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).

Figure 1:

An eight-channel electromyography

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.

Figure 2:

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

Figure 3:

a) Balanced clench b) Unbalanced clench c) Normal swallow d) Aberrant swallow

Electromyography

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.

Figure 4:

T-scan II

The simplest solution to the problem of evaluating the timing and force of occlusal contacts is the T-scan II.21-23 It provides a very sensitive measure of contact force and a moving picture of the order in which the contacts occur.24-32 It is the only technology available to the clinician that can show precisely the order in which contacts occur and simultaneously, the relative force of each distinct contact. The new high density sensors are flexible, more precise and very durable (usable for up to 30 registrations).

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.

Figure 5:

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.

Figure 6:

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’.

However, as we analyse the tracing above, as clear as the picture of occlusion of this case is, we realise that we do not and cannot from this information understand what the musculature is doing to accommodate. But there is a way to do both.

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

Previous studies have attempted to correlate T-scan data with EMG data.33,34 Recently the two companies who separately manufacture the T-scan II and the Bio EMG II have created a milestone by making their programs talk to each other.35 The synergy created offers an opportunity for dentists to more clearly understand their patients’ occlusal conditions comprehensively.
The reason that the programs needed to talk to each other was to synchronise their respective data streams. This is accomplished by having either program act as a ‘master’ while the other program acts as a slave to it. This is true in recording as well as in playback analysis.

Figure 7:

One high force point on the left bicuspids, right anterior temporalis hyperactivity

The simultaneous recording of occlusal force, timing and muscle activity

Analysing the combined traces

When we see that the highest force of contact is on the left can we assume that the greatest muscle activity will be the same? Not at all. Figure 7 shows an example of a patient with a higher force level on the left side (63% of total), focused in the bicuspid area. At the same time we clearly see that the right anterior temporalis is firing at nearly twice the level of the left one.
It is also apparent that the combined activities of the right masseter and temporalis are far greater than the same muscles on the left. How is this possible?
Not one of the muscles of mastication that elevates the mandible is positioned such that there is a straight vertical relationship between the origin and the insertion. Each elevator muscle has a horizontal component to its direction of applied force.
Due to the ginglymo-arthroidial structure of the TM joints, the mandible is able to move freely forward and back, left and right. The same ‘elevator muscles’ that apply vertical forces can – and do – apply horizontal forces to the mandible as needed for function.
In Figure 7 then, we can see that while the left side muscles are applying more force in the vertical direction, the right side temporalis must be applying a significant amount of its force in a non-vertical (horizontal) direction. However, with some extra effort, it is possible to achieve a muscle and force balanced occlusion. See Figure 8.

Figure 8:

Both the forces and the activities of the muscles are balanced in this patient

Figure 9:

Balanced forces do not guarantee balanced muscles

By the time the total force has reached 93.5% of maximum, the centre of force has returned to the midline and the vertical muscle forces are even between left and right sides. However, it is clear that the temporalis muscles are ‘overloaded’ compared to the masseters

References

For the full list of references please email [email protected]

 

Leave a Reply

Your email address will not be published. Required fields are marked *

You Might Also Like