Ramirez-Yañez GO1, Farrel C2. Soft tissue dysfunction: a missing clue when treating malocclusions. Ortop Rev Int Ortop Func 2005; 1(5/6):483-94.
11DDS, MDSc, PhD (in course); Myofunctional Research Co.; Helensvale, QLD; Australia; Oral Biology & Pathology; The University of Queensland; School
of Dentistry; Brisbane, QLD; Australia; Oral Biology & Pathology; School of Dentistry; St Lucia Campus; Brisbane, QLD 4072; Australia; e-mail: german@ myoresearch.com
2 BDSc; Myofunctional Research Co.; Helensvale, QLD; Australia
Contemporaneous treatment of malocclusions concerns about the effect of the muscular activity in the occlusion. Treatment of malocclusion involves dental alignment, jaw repositioning, stabilization of the activity of the masticatory muscles, the muscles of the tongue and the muscles of the cheeks and lips in order to achieve a balance between the forces delivered on the arches and teeth by all the muscles involved in oral functions. Any alteration in the muscular activity of these muscles is what the authors term “soft tissue dysfunction”. It is necessary that dentists treating malocclusion be aware of this dysfunction and acquire the required knowledge to recognize it. The current paper reviews the biology of the muscles of the cheeks, lips and tongue and how their action may affect the occlusion. The goals for an ideal treatment of malocclusions and how appliances based in the oral physiology must be designed to get better success and more stability in the treatment are also discussed.
Dentistry focuses in the study of the physiology and pathology of the oral system. The oral system performs several functions with three of them rising as the most frequently performed: mastication, speech for communication and swallowing. Over the years, dental professionals have dealt with those factors affecting the position of the teeth, and so, altering the normal functional pattern which is described as a dysfunction. Numerous studies have shown that oral dysfunction is not only caused by an incorrect position of the teeth and jaws, but also highly associated to hyper- and/or hypo-activity of the musculature involced in the function of the oral systemiiiiiiivv.
In general, the evaluation of a patient with malocclusion seeks for alterations in the mineralized tissues (bone and teeth), and do not evaluate alterations in the function of the soft tissues (cheeks, lips and tongue). How to evaluate alterations in the mineralized tissues has been extensively explained in the literature through different measurements on cast models, orthopantomograms and lateral cephalograms. However, the evaluation of altera- tions in the function of the soft tissues is based on clinical observations and requires a deep knowledge in oral physiology. For example, dentists interact with patients who may have speech disorders, and it is important for us to realize that the structures for which we are responsible play a vital role not only for chewing and swallowing but also for speech communicationvi, and that a speech disorder may worsen the problem that we are evaluating. So, a high level of observation of the patient’s facial expression is required to detect any deviation in the muscular pattern that may be associated with the oral dysfunction.vii
An ideal treatment must address the underlying muscular problems associated to the malocclusionviii. This paper intends to describe what authors term “soft tissue dysfunction” and, how this dysfunction alters the shape of the arches and the position of the teeth.
The buccinator is the major muscle of the cheeks. This muscle is attached to both the maxilla and the mandibleix. At rest, the pressure exerted by the muscular activity of the cheeks on the buccal side of the mandibular posterior teeth is similar to that exerted by the muscular activity of the tongue on the lingual side, about 2g/cm2. However, the pressure on the maxillary posterior teeth is higher on the buccal side than that on the lingual side, 2.7 and 1.0g/cm2 respectivelyx. This situation changes during chewing and swallowing, when the pressure on the lingual side of the maxillary posterior teeth is greater than that on the buccal sidex. Thus, equilibrium is reached and the position of the posterior teeth is maintained by the cheeks and the tongue.
This normal pattern is altered in those patients with oral habits. For example, in dummy sucking patients, the buccal pressure on the posterior teeth may raise to 21 g/cm2, and the cheek/lip pressure on the canine may be 3-4 times higherxi. Thus, an increase in pressure on the arches with a cheek/lip higher pressure on the posterior teeth with an even higher pressure on the canine region produces a narrow upper arch with the triangular shape frequently observed in those malocclusions associated to oral habits (Figure 1).
FIGURE 1: Fifteen years old girl with a soft tissue dysfunction associated to malocclusion. Note narrowness and the triangular shape of the upper arch (B) due to hyperactivity of the buccinator muscles delivering high forces on the posterior teeth with even higher forces on the canine area. Incompe- tence for lip sealing collaborates to the triangular shape and crowded teeth (C) as the force that should be delivered by the upper lip on the incisors is not present in this patient.
