Investigating the Contraction Pattern of the Zygomaticus Major Muscle and its Clinical Relevance: A Functional MRI Study

Abstract

Background: Our understanding of facial anatomy has significantly evolved, yet the detailed contraction patterns of facial muscles and their presentation during clinical imaging remain largely unexplored. Understanding the contraction patterns and visual presentation of these muscles, particularly the zygomaticus major could enhance pre-surgical facial assessments and the development of new treatment strategies.

Methods: A total of 34 healthy young individuals (17 female, 17 male) with a mean age of 23.6 (2.4) years [range: 20-30] were investigated regarding the length, thickness, width, and angle of the zygomaticus major muscle in five different facial expressions (i.e., repose, anger, joy, surprise, and sadness) utilizing MR imaging.

Results: Joyful expressions caused a reduction in muscle length to 85.6% of its original length and an increase in width (103.4%), thickness (108.4%), and facial angle (2.72°) when compared to that in repose, suggesting isotonic contraction. Conversely, expressions of anger, surprise, and sadness generally led to muscle stretching, seen through changes in length (98.9%, 104.3%, and 102.7%, respectively), width (98.8%, 96.5%, and 99.4%, respectively), and thickness (91.2%, 91.0%, and 102.7%, respectively), with variable alterations in facial angle (0.55°, 1.85°, and 1.00°, respectively) depending on the specific expression.

Conclusion: This MRI-based study indicates that the zygomaticus major muscle experiences isotonic contraction, characterized by decreased length and increased width and thickness. The findings underline the importance of muscle thickness as a reliable parameter in assessing facial muscle function and offer valuable guidance for practitioners in accurately evaluating muscle performance during different facial expressions.

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Keywords: Contraction pattern; Facial anatomy; Facial muscle; Muscle physiology; Zygomaticus major muscle.

Fig. 1

Three-dimensional (3D) facial models illustrating the five facial expressions investigated in this study: repose, anger, joy, surprise, and sadness. Participants were trained on these virtual models before their MR scanning procedure. Images were sampled from the free web portal: www.unrealengine.com

Fig. 2

Three-dimensional (3D) facial reconstruction of one of the study participants. An extracted ZMa superimposed in the corresponding topography on the facial model B Representation of subcutaneous soft tissue structures C Color map of selected facial muscles. Red and white arrows represent the zygomaticus major muscle

Fig. 3

Representation of the multiplanar reconstruction from which the slice was highlighted and measured for the zygomaticus major muscle. The location and method of MRI slice acquisition are shown by the red rectangle superimposed on a facial model

Fig. 4

Representation of facial angle measurement methodology—the arms of the angle include the long axis of the face and the long axis of the zygomaticus major muscle

Fig. 5

A visual comparison of the zygomaticus major muscle contractility in various expressions of the face depending on several factors—muscle length, width, thickness, and facial angle. Green arrows represent an increase, while red arrows represent a decrease in value