An Electromyographic Investigation of the First Six Months of Progressive Mandibular Advancement of the Herbst Appliance in Adolescents
The effects of the progressive activation of the Herbst appliance on the activity of the masseter and temporalis elevator muscles of the mandible were monitored in a group of 14 consecutively treated 10–15-year-old subjects with an Angle Class II, division I malocclusion. A cast silver splint Herbst appliance was activated in multiple stages at a rate of 2 mm/2 mo. The functionality of the superficial masseter and anterior portion of the temporalis muscles was monitored at maximum bite force using surface electromyography (EMG). The EMG recordings were taken at an incisal edge-to-edge position and a retruded mandibular position, both at a vertical interincisal separation of 3 mm using an acrylic bite plate. Measurements of maximum voluntary isometric clenches were taken during adaptive functional changes at pretreatment (baseline) and during the first 6 months of Herbst appliance therapy. Results showed great individual and inter- and intrasessional differences in electromyographic activity of the muscles before and during treatment. At the retruded position, the masseteric activity increased by the sixth month while temporalis activity remained at the same level. Following treatment, the masseteric imbalance was reduced, but the temporalis imbalance was unchanged. At the edge-to-edge position, masseteric activity increased by the sixth month, while temporalis activity remained unchanged. The masseteric imbalance was reduced by the sixth month, while the temporalis imbalance was reduced from the fourth month into treatment. The results imply a favorable muscular response to a progressive regime of Herbst appliance activation.Abstract
INTRODUCTION
Moyers1 pioneered the electromyographic study of the masticatory muscles and was the first to investigate muscle activity in patients with an Angle Class II, division I malocclusion. The clinical use of electromyography (EMG) for orthodontic diagnosis and for monitoring treatment was soon introduced.2,3 Ahlgren4 utilized the technique to investigate the mechanism of mastication, and Möller5 reported on its correlation with facial morphology. During the past 3 decades, in both research and clinical settings, surface electromyography has been utilized to aid in the detection, diagnosis, and treatment of muscle hyperactivity and hypoactivity, muscle imbalance, rest position, and spasm and fatigue of the muscles of mastication.6–11 The pattern of masticatory muscle activity was found to be distinct or highly correlated with respect to the age, type of malocclusion, and type of and stage in orthodontic treatment.12–16
Experimental electrode implant studies as well as clinical studies have been reported for the electromyographic effects on the functional and postural activities of the masticatory muscles during functional appliance treatment of Class II malocclusions.14,17–21 While general initial reductions of functional and postural activities were apparent, a posttreatment pattern of muscle activity similar to that of patients with normal occlusion was often accepted as an indicator of normality or optimized function.
Despite its widespread use as a quantitative measurement tool, the EMG recording's reliability and indications for use have not been unequivocally established due to conflicting available evidence.6,11,22–25 Meanwhile, advancements in hardware technology currently allow for at least a 1000-fold increase in multichannel EMG data to be gathered and processed in real time, with the direct effect being an increase in statistical precision.
In functional appliance treatment of Class II skeletal relationships, the Herbst appliance is designed to minimize the need for patient cooperation.26 While the general treatment objective is to maximize the desirable skeletal effects while limiting the dental effects, the conventional Herbst appliance activation method requires that the mandible be brought forward to an incisor edge-to-edge position where possible (or if this is not possible, as far as the mandible can be moved forward voluntarily).26,27 The side effects of this approach include a tendency for significant dental movement, and this effect seems to be independent of the type and design of Herbst appliance utilized.28–30
Based on positive results from animal studies, it has been suggested that a progressive activation method to bring the mandible gradually forward (thus placing less stress on the investing soft tissue matrix) might reduce the undesirable dental effect while maintaining the skeletal effect.31,32 The effect of the Bass appliance (which used progressive jumping of the mandible), when compared against the Herbst appliance, was shown to have a tendency for a larger effect on the mandibular jaw base position33 and restraining of the maxilla.34 This might well have been due to the method of progressive activation.
This study aims to explore the effect of the progressive activation modality with respect to its functional influence on the elevator muscles of the mandible.
MATERIALS AND METHODS
Subjects
The experimental group consisted of 14 consecutive treated orthodontic patients (11 males and 3 females, age 10–15 years) from the University of Hong Kong's Faculty of Dentistry at the Prince Philip Dental Hospital. All patients had a dental Class II, division I dental malocclusion existing on a skeletal Class II base (Table 1) and were diagnosed as having sufficient remaining growth potential that they might benefit from appropriate functional appliance therapy. Hand-wrist radiographs showed that their developmental stage ranged from before to just past the peak of the pubertal growth spurt as assessed by the patients' skeletal maturity.35 All subjects had symmetrical dental arches and were free from abnormal functional habits or clinical signs of temporomandibular joint disorders.
