Refining occlusion with muscle balance to aid long-term orthodontic stability
The primary objective of orthodontic treatment is the movement of teeth into a more ideal relationship, not only for aesthetic, but also for functional considerations. Another very important objective, often not given enough consideration, is the need to finish the case with the muscles of mastication in equilibrium. If muscle balance is not achieved, an endless procession of retainers, is required for retention.
In simple terms, if the occlusal forces in maximum intercuspation are unevenly distributed around the arch, tooth movement will most likely occur. However, today it is possible to precisely measure the relative force of each occlusal contact, the timing of the occlusal contacts and specific muscle contraction levels, all simultaneously. This technological breakthrough represents a new opportunity for orthodontists everywhere.
Muscle balance and occlusion
Many well-respected orthodontists agree that there is more to occlusion than just teeth. Temporomandibular (TM) joint function and the maxillo-mandibular relation are as much a part of occlusion as are the teeth. Consequently, when a malfunction occurs within the TM joints or a maxillo-mandibular mal-relation exists, a compensatory response is elicited from the stomatognathic musculature. Most often that response can be measured through electromyography (EMG).
An eight-channel electromyography
Over 50 years ago, one orthodontist began to record muscle activity through surface electromyography in an effort to better understand the functions of the muscles of mastication.1 In the intervening years since, surface EMG has revealed several key facts about the relationship between the muscles and a patient’s occlusion. Today we can routinely record up to eight channels of EMG data, right in the clinic. And, data interpretation can lead us to a better understanding of our patient’s specific condition.
a) Relaxed, quiet muscles b) Hyperactive muscles c) Large motor-unit firing
In Figure 2 we see muscles that are; a) relaxed at rest (the normal condition), b) hyperactive at rest (indicating a maxillo-mandibular malrelation), or 3) exhibiting a neurological abnormality (large motor-unit firing). While these factors routinely go unmeasured, their contribution to a precise diagnosis can be highly significant, even to the long-term outcome of a particular case.2-5
Determining muscle balance in function is an easy task for EMG.6-13 Typically, the patient is asked to clench in maximum intercuspation and then swallow. The clench will appear balanced (Figure 3a) or unbalanced (Figure 3b). The swallow will either be with the teeth together (Figure 3c) or with a tongue-thrust (Figure 3d) Then, if an appliance is utilised, muscle activity can be recorded before, during and after adjustment of the appliance. This will immediately demonstrate the effectiveness of the appliance.14-20
a) Balanced clench b) Unbalanced clench c) Normal swallow d) Aberrant swallow
If we see that the muscles are balanced, we know we have a result that will remain stable. But, if the muscles are not in balance, we can’t tell from the EMG recordings exactly what to do about it. While much has been learned about muscle hyperactivity and the various conditions of imbalance that can exist within the masticatory musculature, EMG is not, nor will it likely ever be, adequate to the task of directing case treatment by itself. While surface EMG is a fast, easy and reliable way to record the relative contraction levels of the muscles at rest or in function, it has a low sensitivity to occlusal force locations and the timing of tooth contacts.
A bite-force recording is taken by having the patient bite down several times on the T-scan wafer to condition it. This allows it to conform to the shape of the arch. Then a recording is taken with the patient closing from rest position into the intercuspal position, followed by a clench. Other recordings can also be taken in centric relation, lateral excursions and protrusion.
a) 1st contact b) 1st posterior contact c) 1st left posterior contact
A map of the sequence from initial anterior contact to bilateral contact
In the recording in Figure 5 the initial contact points occur only on the incisors. As the patient continues to close, a contact appears on the right area of the second molar. Eventually a contact appears on the left second molar creating a tripod effect.
Force movie frames
When the recording is replayed as a ‘force movie’, a 3D graph is displayed showing the relative force at each point of contact. Again we see that the initial contacts are on the incisors, then the right posterior and finally the left second molars. What is also evident is that in full closure, the highest contact force is actually on the left second molar, (indicated by the tallest spike) despite the lateness of the contact.
Further inspection clearly suggests that the reason the excessive force is being born by the left second molar is due to a lack of solid contacts on the left first molar and bicuspids. In spite of the large number of contact points around the arch, this is an occlusion badly in need of adjustment.
Why the T-Scan wafer at 85 microns is not too thick:
According to the latest research on mandibular function (Gallo et al) we now know that the sagittal path of closure is more complicated than a simple hinge movement. In fact, the ‘helical axis of rotation’ moves from the vicinity of the angle of the mandible (early in opening) to about mid-ramus (late in opening) in close proximity to where the inferior alveolar nerve enters the mandibular foramen.
For a voluntary closure between rest and occlusion (2-3 mm) the average amount of rotation has been measured at 0.7 degrees (Lewin A. and Moss C). For an 85 micron change that’s about 0.02 degrees of rotation (about 1.5 minutes of arc). If the A/P distance between the incisors and the second molars is 40 mm, 1.5 minutes of arc translates to an 18 micron difference in vertical change (more in the anterior, less posterior) between ‘wafer in’ and ‘wafer out’.
This is a very small difference in comparison to the size of an occlusal adjustment being made and well within the adaptive capacity of the system. Another benefit of placing the T-Scan wafer between the arches … it that it reduces the acuity of proprioception, which reduces, but doesn’t eliminate, the ability of the central nervous system to avoid any existing prematurities.
T-scan II – Bio EMG II
One high force point on the left bicuspids, right anterior temporalis hyperactivity
Analysing the combined traces
Both the forces and the activities of the muscles are balanced in this patient
Balanced forces do not guarantee balanced muscles
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