Muscle & malocclusion

Muscles of mastication and its role in orthodontics Rajeev Kumar Mishra 1ContentsIntroductionApplied anatomyMuscle activity in malocclusionsMuscles and craniofacial morphologyChanges in muscle with treatmetntSpecific aspects of orthodontic treatment with reference to the mandibular muscles

Introduction

It has been widely accepted that function of the masticatory muscle has a considerable influence on craniofacial morphology.

Abnormal pattern of orofacial muscle function as well as abnormal position of lips and tongue can cause orthodontic problem.

The close relation between the muscle and the bone is due to the fact that the muscle influences the growth of the latter both as a tissue affecting the vascular supply of bone and as a force element

Masticatory muscle function is considered to be a local environmental factor.Wolffs law points out that the internal structure and the shape of the bone is closely related to function, and defines a relationship between bone shape and muscle function .

Moss and others have noted that muscle function is one of the most important epigenetic factors involved in guiding facial bone growth

Muscles are inserted into the bone by means of their aponeuroses through the periosteum or tendons, and through them they exert tensile forces on the bone.

Thus, the effect of muscle contraction is both local-that is, exerted in the area of the muscle insertion and general, changing the loading level of the bone which acts as a lever or beam.

Equlibirium theory suggests that size and shape of muscular process of jaws reflects muscle size and activity.

Applied anatomyMuscleOrigin InsertionActionsMasseter Zygomatic arch & Zygomatic process of maxillaLateral side of angle of mandibleElevates the mandibleTemporalisBeneath the fascia of lateral side of temporal boneCoronoid process of mandible1.Anterior & middle section elevate the mandible.2.Posterior section retract it.Medial PterygoidMedial surface of pterygoid plate,pyramidal process of palatine bone,tubercle of maxillaMedial side of angle of mandibleElevates the mandibleLateral PterygoidSuperior head-infratemporal surface of sphenoid boneInferior head-lateral surface of pterygoid plate

Superior head-TMJ capsule & discInferior head-pterygoid fovea on neck of mandibleProtrusion, depression and lateral movement of mandibleParameters of skeletal muscle function which have been found to correlate with facial morphology:Electromyographic activity Occlusal /Bite force

Parameters of skeletal muscle size which correlate with facial morphologyCross-sectional areaVolumeThicknessLength

Muscle activity in Angles malocclusionMuscle function is usually normal in Angles Class I malocclusion.(except open bite cases)

Class II division 1 malocclusions involve abnormal muscle activity

In Class II division 2 malocclusion, there is compensatory muscle activity, with dominance of posterior fibers of both the temporalis and masseter muscles.

In Class III and Class II division 1 malocclusions, the problem is that of dominant bone dysplasia with adaptive muscle function and tooth irregularity reflecting a severe basal dysplasia

Muscle activity in Angles malocclusionComparision of EMG activitiesElectromyographic activity: defined as the recording and study of the intrinsic electrical properties of skeletal muscle

The first effort to apply electromyography to dentistry was made by Robert E. Moyers

`Moyers investigated electromyograms of children with Class II division 1 malocclusion and found increased activity in the posterior part of the temporal muscle in habitual occlusion and at rest.

He asserted that this dysfunction might be an etiologic factor of post-normal occlusion

Ahlgren studied the EMG activity during chewing in eighty children but found no significant difference in integrated EMG activity between normal occlusion and Angle Class II malocclusion.

Pancherz (1980) analysed the electromyographic activity in the masticatory muscles of patients with Class II division1 malocclusion and normal occlusion. Recordings were made during maximal biting in centric occlusion and during chewing

During maximal biting in intercuspal position, the Class II subjects exhibited less electromyographic activity in the masseter and temporal muscles than the controls.

The reduction in electromyographic activity in the study group was most apparent for the masseter muscle.

During chewing the Class II subjects showed less electromyographic activity in the masseter muscle than in the normals. For the temporal muscle, no differences were found between the two groups.

High positive correlations were found between the electromyographic activity during maximal biting and chewing for both muscles of the two groups.

Miralles R (1991) showed that postural activity for masseter and anterior temporal muscles muscles was higher in Class III subjects than in Class I and Class II.

During swallowing, masseter muscle activity in Class III subjects was higher than Classes I and II, whereas anterior temporal muscle activity was not different between Classes III and I.