The upper and lower orbicularis oris are the main structure of the lips. Their mutual activity permits a lip seal, which closes the entrance of the oral cavity. At rest with a lip seal, the mentalis activity is negligible in the patients with normal occlusionv. Therefore, lip seal must show muscular activity of the orbicularis oris, but should show no activity in the mentalis. The force produced by the orbicularis oris is light and continuous, and has been estimated by means of the force transmitted to lip bumpers. It has been calculated 12-14 g at rest and 33-36 g over swallowingxii. The force of the tongue on the lingual surface of the incisors has been calculated as 0.8-1.6 g/cm2 at restxiiixiv. However, this force may increase to 3-7 g/cm2 in oral breathing patientsxv. Studies have shown that the orbicularis oris may exert a force on a lip bumper up to 300 g and the tongue may exert forces up to 500gxvixvii. Some authors have suggested that the tongue pressure on the incisors is greater than forces produced by the lip musculaturexviiixixxx. A recent study has shown that at rest, the pres- sure exerted by the lips directly on the upper and lower incisors is very low, even negative pressurev xxiii. Thus, equilibrium is reached and a correct position of the incisors is maintained because a higher but non-continuous force is delivered by the tongue on the lingual surface of the incisors, whereas a lower but continuous force is delivered by the lips on the buccal surface of the anterior teethxxixxii.
In the incompetent lip patient, lip contact produces greater muscular activity of the orbicularis oris, whereas at rest with the lips separated, there is no activity in the upper or in the lower orbicularis orisv. In addition, mentalis muscular activity increases to moderate during saliva deglutition, and it is greater during water deglutition. Conversely, the orbicularis oris present slight to moderate activity during saliva and water deglutition in incompetent lip patientsxxiii. Nevertheless, as the most of the time the lips remain separated in the incompetent lip patient, no muscular activity of the orbicularis oris is the most frequent situation in these patients. Thus, the equilibrium between the tongue/lip muscular activities is lost and the pressure of the tongue moves the incisor buccal-ward. In addition, during swallowing, which occurs between 600 and 2400 times each dayxxiv, the lip incompetent patient produces compression on the anterior part of the mandible, which results in that convex profile frequently observed in patients with lip incompetence due to an undeveloped chin (Figure 2).
FIGURE 2: Eleven years girl with soft tissue dysfunction associated to disto-occlusion (B). This malocclusion results from hyperactivity of the mentalis which produces a backward force on the anterior area of the mandible, and so, opposites to normal mandibular growth (C).
The tongue is a structurally complex and extremely flexible organ occupying a large portion of the oral cavity. The tongue resting position refers to a reproducible physiologically relaxed position of the organ observed when the lips are smoothly sealed, or slightly separated, and the mandible is slightly openedxxv. At this position, the tongue produces the lowest pressure on the surrounding structures, whereas during its movements to participate in the oral functions, the pressure increases or reduces at different sites at different timesx.
The most performed activity of the tongue is that observed during swallowing. This oral function is divided into six stagesxxvi, starting from a resting position where the tip of the tongue is positioned behind the lingual surface of the upper incisors, slightly touching the anterior palate, the dorsum running parallel and near to the hard palate and the posterior part of the tongue mostly in contact with the soft palate. At the second stage, the tip of the tongue locates at its most backward position. Stage three is characterized by the fixation of the tongue tip on the anterior palate and a descending of the dorsum of the tongue. At stage four, the soft palate rises to the highest level with the dorsal surface of the tongue mostly in contact with the palate. At stage five, the soft palate descends to the lowest position, and at the last stage, the tongue returns to the initial position.
The tongue moves in a precise manner contributing highly to the complex orofacial movementsxxvii. All these movements are produced by a coordinated performance of two sets of muscles, the extrinsic and the intrinsic muscles. The extrinsic muscles are the genioglosus, which is the principal tongue protruder, and the styloglosus and the hyoglosus, which act as tongue retractorsxxviiixxix. However, the action of the tongue extrinsic muscles is not as simple. Co-activation of the protruder and retractor muscles produces tongue retractionxxx, and co-contraction of the genioglosus (protruder) and styloglosus (retractor) during jaw-opening, contributes to shape the tongue for gathering the food stuffxxxi. On the other hand, the intrinsic muscles are classified as transverses, verticalis and longitudinalisxxviii. The intrinsic muscles have a characteristic architecture and fiber composition which differs from those of orofacial and masticatory musclesxxxiixxxiii. The fibers in the intrinsic are responsible for the fast and flexible actions in positioning and shaping the tongue during the oral functionsxxxiii, and enable the dorsum of the tongue to harden for pressing food during mastication and shifting the food posteriorly for swallowingxxxii.
