ABSTRACT
Little is known about the nature of masseter muscle hypertrophy. We investigated the masseter muscle stiffness change after a single intra-masseteric session of Botox injections in people with benign bilateral masseter hypertrophy and the effect of the treatment on the stiffness of the temporalis muscle. Stiffness of the muscles was measured with shear-wave elastography at baseline and 3 weeks after Botox injections in 22 otherwise healthy people. Before the treatment, the stiffness of the masseter was lower than of the temporalis muscle (10.18 ± 1.67 kPa vs. 11.59 ± 1.54 kPa; p = 0.002). After the treatment, this difference increased (6.38 ± 1.34 vs. 13.10 ± 1.92; p < 0.0001). The drop in the stiffness of the masseter muscle was symmetrical (left side by 3.78 kPa; right side by 3.83 kPa). No differences between the left and right sides of the face in terms of muscle stiffness were observed. The study shows that Botox injections reduce stiffness of the masticatory muscles which, in turn, increases the stiffness of the temporalis muscles. Due to the knock-on effect of the change in the masseter function on the other masticatory muscles, simultaneous evaluation and treatment of the temporalis muscle may be required to ensure the desired functional and cosmetic effect.
ABSTRACT
BACKGROUND: Masseter muscle pathologies include hypertrophy and the experience of pain, which clinically manifest with increased stiffness and tension. Assessment of muscle stiffness has been gaining importance among physicians dealing with temporomandibular disorders (TMD). Currently, shear wave elastography (SWE) is still often performed by radiologists, while dentists diagnose, treat and monitor TMD. OBJECTIVES: In this cohort study, we investigated whether dentists trained to use SWE can obtain reliable measurements of masseter muscle stiffness following participation in a short training program and hands-on workshop. MATERIAL AND METHODS: A group of healthy volunteers was examined by an experienced radiologist and a novice dentist before and after the training. RESULTS: The mean values of stiffness obtained by the operators were consistent and ranged from 10.20 kPa to 10.84 kPa. Intraobserver agreement was excellent for measurements of the radiologist (intraclass correlation coefficient (ICC) 0.92 and 0.93, respectively). The training improved the agreement between measurements made by the dentist from poor before the training (ICC = 0.46) to good after the training (ICC = 0.89). Also, the operator agreement between the radiologist and dentist increased from poor (ICC = 0.48) before the training to good (ICC = 0.84) after the training. CONCLUSIONS: The diagnostic accuracy of measuring masseter muscle stiffness was acceptable among dentists after the training. For this reason, the patient can be diagnosed by a single TMD specialist. This can shorten the diagnostic process and reduce treatment costs.
