding) [12,13].
There is an extensive link between TMJ and cervical region via anatomical, biomechanical and neurophysiological relation.Biomechanical bases ensured that additional jaw either closure or opening requires extra flexed or extended neck with interaction between trigeminal and cervical motor patterns among healthy population; thus, no doubt any related restrictions lead to vice versa impacts [14].Neurophysiological explanations enhance mutual influences between cervical and jaw under upper cervical afferents' convergence, those addressed as neck sensory inputs (C1, C2, and C3) and trigeminal mandibular branch that is recognized as trigeminocervical complex (TCC).Due to TCC convergence central restrictions for interpreting specification of original source, so particular pathology at one area may affect the other [15,16].
There were many studies that reported other methods to assess TMJ pain and cervical proprioception, and Alstergren et al. reported using visual analog scale [VAS] and numerical rating scale to assess TMJ pain severity [11].Menabde et al. reported that the "UPAT" (Universal Pain Assessment Tool) was addressed as an investigating tool for ensuring restricted jaw function and pain among TMD individuals [17].Another study by Gucmen et al. demonstrated that commonest cervical positional error test is laser pointer for clarifying proprioceptive affection [18].As well, Reddy et al. reported that cervical range of motion (CROM) device validity for quantifying proprioceptive errors [19].
There was a gap in literature concerning relation between bruxism and cervical function by using pain pressure algometer for pressure pain threshold evaluation and bubble inclinometer for assessing cervical proprioceptive sense.Therefore, the present trial is attempted to examine bruxism impacts and its relation with cervical function.

Participants
Sixty-seven participants of both genders, diagnosed with bruxism, were recruited for current study from the outpatient clinic of health unit in 6th of October City, and IPC clinic, Giza, Egypt, through a period from December 2022 to April 2023.
Patients were enrolled according to bruxism questionnaire; their age range was 18-30 years old, because the temporomandibular pain occurs in about 10% of the population over than 18 years, and its prevalence is more in middle-aged adults [13,16], with BMI 18-25 kg/m 2 .Criteria for inclusion were clenching, grinding or associated sounds for bruxism, or even discomfort jaw musculatures or painful complains at ear front, jaw, or temple, and those influence motion or jaw functioning and pain in the temporalis or masseter muscles [20].Criteria for exclusion were traumatic facial history or undergone any orthodontic management, pulsatile tinnitus, or degenerative lesions, as well as TMJ systematic articular arthritis or osteoarthritis, and muscular lesions, and/ or neurologic disorder involving trigeminal neuralgia, in addition to any neck or head malignancies were excluded from this study [20,21].
G*Power software (3.0.10 version) was used for calculating size of current study sample via correlation bivariate normal model.The sample size was calculated using the G*Power software (version 3.0.10).Correlation Bivariate normal model was selected.Considering a power of 0.80, (one tail) and correlation p H1 0.3, one group, a generated sample size of 67.
Human participant clinical trial has complied with all national relevant rules and institutional procedures, and it has been approved by the Faculty of Physical Therapy Ethical Committee, Cairo University (No.: P.T.REC/012/003843).Registration of clinical trials is as follows: NCT05657353.Each participant has been assigned a consent form, initially.Also, measurements were done for once.The patients were assessed by using pain pressure algometer for pain pressure threshold and using bubble inclinometer for cervical proprioception.

Bruxism questionnaire
Questionnaires are being the most commonly used method because it is not expensive as other methods [22].Bolarina has stated its validity and reliability for clinical and academic purposes [23].For checking possible bruxism, initial participant awareness for bracing, clenching, or thrusting meaning is easily recognized by touching the teeth for non-swallowing target.After then, the patient is asked to keep an eye on their behavior for a week or two, therefore recorded participant response regarding bruxism [24].
Use two self-reporting questionnaires as follows: categorical participants' response "I do not know, No or Yes." For questionnaire 1, positive result bruxism for positive responded to first couple of questions or one of them with at least one manifestation enumerated by third question.For questionnaire 2, positive bruxism for double response of four questions [25].

Pain pressure algometer
It is used for the measurement of pressure pain threshold (PPT) for the trigger points of muscles [26], e.g., temporalis, masseter, sternocleidomastoid, and upper trapezius.Pressure pain thresholds were used to evaluate the mechanosensitivity of the orofacial structures, masticatory muscles, and cervical muscles [16].Knapstad et al. ensured the validity and reliability of algometer tool for pressure pain threshold in evaluating of muscular trigger points [27].
Well-supported participant sat on a chair with hands over knees' top, with forehead facing forward.During assessment, perpendicular algometer placement, plus instructed participant for immediate response for initial pain, felt to applied pressure.Three repetitions were conducted with a 30-s interval and then recorded the average [28].

