Morphological Observation of the Medial Pterygoid Muscle by the Superimposition of Images Obtained by Lateral Cephalogram and MRI

Objective

To observe the morphological relationship between the maxillofacial skeleton and medial pterygoid muscle by superimposing images constructed by MRI on a cephalogram.

Design

Cross-sectional study.

Setting

The Departments of Maxillofacial Orthognathics and Orthodontics, Tokyo Medical and Dental University.

Participants

Sixteen patients (5 males and 11 females, aged between 13.5 and 27.5 years) with various craniofacial skeletal patterns, who were about to start orthodontic treatment.

Methods

Lateral cephalometric radiographs and MRI scans were obtained and their images uploaded to a computer using a digitizer. The area of the medial pterygoid muscle was selected by binarization from the MRI. The mid-sagittal-plane MRI with a complete superimposed sagittal image of the medial pterygoid muscle was superimposed on the cephalogram using anatomical structures situated in the mid-sagittal plane of the head and shapes that could be identified from both the radiograph and the MRI image.

Results

These combined images showed various shapes of the medial pterygoid muscle. The inclination axis of the medial pterygoid muscle was correlated with various cephalometric variables including SNB (r=0.658), Facial angle (r=0.601), ramus inclination (r=0.676) and Ba-Po% (r=0.585).

The volume of the medial pterygoid muscle was also correlated with cephalometric variables such as ramus inclination (r=0.453), Ba-Nmm (r=0.676), Ba-Po% (depth) (r=0.447), Ar-Go% (depth) (r=0.444) and Ar-Go% (actual length) (r=0.532).

Conclusions

Morphometric analysis using a superimposed image of the medial pterygoid muscle produced from a cephalogram and MRI may help explain the influence of the medial pterygoid muscle inclination axis and volume on the shape of the mandibular bone, especially the shape of the ramus.