Thus, the action of the muscles of the tongue is coordinated within them, but furthermore, tongue action is also associated to function of the masticatory muscles. During jaw opening, the genioglosus increases its activity in correlation to the increase in the digastrics activityxxxi xxxiv. During jaw closing, an increase in styloglosus activity is observed in correlation to the increase in the activity of the masseterxxxiv. Both masticatory and extrinsic muscles are active and coordinated during mastication. An increase in the activity of one group (genioglosus/digastrics or styloglosus/masseter) inhibits the activity of the antagonist groupxxxiv. But, this is not analogous between the two muscle groups. The inhibitory effect of the masseter on the digastrics is higher than that produced by the genioglosus on the styloglosusxxxiv. This interaction between the tongue and the masticatory muscles is referred as the jaw-tongue reflexxxxv.
At birth, all subjects posses a normal tongue position and the abnormal one is developed by aging accompanied by abnormal swallowing habitsxxxvi. The movements of the tongue transform from undifferentiated movements in the infant, to differentiated
and refined movements in the young childxxxvii. Tongue- base position is associated with maxillary position and vertical mandibular rotation, and so, influences maxillo-mandibular growthxxxviii. Therefore, tongue activity defines the position of the teeth, as referred above, and also determines the position of the mandible. Thus, a balance between tongue pressure from the inside and labio-buccal pressure from the outside must existxxxix associated to a correct position of the tongue-basexxxviii, in order to develop correct patterns during the dento-maxillo-mandibular growth.
The developmental process of the tongue movements does not always occur in individuals exhibiting orofacial myofunctional disordersxxxvii, and the movement of the tongue may affect the development of the craniofacial morphologyxl. Patients with open-bite present characteristic tongue movements associated to their morphological featuresxli. Tip and dorsum of the tongue are positioned anteriorly and inferiorly at rest and during the build up of negative intraoral pressure in anterior open-bite patientsxxvi. Furthermore, tongue-tip position is more protrusive during deglutition in these patientsxlii. The tongue posture observed in open-bite patients pushes the maxillary and mandibular anterior teeth forwardxxvi. In general, anterior open-bite and tongue thrust swallowing (Figure 3) produce longer face morphology, greater upper incisor proinclination, less consistent production of closures during speech, more posterior pattern of electropalatography contact, and relatively sparse contact during swallowingxliii.
Current Concerns in Orthodontics
During the last 20 years, clinicians treating malocclusion have asked: Does contemporary treatment have no satisfactory solution to the problem of achieving long-term stability?xliv; Retention is necessary for a prolonged period of time or even the rest of the patient’s life to avoid relapse? The possible answer is that perhaps several additional factors may have an important bearing on malocclusion treatment stabilityxliv. In other words, clinicians are probably not realizing that moving the teeth does not always means altered muscular patterns are corrected. In the oral system, change is a constant factor and the stability of the system may be affected by changes in the neighbor systems. The mode of breathing, airway resistance, head and body position play a great part in the function of the oral system and can lead to eventual changes in the peri- and intra-oral muscular activity as well as in tongue position, which may cause eventual changes or even recurrence of the original disordersxlvxlvixlviixlviii .
Although contemporaneous treatment of malocclusions with fixed appliances moves teeth and at some instances reshapes the arches, it has no effect on tongue movements, in the force produced on the lingual surface of the teeth, in the muscular activity of the cheeks and the lips, or in the compression exerted by these structures on the buccal side of the teeth and arches. It has been demonstrated that rapid expansion by means of a fixed screw to the maxillary arch produces a change in the forces produced by the buccinator on the posterior teeth, but the values after three months return to that before the expansionxlix. So, if the width of the arch is increased, but the forces delivered on the arch by the musculature of the cheek is not controlled and reduced, a relapse would be expected. Early over the last century, orthopedic appliances were proposed to treat malocclusions (eg. Monoblock and Bionator; reads rigid orthopedic appliances). These appliances were designed to produce changes by acting on the muscles, and so relocate the mandible. Although muscle tends to adapt to gradual advancement of the mandiblel, and masticatory muscles tend to adapt to the new mandibular location, changes in the activity of the musculature of the cheeks, lips and tongue when the mandible is relocated by rigid orthopedic appliances have not been reported.
