The effect of incremental lower lip advancement on oral pressure and EMG activity of the lower lip

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1 European Journal of Orthodontics 36 (14) doi:.93/ejo/cjt094 Advance Access publication 2 January 14 The Author 14. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please journals.permissions@oup.com The effect of incremental lower lip advancement on oral pressure and EMG activity of the lower lip Hannah C. Jack*, Jules Kieser**, Joseph S. Antoun* and Mauro Farella* *Department of Oral Sciences and **Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, New Zealand, Dunedin, New Zealand Correspondence to: Mauro Farella, Discipline of Orthodontics, Department of Oral Sciences, Faculty of Dentistry, University of Otago, New Zealand, PO Box 647, Dunedin 9054, New Zealand. mauro.farella@otago.ac.nz summary Objectives: During orthodontic arch expansion, the teeth are displaced against the perioral soft tissues. This can affect the equilibrium of forces that are thought to act on teeth, with consequent implications for long-term stability. The aim of this study was to investigate the effect of incremental lower lip advancement on intraoral pressure and electromyographic (EMG) activity of the lower lip. Materials and methods: Intraoral pressure and EMG activity was measured in participants (2 males, 8 females; 22 years ± 7 months) as the lower lip was incrementally advanced using three custom-made vacuum-formed stents of differing labial thicknesses (0.5, 2.5, and 4.5 mm). A task paradigm including at rest recordings and maximal voluntary contraction was completed for each tray. Results: Resting lip pressure generated increased as the lower lip was advanced (P < 0.001). The EMG activity of the lower lip increased significantly (P < 0.001) only once the lip was advanced from 2.5 to 4.5 mm. For the pressure recordings, the response to incremental lip advancement showed considerable individual variation. Conclusions: These findings suggest that the initial pressure increase on the lower incisors was mostly likely due to the inherent viscoelastic properties of the lower lip, while the pressure increase between the 2.5 and 4.5 mm advancement was due to increased muscle activity. Each individual also responded to lower lip advancement in a different yet subject-specific manner. Introduction Arch expansion is commonly used for gaining space during orthodontic treatment, but the long-term stability of the correction may be unsatisfactory (Schiffman and Tuncay, 01; Lagravère et al., 05; Zuccati et al., 13). Although the exact cause of orthodontic relapse still remains unknown, the neuromuscular environment surrounding the teeth appears to play an important role (Jacobs, 1969; Ackerman and Proffit, 1997). During arch expansion, the teeth are displaced against the perioral soft tissues, thus affecting the balance between labial and lingual forces said to be acting on the dentition (Gould and Picton, 1964; Proffit, 1978). Experimental evidence has shown that the pressure on the buccal surface of the teeth increases immediately post-expansion (Weinstein et al., 1963; McNulty et al., 1968). Over time, these increased pressures gradually reduce towards pre-expansion values indicating an adaptive response of the labial musculature to the new position of the teeth (Weinstein et al., 1963; McNulty et al., 1968; Soo and Moore, 1991; Moawad et al., 1996; Shellhart et al., 1997; Küçükkeleş and Ceylanoğlu, 03). Muscle tone is defined as the resistance of muscle tissues to passive stretch during resting state (Simons and Mense, 1998). In a similar way to other skeletal muscles, the pressure response of the labial musculature to stretch, such as that occurring during arch expansion, is the result of both postural muscle activity and the viscoelastic properties of the perioral soft tissues (Ho et al., 1982; Yabushita et al., 06). While the postural activity of skeletal muscles generally adapts quickly to a new resting length (Yemm and Nordstrom, 1974), this may not be the case for the viscoelastic tone, which represents an inherent muscular property that is unique to each individual. To the best of our knowledge, no study has simultaneously investigated intraoral pressure and electromyographic (EMG) activity of the labial musculature following expansion. A better understanding of their relationship may provide new insights into our current understanding of the mechanisms responsible for orthodontic relapse. The objectives of this study were to determine the changes in oral pressures both at the midline and adjacent to the canine as the lower lip is incrementally advanced, while also recording EMG activity within the lower lip. Material and methods Subjects A convenience sample was recruited via an advertisement on a student job website. A total of 21 participants expressed an interest in participating in the experiment. To be included in the study, the participants had to be adult age (>18 years),

