Force of mouth opening

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SUBJECTS WITH ANTEROPOSTERIOR MANDIBULAR DYSPLASIA

Dissertation submitted to

THE TAMILNADU DR. M.G.R. MEDICAL UNIVERSITY For Partial fulfilment of the requirements for the degree of

MASTER OF DENTAL SURGERY

BRANCH - V

ORTHODONTICS AND DENTOFACIAL ORTHOPEDICS THE TAMILNADU DR. M.G.R. MEDICAL UNIVERSITY

CHENNAI – 600 032 2017 – 2020

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This is to certify that Dr.G.AADHIRAI Post graduate student (2017-2020) in the Department of Orthodontics and Dentofacial Orthopaedics, Tamil Nadu Government Dental College and Hospital, Chennai – 600003 has done this dissertation titled “Estimation of jaw opening forces in female subjects with anteroposterior mandibular dysplasia” under my direct guidance and supervision for partial fulfilment of the M.D.S. degree examination in April 2020 as per the regulations laid down by The Tamil Nadu Dr. M.G.R. Medical University, Chennai – 600032 for M.D.S., Orthodontics and Dentofacial Orthopaedics (Branch – V) degree examination.

Guided by

Dr. SRIDHAR PREMKUMAR, M.D.S.

Head of the Department

Professor of Orthodontics and Dentofacial Orthopaedics, Tamil Nadu Government Dental College &

Hospital, Chennai - 600003

Dr. Sridhar premkumar, M.D.S., HOD and Professor,

Dept. of orthodontics and DentofacialOrthopedics Tamil Nadu Government Dental College &

Dr. G.Vimala, M.D.S., Principal,

Tamil Nadu Government Dental College &

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I, Dr G.AADHIRAI, do hereby declare that the dissertation titled “Estimation of jaw opening forces in female subjects with anteroposterior mandibular dysplasia” was done in the Department of Orthodontics, Tamil Nadu Government Dental College &

Hospital, Chennai-600003. I have utilized the facilities provided in the Government Dental College for the study in partial fulfilment of the requirements for the degree of Master of Dental Surgery in the specialty of Orthodontics and Dentofacial Orthopaedics (Branch V) during the course period 2017-2020 under the conceptualization and guidance of my dissertation guide, HOD and Professor Dr. SRIDHAR PREMKUMAR, MDS.,

I declare that no part of the dissertation will be utilized for gaining financial assistance for research or other promotions without obtaining prior permission from The Tamil Nadu Government Dental College & Hospital.

I also declare that no part of this work will be published either in the print or electronic media except with those who have been actively involved in this dissertation work and I firmly affirm that the right to preserve or publish this work rests solely with the prior permission of the Principal, Tamil Nadu Government Dental College & Hospital, Chennai 600 003, but with the vested right that I shall be cited as the author(s).

Signature of the PG student Signature of the HOD

Signature of the Head of the Institution

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I seek the blessings of the ALMIGHTY GOD without whose benevolence this study would not have been possible.

With my heartfelt respect, immeasurable gratitude and honour, I thank my benevolent guide, Dr. SRIDHAR PREMKUMAR, M.D.S., Head of the department, Professor, Department of Orthodontics and Dentofacial orthopaedics, Tamil Nadu Government Dental College and Hospital, Chennai – 3, for his astute guidance, support and encouragement throughout my post graduate course and to bring this dissertation to a successful completion.

My sincere and heartfelt thanks to Dr. G. VIMALA, M.D.S., our Principal, Tamil Nadu Government Dental College and Hospital, Chennai – 3, for her continuous and enormous support in allowing me to conduct this study and for his constant encouragement and advice during my tough phases in curriculum.

I owe my thanks and great honour to Dr. BALASHANMUGAM, M.D.S., Professor, Department of Orthodontics and Dentofacial Orthopaedics, Tamilnadu Govt. Dental College and Hospital, Chennai - 3, for helping me with his valuable and timely suggestions and encouragement.

I sincerely thank Professor Dr. G. USHA RAO, and Associate Professors Dr. M. VIJJAYKANTH, Dr. M.D. SOFITHA and Senior Assistant professors Dr. K. USHA, Dr. M.S. JAYANTHI, Dr. D. NAGARAJAN, Dr. MOHAMMED IQBAL,

and Dr. R. SELVARANI for their continuous support and encouragement.