Bubble inclinometer for cervical proprioception
It is used for examining cervical proprioceptive sense using both target head position (THP) and neutral head position (NHP) [29].Vafadar has reviewed that the inclinometer joint position sense (JPS) measurement method is reliable and valid [30].
The inclinometer was set at zero, and the participant sat on a chair with lower extremities bent at 90° and both feet on the ground with closed eyes for THP/NHP in flexion, extension, and bilateral side bending and then in supine for bilateral rotation THP.Looking straight ahead, the bubble inclinometer was strapped in place just above the ear for flexion/extension THP [31], for flexion/extension THP, on forehead center for bilateral side bending, and overhead vertex for bilateral rotation THR [32].
To evaluate THP, participants were checked couple of times for exact adopting of flexion and extension at 35°, bilateral lateral flexion at 30°, and bilateral rotation of 45° with clear orientation for neutral neck positioning in a passive maneuver.Then, each participant has to remember and adopt each position mentioned across active approach without instructions from the therapist.Three repetitions for THP test per each position were recorded, and the difference between each position's measured and correct values was in degrees [33].And sustained for 3 seconds, the subjects received neither verbal nor visual feedback Throughout the test [32].

Statistical analysis
Pearson correlation coefficient relation used for parametric data (cervical pain) and Spearman correlation coefficient were used for nonparametric data (proprioception) to determine the relation between variables in bruxer patients.SPASS Inc. version 20 for PC Statistical Package for Social Sciences computer program (Chicago, IL, USA) was used.P ≤ 0.05 was recognized as significant.

Demographic features
Whole 67 participants demographic analysis revealed their age mean value was 24.3 ± 5.6 years, weight mean value was 63.4 ± 7 kg, height mean value was 164.7 ± 7 cm, and body mass index (BMI) mean value was 23.3 ± 1.6 kg/ m 2 .Males number was 27 (40.3%),and females were 40 (59.7%) as shown in Table 1.

Normality test
Data were screened for normality assumption, homogeneity of variance, and presence of extreme scores.Shapiro-Wilk and Kolmogrov-smirnov tests for normality showed that PPT variable is normally distributed, while Proprioception error of cervical movements is not normally distributed.

Discussion
Current study focused on investigation of correlation between bruxism and cervical function by using pain pressure algometer for pressure pain threshold evaluation and bubble inclinometer for assessing cervical proprioceptive sense.
Anatomical, biomechanical, and neurophysiological evidence links the craniomandibular region and upper cervical spine; however, the neurophysiological relationship between both regions of the trigeminocervical nucleus may provide the most compelling explanation.The cervical region would become more sensitive to painful afferents from the temporomandibular region [16].So, bruxism can decrease the threshold for pain in the orofacial and the cervical musculature and produce pain in the masticatory and cervical muscles [7].
Pain due to bruxism may take place at masticatory musculatures, or in cervical region or craniofacial complex, where painful hyperactive musculatures are due to existence of trigger points with taut bands.In addition, painful low orofacial or neck musculatures with bruxism could be explained by localized hyperexcitability [7].
It has been postulated that abnormal cervical alignment may result in painful manifestations because of muscular imbalance within craniocervical area.It has also been postulated that the neck posture influences the mandibular position and both cervical musculatures and also masticator activities [34].
The increased magnitude of altered positional sense may be a negative pain impact based on painful chemical mediators; also, altered afferents form free neural endings, and those result in altered proprioceptive data.As well, many articles concluded that positive association between pain severity and elevated cervical proprioceptive sense errors [35], so pain, tissue damage, and degenerative joint conditions all depend on impaired proprioception [36].
Current trial findings agreed with [16] who evaluate neck impairment with TMD patients in relation to jaw impairment, craniocervical location, cervical alignment, and sensorimotor deficits; the findings demonstrated a correlation between cervical and mandibular disability in TMD patients, and [1] pointed out that increased tension in the masticatory muscles may be the cause of discomfort in the temporomandibular joints, cervical spine, and craniofacial region.
Also our study agreed with [37] which examined that a significant positive correlation between neck pain intensity and position sense and proved that extra painful complains was correlated with additional altered static cervical proprioceptive sense.Also, [38] found the relation which said that impaired cervical joint position sense among whom have painful neck complains.In addition, [36] had ensured that altered proprioceptive sense is a vital issue for persisted pain for degenerative or acute disorders.
But [37] mentioned that Lee et al. had investigated temporal pain aspects (frequency, intensity, and extend),

Conclusion
Bruxer subjects showed remarkable cervical joints' position errors in all tested directions.Hence, the current study revealed that elevated pain severity in bruxism was correlated to impaired proprioception and obvious cervical positional error in whom with bruxism.

Fig. 1
Fig. 1 Correlation between proprioception error of cervical flexion and PPT of M1

Fig. 2 Fig. 3
Fig. 2 Correlation between proprioception error of cervical flexion and PPT of

Fig. 4
Fig. 4 Correlation proprioception error of cervical left rotation and cervical left bending

Table 2
Spearman correlation between joint position error and pain pressure threshold in Bruxer

Cervical flexion Cervical extension Cervical right bending Cervical left bending Cervical right rotation Cervical left rotation PPT of T1 PPT of M1 PPT of M2 PPT of SM PPT of trapezius Error of flexion
correlating to sense of cervical position among subclinical individuals with cervical pain.Cervical pain severity did not reveal any consistent influence on cervical proprioceptive sense.Lee may not have consistently observed a relationship between each subject's pain severity and proprioceptive skill.