2 Changes in Pressure and EMG Activity During Lip Advancement 673 had to be Caucasian, and had to have no underlying medical conditions. Exclusion criteria were an overjet 5 or 1 mm, an overbite less than per cent or greater than 80 per cent, a severe skeletal jaw discrepancy, and the absence of more than two teeth (with the exception of third molars). The participants included eight females and two males with a mean age of 22 years ± 7 months. All participants had a class I incisal relationship and less than 3 mm of crowding in either arch. The study protocol fully complied with the principle of the Helsinki Declaration and was approved by the University of Otago Ethics Committee (/116). Intraoral pressure equipment Impressions of each subject s maxillary and mandibular teeth were taken using alginate impression material (Kromopan ISO 1563, Class A, type 1, Italy) from which stone casts (Type IV stone, GC Fujirock EP, Belgium) were poured. Three custom-made vacuum-formed stents were constructed for each participant s lower arch. Each stent was constructed of 0.5 mm thick acrylic (Duran, Scheu Dental, Germany) and was extended to cover the coronal surfaces of all teeth except the second and third molars. Layers of acrylic (Vertex Castapress, Vertex, the Netherlands) were added to the lower labial surface on two of the stents to increase the thickness to 2.5 mm for the second stent and 4.5 mm for the third stent (Figure 1). Four miniature pressure sensors (type 5S, Precision Measurement Company (PMR), Michigan, USA) with stainless steel diaphragms were then attached to each stent using pink baseplate wax (Kemdent, UK). The sensors were small with a low profile (thickness of mm and a diameter of 2.67 mm) and were used to measure absolute pressure (0 4 kpa). Sensors were placed adjacent to the lower left canine and at the lower midline on both the labial and lingual surface. Pressure signals were sent to an 8-channel bridge amplifier (PowerLab Octal bridge amplifier ML228, ADInstruments, New South Wales, Australia) and analogue-to-digital converted at 1 khz (PowerLab ML880, ADInstruments, New South Wales, Australia). The digitized signals were then transmitted to a PC laptop (Tecra A11, Toshiba, Japan) and displayed by software (Chart and Scope, ADInstruments, Pty Ltd, New South Wales, Australia). EMG equipment Two surface pre-gelled self-adhesive EMG electrodes with a 15 mm pad (model 9013S0212, Alpine Biomed ApS, Skovlunde, Denmark) were positioned on the lower lip on either side of the dental midline after cleaning the skin with an alcohol swab (Briemarpak, Briemar Nominees Pty Ltd, Australia). The electrodes were positioned slightly below the lower vermilion border, with 2 cm electrode centre separation. A third EMG electrode (MLA, ADInstruments Pty Ltd, New South Wales, Australia) was also placed behind the right ear in the mastoid area to act as reference electrode. The lower lip electrodes were secured to the skin using adhesive porous tape (Leukoplast 1.25 cm; Smith & Nephew, UK), while taking care not to impair lip mobility. The electrodes were connected to the recording unit using MLA 03 unshielded lead wires (ADInstruments Pty Ltd, New South Wales, Australia). EMG signals were amplified (Octal Bioamp, ML138, ADInstruments, New South Wales, Australia), analogueto-digital converted at 1 khz (PowerLab ML880, ADInstruments, New South Wales, Australia), band-pass filtered between and 00 Hz, and band-stop filtered between 40 and 60 Hz via software (Chart and Scope software, ADInstruments, Pty Ltd, New South Wales, Australia). Raw EMG signals were then rectified using a root mean square function (window = 125 ms) and stored as ASCII files for subsequent analysis (LabChart Reader software ADInstruments Pty Ltd, New South Wales, Australia). Further details about signal processing have been detailed elsewhere (Farella et al., 09) Procedure Before each experiment, participants were requested to switch off any personal electronic devices (phones, etc.) and sit in a comfortable, upright position with their heads Figure 1 Three custom-made acrylic stents were constructed for each participant with different thickness of acrylic on the labial surface: (A) 0.5 mm stent, (B) 2.5 mm stent, and (C) 4.5 mm stent.