Cast silver splint Herbst appliances were used, with multistage bilateral activation of the appliance at a rate of 2 mm/2 mo until either a Class I or overcorrected Class I incisal relationship was reached. All subjects' Herbst appliance treatments progressed beyond the 6-month stage.
EMG registration schedule and setup
Baseline EMG readings were taken at 3 separate sessions (5–14 days apart) within 4 weeks before appliance. Thereafter, EMG registrations were taken at 1 week, then (just before appliance activation) at 2, 4, and 6 months into Herbst appliance therapy.
Direct and integrated EMG registrations were obtained from the superficial masseter and anterior temporalis muscles bilaterally using the Myo-tronic K6-I Diagnosis System's (Myo-tronics Inc) in-built Scan 11 protocol, specifically for testing these 2 muscle groups. Data were gathered using disposable silver/silver chloride bipolar surface disk electrodes (Duo-trode, Myo-tronics Inc) and was input through 8 double and 1 reference electromyographic channels to the system's preamplifier. The data were transmitted with no significant amplification bias. The muscle action potentials recorded were amplified and full-wave rectified and integrated (FWRI). Integral values of 50 millisecond epochs were calculated and the corresponding analog signals were displayed in real time on a color computer monitor.
EMG activities were measured during maximum voluntary isometric contraction (MVIC) with the patient biting onto a customized acrylic bite plate (Figure 1). Where Herbst appliance treatment had started, the appliance minus the plungers would be left in place during registration sessions.
Citation: The Angle Orthodontist 71, 3; 10.1043/0003-3219(2001)071<0177:AEIOTF>2.0.CO;2
Interarch relationship adopted for registration
EMG recordings of the MVIC were taken with the central incisors in an edge-to-edge sagittal position and at the most retruded passive mandibular resting position during pretreatment (or as was dictated by the then-current amount of Herbst appliance activation after the plunger assembly was removed at each visit). Both recordings were taken with a vertical separation of 3 mm between the central incisors, maintained by having the patient bite on a customized 3-mm-thick bite plate of clear acrylic. Only 1 bite plate per patient (with 2 different acrylic imprints of lower incisal positions) was used in order to minimize chairside time as well as experimental error.
Procedure
Using a dental chair with armrests, the subject was seated upright and at ease with the head in a normal and relaxed position facing forward. A color computer monitor was placed 5 feet in front, slightly below eye level, and off to the right by about 10° to provide visual feedback.
A blank acrylic bite plate was customized to the then-current occlusal relationship with addition and modification of self-curing acrylic (Simplex Rapid, Howmedica Inc) cured in a hydroflask.
Electrode placement
The skin surface was cleansed with 70% isopropyl alcohol and pumice and dried to reduce impedance. The standard placement of electrodes was modified as follows.
The electrode site on the superficial masseter was defined as an area midway along a line connecting the inferior border of the zygomatic arch at the zygomatico-temporal suture to the gonial angle. The electrodes were placed centrally, about 1 cm distal to the anterior border of the muscle. The site on the anterior temporalis was defined by palpation during clenching of the mandible,36,37 and the electrodes were placed 1–1.5 cm from the anterior border of the muscle. A reference electrode was placed inferior and posterior to the right ear. Reference marks to electrode positions and orientations were made and recorded. Polaroid photographs were taken during the first session as reference for future electrode placement.
Alignment of the electrodes was facilitated by palpation of the muscles, with the individual requested to clench submaximally and relax at the position of maximum intercuspation (prior to any treatment) and at the most retruded mandibular position (while under treatment). To the best clinical estimation, all bipolar electrodes were placed parallel to the muscle fibers and about halfway between the origin and insertion of the muscles. Following placement, the electrodes were left to stabilize for 20 minutes.
EMG registration
Recordings of trial clenches at submaximal forces, with and without the acrylic bite plate, were performed at the beginning of a session as well as before each electrode stabilization period for system validation.
Each EMG registration was preceded by at least 1 second of data at the resting state and separated by 2 seconds of rest (trial). Each registration consisted of 3 periods of maximum voluntary isometric contractions lasting 2 seconds each. Three trials were taken with each of the 2 prescribed incisal bite positions (set). Following completion of the 2 sets of EMG registrations (part 1), the electrodes were removed. Fresh electrodes were reapplied according to the standard protocol, and the validation and registration routine was repeated with the sequence of bite positions reversed (part 2).
Design and construction of acrylic bite plate
Transparent acrylic bite plates were used to ensure a reproducible, predetermined, and stable occlusal relationship during (and to an extent, between) EMG registration of MVIC. Flat acrylic blanks were constructed in the laboratory and customized at chairside with the addition of self-curing acrylic to register both the retruded and edge-to-edge incisal relationships (both at 3-mm vertical separation) of the upper incisors and second premolars with respect to the lower incisors. Otherwise, the plate was relieved from all tooth contact.
