Evaluation Of The Effect Of Low-Level Laser Therapy On Masseter Muscle Versus Therapeutic Ultrasound On Patients With Myofascial Pain Disorder : A Randomized Clinical Trial

Myofascial pain dysfunction (MPD) is one of the most common causes of chronic orofacial pain, typically involving the masticatory muscles such as the masseter and temporalis. It is characterized by localized muscle tenderness, trigger points, and functional limitation of jaw movements. The condition affects a significant percentage of adults and can lead to persistent discomfort and impaired quality of life.

The etiology of MPD is multifactorial, including parafunctional habits such as bruxism, psychological stress, trauma, malocclusion, and muscle overuse. Because of this complex etiology, management often requires a multimodal approach combining behavioral, pharmacological, and physical therapies.

1. Myofascial Pain and Physical Modalities

Conservative physical modalities are widely used as first-line treatments for MPD because they are non-invasive, safe, and easily applicable. These modalities include ultrasound therapy, low-level laser therapy, transcutaneous electrical nerve stimulation (TENS), and thermotherapy. Among these, therapeutic ultrasound (US) and low-level laser therapy (LLLT) have gained increasing attention for their effectiveness in pain reduction and tissue healing without pharmacological side effects.

1. Low-Level Laser Therapy (LLLT)

Low-level laser therapy, also referred to as photobiomodulation therapy, uses light energy at specific wavelengths to produce photochemical and photothermal effects in tissues. These effects lead to enhanced cellular metabolism, increased microcirculation, improved oxygen supply, and stimulation of ATP production, which collectively accelerate healing and reduce inflammation.

Several studies have explored the role of LLLT in treating myofascial pain dysfunction.

Azizi et al. (2007) evaluated the effect of LLLT (780 nm, 10 mW) applied to the masseter, temporalis, medial, and lateral pterygoid muscles. Each region received 1200 mJ of energy per session, with twelve sessions performed over four weeks. The study demonstrated a significant reduction in pain and muscle tenderness and an improvement in mandibular function. Importantly, these therapeutic effects persisted up to three months after the completion of treatment, indicating both short-term and long-term efficacy.

Other research supports these findings, confirming that LLLT promotes analgesia by modulating nerve conduction velocity, reducing bradykinin and histamine release, and stimulating mitochondrial cytochrome oxidase activity. The cumulative evidence suggests that LLLT provides a safe, effective, and sustainable method for managing myofascial pain.

1. Therapeutic Ultrasound (US)

Therapeutic ultrasound is a commonly used modality in physiotherapy and dentistry. It transmits acoustic energy through tissues, producing both thermal and mechanical effects. These effects enhance blood flow, increase cell membrane permeability, and promote tissue repair while reducing muscle spasm and pain.

Khairy (2018) conducted a randomized clinical study comparing phonophoresis and ultrasound therapy in myofascial pain dysfunction syndrome. The study applied pulsed ultrasound at 1.5 W/cm² for 5 minutes per session, over ten sessions spanning three weeks. The ultrasound head was moved in circular motions across the affected masseter muscle using coupling gel.

Results showed significant improvements in pain intensity (VAS), maximum mouth opening (MMO), and muscle tenderness compared with baseline values. This confirms the beneficial role of pulsed ultrasound in decreasing pain and improving masticatory muscle function.

The therapeutic benefits of ultrasound are attributed to its mechanical micro-massage effect, which enhances cellular exchange, reduces edema, and improves local metabolism. The pulsed mode, in particular, minimizes thermal buildup and focuses on non-thermal biostimulation, making it suitable for inflamed or tender tissues such as the masseter.

1. Comparison between LLLT and Ultrasound Therapy

Both LLLT and ultrasound therapy share common goals-reducing pain, decreasing muscle stiffness, and improving jaw function-but act through different biophysical mechanisms. LLLT works primarily through photochemical cellular activation, while ultrasound acts via mechanical micro-vibration and mild thermal stimulation.

Although both modalities have been proven individually effective, few studies have directly compared their outcomes in myofascial pain dysfunction.

Combining evidence from Azizi (2007) and Khairy (2018) indicates that both approaches are effective within short treatment durations (3-4 weeks) and produce measurable improvements in pain and muscle performance. However, the relative superiority of one modality over the other remains undetermined, justifying the need for a direct head-to-head comparison using standardized settings.