The aim of this study was to evaluate the clinical course in terms of pain, function and quality of life of patients with TMD-related myalgia during six months after receiving BTX-A injections. The research question was: What are the outcomes in pain, quality of life and function of patients with TMD-related myalgia receiving BTX-A during 6 months? The hypothesis to be tested was that patients receiving a BTX-A injection due to TMD-related myalgia will have a reduction in pain and increase in quality of life and function after one, three- and six- months.

This is a prospective and observational cohort study. The Medical Research Ethics Committees United provided a non-WMO (Medical Research Involving Human Subject Acts) waiver (W23.212) prior to commencement of this study. All participants will be informed and most give their written consent prior to participating, according the requirements of the Declaration of Helsinki (World Medical Association, 2013).

Participants Participants in this study are adults with TMD-related myalgia according to the DC/TMD treated with a BTX-A injection. All participants are recruited at the department of Oral and Maxillofacial Surgery (OMS) of the Diakonessenhuis, Utrecht, The Netherlands.

Inclusion criteria for participation are:

• TMD-related myalgia in accordance with the DC/TMD;
• Accepted BTX-A injections as treatment;
• Age ≥ 18 years;
• Understanding of the Dutch language to fill in the questionnaires.

Exclusion criteria for participation are:

• Systemic inflammatory and connective tissue diseases (e.g., rheumatoid arthritis, ankylosing spondylitis, psoriatic arthritis);
• Surgery in the TMJ area < 12 months ago;
• Pain of dental origin;
• Limited cognitive functioning (not allowed to make medical decisions);
• Use of muscle relaxants, or aminoglycoside antibiotics;
• A history of allergic reactions to BTX;
• Pregnancy and lactation.

Study procedure Participants willing to enroll in the study will recieve an information letter. After receiving all information participants will have two weeks to consider participation. When willing to participate they can contact the researcher for an appointment to sign the informed consent form and check for the inclusion/exclusion criteria.

During the first visit, participants will fill in their demographics, health history, previous treatments concerning TMD-related myalgia and presentation of current symptoms on a standard form. The researcher will check the presence of a TMD-related myalgia according to the DC/TMD.

Prior to the BTX-A injection, participants rate their pain on a Visual Analogue Scale (VAS). Next, the Oral Health Impact Profile-14 (OHIP-14) and the Mandibular Function Impairment Questionnaire (MFIQ) were administered. Then, the researcher will measure the maximum active and passive mouth opening according to the Dutch translation of the DC/TMD.

BTX-A injections All participants will recieve a solution of BTX-A (Xeomin, Merz Pharma GmbH, Frankfurt am Main, Germany) in the masseter and temporalis muscles (total of 100 units reconstituted with 4cc unpreserved 0.9% sodium chloride). The injection will be provided by the same maxillofacial surgeon. The participants are asked to clench their teeth to determine the site of the most intense muscle activity in the masseter and temporal muscles with palpation. Intramuscular BTX-A injections are administered on both sides at the location of the most intense muscle activity of the masseter (80 units) and temporalis (20 units). This technique is similar to other studies using BTX-A for the masseter and temporal muscles (Von Lindern et al., 2003; Guarda-Nardini et al., 2012; De la Torre Canales et al., 2021)

Description of measuring instruments and questionnaires Visual Analogue Scale (VAS) The VAS will be used to measure pain intensity. The VAS consists of a 100 mm line, with two set points, 0 (indicating no pain) and 10 (indicating worst imaginable pain). The participants rate their current pain by placing a mark on the line. Data are classified as metric. The test-retest reliability is good to excellent, with intraclass correlation coefficients (ICC's) ranging from 0.71 to 0.99 (Kahl & Cleland, 2005). In comparison with the numeric pain rating scale the VAS has up to 0.95 convergent validity (Kahl & Cleland, 2005). A study by Emshoff et al. (2010) showed an estimated minimal clinical difference of 19,5 mm in chronic TMD-related pain.

Oral health-related quality of life evaluation (OHIP-14) The OHIP-14 questionnaire is used to assess oral health-related quality of life on seven subscales. Every subscale includes two items. The subscales are: functional limitation, physical pain, psychological discomfort, physical disability, psychological disability, social disability and social handicap. The answer options are on a 5-point Likert scale ranging from 0 (never) to 4 (very often). The total score ranges from 0 to 56 (metric), with a higher score indicating a more negative outcome. The test-retest reliability is moderate to high, with ICC's ranging from 0.78 to 0.90 (Van der Meulen et al., 2008). The smallest detectable difference (SDD) in dental disorders is measured at 5 points (Locker et al., 2004).

Mandibular Function Impairment Questionnaire (MFIQ) Mandibular function is assessed by the sum score of 17- item Dutch translation of the MFIQ (Kropmans et al., 1999). Each item is scored on a 5-point Likert scale with 0 indicating no difficulty and 4 being extremely difficult to perform a mandibular task, resulting in a total score ranging from 0 to 68 (metric). A higher score indicates higher levels of functional impairment. The test-retest reliability is moderate to high, with ICC's ranging from 0.69 to 0.96. The SDD is 10 points (Kropmans et al., 1999).

Maximal mouth opening Maximal mouth opening is measured at the distance between the incisal edges of the central incisors in millimeters. The measurement will be conducted with a metal ruler unassisted for the active mouth opening (AMO) as well as assisted for the passive mouth opening (PMO) according to the Dutch translation of the DC/TMD (Schiffman et al., 2014). Intra-rater reliability varied from 0.70 to 0.99. Interrater reliability varied from 0.90 to 1.00 (Walker et al., 2000). The mean of three measurements was used. The SDD in patients with painful restriction of the TMJ is 3 mm (Kropmans et al., 1999).

All outcome measures are assessed prior to the BTX injection (T0) and re-assessed at one (T1), three months (T2) and six months (T3) post-treatment by the same researcher. The researcher was external from the department of OMS and was not involved in any treatment session or treatment contact. All measurements are conducted without the presence of the healthcare practitioner providing the injections to avoid observer bias due to the patient-doctor relation. At all post treatment measurements, participants are asked if they had other treatments for their TMD-related myalgia and the exclusion criteria were reevaluated.

Inclusion Criteria:

• TMD-related myalgia in accordance with the DC/TMD;
• Accepted BTX-A injections as treatment;
• Age ≥ 18 years;
• Understanding of the Dutch language to fill in the questionnaires.

Exclusion Criteria:

• Systemic inflammatory and connective tissue diseases (e.g., rheumatoid arthritis, ankylosing spondylitis, psoriatic arthritis);
• Surgery in the TMJ area < 12 months ago;
• Pain of dental origin;
• Limited cognitive functioning (not allowed to make medical decisions);
• Use of muscle relaxants, or aminoglycoside antibiotics;
• A history of allergic reactions to BTX;
• Pregnancy and lactation.

Participants for this study were adults with TMD-related myalgia according to the DC/TMD treated with a BTX-A injection. All participants were recruited at the department of Oral and Maxillofacial Surgery (OMS) of the Diakonessenhuis, Utrecht, The Netherlands.