I thank Dr. RAVANAN for helping me in statistical analysis.

I thank the almighty god in the form of my father- V.R.GOPINATH, my mother VASANTHI GOPINATH for their blessings, unconditional love, affection, care and prayers. Without them, nothing would have been made possible.

I also thank my post graduate colleagues for their help and constant support.

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This agreement herein after the “Agreement” is entered into on this... day of December 2019 between the Tamil Nadu Government Dental College and Hospital represented by its Principal having address at Tamil Nadu Government Dental College and Hospital, Chennai-03, (hereafter referred to as, “the college”)

Dr. G.AADHIRAI aged 26 years currently studying as postgraduate student in Department of Orthodontics in Tamil Nadu Government Dental College and Hospital (Herein after referred to as the ‘PG/Research student and Principal- investigator’).

And

Dr. SRIDHAR PREMKUMAR aged 52 years working as Head and professor at the college, having residence at B-3, Block-2, Jains Ashraya Phase III, Arcot road, Virugambakkam, Chennai-92 Tamil Nadu. (Herein after referred to as the ‘co- investigator’)

Whereas the ‘PG/Research student as part of his curriculum undertakes to research

“Estimation of jaw opening forces in female subjects with anteroposterior mandibular dysplasia” for which purpose the PG/Principal investigator shall act as principal investigator

and the college shall provide the requisite infrastructure based on availability and also provide facility to the PG/Research student as to the extent possible as a Co-investigator.

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including in particular the copyright and confidentiality issues that arise in this regard.

Now this agreement witness as follows:

1. The parties agree that all the Research material and ownership therein shall become the vested right of the college, including in particular all the copyright in the literature including the study, research and all other related papers.

2. To the extent that the college has legal right to do go, shall grant to license or assign the copyright do vested with it for medical and/or commercial usage of interested persons/entities subject to a reasonable terms/conditions including royalty as deemed by the college.

3. The royalty so received by the college shall be shared equally by all the parties.

4. The PG/Research student and PG/Principal Investigator shall under no circumstances deal with the copyright, Confidential information and know – how generated during the course of research/study in any manner whatsoever, while shall sole vest with the manner whatsoever and for any purpose without the express written consent of the college.

5. All expenses pertaining to the research shall be decided upon by the principal investigator/Co-investigator or borne sole by the PG/research student (co- investigator).

6. The college shall provide all infrastructure and access facilities within and in other institutes to the extent possible. This includes patient interactions, introductory letters, recommendation letters and such other acts required in this regard.

7. The principal investigator shall suitably guide the student Research right from selection of the Research Topic and Area till its completion. However the selection and conduct of research, topic and area research by the student researcher under

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recommendations and comments of the Ethical Committee of the college constituted for this purpose.

8. It is agreed that as regards other aspects not covered under this agreement, but which pertain to the research undertaken by the student Researcher, under guidance from the Principal Investigator, the decision of the college shall be binding and final.

9. If any dispute arises as to the matters related or connected to this agreement herein, it shall be referred to arbitration in accordance with the provisions of the Arbitration and Conciliation Act, 1996.

10. In witness where of the parties herein above mentioned have on this the day month and year herein above mentioned set their hands to this agreement in the presence of the following two witnesses.

Principal PG Student

Witnesses Student Guide 1.

2.

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CERTIFICATE II

This is to certify that this dissertation work entitled research “Estimation of jaw opening forces in female subjects with anteroposterior dysplasia” – of candidate G.AADHIRAI with registration number 241719001 for the award of MASTER OF DENTAL SURGERY in the branch -V.I personally verified the urkund.com website for the purpose of plagiarism check. I found that uploaded thesis file contains from introduction to conclusion pages and result shows 9% only of plagiarism in the dissertation.

Guide and supervisor sign with seal

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SL. NO. TITLE PAGE NO.

1. INTRODUCTION 1

2. AIMS AND OBJECTIVES 4

3. REVIEW OF LITERATURE 5

4. MATERIALS AND METHOD 29

5. RESULTS 39

6. DISCUSSION 47

7. SUMMARY AND CONCLUSION 51

8. BIBLIOGRAPHY 54

9. ANNEXURE 60

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SL.

NO.