Deguchi T (1995) also concluded that compared with normal subjects, patients with a Class III malocclusion have a demonstrably abnormal masticatory muscle activity which is well characterized by the DL-EMG method.

However Amorim MM (2010) concluded that the electromyographic activity of the temporal muscle is not influenced by the skeletal class in Class III individuals.

EMG activity in skeletal or dentoalveolar open bite Faria et al compared the electromyographic (EMG) characteristics of masticatory muscles in children with either a skeletal or dentoalveolar open bite and compared with a control group

They found that EMG activity of the masseter and temporal muscles during clenching and chewing was significantly higher in the control group compared with the dentoalveolar antrior open bite (DAOB) and skeletal anterior open bite groups(SAOB).

The DAOB group had a higher mean EMG than the SAOB group during clenching and biscuit chewing.

EMG activity in cross-bite

Troelstrup et al investigated ,in 12 children of 811 years of age with unilateral cross-bite, the action of the temporal and masseter muscles which was studied (1) with the mandible at rest, i. e. without tooth contact, and (2) during maximal bite in the intercuspal position reported that with the mandible at rest.

They reported the activity in the anterior temporal muscle was strongest on the side of normal occlusion and in the posterior temporal muscles it tended to be strongest on the side of the cross-bite. During maximal bite the action of the posterior temporal muscles was reduced on the side with normal occlusion as compared with the side of the cross-bite.

The activity in the masseter muscles was not modified.

As a whole the pattern of activity at rest and during full effort indicated adaptation to the transverse occlusion

EMG activity in cleft lip and palate patientsLi et al. evaluated the characteristics of masticatory muscle activity in operated unilateral cleft lip and palate patients with anterior crossbite and compared with normal individuals.

Electromyographic activity of the masseter and anterior temporalis muscles were recorded bilaterally.

They found that the rest activity of temporalis and masseter muscles in cleft patients was higher than normal.

The hyperfunction of masseter and temporalis muscles in the rest position suggested that there was muscular strain in that position

However lower activity of masseter and temporalis muscle was found during maximum clenching suggesting that muscle were in hypofunction

Relationship with craniofacial morphologyForm of the face partly depends on the strength of the mandibular muscles.

Individuals with strong masticatory muscles have a more homogeneous facial morphology, in contrast to individuals with weak masticatory muscles who show great interindividual variation in their vertical facial dimensions

Subjects with strong or thick mandibular elevator muscles have wider transverse head dimensions.

Tendencies toward parallelism between jaw bases and between occlusal and mandibular lines, smaller gonial angles, smaller lower facial heights, and more rectangular shape of the face were also observed in some of the studies.

Increase in the function of the masticatory muscles is associated with anterior growth rotation pattern of the mandible and with well-developed angular, coronoid,and condylar processes.

Elevator muscles of the mandible influence the transversal and the vertical dimensions of the face and dentition

Masseter and Medial pterygoid muscles have large cross-sections in people with short anterior face heights and small gonial angles.

Van Spronsen et al reported no significant correlations between either anterior facial height or posterior facial height and any jaw elevator crosssectional areas in 32 men with normal skull shapes, when studied with magnetic resonance imaging.

Kiliardis et al reported that thickness of masseter muscle is related to facial morphology mainly in women , not in men,and women with thin masseter muscle has proportionlly longer face.

Benigton et al measured volume,cross sectional area,thickness and length of masseter muscle with 3D ultrasound in adult males and females.

They found that muscle volume showed negative correlation with mandibular inclination including gonial angle and positive correlation with total posterior face height and ramus height.Gionhaku and Lowe calculated masseter and medial pterygoid muscle volume and CSA from CT scan in adults with OSA They reported that masseter muscle volume showed negative correlation with steepness of mandibular plane ,size of gonial angel and positive correlation with ramus height ,posterior facial height and ratio of ramus height to anterior facial height.

Similarly Eckhardt and Harzer also used CT to measure volume of masseter, medial pterygoid and lateral pterygoid. They concluded that increasing volume of masseter muscle was related to anterior growth direction of mandible.

Bakke et al found a negative correlation between maximal thickness of superficial portion of masseter muscle and anterior facial height,vertical jaw relation and mandibular inclination.Position, orientation and mechanical advantage ofthe mandibular musclesA short posterior face height with steep mandibular plane and large gonial angles is often associated with an anteriorly inclined superficial masseter relative to the occlusal plane and a superior positioning of its insertion on the mandible.

