Hyperactivity of the lateral pterygoid muscle (LPM) is one of the presumed mechanisms of the onset of temporomandibular disorders (TMD) and explains why intramuscular injection of botulinum toxin (BT) may be indicated for treatment. Intramuscular injection without guidance is difficult because of its deep location. The objectives of this study were to determine the feasibility of MR-guided navigation for BT injection in the LPM in TMD and to report about our first results.

Six patients suffering from persistent myogenic TMD were enrolled in a prospective study and treated with intramuscular injection of BT A: 20 UI in each LPM with MR-guided navigation using the Brainlab plateform (Kolibri^®, Brainlab^®, Munich, Germany) the target being the center of the upper head, 30 UI in each masseter and 20 UI in each temporal with clinical guidance. The investigated outcomes were: pain intensity, maximum interincisal opening and joint sounds.

MR-guided navigation could be used in all patients and the target could constantly be reached. Pain improvement [mean reduction of 4.4 on a numeric scale (p = 0.0579)] was observed in 66.7% of the patients. Significant improvement of maximum interincisal opening was found (p = 0.0360) and joint sounds tend to disappear (p = 0.5594).

MR-guided navigation is an effective method for tracking the upper head of the LPM and allows for a precise injection of BT. Injection of BT in the upper head of the LPM, the masseter and the temporal muscles is effective in refractory TMD. A combination of this morphologic guidance with electromyographic tracking would allow to select more carefully the muscles to be injected.