TOPIC

PAGE NO

1 Test for normality using Kolmogorov Smirnov test 39

2

Comparison of groups between class I, class II and class III KRUSKAL-WALLIS

40

3 Comparison of the class I and class II -MANN-WHITNEY 41 4 Comparison of class Iand class III-MANN WHITNEY 42 5 Comparison of class II and class III-MANN WHITNEY 43

LIST OF FIGURES

FIGURE NO. TITLE

1 Muscle chain

2 Force sensor

3 Instruments for detection mouth opening force

4 The circuit for detecting force

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6 Class II patient with the device in open mouth position 7 Class III patient with the device in rest position 8 Class III patient with the device in open mouth position 9 Qualitative assessment of force- low with green light 10 Qualitative assessment of force- medium with yellow light 11 Qualitative assessment of force- high with red light

LIST OF BAR DIAGRAMS

1 Force levels between the three groups 2 Force in class I

3 Force in class II 4 Force in class III

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INTRODUCTION

Muscles play an important role in the aetiology, treatment planning and stability of orthodontic treatment. It is essential to describe the muscle in an orthodontic perspective as the bony contours get shaped up according to the muscle pull. Therefore, it is imperative to have a thorough knowledge on the normal muscle function and an increased activity of which may be a possible aetiology in a malocclusion. With the identification of muddled muscles, treatment planning eventually would involve establishing the proper balance, which would result in stable outcome.

Many factors that have been linked to influence the facial type include the condylar position, muscle force, gravity, ramal height, body length and gonial angle.

Growth studies ^1,2 gives the impression that as the face enlarges it grows downwards and forwards away from the cranial base. However, it is now known that growth of the craniofacial region is much more complex than this, with the calvaria, cranial base, maxilla and mandible experiencing differing rates of growth and differing mechanisms of growth at different stages of development, all of which are under the influence of a variety of factors- muscles being crucial amongst them . The overall pattern of facial growth results from the interplay between them and they must all harmonize with each other if a normal facial form is to result. Small deviations from a harmonious facial

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growth pattern will cause discrepancies of facial form and jaw relationships which are of major significance to the dentists.

The differences in muscle morphology, muscle force and the differential craniofacial growth pattern is also a less probed topic which would be of much interest to an orthodontist. Not only the malocclusions but also the facial changes possibly occur with the changes in muscle. There is a vacuum in this aspect.

Its already been proven that bite force or the muscles involved in closing the mouth does play an important role in the facial type or malocclusion of the patient either as a cause or effect. The objective of determining the bite force which is said to be an indicator of functional status of masticatory system has been dealt in many studies and no concentration on mouth opening forces which is the combatant to the former was given.

On identifying the mouth opening forces, which would contribute to stability of the jaw- orthodontist would have a holistic approach to diagnosis, stability of orthodontic treatment and prevention of post-treatment relapse tendencies. Jaw opening forces or the muscles involved will affect the stomatognathic system and therefore may potentiate malocclusion. To our knowledge no study has been done regarding estimation of jaw-opening force and its significance in malocclusion.

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Little is known about the properties of the jaw depressor muscles, namely the digastric, geniohyoid and mylohyoid. These muscles act against the hyoid bone to pull the mandible downwards. In this process, these muscles are also assisted by the stylohyoid, infrahyoid and lateral pterygoid muscles, which assist in stabilizing the hyoid bone and promoting the anterior sliding movement of the mandibular condyle respectively.

Investigation of the properties and biomechanics of jaw depressor muscles will further enhance our understanding of the masticatory apparatus function, in addition to contributing to applied clinical studies. These include the diagnosis and management of diseases and conditions which require knowledge of the forces to be resisted by intermaxillary fixation and other dental devices.

Hence this study is undertaken which can throw light on probable aetiology of malocclusion or an approach to maintain a natural balance in the musculature.

Relapse being one of the biggest demerits in orthodontics, can be dealt in with caution if the muscle factors are probed to perfection.

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AIMS AND OBJECTIVES

AIM OF THE STUDY:

To compare the force interplay in maximum mouth opening in female subjects with anteroposterior mandibular dysplasia.

OBJECTIVES:

1. To compare the force interplay in maximum mouth opening in subjects with retrognathic mandible and normal maxilla

2. To compare the force interplay in maximum mouth opening in subjects with mandibular prognathism with normal maxilla

3. To compare the force interplay in maximum mouth opening in subjects with normal maxilla and normal mandible.

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REVIEW OF LITERATURE

According to Horowitz 1955- Exercising masseter muscles unilaterally in growing rats resulted in severe modification of the skull, dento-alveolar complex and in malocclusion on the affected side^.³

According to Graber 1963- An analysis has been made of muscles and their relationship to structural configuration in Class I, Class II, and Class III malocclusions.