Statistical methods
Means and standard deviations were calculated for total mean MVIC generated for each muscle pair at each incisal position for individuals and as a group for every session. The 3 baseline sessions' data were combined to give a baseline mean and standard deviation for each individual and as a group. ANOVA analyses were performed with respect to first/second electrodes (ie, parts 1 and 2), with trial and period as variables. R values for all sessions (P > F @ .0001) were calculated (Table 2). Muscle imbalance data for each muscle group were derived by having the higher EMG output side minus its lesser counterpart during the baseline period of each individual, which was then averaged for the group mean. During subsequent recordings, the order of the subtraction between the sides was maintained for each individual.
RESULTS
Mean microvoltage values per session among individuals varied over 3-fold. The individual and group's mean values could be statistically related to confidence intervals (CI), which were taken in this study as equivalent to 1.96 times the standard error (SE). There were wide variations and fluctuations of mean and CI values among individuals. The CI within each individual at the baseline was relatively smaller due to the larger sample size (by 3 times) despite intersessional deviations. The magnitude of subsequent CIs varied from session to session, with no particular trend observed.
MVIC EMG performance at retruded position
Masseter and temporalis (Figure 2)
Citation: The Angle Orthodontist 71, 3; 10.1043/0003-3219(2001)071<0177:AEIOTF>2.0.CO;2
The group's mean MVIC masseter values were reduced at the start of treatment and at the second month but returned to the pretreatment level by the fourth month and surpassing it at the sixth month. In general, an individual performance was less variable (in terms of microvolt variation from session to session as well as the general conformity to the group's mean). The group's mean temporalis MVIC values remained at the same level throughout the treatment period.
Muscular imbalance (Figure 3)
Citation: The Angle Orthodontist 71, 3; 10.1043/0003-3219(2001)071<0177:AEIOTF>2.0.CO;2
The group's mean MVIC values for both muscle groups were reduced following treatment, but these reductions were only statistically significant for the masseter.
MVIC EMG performance at edge-to-edge position
Masseter and temporalis (Figure 4)
Citation: The Angle Orthodontist 71, 3; 10.1043/0003-3219(2001)071<0177:AEIOTF>2.0.CO;2
Total masseteric activity was reduced at the beginning of treatment but increased to above baseline level by the sixth month. Total temporalis activity remained unchanged.
Muscular imbalance (Figure 5)
Citation: The Angle Orthodontist 71, 3; 10.1043/0003-3219(2001)071<0177:AEIOTF>2.0.CO;2
The magnitudes of muscular imbalance for both muscle groups were similar to that at the retruded position, with the temporalis imbalance again over a relatively minor range. For group mean values, the masseteric imbalance was reduced by the sixth month into treatment, while the temporalis imbalance was reduced at the start and by the fourth and sixth month of treatment.
DISCUSSION
Little data exist in the literature about the distribution of surface EMG activity over the masseter or the anterior temporalis,38 the contractile behavior of fast and slow motor units in jaw muscles,39 or intramuscular activation patterns.40 Distribution of fiber types varies among the masticatory muscles and in different parts of the muscles,41 with the elevator muscles' strength during biting influenced by fiber size and distribution.42 Despite a large electrode area, the potentials recorded by surface electrodes are usually summed potentials from a small number of motor units comprising only a small fraction of the whole muscle, with the largest signals from the closest fibers.43 Thus, surface EMGs may not accurately reflect activity of the entire muscle, especially when the recordings are from large muscles with uneven distribution and recruitment of motor-unit types. Functionally, the superficial masseter's and anterior temporalis's activities overlap that of other parts of the same muscles and cannot be electrically isolated in clinical situations. These would contribute significantly to the variations observed as well as limiting the interpretation of the results.
A common method used to quantify direct surface EMG is to average the rectified signals and filter them into a smooth mean voltage curve.5,44 The mean voltage of the surface electromyogram gives an indication of the amplitude of the muscle activity.5 Although considerable variability of EMG recordings existed even with careful and systematic procedures and computerized EMG measurement and analysis of systems,45 a quantitative assessment of static and dynamic activities of the masticatory muscles would still be possible with thorough and statistical evaluation of EMG recordings, if necessary in relation to reference data.
EMG is a measurement of the electrical activity of muscle and not a measure of bite force. The use of multiple electrodes permits MVIC recordings from groups of muscles but cannot be used to compare the relative forces developed by individual muscles, nor is the data appropriate for indicating whether a muscle is contracting isometrically or isotonically.
