This study investigates whether repeated botulinum toxin type A (BoNT-A) injections used to manage sleep bruxism lead to structural changes-specifically intramuscular fat accumulation-in the masseter muscle. Adults with probable sleep bruxism receive BoNT-A injections every six months and undergo ultrasound evaluations before each injection. By tracking fat percentage in the masseter muscle over multiple treatment cycles, the study aims to determine whether long-term BoNT-A use causes progressive structural alterations or remains safe for repeated clinical use.

Botulinum toxin type A (BoNT-A) is commonly used for the management of sleep bruxism, yet concerns persist regarding its long-term effects on muscle structure, including the possibility of intramuscular fat accumulation in the masseter muscle. This prospective longitudinal study follows adults with probable sleep bruxism who receive bilateral BoNT-A injections at six-month intervals. Ultrasound imaging is performed immediately before each injection, resulting in four injection cycles and five standardized ultrasound assessments.

Intramuscular fat percentage is quantified from B-mode images at rest and during maximum voluntary contraction using binary segmentation techniques. By examining changes over repeated treatment cycles, the study evaluates whether BoNT-A leads to progressive fat infiltration or instead demonstrates a stable or adaptive pattern. Findings will help clarify the structural safety of long-term BoNT-A use in non-spastic masticatory muscles and guide clinical decision-making for ongoing bruxism management.

Inclusion Criteria:

• Adults aged 18 years or older.
• Clinical diagnosis of probable sleep bruxism according to the international consensus criteria proposed by Lobbezoo et al.
• Indication for BoNT-A injection in the masseter muscle due to bruxism-related pain or muscle hypertrophy.
• Willingness to undergo repeated BoNT-A injections at six-month intervals.
• Willingness to participate in ultrasonographic evaluations at each follow-up visit.

Exclusion Criteria:

• Presence of neurological or neuromuscular disorders affecting the masticatory muscles (e.g., dystonia, myasthenia gravis).
• History of orofacial trauma, temporomandibular joint dysfunction, or maxillofacial surgery.
• Systemic diseases that may affect neuromuscular transmission or muscle metabolism.
• Use of medications known to interfere with neuromuscular function, including aminoglycoside antibiotics or corticosteroids.

Due to ethical and confidentiality considerations, the dataset generated and/or analyzed during the current study is not publicly available. However, data may be made available from the corresponding author upon reasonable request and subject to institutional approval.

The radiologist who acquired the ultrasonographic data and the researcher who performed the cross-sectional fat infiltration analysis were completely blinded to each other's procedures.