The effect of muscle forces is three-dimensional, although most orthodontists have considered it only in one vector—that of expansion. Whenever there is a struggle between muscle and bone, bone yields. Muscle function can be adaptive to morphogenetic pattern. A change in muscle function can initiate morphologic variation in the normal configuration of the teeth and supporting bone, or it can enhance an already existing malocclusion. In the latter instance, the inherent structural malrelationship calls for compensatory or adaptive muscle activity to perform the daily functions. The structural abnormality is increased by compensatory muscle activity to the extent that a balance is reached between pattern, environment, and physiology. At times it is impossible to assign a specific cause-and-effect role to any one factor. It is imperative that the orthodontist appraise muscle activity and that conducts his orthodontic therapy in such a manner that the finished result reflects a balance between the structural changes obtained and the functional forces acting on the teeth and investing tissues at that time

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Vertical dimension, diagrammed to demonstrate role of muscles in maintaining the balance of the head on the vertebral column. Post- and prevertebral muscles and masticatory, facial, and hyoid group muscles all contribute to the establishment of the relatively constant postural resting position. ⁴

Fig 1: Muscle chain

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Nanda 1967 - employed a surgical procedure to reposition the masseter muscle unilaterally on nine young dogs: this produced angular changes and loss of antegonial notch on the operated side. More recently, similar effects have been obtained by altered expression of myostatin (GDF-8) which is the principal factor that negatively regulates muscle fiber growth.⁵

Sassouni 1969 outlined the concept that vertical alignment (and subsequent force) of jaw-closing muscles directed skeletal growth toward a shallow mandibular plane angle, an acute gonial angle, and deep bite, whereas obliquely aligned jaw-closing muscles (with subsequent diminished force) permitted a steep mandibular plane, an obtuse gonial angle, and open bite. Bite force studies have documented diminished occlusal force at the molar occlusal plane in long-faced adults. These force differences might not be due to intrinsic muscle differences but, rather, to mechanical advantage loss in obliquely applied force. ⁶

Yildrim and DeVincenzo 1971 jaw- opening and jaw- closing forces in 45 individuals with varying degrees of open and closed bites. These authors reported higher values for opening forces for males compared to females; however, there was no significant difference between people with an open and closed bite.⁷

Moore 1973 Masseteric muscle fibers are inclined from posterior mandible to anterior zygomatic arch; when muscle was brought anterior, the fibers became more vertical and shorter. Compared with a brachyfacial specimen, the superficial masseteric muscle

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is angled considerably more anteriorly, with a much more acute angle to the occlusal plane in a dolichofacial specimen.

In dolichofacial subjects, significantly smaller maximum molar bite forces have been found during maximum effort than in mesofacial and brachyfacial subjects. This implies a correlation between bite force and facial morphology, and these findings have been used to support the theory that the form of the face partly depends on the strength of the mandibular muscles.

Variation between people with weak muscles is therefore wide, and those with weak mandibular muscles can belong to either the mesofacial or the dolichofacial group. 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. ⁸

Astrand 1977 -The maximum isometric contraction force of a muscle is proportional to its cross-section area (Astrand and Rodahl, 1977). As muscle area is related to the size of the bony attachments, muscle force also tends to be so related. Thus, the strength of jaw depressor muscles might similarly be related to the size of their bony attachments^.9

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Finn 1978 reported that maximum biting force in the molar region was twice as great for normal as compared to dolichofacial (long-face) subjects (128 vs. 66 pounds), while brachyfacial (short-face) subjects had still higher maximum force. These data were obtained with a quartz crystal transducer producing about 10 mm of jaw separation. ¹⁰

Throckmorton 1980 A two-dimensional model which allows calculation of mechanical advantage of the human temporalis and masseter muscles is presented. The model is manipulated to demonstrate how selected differences in facial morphology affect the mechanical advantage of the muscles. The model is then used to evaluate the differences in mechanical advantage between patients with the long face syndrome and those with the short face syndrome. Differences in facial morphology between these two groups result in significant differences in the mechanical advantages of their muscles. Mechanical advantage explains observed differences in bite force between the two groups. The model suggests that some surgical procedures used to correct facial disharmonies may have a significant effect on the mechanical advantage of the jaw muscles. The adductor muscles have mechanical advantage when the ramus is more vertical and gonial angle acute. Thus, proved that bite force is reflection of form. ¹¹

Proffit 1983- The square jaw (relatively acute gonial angle) and short lower face height of the "powerful individual" are part of the usual caricature, despite a lack of data to demonstrate that occlusal forces are greater in individuals with this facial

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morphology. Case reports have demonstrated a lengthening of facial dimensions in individuals with muscular weakness.

