Table of Contents
- Key Points
- Background: Why This Research Matters
- What Is Trismus and Why Does It Happen?
- How the Study Was Conducted
- The Laser Treatment Protocol in Detail
- How Researchers Measured Results
- Who Took Part in the Study
- Key Findings: What the Study Showed
- Mouth Opening Results
- Muscle Pain Results
- Quality of Life and Nutrition Results
- What This Means for Patients
- Limitations: What the Study Could Not Prove
- Recommendations for Patients
- Frequently Asked Questions
- Source Information
Key Points
- In a 46-patient trial, daily low-level laser therapy during head and neck radiotherapy reduced mouth opening loss compared with placebo.
- Laser-treated patients lost 0.15 mm of mouth opening on average versus 3.75 mm with placebo, a difference of 3.50 mm.
- The laser group had less masticatory muscle pain, and daily trismus scores showed no trismus (grade 0) in 87.3% of scores in the laser group versus 85.6% with placebo.
- No significant differences appeared in quality-of-life scores, dental health scores, or nutrition, and mouth opening was similar at six months.
- The trial was small, single-center, and used one specific laser protocol, so larger studies are needed to confirm the findings.
Background: Why This Research Matters
Radiotherapy (RT, the use of high-energy radiation to kill cancer cells) is a cornerstone of treatment for head and neck cancer (HNC). It saves lives, but it can cause lasting side effects. One of the most troublesome is trismus, a condition in which the jaw muscles stiffen and the patient can no longer open their mouth fully.
Trismus can affect up to 69% of head and neck cancer patients after cancer treatment. Radiation therapy is one of the main causes. This side effect is often underreported, yet it can begin during or after radiotherapy. It typically becomes noticeable within the first six months and may persist for up to ten years after treatment ends.
Photobiomodulation therapy (PBMT, also called low-level laser therapy) has been explored as a possible treatment for trismus. The therapy is believed to promote anti-inflammatory and analgesic (pain-relieving) effects, which may improve mouth opening. Earlier studies suggested that PBMT might help reduce pain and inflammation in the masticatory muscles (the muscles used for chewing) and the temporomandibular joint (TMJ, the jaw joint).
However, evidence about whether PBMT can prevent trismus during radiotherapy remained limited. That gap in knowledge is exactly what this study set out to fill.
What Is Trismus and Why Does It Happen?
Trismus refers to a restriction in mouth opening. It is most commonly defined as a maximum interincisal opening (the distance between the upper and lower front teeth) of less than 35 millimeters (mm).
The exact mechanism behind radiation-induced trismus is not fully understood. However, several studies suggest it is strongly associated with the formation of free radicals (unstable molecules that damage cells), tissue damage, and hypoxia (low oxygen levels in tissues). Several studies also suggest it is strongly associated with an increase in inflammatory cytokines and chemokines (signaling molecules that promote inflammation). Together, these processes cause muscle tissue damage and lead to fibrosis (scarring and stiffening) of the masticatory muscles.
Several factors increase the risk of developing trismus in head and neck cancer patients:
- Advanced age
- Tumor location close to the chewing muscles
- High radiation doses (greater than 40 Gy, where Gy stands for gray, a unit of radiation dose)
- Prior surgical procedures
- Use of concomitant chemotherapy and radiotherapy (CCP RT, meaning chemotherapy given at the same time as radiation)
Fibrosis caused by radiation significantly reduces how far the jaw can move. This impairs oral function and overall quality of life. In more severe cases, trismus can lead to serious complications, including malnutrition, aspiration pneumonia (lung infection caused by inhaling food or liquid), and an increased risk of death.
How the Study Was Conducted
This was a phase II, randomized, triple-blind, placebo-controlled clinical trial. "Triple-blind" means that the patients, the evaluators, and the statistician all did not know which treatment each patient received. This design is considered the gold standard for reducing bias in medical research.
The study was registered in the Brazilian clinical trials registry (www.ensaiosclinicos.gov.br) under protocol number RBR-67qxnt9. It was approved by the Ethics Committee of the Haroldo Juaçaba Hospital (HHJ) Cancer Institute of Ceará (ICC) under protocol 5.432.685. All ethical aspects of Resolution 466 of 2012 of the National Health Council/Ministry of Health were respected.