3 674 unsupported and in the natural head position (NHP). They were asked to remain as relaxed and still as possible during the tasks to minimize movement artefacts in the recordings. All recordings were performed in a well-warmed environment between 5 and 7 pm to minimize the effect of environmental factors on the recordings. The 0.5 mm stent was connected to the recording equipment and zeroed to atmospheric pressure. The stent was then placed in a custom-made sealed chamber (Emtech Laboratories, University of Otago, New Zealand), which was used to calibrate the pressure sensors, using nitrogen at 50 kpa. The appliance was then removed from the chamber, and re-zeroed before being inserted in the volunteer s mouth (Figure 2). Prior to beginning of the recording session, the stent was left intraorally for at least 5 minutes to allow for participant accommodation as well as to warm up the sensors to oral temperature. The participants were then asked to perform a task paradigm, which consisted of at rest recordings, speech sounds, and maximal lower lip pressure recordings. The speech sounds and maximal lower lip pressure recordings were used to calculate maximum voluntary contraction (MVC) of the lower lip; the pressure recordings for these tasks were not included in the analysis. The MVC for each individual H. C. JACK ET AL. needed to be calculated, as the absolute measurement of muscle activity varies markedly across individuals due to a number of factors (gender, skin fold thickness, skin impedance, etc.). This often makes comparing absolute EMG activity across individuals meaningless. For this reason, individual EMG activity was presented as a percentage of MVC rather than as an absolute measurement. For the at rest recordings, the participant was asked to sit with their teeth in light contact for 30 seconds, and then to move their teeth slightly apart to mandibular postural position for 30 seconds while keeping their lips in light contact. This was repeated three times. To determine the MVC of the lower lip, the participants were first asked to enunciate three sets of p-, b- and f-sounds as clearly as possible. Following this, they were asked to pull their lower lip back against their lower incisors as firmly as possible. The maximum EMG activity recorded during either of these tasks was recorded as the MVC. Following completion of the task paradigm, the 0.5 mm stent was removed. The above procedure was then repeated using the 2.5 and 4.5 mm stents, consecutively. Data analysis The pressure and demodulated EMG signals were displayed in the time domain using the Chart and Scope software (ADInstruments Pty Ltd, New South Wales, Australia). Data collected from the lingual sensors were not analysed in this study. Lip pressure at each recording site was calculated as the average pressure over a second window. This window was taken from the middle of each task recording, so that the activity ramps indicating task onset and cessation were discarded. As each task (teeth apart and teeth together) was repeated three times for each of the three trays, there were 360 separate tasks to be analysed ( participants 2 sites 2 tasks 3 trials 3 trays). The mean EMG amplitude was calculated using the same window of the pressure data. The EMG data found during the rest tasks were normalized to the maximum pressure found during the entire task paradigm (%MVC). Data were first analysed using conventional descriptive statistics. The outcome measures (i.e. pressure and EMG activity) were then analysed by means of a linear mixed-effects model: participant (n = ; random factor), tray (n = 3; fixed factor), task (n = 2; fixed factor), and their first-order interactions. All analyses were performed using Statistical Package for Social Sciences (SPSS IBM.0, Chicago, Illinois, USA). Statistical significance was accepted at P < Figure 2 Participant with EMG electrodes attached and 0.5 mm stent in place. Results The mean midlabial pressure at rest was 14.6 ± 6.4, 24.3 ± 8.6, and 28.2 ± 8.0 g/cm 2 for the 0.5, 2.5, and 4.5 mm trays, respectively (Figure 3A). This increase in pressure with incremental

