Masticatory Muscle Function: a Multichannel Electromyographic Investigation

The aim of this electromyographic study was to obtain insight in the normal function of the lateral pterygoid muscle and its relation to other masticatory muscles in test persons with a normal masticatory system.
The lateral pterygoid muscle consists of two heads: a superior head and a larger inferior head. The muscle is located in front of the ear and behind the zygomatic arch. The inferior head attaches to the mandibular condylar head and the superior head partially inserts into the joint capsule and the articular disc too.
Until the early sixties both heads of the muscle were supposed to act synchronously. Bilateral synchronous contraction was supposed to assist in opening of the mouth and protrusion of the mandible: unilateral contraction would result in lateral movement of the mandible to the contralateral side. Since that time, however, evidence developed that the two heads worked independently. The superior head seemed to be active during closing and the inferior head mainly during opening and lateral movements. Not all research projects, however, could support this concept.
Insight in the functions of the lateral pterygoid muscle is not only of interest from a functional anatomic point of view, but seems also to be of clinical importance. This muscle is supposed to be involved in symptoms of temporomandibular dysfunction. Pain, clicking in the joint, deviations, deflections and some limitations of mandibular movement are attributed to dysfunction of this muscle.
In electromyographic research two different techniques are used. Surface electrodes are used on muscles, wich are situated closely under the skin. The technique is painless and gives an impression of the activity of the whole muscle or a large part of it. muscle. Independently functioning parts of a muscle, but also activity of nearby muscles can be recorded at the same time and may disturb the reliability (cross-talk).
In the second technique, the intramuscular technique, recording electrodes are situated within a muscle. This technique gives information on a limited part of a muscle. The method is mainly used in muscles not accessible for surface electrodes, for example the lateral and medial pterygoid muscles. The introduction of the electrodes (almost to the end insulated fine wires) is painful. After a period of five minutes, however, they are not noticed anymore. The presence of the wires limits the maximal excursions of the mandible to a certain extent.
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Another disadvantage is that the occurrence of an hematoma cannot be excluded. All participants were informed about the techniques to be used and probable risks; all persons gave their consent.
The interpretation and analysis of the analogue signal (raw-EMG) is of a limited value for static contractions and dynamic contractions, for example occurring during movements and chewing.
In order to gain more insight in the muscular behavior, the analogue signals were digitized and fed into a computer.
Integrated electromyograms (IEMG) were made of static contractions. Mean muscle activities (IEMG) could be calculated for each muscle during mean cycles of movements and chewing.
A second problem of the use of two different types of electrodes (large and small areas of recorded activity) is that in the superficial technique the higher frequencies are lost due to filtering of the skin and its underlying structures (impedance).
From our study of the literature was learned that electromyographic research of the muscles of mastication may lead to heterogeneous results. An explanation of these differences could be found in the selection of the test persons. The groups in the literature consisted of patients with symptoms of TMD, young persons with orthodontic problems and groups with participants with not confirmed differences in occlusion and articulation of the dentition.
This information lead us to three other investigations in the following order:
1. The development of an aiming device and a method to insert the intramuscular electrodes in the two separate heads of the lateral pterygoid muscle (Part II, Chapter 9.4)
2. The selection of a group of volunteers with optimal occlusal conditions and optimal health of the stomatognathic system.
3. A comparative electromyographic study of the masticatory muscles synchronously recorded by the two different types of electrodes (Part I). After the actual experiments the taped raw EMG signals had to be transformed into IEMG values. The most efficient sample frequency had to be found to collect a representative amount of the recorded frequencies of the signals of both techniques.
We also looked at the differences connected with the process of chewing of a piece of carrot from the first crushing strokes to the strokes immediately before the actual swallowing of the bolus (Part 1, Chapter 7).