Piezo-electric force transducers using quartz crystals as the active element are now readily available commercially and can be used quite satisfactorily for measuring occlusal forces at several millimetres of jaw separation. Recently, it has become possible to manufacture piezo-electric polymers which can be produced as thin foil sheets. Using piezo-foil as the active element, we have been able to fabricate an occlusal force transducer less then 2 mm in thickness, which allows us to measure occlusal forces with much less jaw separation than in previous experiments. The sensitivity of both types of piezo-electric transducers makes it possible to study not only maximum biting force, but also forces generated both during simulated chewing and when the jaw is stabilized for swallowing by contraction of the elevator muscles. Stronger relationships exist between skeletal hyper divergence and masticatory function, including reduced jaw muscle size, lower maximum bite force, lower electromyography (EMG) activity and reduced muscle efficiency.¹²

Sharkey 1984- demonstrated maximum mouth opening forces with extraoral gnathodynamometer and found out that mandibular plane angle had a correlation with MMO forces.¹³

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Weijs 1984 Cross-sectional areas of the masseter, temporalis and medial, and lateral pterygoid muscles were determined in 16 subjects by means of computer tomography.

In each subject three scans were made, intersecting the thickest part of the muscles at right angles to the fiber direction. The masseter and medial pterygoid muscles are large in persons with brachycephalic skulls, short faces, and a small jaw angle. The cross- sectional areas of the temporalis and lateral pterygoid muscles showed no correlation with facial dimensions. The lack of correlation between the cross-section of the temporalis, the largest jaw muscle, and head shape is striking. The temporalis varies less in cross-section than do the other jaw muscles and shows a relatively weak correlation with the total jaw muscle cross-section.¹⁴

Takada 1984 a 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.

In order to identify associations between the orientation of the superficial masseter and temporalis muscles and dentoskeletal morphology were evaluated from lateral head films. Three canonical correlations were identified between the muscle orientation and dentoskeletal variable groups at the 0.05 level of significance and canonical loadings were determined for each set of variables. The first canonical correlation (r1 = 0.931) represents a growth-related correlation factor between the masticatory muscle insertion positions relative to the occlusal plane and the dimensional and positional changes of craniofacial structures during growth. The second canonical correlation (r2 = 0.846) may account for an operational artefact in the geometric measurement process, and the

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third canonical correlation (r3 = 0.813) suggests an association between superficial masseter muscle orientation relative to the occlusal plane and mandibular form. This report confirmed the usefulness of multivariate statistical methods to extract latent associations between muscle orientation and craniofacial morphology; the results obtained suggested a contribution from the geometric orientation of the masticatory muscles to the development and maintenance of the dentoskeletal system.¹⁵

Bolt and Orchardson 1986- No significant relationship was demonstrated between OF and height, weight, lean body-mass or maximum gape; however there was a significant relationship between MOF and mandibular base lengths Ar-me and Ar-GT. Significant relationships also existed between both MOF and AOF and each of gonial angle, angle (Go-Ar-GT) and the ratio of mandibular-base length to mandibular-body length. A significant positive correlation existed between OF and angle NS-ML, and there was a significant negative correlation.¹⁶

Gionhaku 1989 Masseter muscle volume had a negative correlation with mandibular plane and gonial angle, and a positive correlation with posterior face height, ramus height, posterior face length, condylar center to first molar point length, gonion to pterygomaxillary fissure length, and the ramus height/anterior face height ratio. Medial pterygoid muscle volume showed a positive correlation with posterior face height, ramus height, posterior face length, and the lengths between condylar centre to first

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molar contact point, gonion to pterygomaxillary fissure, and ante-gonion to key ridge.