Who was included: Patients over the age of 18 with head and neck cancer in the nasopharynx, maxillae, oropharynx, hypopharynx, glottis, mouth, or salivary glands. Their cancer was at stages I, II, III, or IV. All patients had a mouth opening of 35 mm or greater and had been indicated for radiotherapy. Radiotherapy could be adjuvant (given after surgery), palliative (to relieve symptoms), or curative. It could be combined with chemotherapy, immunotherapy, or biological therapies.
Who was excluded: Patients with tumor recurrence, a previous history of radiotherapy to the head and neck region, tumor involvement in the masticatory muscles, or a radiotherapy dose below 40 Gy. Patients who withdrew from treatment, required a change in their therapeutic protocol, developed extreme toxicity, or died were also excluded from the study.
All patients were treated at the dental outpatient clinic in the radiotherapy department of the Haroldo Juaçaba Hospital, Ceará Cancer Institute.
Sample size calculation: Based on a previous study by Borges et al. (2023), researchers estimated that low-power laser photobiomodulation could increase daily mouth opening by 1.69 ± 2.15 mm in patients with trismus related to head and neck radiotherapy. Using 90% power and 95% confidence, they calculated that 19 patients per group would be needed. To account for possible dropouts, they added 20%, giving a target of 23 patients per study group.
Randomization and blinding: Simple randomization was performed by an independent collaborator using the "=RAND()" function in Microsoft Excel. Participants were allocated into two groups (Groups A and B). The randomization numbers were printed on sealed envelopes and opened only by the study's principal investigator at the time of treatment.
The principal investigator needed the help of two collaborators who were blinded to group assignment for clinical evaluation and questionnaire collection. The laser protocol was applied equally to both groups. However, in the control group, the principal investigator simulated the application by turning the device on and off immediately. This blinded the patients to their group assignment. The evaluator and the supporting statistician were also unaware of each patient's group. Therefore, only the principal investigator knew the group assignments, making the study triple-blind.
The Laser Treatment Protocol in Detail
A Therapy XT laser (DMC, São Carlos, SP, Brazil) was used. The device produced 100 milliwatts (mW) of continuous wavelength light output at 660 ± 10 nanometers (nm, red light) and 820 ± 10 nm (infrared light). The device has a tip with an area of 0.28 mm² (or 0.0028 cm²), which was kept in light contact with the treated area during applications.
Patients were treated daily from D0 (the day before the first radiotherapy session) until DF (the last day of radiotherapy).
The photobiomodulation therapy protocol followed the recommendations of the Multinational Association for Supportive Care in Cancer and the International Society of Oral Oncology (MASCC/ISOO). These recommendations concern supportive care in head and neck cancer patients. Laser parameters, including wavelength, power, energy density, and application time, were defined based on the guidelines of the World Association for Photobiomodulation Therapy (WALT).
The protocol was conducted according to the predefined energy density and application points described by Borges et al. The specific settings were:
- Wavelength: Infrared laser at approximately 808 nm
- Power: 0.1 watts (W)
- Energy: 3 joules (J) per point
- Time: 30 seconds per point (107 J/cm²)
- Total extraoral application time: 270 seconds, with 24 J of energy per side
- Total intraoral application time: 30 seconds, with 3 J of energy per side
- Total application points: 18 (applied bilaterally, meaning on both sides)
Extraoral application points (applied outside the mouth, close to the temporomandibular joint):
- Point A: Superior to the mandibular condyle (the jaw joint bone)
- Point B: Posterior to the mandibular condyle
- Point C: Anterior to the mandibular condyle
- Point D: Applied intra-auricularly (inside the ear) toward the tragus, with the patient asked to open their mouth
Masticatory muscle application points:
- Point E: Upper masseter muscle
- Point F: Middle masseter muscle
- Point G: Lower masseter muscle
- Point H: Anterior portion of the temporal muscle
Intraoral application point:
- Point I: Medial pterygoid muscle, using the infrared laser at approximately 808 nm, 0.1 W of power, 3 J of energy, 30 seconds (107 J/cm²) per point
How Researchers Measured Results
Trismus and pain evaluation: The interincisal distance between the upper and lower central incisors was measured daily in millimeters using a caliper. This measured maximum mouth opening and the presence of trismus. On the same days, patients were asked about their perception of pain in the masticatory muscles using the Visual Analog Scale (VAS). The VAS ranges from 0 to 10, where zero corresponds to no pain and 10 corresponds to maximum pain.