Factors such as age, gender, composition and shape of the face, connective tissue, and fat content may all affect the type and magnitude of signals recorded.46 The conventional use of a force transducer introduced between antagonizing teeth is uncomfortable and necessitates a significantly larger interincisal separation. These may prevent the subject from clenching as hard as possible. The use of the customized 3-mm-thick acrylic bite plates offered a standardized number of tooth-acrylic contacts adequate for maximal clenching, physiological interincisal separation, and a reproducible intermaxillary position as means to reduce some of the variables encountered with EMG registration. Beyond the 6-month treatment period, the 2 incisal registration positions of a growing number of subjects were beginning to approximate each other and the inclusion of EMG data taken at these times might have given a biased result.
Proportional change in muscular activity with respect to the individual's baseline values might be a more meaningful indicator when compared with absolute changes of magnitude, especially when the sample is less uniform. As intragroup MVIC values (both at baseline and for the same stage of treatment) may differ significantly in magnitude, individuals with generally a higher MVIC reading would exert a more pronounced effect on group statistics compared with another group with similar proportional variation.
Due to the diversity of samples, different recording techniques, and types of electrodes used, it is difficult to compare the various reported findings.47 Changes in muscle activity during treatment may be due to discomfort or pain, changes in the occlusal relationship between the maxillary and mandibular dentitions produced by tooth movement, the orthodontic appliance itself,48 unstable occlusal conditions affecting muscle contraction patterns,49 the dentition changing to a more stable occlusion,50 or a skeletal configuration with a different mechanical advantage.
Boys with Class II malocclusions exhibited less EMG activity in the masseter and temporalis muscles than boys with normal occlusion during maximal biting at intercuspal position (centric occlusion), with the reduction in masseteric activity most apparent.47 Pancherz and Anehus-Pancherz51 showed that treatment by maximum bite jumping with the Herbst appliance would result in an increase and an equalization of temporalis and masseter EMG activity. Our results on both bite positions support the reported increase in masseteric activity following 6 months of Herbst appliance treatment. While results for our anterior temporalis activity did not change significantly (this might be due to the fact that the malocclusion wasn't totally corrected at this time or the temporalis was less affected by this treatment technique), the 2 investigations cannot be strictly compared because the electrodes and positioning used were different. Equalization of the muscles' EMG data following treatment could simply be incidental. Pancherz and Anehus-Pancherz also reported that the EMG activity from the 2 muscles during maximum biting was markedly reduced when the appliance (activated at maximum bite jumping) was inserted. The sudden change to a true edge-to-edge incisal contact (in contrast with the 2-mm progressive advancement adopted in this study, which might place less physiological stress on the tissues involved) would not be surprising, nor should it be taken as indicative of the physiological state or of the muscles' full capability at that stage. Our registration at 1 week following appliance insertion (allowing the subject to acclimatize to the appliance's functional limitations) indicated a much less significant drop (at one-fifth for retruded position and one-eighth for edge-to-edge position) in masseteric MVIC. There was no significant change in anterior temporalis MVIC. Change in the basal relationship to a more mechanically advantageous position and a more stable occlusion might explain the increase in masseteric MVIC and the reduction of both muscles' imbalance during treatment, thus indicating the multistep approach to be a more physiologically favorable effect in terms of muscular response with respect to the conventional 1-step advancement technique.
Many studies have questioned the reliability and usefulness of surface EMG as a quantitative tool in diagnostics and treatment outcome assessment. This is notably due to great individual variations,25 but it can nevertheless be considered a research tool for treatment outcome assessment of a group (rather than for individual cases). This is true provided that the case selection criteria, EMG sample size, and data manipulations are appropriately managed.
CONCLUSIONS
There were great individual and inter- and intrasessional differences in electromyographic activity of the muscles investigated before and during treatment with progressive activation of the bonded Herbst appliance.
At both the retruded and the edge-to-edge positions, masseteric activity was initially reduced but increased by the sixth month. The temporalis activity, however, remained at the same level. Muscular imbalances for both muscle groups at the retruded position were reduced following treatment. At the edge-to-edge position, masseteric imbalance was reduced by the sixth month into treatment, while temporalis imbalance was reduced at the start and by the fourth and sixth month of treatment. The results imply that the muscle response was more favorable with a progressive regime than with a 1-step advancement of Herbst appliance activation.
Acrylic bite plates in various states of customization. Blank (left); customization completed in laboratory (center); chairside customization completed with modification and addition of cold-cured acrylic (right)
MVIC activity at retruded position. Error bars indicate CI of 1.96 times SE
Muscular imbalance at retruded position. Error bars indicate CI of 1.96 times SE
MVIC at edge-to-edge biting position. Error bars indicate CI of 1.96 times SE
Muscular imbalance at edge-to-edge position. Error bars indicate CI of 1.96 times SE