Subjects with large masseter and medial pterygoid muscle volumes had flat mandibular and occlusal planes, and small gonial angles. ¹⁷

Minamitani 1990 Excessive muscle contraction can also restrict craniofacial growth.

This effect is seen most clearly in torticollis, a twisting of the head caused by excessive tonic contraction of the neck muscles on one side, primarily the sternocleidomastoid.

The result is facial and cranial asymmetry because of growth restriction on the affected side, which can be quite severe unless the contracted neck muscles are surgically detached at an early age.¹⁸

Van spronsen 1991 Serial MRI scans of the jaw muscles were taken approximately perpendicular to the mean fibre direction of the jaw muscles to determine their cross- sectional areas. These areas are proportional to the maximal isometric strength of a muscle. To describe facial skeletal variation, nine angular and 21 linear cephalometric measurements were recorded, and statistically reduced by means of multiple regression and principal component analysis. Six components were extracted, rotated, and subsequently correlated with the maximal cross-sectional areas of the jaw elevators and anterior digastric muscle. Positive significant correlations were found between a linear combination of several transversal skull dimensions on the one hand, and the maximal temporalis and masseter cross-sections on the other. A negative significant correlation

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was found between the flexure of the cranial base and the temporalis cross-section.

Surprisingly, no significant correlations were found between either anterior facial height or posterior facial height and any of the jaw muscles cross-sections. It was concluded that, in adult males with normal skull shape, relationships exist to a limited extent between craniofacial morphology and the cross-sectional areas of the jaw muscles.

Masticatory performance is a direct quantitative evaluation of masticatory function. It scales one’s ability of breaking down food according to particle sizes after a specific chewing cycle number. Many factors like age, gender, BMI, bite force and occlusal contact areas have been proved to have an impact on masticatory performance Malocclusion, to some extent, is regarded as a form of mutilated dentition, which could negatively affect masticatory function based on mechanical disadvantage.¹⁹

Bakke 1991- Activity in temporalis and masseter muscles, and traits of facial morphology and occlusal stability were studied in anterior open bite cases and symptoms and signs of craniomandibular disorders. Facial morphology was assessed by profile radiographs, occlusal stability by tooth contacts, and craniomandibular function by clinical and radiological examination. Electromyographic activity was recorded by surface electrodes after primary treatment with a reflex-releasing, stabilizing splint. Maximal voluntary contraction was reduced compared to reference values, particularly in subjects with muscular affection, but maximal activity increased significantly when biting on the splint. Maximal voluntary contraction was positively correlated to molar contact and negatively to anterior face height, mandibular

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inclination, vertical jaw relation and gonial angle. Relative loading of the muscles was markedly increased during resting posture. It was concluded that reduced occlusal stability and long-face morphology were associated with weak elevator muscle activity with disposition overload and tenderness. The results also indicated that increase of occlusal stability might lead to increased muscle strength and possibly reduce risk of physical strain. It is generally accepted that a relationship exists between the form and function of the craniofacial skeleton. Weaker maximum bite forces have been related to increased malocclusions, especially in subjects with open bite tendencies and posterior crossbites with narrow maxillary arches, and incisor crowding.²⁰

Muto 1994-The positional change of the hyoid bone in both closed and maximal mouth-opening positions of the mandible was investigated by cephalometric measurements. The following results were obtained: (1) With the increase in mouth opening the hyoid bone moved downward and backward. At maximal mouth opening the head posture changed posteriorly compared with that of occluded mouth position.

(2) By superimposing films of the S-N plane, it became apparent that the hyoid bone was displaced downward by sagittal opening movement of the mandible and backward by the posterior change of the head posture. (3) Significant correlations were found between the degrees of sagittal rotation of the mandible and the position of the hyoid bone. (4) These results suggest that the posterior change of the head posture and inferior shift of the hyoid bone with mouth opening are important factors in obtaining maximal mouth opening.²¹

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Eckardt 1996 Progressive weakness of the mandibular elevator muscles (as in Duchenne muscular dystrophy) leads to a downward and backward rotation of the mandible away from the maxilla with a resultant long face deformity and open bite.²²

Muto and Kanazawa 1996- It is thought that the distance between upper and lower incisors at maximal mouth opening correlates more to the size of the mandible than to body height. During mouth opening the condyle rotates and translates, often to the summit of the articular eminence. In the study, the distance of the condylar part was 20.5 mm in men and 18.1 mm in women, respectively. This difference was significant and may be explained by the size of the skeleton, and there was a strong correlation between condylar path and maximal vertical movement of the condyle.²³