Haskell et al reported that the superficial masseter was angled considerably more anteriorly with a much more acute angle to the occlusal plane in a dolichofacial specimen when compared with a brachyfacial specimenVan Spronsen et al however, observed that the orientation of the mandibular muscles in mesofacial and dolichofacial subjects were similar.Bite force and craniofacial patternThe bite force does not seem to vary between Angle malocclusion types

An important determinant of the maximum force that can be produced by a muscle is apparently its cross-sectional area.

Significant positive correlations have been reported between the cross-sectional areas of the masseter and medial pterygoid and the maximum molar bite force.

The maximum bite force also varies with skeletal craniofacial morphology, decreasing with increasing vertical facial relationships, the ratio between anterior and posterior facial height, mandibular inclination, and gonial angle

In dolichofacial subjects, significantly smaller maximum molar bite forces have been found during maximum effort than in mesofacial and brachyfacial subjects

Throckmorton et al prorposed that the significantly smaller bite force of dolichofacial subjects might to a large extent be due to the reduced mechanical advantage of the mandibular muscles.

However Hannam and Wood and Sasaki et al found a statistically significant correlation between masseter and medial pterygoid cross-sectional areas and molar bite force but no correlation between cross-sectional areas and the muscle moment arms.Human mandible as a lever

Cause and effect relationship between musclefunction and craniofacial morphologyThere is often a relationship between form and function, but it is not known whether a genetically determined facial morphology dictates the strength of the mandibular muscles or whether a strong musculature influences the form of the face.

Specific aspects of orthodontic treatment with reference to the mandibular muscles and vertical facial patternExtrusive mechanicsThere is a greater potential for the undesirable extrusion of molars in dolichofacial subjects compared with brachyfacials, who have stronger musculature that tends to resist extrusive forces during orthodontic treatment

Even if molar extrusion does occur during treatment in brachyfacial patients, there is likely to be a strong tendency toward reintrusion through the influence of the muscles during swallowing and chewingMuscular anchorageBench et al (1978) introduced the concept of muscular anchorageAccording to this concept teeth would be controlled with natural anchorage in a brachyfacial pattern, where the musculature is strong, but there would be less muscular anchorage in dolichofacial subjects with weak mandibular musculatureIt has been suggested that brachyfacial patterns might allow greater expansion of the arches during treatment, in contrast to dolichofacial patterns with generally weaker mandibular muscle forces that might allow less expansion during treatmentMuscles of mastication as predictor of treatment outcomeKiliaridis et al treated twenty-two children, aged 812, with skeletal and dental class II relationships and increased overjet were treated with twin-block appliances.

They found that treated children with thinner pre-treatment muscles showed greater mandibular incisor proclination, distalisation of maxillary molars, and posterior displacement of the cephalometric A point during treatment.Effect of treatmentKiliardis et al reported a decrease in thickness of Masseter muscle in patients treated with twin block appliance while muscle thickness increased in control subjects.

Pterygoid responseModifiation of functional position of mandible resulted in immediate alteration of the neuromuscular activity of the orofacial muscles, particularly noticable in the lateral pterygoid muscle which was called ptrygoid response.Studies done at Department of Orthodontics,AIIMSEMG studies:Aggarwal P et al conducted an electromyographic study on 10 young growing girls in the age group of 9 to 12 years with Class II Division 1 malocclusion and retruded mandible, who were under treatment with Twin-block appliances.

EMG activity while swallowing and maximal voluntary clenching was observed for a period of 6 months. Each patient underwent 4 EMG registration sessions both with and without the Twin-block appliance in the mouth.Before and immedialtey after before and immediately after fitting the Twin-block Within one month of fitting the applianceAt the end of 3 monthsAt the end of 6 monthsInsertion of the Twin-block appliance in the mouth caused a change in the EMG pattern of both the anterior temporalis and masseter muscles during the 6 months observation period. Both the muscles were stimulated; however, the masseter showed a more definite pattern of change

Although temporalis did show an increased activity this change was not significant

The masseter activity increased during postural rest position and clenching, whereas during the act of swallowing there was no change in EMG activityRajiv B. (2011) conducted EMG study in 7 growing children with midface deficiency within the age group of 7-13 years.