The CTCAE v5.0 adverse event criteria scale was used to classify the degree of trismus clinically:
- Grade 1: Decreased range of motion of the masticatory muscles with adequate oral intake
- Grade 2: Decreased range of motion requiring small bites, soft foods, or purees
- Grade 3: Decreased range of motion with inability to eat adequately or hydrate orally
These assessments were also carried out every month up to 6 months after the end of radiotherapy treatment.
Clinical and sociodemographic data: The Electronic Patient Record (EPP) was evaluated to collect clinical and pathological data. This included age, tumor location, co-morbidities (other health conditions), drug use, psychological support, and nutritional guidance. Tumor characteristics were also collected, including pathological classification of tumor-nodule-metastasis (pTNM), histological type, and immunohistochemical analysis for p16 (a protein marker). Sociodemographic data included race, education level, occupation, geographic origin, place of birth, and family income.
Patients were weighed on a conventional scale, and their weight was divided by their height squared to calculate their body mass index (BMI = mass / height²).
Quality of life analysis: The OHIP-14 questionnaire was administered on day D0 (start of radiotherapy) and day DF (last radiotherapy session). The OHIP-14 is a subjective indicator that measures disability, discomfort, and handicap attributed to oral conditions through self-assessment. It consists of 14 questions and is a reduced version of the OHIP-49. Answers range from 1 (never), 2 (rarely), 3 (sometimes), 4 (repeatedly), to 5 (always).
The OHIP-14 is divided into seven domains:
- Functional limitation (questions 1 and 2)
- Physical pain (questions 3 and 4)
- Psychological discomfort (questions 5 and 6)
- Physical limitation (questions 7 and 8)
- Psychological limitation (questions 9 and 10)
- Social limitation (questions 11 and 12)
- Disability (questions 13 and 14)
The seven domains comprise the overall quality of life scale, ranging from 14 to 70.
Food intake analysis: Food intake was analyzed at the beginning of radiotherapy (D0) and on the last day of radiotherapy (DF). Patients answered the Portuguese version of the patient-generated subjective global assessment (PGSGA). The ASGP is an inventory developed to assess the nutritional status of cancer patients, previously validated in Brazilian Portuguese. It is composed of two blocks: one containing questions for the patient and the other containing assessments to be made by a health professional. The scale allows simple summations to obtain a nutritional status score. As it is a subjective scale, it was applied by the same professional to reduce observation bias.
Statistical analysis: Categorical data were expressed as absolute and percentage frequencies and analyzed using Fisher's exact or Pearson's chi-square tests. Quantitative data were expressed as mean and standard deviation, subjected to the Kolmogorov-Smirnov normality test, and compared using Student's t-tests (for parametric data) or Mann-Whitney tests (for non-parametric data). Linear regression analysis was used to evaluate the variation in mouth opening and pain during mouth opening and on palpation of the masticatory muscles and TMJ during radiotherapy cycles. Cronbach's alpha coefficient was used to analyze the internal consistency of the OHIP-14 quality of life questionnaire. All analyses were conducted using SPSS software, version 20.0 for Windows, with a 95% confidence level.
Who Took Part in the Study
A total of 46 patients were included and divided equally into two groups: the PBMT group (23 patients) and the placebo group (23 patients).
Most participants were male (n = 19, 82.6% in each group). The mean age did not differ significantly between the groups (p = 0.421). The majority of patients self-identified as brown (PBMT: n = 17, 73.9%; placebo: n = 15, 65.2%; p = 0.522) and had incomplete primary education (PBMT: n = 15, 65.2%; placebo: n = 9, 39.1%; p = 0.133). Most participants reported a family income of up to one minimum wage (PBMT: n = 20, 87.0%; placebo: n = 21, 91.3%; p = 0.636).