Henrikson 1998 who detected that children with normal occlusion had better masticatory performance than those with Class II and Class III malocclusions.²⁴

Fitts 2001- Studied on both rats and humans demonstrate a rapid loss of cell mass with microgravity. In rats, a reduction in muscle mass of up to 37% was observed within 1 week. For both species, the antigravity soleus muscle showed greater atrophy than the fast-twitch gastrocnemius. However, in the rat, the slow type I fibers atrophied more than the fast type II fibers, while in humans, the fast type II fibers were at least as susceptible to space-induced atrophy as the slow fiber type. Space flight also resulted in a significant decline in peak force. The reduced force can be attributed both to muscle atrophy and to a selective loss of contractile protein. The former was the primary cause

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because, when force was expressed per cross-sectional area (kNm(-2)), the human fast type II and slow type I fibers of the soleus showed no change and a 4% decrease in force, respectively. Microgravity has been shown to increase the shortening velocity of the plantar flexors. This increase can be attributed both to an elevated maximal shortening velocity of the individual slow and fast fibers and to an increased expression of fibers containing fast myosin. Although the cause of the former is unknown, it might result from the selective loss of the thin filament actin and an associated decline in the internal drag during cross-bridge cycling. Despite the increase in fiber V(0), peak power of the slow type I fiber was reduced following space flight. The decreased power was a direct result of the reduced force caused by the fiber atrophy. In addition to fiber atrophy and the loss of force and power, weightlessness reduces the ability of the slow soleus to oxidize fats and increases the utilization of muscle glycogen, at least in rats.

This substrate change leads to an increased rate of fatigue. Finally, with return to the 1g environment of earth, rat studies have shown an increased occurrence of eccentric contraction-induced fiber damage.²⁵

Gedrange 2004 The postnatal craniofacial development is determined by exogenous and endogenous factors that may result in morphological and functional muscle changes and influence the dentoskeletal region in terms of a physiologic or dysgnathic development. Using functional appliances, efforts are made to treat skeletal malocclusions through targeted exercise and to prevent an undesirable development of the dentition and the craniofacial structures. However, the success of the treatment and the stability of the outcome are not always adequate. To illustrate the treatment processes, clinically relevant measures for diagnosing muscle function and morphology have been developed in recent years. Electromyographic investigations and bite-force

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measurements show an excessively high variability and the histologic examinations applied to date are restricted in their suitability for analysis of the human masticatory muscles. Animal experimental studies have meanwhile succeeded in simulating functional jaw orthopaedics and in demonstrating muscle remodelling processes at the genetic level. Despite some invasiveness, the time and the small quantity of muscle tissue involved permit molecular biological measuring in the orofacial system.

Although a number of findings relating to the problem of muscular reaction to functional jaw orthopaedics have been published, they are by no means sufficient for conclusions to be drawn on a more targeted procedure or for causes of therapeutic failure to be deduced. On the other hand, it is clear that only a neuromuscular system adapted to a changed sagittal jaw relationship can guarantee the stability of the treatment outcome. The challenge facing clinical activity is the need and/or the opportunity for a more highly differentiated functional diagnosis based on modern methods of diagnosing neuromuscular function. The methods currently available are either too unspecific or have still to be perfected.²⁶

Rowlerson 2005 One constant feature of masseter muscle is the predominance of type I fibers: they have the largest mean area and often are the most numerous types. At normal vertical dimension, type I fibers occupy approximately half the tissue volume of masseter muscle and type II fibers only approximately 15%. In open-bite subjects, there is little difference in the size and occupancy of type I fibers, but the occupancy of type II fibers drops to approximately 8%, with an increase in hybrid fibers. In deep bite subjects, there are significant changes in percent composition of all 3 of these fiber types; that is, type I and hybrid fibers are substantially decreased in occupancy and type II fibers substantially increased in occupancy. The neonatal/atrial group typically

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occupies a modest area of masseter muscle. Although much speculation is possible regarding these variations, a striking observation is the nature of variation in the type II fiber group. In open-bite subjects, type II fibers occupy the smallest area of masseter muscle, but, at the other extreme, in deep bite subjects, they are approximately equal in occupancy to the type I fibers.²⁷