Patients were treated with a protocol of Rapid Maxillary Expansion and Delaire facemask for maxillary protraction

EMG recordings were mde at six stages of treatment

EMG activity (in V) of Masseter muscle during various activities in left and right side

EMG activity (in V) of Temporalis muscle during various activities in left and right side He concluded that RME and facemask therapy significantly increases EMG activity of the masseter and temporalis muscles initially in the clinical conditions of rest, dental clenching, and swallowing followed by gradual decrease towards baseline.

MRI studiesChaudhary P.(2010) studied masseter muscle dimension during active phase of twin block therapy over 6 month using MRI . Same cases were followed upto a duration of 18 months by Dongre S.Due to mild atrophy of masseter muscle, there was an initial decrease in volume, cross-sectional area and thickness of masseter muscle in response to altered functional environment by Twin Block appliance therapy.

As adaptation to altered environment continued, muscle showed a tendency towards normalization in its size but remains less than baseline at 6 months.

At 12 months and 18 months, all parameters were slightly more than pretreatment values but the change was not statistically significant.

That may be attributed to the anterior growth direction of mandible (Eckhardt and Harzer, 1993*) as well as continuing adaptation to the muscle stretch imposed by the Twin-block.

* Eckardt L, Harzer W .Computertomographic volume registration of jaw and tongue muscles in consideration of relationship between function of muscle and skeletal configuration. Eur J Orthod (1993). 15:442

ResultsComparison of changes in Masseter muscle dimensions (MMD) at various stages of Twin-block Therapy

17.14+3.8715.92+3.55*16.52+3.7017.913.74 18.013.75

3.53+0.64

3.15+0.56*

3.23+0.603.460.57 3.490 .62

1.35 0 .13

1.280 .13 1.300.111.330 .11 1.330.12

5.98+0.45

6.280.676.25+0.54 6.160.326.120.32Muscle adaptation occurs with functional appliance therapyInitial reduction in masseter volume(MV) and cross sectional area(CSA) with trend towards normalization thereafter.Muscle volume and CSA affected by Functional Appliance Therapy with minimal effect on length and thickness.

Ultrasound & EMG studiesMankar M. evaluated masseter muscle changes after twin block appliance therapy by USG and EMG.

In Twin block therapy there was initial increase in length and decrease in thickness of masseter muscle but the values normalized at 6 months and follow up upto 18 months.The thickness at 12 & 18 months was less than base line but not statistically significant.

The EMG activity decreased immediately on insertion of appliance but approached pre-treatment value at 3 months. At 6 months the value was higher than pretreatment. At end of 12 and 18 months the activity was still higher than pretreatment value but not significant.

Mean masseter muscle measurements using different imaging techniques at different Stages Observation timeEMG*Ultrasonography*MRI(Present Study)PRPSwMCLengthThicknessA-P diameterVolumeCross Sectional AreaLengthThicknessStage 0 (Pre T/t)41.7495.20504.515.250.884.2816.973.475.981.35Stage I (Twin Block insertion)40.8792.75500.135.530.813.93----Stage II (One month)28.3969.12290.025.570.783.88----Stage III (3 months)38.0891.55476.755.490.773.9015.923.156.281.23Stage IV (6 months)40.9199.55524.465.370.813.8716.523.236.251.28Stage V (12 months)41.2097.93521.625.330.823.9717.913.466.161.33Stage VI (18 months)42.27102.14553.075.300.844.0718.293.496.121.33* Mankar M, MDS thesis June 2010 References Andrew P,Michael W,Christopher B. The mandibular muscles and their importance in orthodontics: A contemporary review. Am J Orthod Dentofacial Orthop 2005;128:774-80.Kiliaridis S. Masticatory muscle influence on craniofacial growth. Acta Odontol Scand 1995;53: 196-202Kiliaridis S, Mills CM, Antonarakis GS.Masseter muscle thickness as a predictive variable in treatment outcome of the twin-block appliance and masseteric thickness changes during treatment Orthod Craniofac Res 2010;13:203213.Aggarwal P, Kharbanda OP, Mathur R, Duggal R, Parkash H. Muscle response to the twin-block appliance: an electromyographic study of the masseter and anterior temporal muscles. Am J Orthod Dentofacial Orthop. 1999 Oct;116(4):405-14.Pancherz H. Activity of the temporal and masseter muscles in Class II Division 1 malocclusions -An electromyographic investigation. Am.J.Orthod 1980;77(6):679-88.

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