The most frequent clinical stages were T3 and T4 in both groups, with no significant difference between them (PBMT: T3 n = 7, 30.4%, T4 n = 8, 34.8%; placebo: T3 n = 9, 39.1%, T4 n = 6, 26.1%; p = 0.911). Regarding lymph node status, N0 disease was more frequent in the placebo group (n = 14, 60.9%) than in the PBMT group (n = 9, 39.1%), although this difference was not statistically significant (p = 0.068).
Most patients received radiotherapy doses of 69 Gy or higher, delivered in 31 to 35 fractions. No significant differences were observed between the groups in terms of total radiotherapy dose (p = 0.459), treatment technique (IMRT vs. VMAT; p = 0.768), treatment interruptions, or radiotherapy-related adverse events (p = 0.437).
IMRT stands for Intensity-Modulated Radiotherapy, and VMAT stands for Volumetric Modulated Arc Therapy. Both are advanced techniques that shape the radiation beam to match the tumor's shape more precisely.
The mean duration of radiotherapy was significantly shorter in the PBMT group (47.70 ± 9.00 days) than in the placebo group (52.39 ± 8.91 days; p = 0.041).
Key Findings: What the Study Showed
Patients in the PBMT group showed a significantly smaller reduction in mouth opening during radiotherapy compared to the placebo group (p = 0.005). They also had a lower prevalence of masticatory muscle pain and grade 1 trismus (p = 0.002).
Regarding trismus severity assessed by CTCAE between days 1 and 35, higher-grade scores (grades 1 and 2) were more frequently observed in the placebo group (11.1% and 3.4%, respectively). Higher-grade scores were observed less frequently in the PBMT group (7.0% and 5.7%, respectively; p = 0.002).
The frequency of CTCAE grade 0 trismus (meaning no trismus) was 1.31 times higher in the PBMT group (95% CI: 1.06–1.62). In plain language, patients receiving PBMT were about 31% more likely to have no trismus at all during treatment.
No significant differences were observed between the groups regarding the decayed, missing, and filled teeth index (DMFT). No significant differences were observed regarding the Oral Health Impact Profile-14 (OHIP-14) or the Anderson Symptom and Overall Performance scores (p > 0.05).
Mouth opening did not differ between the groups at the six-month follow-up.
Mouth Opening Results
The mean initial mouth opening of patients in the PBMT group was 42.89 ± 6.10 mm, while in the placebo group, it was 46.17 ± 7.28 mm (p = 0.104) on the first day (D1). There were no statistically significant differences in mouth opening between the groups on any of the days evaluated.
However, both groups showed a slight reduction in mouth opening over the days. This reduction had a significant negative correlation with time that was stronger in the placebo group (p < 0.001, r = -0.909) than in the PBMT group (p < 0.001, r = -0.777). A correlation coefficient closer to -1 indicates a stronger relationship between time and mouth opening loss.
The mean change in mouth opening from the start to the end of radiotherapy was significantly smaller in the PBMT group (-0.15 ± 4.51 mm; p = 0.005) compared to the placebo group (-3.75 ± 3.69 mm). The difference between the two groups was 3.50 ± 1.21 mm.
Only one patient in the PBMT group showed a reduction in mouth opening. In contrast, six patients in the placebo group had negative results in terms of mouth opening reduction.
During the first seven days of follow-up (D1–D7), patients in both groups reported no pain (0.00 ± 0.00). From D8 onward, mean pain scores began to fluctuate.
Muscle Pain Results
PBMT resulted in a reduction of pain on specific days in the masseter muscles when compared to the placebo group. For the right masseter, PBMT reduced pain scores on D7 (p = 0.039). For the left masseter, no significant differences were found on specific days. However, the average scores in the PBMT group remained consistently lower throughout the evaluation days compared to the placebo group.
In the right masseter, both groups showed an increase in pain scores. In the PBMT group, this increase was 9.5% (p = 0.584), with values ranging from 0.00 ± 0.00 to 0.22 ± 0.74. In the placebo group, the increase was 46.6% (p = 0.005), with scores ranging from 0.00 ± 0.00 to 0.78 ± 1.68.
For the left masseter, there was a reduction of -13.5% (p = 0.439) in the PBMT group, ranging from 0.26 ± 1.25 to 0.22 ± 0.74. In the placebo group, there was a significant increase in pain of 49.3% (p = 0.003), with scores ranging from 0.35 ± 1.67 to 0.74 ± 1.68.