4 Changes in Pressure and EMG Activity During Lip Advancement 675 Mean pressure (N/cm 2 ) A B C Tray thickness (mm) Mean pressure (N/cm 2 ) Tray thickness (mm) Tray thickness (mm ) EMG activiy (%Max) 15 Figure 3 (a) Mean lip pressure (N/cm 2 ) at the midlabial site increased as the lower lip was incrementally advanced. Note that the pressure increase in the first 2 mm advancement was greater than in the second 2 mm of advancement. (b) Mean lip pressure (N/cm 2 ) at the canine labial site increased as the lower lip was advanced. The overall pressure increase at the canine site was less than at the midlabial site. (c) EMG activity (as a percentage of MVC) at the midline of the lower lip did not increase significantly (P > 0.05) in the first 2 mm of lip advancement. There was a significant increase in EMG activity in the second 2 mm of advancement (P <0.01). In all three graphs, the error bars depict the 95% confidence interval. lip advancement was highly significant (F = 1.5, P < 0.001). There was no difference in pressure recorded with the teeth together or apart (F = 0.14; P = 0.71). The analysis of curves obtained from each participant showed that the response of the lower lip to incremental advancement at the midlabial site varied markedly across individuals (Figure 4), with considerable differences in their gradient. The mean canine labial pressure at rest was 17.6 ± 6.9 g/ cm 2 for the 0.5 mm tray, which increased to 19.3 ±.6 and 22.3 ± 9.3 g/cm 2 for the 2.5 and 4.5 mm trays, respectively (Figure 3B). These changes were statistically significant (F = 13.7; P < 0.001), whereas there was no significant difference between pressure recorded with the teeth together or apart (F = 0.30; P = 0.582). The mean EMG activity at rest was 9.8 ± 6.7,.1 ± 6.5, and 13.3 ±.7 %MVC for the 0.5, 2.5, 4.5 mm trays, respectively (Figure 3C). The overall effect of tray was statistically significant (F = 17.0; P < 0.001). Post-hoc tests showed that the change in EMG activity between the 0.5 and 2.5 mm trays was not significant (P > 0.05); however, the difference between the 0.5 and 4.5 mm and the 2.5 and 4.5 mm trays were both highly significant (P < 0.001). There was no difference in lip EMG activity recorded with the teeth in contact or apart (F = 1.7; P = 0.195). Discussion The aim of the present study was to investigate the response of the lower labial musculature to incremental advancement. Oral pressure was recorded on the labial tooth surface at the midline and adjacent to the canine, with lower lip EMG activity measured concurrently at the midline. To the best of our knowledge, this is the first study to advance the lip incrementally and investigate how lip behaviour changes at each advancement. At both intraoral sites, the pressure recorded increased significantly as the lower lip was advanced. This is in agreement with a number of authors who have shown that the pressure on the buccal tooth surface increases with expansion (McNulty et al., 1968; Soo and Moore, 1991; Moawad et al., 1996; Shellhart et al., 1997; Küçükkeleş and Ceylanoğlu, 03). Mean pressure recorded at the midline was greater than at the canine for all three at rest recordings. This observation agrees with Moawad et al. (1996) but is in contrast to the findings of Gould and Picton (1964) and Soo and Moore (1991). This finding is most likely ascribed to differences in methodology and recording techniques between authors. The site-specific pressure profiles were also of different gradients, with the overall pressure increase at the midline site being greater than at the canine site. The reason for the differences in pressure profiles may be related to the different muscular anatomy at each site. More research is required in future to further investigate this finding. By recording both EMG activity of the lower lip musculature and the pressure on the lower incisors at the midline concurrently, it was possible to infer the relative contribution to lip pressure force of muscle contractile activity and passive viscoelastic tone as the lower lip was advanced. In this study, a significant increase in pressure was recorded at the midline as the lip was advanced from 0.5 to 2.5 mm. In contrast, there was no significant increase in EMG activity during the first 2 mm advancement of the lower lip. As the lower lip was advanced a further 2 mm, a significant increase in both pressure and EMG activity at the midline was recorded. This indicates that the initial pressure increase on the lower incisors was likely to be derived from the viscoelastic properties of the lip, while the pressure increase recorded during the second advancement was largely from the increased muscular activity. Large inter-individual variation is a common finding in lip pressure studies (McNulty et al., 1968; Proffit et al., 1975; Moawad et al., 1996; Shellhart et al., 1997; Küçükkeleş and Ceylanoğlu, 03). This has most commonly been ascribed to inherent differences in muscle characteristics between individuals (Thüer and Ingervall, 1986; Moawad et al., 1996; Shellhart et al., 1997) although research investigating the exact nature of these differences is sparse. In the present study, the analysis of individual