Nevertheless, over the last decades several appliances have been designed permitting to treat malocclusions in a more physiological way (eg. Bimler, Frankel, Simoes Networks, Trainers). These functional orthopedic appliances correct alterations in the muscular function (masticatory and facial muscles) and some of them have a direct impact on the tongue functionlilii. Furthermore, changes in the activity of the musculature of the cheeks, lips and tongue when the mandible is relocated by the appliance have been largely reported when maloc- clusions are treated with functional orthopedic appliancesxi liiilivlvlvilvii. Thus, functional orthopedic appliances have risen as the better choice to treat malocclusions as they are able to correct the muscular problems, and so, permit to achieve a better oral function and align the teeth. A critical point with functional orthopedic appliances is the position of the elements in the appliance. A small deviation from the correct position of one of the elements during the construction of the appliance may deliver undesired forces by the tonguelviii, and so to produce results which do not match with the treatment goals. Therefore, if a dentist intents to use functional orthopedic appliances to treat malocclusions, it is important that the professional understands the modus operandi of the appliance, as well as the correct position of every element in the appliance.
Thus, any appliance with the intention of treating malocclusions must be constructed in such a way that maintain the normal function of the tongue (if present), or re-educate the tongue when a tongue dysfunction have developed. However, relocating the tongue is a complex process. The functional performance of the tongue is determined by its size, strength, movement patterns and resting position. Any appliance which intends to modify tongue posture, may act on the tongue’s extrinsic muscles and on the masticatory muscles to evoke the jaw- tongue reflex, but at the same time, stimulate the tongue’s intrinsic muscles to re-shape the tongue. Furthermore, such appliance must be built in a way that permits to increase the force of the tongue in some places, while it is reduced where it is required. In other words, must change the function of the extrinsic and the intrinsic muscles associated to a change in the functional activity of the masticatory muscles. Another point that must be considered when treating malocclusions with functional orthopedic appliances is the muscular activity in the cheeks and the lips. Therefore, an ideal appliance to correct malocclusions would be that that at the same time control the pressure of the cheeks and the lips on the buccal side of the teeth, reduce the activity of the muscles at the chin to a minimum or even negligible, correct tongue posture, relocate the mandible forward or backward, expand the maxillary and mandibular arches, and align the teeth. In other words, an ideal treatment must not only move teeth, but also must address the underlying myofunctional problems causing or associated to the disorder.
This is not a new revelation. The fact that the influence of the lips and cheeks, as well as the position of the tongue, modifies the form of the arches and that almost every malocclusion offers some noticeable and varying manifestation of their function has been reported in the literature since long time agolix. Furthermore, it has been stated that it is imperative that those clinicians treating malocclusions appraise muscle activity and that they conduct their therapy in such a manner that the finished result reflects a balance between the structural changes and the new functional forces acting on the arches and teethviii. A treatment based only in moving teeth is treating only part of the problem and relapse could be expected. Some clinicians have included rigid orthopedic appliances in their treatment to improve their results. Although this is a better way to approach the problem, they still have to combine fixed appliances to align the teeth and orthopedic appliances to relocate the mandible and to improve muscular activity. In many cases patients refuse all these devices and do not cooperate with the proposed treatment. Although, functional orthopedic appliances have risen as the better choice to treat malocclusions, up to date it appears that there is a unique appliance which achieves all the goals at once. Every case is different and may respond to the treatment accordingly to the etiology of the malocclusion and the biology of the patient. Therefore, it is necessary to understand the physiology of all the structures involved in the oral functions, how the oral system functions and how it functions in association with the surrounding systems. Thus, little by little we are being able to develop simpler techniques which treat at the same time alterations in the mineralized tissues (jaws and teeth) and alterations in the muscular activity of the soft tissues (masticatory muscles, cheeks, lips and tongue). However, further research on the etiology of malocclusions and on the individuality of the patient’s response are necessary to develop better and simpler techniques to treat malocclusions.
This review has shown that the position of the tongue and the lips as well as the muscular activity of the muscles of the tongue, lips and cheeks during the oral functions significantly affects the shape of the arches and the position of the teeth. Considering that postural and functional alterations anticipate dento-skeletal changeslix lxlxilxiilxiiilxivlxv, the activities of the masticatory muscles, of the tongue, and of the muscles of cheeks and lips are one of the most important factors determining if a malocclusion is or not present. Any alteration in the normal pattern of activity of the muscles acting during the oral functions is what the authors term “Soft Tissue Dysfunction”.
Dentists must be aware and evaluate in each patient if a dysfunction in the muscular activity is present and how this soft tissue dysfunction has affected the occlusion. Furthermore, a plan of treatment intending to correct a malocclusion must include appliances which eliminate soft tissue dysfunction acting on the muscles of the cheeks, lips and tongue at the time it is correcting teeth and jaws position. Therefore, the success and the stability of any treatment which intends to correct a malocclusion are, at least in part, in direct relationship to that achievement in the equilibrium between the forces produced by the tongue and those produced by the cheeks and lips. In this context, functional orthopedic appliances raise as the better choice to correct soft tissue dysfunction and to achieve better and more stable results when treating malocclusions.
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