There was no significant reduction in pain scores on specific days in the right and left temporal muscles. The mean daily reduction in pain scores on palpation of the right temporal muscle in the PBMT group was not statistically significant.
Quality of Life and Nutrition Results
No statistically significant differences were found between the PBMT and placebo groups at any evaluation time point for the OHIP-14 quality of life questionnaire. This means that the laser treatment did not significantly change patients' self-reported oral health-related quality of life during the study period.
Similarly, no significant differences were observed between the groups regarding the decayed, missing, and filled teeth index (DMFT). No significant differences were observed regarding the Anderson Symptom and Overall Performance scores or the food intake analysis using the PGSGA (p > 0.05 for all comparisons).
These findings suggest that PBMT helped preserve mouth opening and reduce muscle pain. However, PBMT did not produce measurable improvements in overall quality of life or nutritional status within the study timeframe.
What This Means for Patients
This study provides strong evidence that photobiomodulation therapy can help prevent trismus in head and neck cancer patients undergoing radiotherapy. The treatment was applied daily during radiotherapy and was well tolerated.
The key clinical implications are:
- Patients receiving PBMT lost only 0.15 mm of mouth opening on average, compared to 3.75 mm in the placebo group. This difference of 3.50 mm could mean the difference between eating normally and needing soft foods or purees.
- Patients receiving PBMT were 1.31 times more likely to have no trismus at all during treatment.
- PBMT reduced pain in the masseter muscles, particularly on the right side.
- The treatment did not significantly improve quality of life scores or nutritional status during the study period.
For patients facing radiotherapy for head and neck cancer, these findings suggest that asking their care team about photobiomodulation therapy may be worthwhile. The treatment is non-invasive, involves no medication, and was applied in this study as a preventive measure starting before the first radiotherapy session.
It is important to note that mouth opening did not differ between the groups at the six-month follow-up. This means the protective effect of PBMT was most apparent during the active radiotherapy period. The long-term benefits may require ongoing treatment or additional strategies.
Limitations: What the Study Could Not Prove
This study has several limitations that patients and clinicians should consider:
- Small sample size: The study included 46 patients (23 per group). While this was based on a statistical calculation, larger studies are needed to confirm the findings.
- Six-month follow-up only: Trismus can persist for up to ten years after radiotherapy. This study could not determine whether PBMT provides protection beyond six months.
- No quality of life improvement: Despite preserving mouth opening, PBMT did not significantly improve OHIP-14 quality of life scores. This may be because the questionnaire captures many aspects of oral health beyond mouth opening.
- Single center: All patients were treated at one hospital in Brazil. Results may differ in other populations or treatment settings.
- Specific laser protocol: The results apply to the specific laser parameters used in this study (808 nm, 0.1 W, 3 J per point, 18 points). Other protocols may produce different results.
- Radiotherapy duration difference: The PBMT group had a significantly shorter mean radiotherapy duration (47.70 days vs. 52.39 days). This difference could have influenced the results, although the authors did not specifically analyze this factor.
The study also could not determine the optimal timing, frequency, or duration of PBMT for preventing trismus. These questions remain for future research.
Recommendations for Patients
Based on this study's findings, patients undergoing radiotherapy for head and neck cancer may consider the following:
- Ask your care team about photobiomodulation therapy. This study showed that PBMT, started before the first radiotherapy session and applied daily, helped preserve mouth opening and reduce muscle pain.
- Monitor your mouth opening. The study measured mouth opening daily using a caliper. Patients can ask their care team to measure their interincisal distance regularly to detect trismus early.
- Report muscle pain. If you experience pain in your jaw muscles or difficulty opening your mouth, tell your care team. The study found that PBMT reduced pain in the masseter muscles.
- Continue follow-up. The study found that mouth opening did not differ between groups at six months. This means that even with preventive treatment, regular follow-up is important to monitor for trismus.
- Discuss nutritional support. Trismus can lead to malnutrition. The study included nutritional guidance for most patients (95.7% in the PBMT group and 91.3% in the placebo group). Ask your care team about nutritional support during and after treatment.
It is important to remember that this is one study, and its findings need to be confirmed by larger trials. However, the results are promising and suggest that photobiomodulation therapy is a non-invasive option that may help prevent trismus during head and neck cancer radiotherapy.