5 676 Mean pressure (N/cm 2 ) Participant #1 Participant #4 Participant #7 Participant #9 Tray thickness (mm) Figure 4 Mean pressure curves showing pressure profiles at the midlabial site for four participants during incremental lip advancement. Note the different dose response curves, which may be due to different soft tissue characteristics among the participants. data revealed that the response to incremental lip advancement showed considerable variation between participants, with some of them showing pressure response curves much steeper than others. It has been shown that muscle tone in a relaxed muscle is dependent on the passive properties of the muscle itself, with the viscoelastic properties of the muscle cells and muscular connective tissue contributing to the overall muscle tension (Brodal, ). Therefore, it is not surprising that the inherent muscle tone varies between individuals (Gallerano et al., 12; Ertekin et al., 13). The findings from this study suggest that the response of each individual to lower lip advancement is variable and may be related to each person s inherent viscoelastic tone. This could have important clinical implications as muscles with higher inherent tone may offer more resistance to expansion than muscles with lower tone. Further research is required to investigate this relationship. No significant difference was seen in pressure or EMG activity when the teeth were held together or kept apart. This is in agreement with previous research, which has shown no significant difference in EMG activity of the orbicularis oris muscle between these two mandibular positions (Lowe et al., 1983). The present findings suggest that slight increases in vertical dimension are likely to have minor impact on lip muscle tone. One limitation of this study is that a relatively small sample size was used. The inter-individual variability in response of the lower lip musculature to advancement may be ascribed to differences in lip muscle tone or the amount of lip protrusion among participants. Due to the limited sample size we could not test for this effect, but it should be further investigated in future research. The present study was a within-subject comparison that investigated individual response to lower lip advancement. As discussed earlier, EMG activity of the lower lip increased as the lower lip was incrementally advanced. The relationship between EMG activity and overjet remains unclear, as other studies have reported no relationship between overjet H. C. JACK ET AL. and EMG activity of the orbicularis oris muscle of the upper lip (Lapatki et al., 02; Kilic, ). This research indicates that the position of the teeth affects the amount of pressure generated by the lower lip and also suggests that in patients with high viscoelastic muscle tone, the lower lip may generate more pressure during initial advancement. It is possible that in future the assessment of the viscoelastic tone of the lip musculature may form an integral part of orthodontic records, which may assist the orthodontist in determining how stable expansion treatment is likely to be in the long term. Conclusion The present study has demonstrated that the initial pressure increase resulting from slight advancement of the lower lip is likely due to the inherent viscoelastic tone, whereas lip muscle activity increases only when the lip is advanced of more than 4 mm. It has also been shown that the response is individual specific. Further research is required to investigate the adaptation response of lip activity and lip pressure during orthodontic expansion and determine whether this adaptation varies among individuals showing a different tone of perioral tissues. Funding Foundation for Orthodontic Research & Education (FORENZAO) Trust via the Education Research and Development Group (ERDG) of the New Zealand Association of Orthodontists (NZAO); Fuller Scholarship through the Sir John Walsh Research Institute (SJWRI) of the University of Otago. Acknowledgments The authors would like to acknowledge the technical support received from Ian van Staden and Peter Fleury. References Ackerman J L, Proffit W R 1997 Soft tissue limitations in orthodontics: treatment planning guidelines. Angle Orthodontist 67: Brodal P The central nervous system. Oxford University Press, New York, p. 297 Ertekin C, Eryasar G, Gürgör N, Arici S, Secil Y, Kurt T 13 Orbicularis oculi muscle activation during swallowing in humans. 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