Botox Therapy for Hypertrophy of the Masseter Muscle Causes a Compensatory Increase of Stiffness of Other Muscles of Masticatory Apparatus

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.

Botulinium toxin applications in the lower face and neck: A comprehensive review

“…Treatment of a wide lower face and masseteric hypertrophy includes BT injection into the masseteric muscle and more invasive methods such as bone resection. BNT injection has been highly effective for this purpose, which is proved in several studies using different methods such as ultrasound, 3‐dimensional scanning, computed tomography, photographs, and patient satisfaction 54,56–62 …”

“…In most cultures, a wide lower face is not desirable and people seek cosmetic interventions to decrease the volume of mandibular prominence as well as masseter muscle and subcutaneous tissue. Masseteric hypertrophy usually occurs between 20 and 40 years and is more common among the Asian population 13,52–55 . Treatment of a wide lower face and masseteric hypertrophy includes BT injection into the masseteric muscle and more invasive methods such as bone resection.…”

“…BNT injection has been highly effective for this purpose, which is proved in several studies using different methods such as ultrasound, 3-dimensional scanning, computed tomography, photographs, and patient satisfaction. 54,[56][57][58][59][60][61][62] Interestingly, this method has been associated with low rates of side effects and a desirable clinical outcome. Therefore, most patients desire to repeat this treatment.…”

Temporalis Muscle Changes Following Botulinum Toxin A Injections in Masseter Hypertrophy Patients: A Randomized Triple-Blinded Trial

“…The primary effect of BoNT-A on the masseter muscle is to diminish its size, potentially causing alterations in the function and/or size of other masticatory muscles, especially the temporalis muscle. Since the masseter and temporalis muscles work in conjunction during mandibular movements and function, changes in one can affect the other [5]. It may also generate a compensatory increase in function, increasing muscle volume and thickness, which may affect aesthetics and functionality [6,7].…”