Frequently Asked Questions
What is trismus and why does it happen after head and neck cancer radiotherapy?
Trismus is restricted mouth opening, usually defined as less than 35 mm between the upper and lower front teeth. Radiation can damage the chewing muscles through free radicals, low oxygen, and inflammation, leading to fibrosis or stiffening. It can affect up to 69% of head and neck cancer patients after treatment and may begin within six months.
Who was eligible to take part in the trial of laser therapy for trismus?
Patients over 18 with head and neck cancer in the nasopharynx, maxillae, oropharynx, hypopharynx, glottis, mouth, or salivary glands, at stages I to IV, were included. They needed a mouth opening of 35 mm or more and a radiotherapy plan. People with recurrent tumors, previous head and neck radiotherapy, or tumor in the chewing muscles were excluded.
How was the laser treatment given in the trial?
Patients received daily low-level laser sessions from the day before radiotherapy until the last day. An infrared laser at about 808 nm, 0.1 watts, and 3 joules per point was applied for 30 seconds at 18 points on both sides, including near the jaw joint and on chewing muscles. Each session took about 270 seconds outside the mouth and 30 seconds inside.
What did the trial find about mouth opening during radiotherapy?
In this trial of 46 patients, those receiving laser therapy lost an average of 0.15 mm of mouth opening during radiotherapy, compared with 3.75 mm in the placebo group. The difference was 3.50 mm. Patients in the laser group were about 31% more likely to have no trismus at all during treatment.
Did the laser treatment reduce jaw muscle pain?
Yes, in this trial the laser group had less masticatory muscle pain. For the right masseter muscle, pain scores were lower on day 7. The placebo group showed significant pain increases in both masseter muscles over the radiotherapy period, while the laser group did not. No significant differences were found for the temporal muscles.
Were there any side effects or risks from the laser therapy?
The trial reported that the laser treatment was well tolerated, and no significant differences in radiotherapy-related adverse events appeared between the laser and placebo groups. The therapy is non-invasive and involves no medication. However, the study did not specifically report on side effects of the laser itself, so patients should discuss any concerns with their care team.
Did the benefits last after radiotherapy ended?
In this trial, mouth opening did not differ between the laser and placebo groups at the six-month follow-up. The protective effect was most apparent during the active radiotherapy period. The authors noted that trismus can persist for up to ten years, and longer follow-up is needed to know whether ongoing laser treatment or other strategies help.
I'm starting radiotherapy for head and neck cancer — when should I get a second opinion about adding low-level laser therapy to prevent trismus?
A second opinion can be useful before radiotherapy begins, since photobiomodulation therapy in this trial started the day before the first session and was applied daily until the last one. Patients receiving it lost about 0.15 mm of mouth opening versus 3.75 mm with placebo, and were 1.31 times more likely to have no trismus during treatment. Mouth opening did not differ between groups at six months, and the laser settings used were specific (808 nm, 0.1 W, 3 J per point, 18 points). Diagnostic Detectives Network provides independent expert second opinions.
Source Information
Original article title: Effects of photobiomodulation on trismus in head and neck cancer patients after radiotherapy: a prospective, randomized, triple-blind, placebo-controlled clinical trial.
Authors: Marcela Maria Fontes Borges Franco, André Alves Crispim, Gabriella Alves Julião Costa, Giulianna Barreto Aparecida, José Fernando Bastos Moura, Cássia Emanuella Nóbrega Malta, Mário Rogério Lima Mota, Paulo Goberlânio Barros Silva.
Affiliations: Federal University of Ceará, Fortaleza, Brazil; Haroldo Juaçaba Hospital, Ceará Cancer Institute, Fortaleza, Brazil; Unichristus, Fortaleza, Brazil.
Publication: Lasers in Medical Science (2026) 41:56. DOI: 10.1007/s10103-026-04834-3.
Clinical Trial Registration: RBR-67qxnt9 (registered on 16/04/2025).
Received: 24 April 2025. Accepted: 6 February 2026.
This patient-friendly article is based on peer-reviewed research. It is intended for educational purposes and should not replace consultation with a qualified healthcare professional. Patients should discuss any treatment decisions with their care team.
