# Anatomical Variations of the Temporalis Muscle: A Magnetic Resonance Imaging–Based Study

## TL;DR

This study uses MRI scans to examine variations in the temporalis muscle, revealing differences in muscle layers and hypoplasia that could impact diagnosis and treatment of jaw-related disorders.

## Contribution

The study identifies new anatomical classifications of the temporalis muscle based on MRI data, highlighting sex-related differences in hypoplasia.

## Key findings

- Significant anatomical variability in the temporalis muscle was observed across participants.
- Hypoplasia in the posterior portion of the superficial layer was more common in males.
- Three distinct anatomical classifications of the muscle were identified based on layer presence and hypoplasia.

## Abstract

The temporalis muscle is a key masticatory muscle involved in jaw movement, specifically in the elevation and retraction of the mandible. This muscle has a complex anatomical structure, which includes both superficial and deep layers that are vital for its function. Variability in the muscle’s morphology can influence its efficiency in chewing and may play a role in several clinical conditions, such as temporomandibular joint disorders (TMDs) and bruxism. Understanding the anatomical variations in the temporalis muscle is essential for improving diagnostic accuracy and treatment approaches.

This study included 34 female participants (age: median 48.5 [36–64] years) and 24 male participants (age: median 47.5 [37–68] years). MRI scans were used to assess the anatomy of the temporalis muscle, focusing on the presence of both superficial and deep muscle layers. The presence of a longitudinal muscle core was also examined to identify anatomical variations. Cases were classified based on the presence of both layers and the extent of hypoplasia in the posterior portion of the superficial layer. Descriptive statistics are reported as median and the first and third quartiles (Q1–Q3), and group differences were assessed with nonparametric tests.

The study revealed that the temporalis muscle exhibited significant anatomical variability across participants. In six cases, only the deep layer was present, with symmetry in one female and one male. In 26 cases, the posterior portion of the superficial layer was hypoplastic, appearing as a thin muscular layer. Hypoplasia was significantly more common in males (p = 0.02168). Based on these observations, the muscle was classified into three groups: (1) muscles with both layers intact (72.4%), (2) muscles with both layers but with a hypoplastic posterior portion of the superficial layer (22.4%), and (3) muscles with only the deep layer (5.2%).

This study provides new insights into the anatomical variability of the temporalis muscle, highlighting differences in the superficial and deep layers and the prevalence of hypoplasia in the posterior portion of the superficial layer. These findings have important clinical implications, as they can improve radiological diagnostics and aid in the accurate identification of muscle variations. Understanding these variations enhances the ability to diagnose and treat temporomandibular disorders and related conditions more effectively, offering more personalized care for patients.