Exploratory Analysis of Upper Facial Muscle Interplay During Emotional Expressions: Magnetic Resonance Imaging (MRI) Insights from Young, Caucasian, Toxin-naïve Individuals

Abstract Background Understanding the interplay of muscle activity in the upper face is crucial as it can significantly impact the effectiveness and safety of aesthetic treatments. Traditional injection algorithms typically focus on the general 2D and 3D anatomy of muscles, often neglecting the areas where muscles exert the greatest force during facial expressions. Objectives To analyze the location of greatest morphological change in upper facial muscles procerus muscle (PM), corrugator supercilia muscle (CSM), orbicularis oculi muscle (OOM) and frontalis muscle (FM) during various facial expressions. Methods A total of 34 healthy young individuals (17 females, 17 males), with a mean age of 23.6±2.4 years [range 20–30], were examined using MRI to assess the length, thickness and width of upper facial muscles (PM, CSM, OOM, FM) for five different facial expressions: repose, anger, joy, surprise, and sadness. Results Facial muscle thickness is a key indicator of activity during expressions like anger, joy, surprise, and sadness. During anger, the PM and CSM decreased in length and width but increased in thickness, while FM passively contracted to stabilize the expression. The OOM showed increased thickness in its medial, inferior, lateral, and superior portions during various expressions, with specific regions activating differently depending on the expression, such as the medial and lateral parts during surprise and the inferior and lateral parts during joy. The medial third of the CSM was the most active region during contraction. Conclusions Upper facial muscles – regardless of them agonists or antagonists - act together during facial expressions to stabilize facial expressions, emphasizing the need to assess both groups in neuromodulator treatments. The medial third of the corrugator supercillii shows the most significant MRI changes, making it the primary target for injections.