Pepicielli 2005- Their review presents an outline of the mandibular muscles and the vertical facial pattern. The different methods by which the mandibular muscles have been investigated were discussed. The potential influence of these muscles on normal morphologic variation in different people was also discussed, along with the implications for contemporary orthodontic treatment and stability.²⁸

Kwon 2007- There was a significant correlation between thick masseter muscles and a longer ramus. Botulinum toxin A (BTXA) has been used widely to induce muscular atrophy, especially for patients with masseteric hypertrophy. Botulinum toxin binds to presynaptic nerve endings and inhibits the release of acetylcholine. Thus, BTXA injection into the masseter leads to temporary partial denervation and a reduction in the cross-sectional areas of muscle fiber. There was a reduction in ipsilateral ramal height in the experimental groups accompanied by a compensatory increase of the body length, thus resulting in symmetry of the mandibular length. This study showed that unilateral masseteric hypofunction affected the local skeletal site at the origin or insertion area of the muscle.²⁹

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Raoul 2011- The aetiology of some forms of human malocclusion, and orthodontic treatment philosophy, were attributed to the anatomic position of jaw closing muscles and subsequent force vectors acting on the skeleton during crucial periods of facial growth, tooth eruption and dental alveolar development. They found significant relationships between anterior facial height and fiber type II myosin isoform in the masseter of orthognathic surgery patients, indicating an association between masseter composition and craniofacial morphology. It was the type II (fast) fiber population that was significantly altered in relation to the craniofacial form. In the previous study by the same author, type II fiber occupancy was increased in patients with deep bite (reduced anterior facial height), and in the patients with mandibular asymmetry described here, it was increased on the side of the deviation (as considered as the short side). This is consistent with the idea that the muscle in such cases was developing more force because type II fibers (in larger motor units) are normally recruited principally at higher contraction strengths and that this may have influenced mandibular form. The opposite condition, jaw muscle weakness, is known to predispose toward open bites.

In a signiﬁcant number of patients with craniofacial dysmorphologies, the cause of the condition is unknown. Bone is readily remodeled during growth in response to a number of factors, which can include muscle action, as encapsulated in the Wolff law.³⁰

Sciote 2013 Vertical facial dimension is related to fiber type composition (with an increase in type-II in deep bite cases); Facial asymmetry is common in dentofacial deformities patients, and shows the same relationship between vertical dimension and fiber type composition (type-II increased on the ‘short’ side);There are some gender differences in masseter muscle fiber sizes; sagittal facial dimension shows little relation

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to masseter fiber type composition. These fiber types and their contractile activity pattern provide a mechanism by which the compressive forces of muscle function can influence mechano-transduction and modelling of bone to produce jaw deformation phenotypes seen in malocclusion.³¹

Choi 2015 Apart from analysing the association in terms of Angle Classifications, utilized objective and subjective methods to examine the masticatory function of individuals with vertical craniofacial discrepancies, i.e. anterior open-bite. The results revealed that masticatory function in patients with non-sagittal discrepancies is significantly reduced both objectively and subjectively.³²

Brunton 2017 aimed to estimate maximum jaw- opening forces in a large sample of healthy participants of broad age range and diverse ancestry and to estimate whether opening forces were associated with sex, age and anthropometric parameters such as height, weight and BMI.

An adjustable extraoral device consisting of a rigid skull and chin caps connected to a 1000 N load cell, developed specifically for this study, was fitted to each participant.

The participants had the measuring device placed on the head in such a way that the skull cap was opposite the chin cap with the jaw in centric occlusion, and the measuring device was tightened to the point where there was the beginning of discomfort. The device was connected to a Power Lab data acquisition system (AD Instruments, Dunedin, New Zealand). A few test attempts at jaw opening were performed, for each subject, prior to each data collection session to ensure the device was correctly placed and that participants were comfortable with the equipment.

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Amongst women, median opening forces were higher with greater ages, height, weights and BMIs. For men, median opening forces were only associated with weight. The effect for age was positive for women but negative for men.³³

STUDIES ON BITE FORCE:

Bakke 1989 At predetermined levels of contraction, temporalis and masseter activity were linearly related. Correlations of bite force and activity in short static contractions were significant with respect to unilateral, but not to bilateral force measurements. Only in the masseter muscle was strength of dynamic contractions during chewing significantly correlated to bite force. With the present method it was demonstrated that unilateral bite force is a simple clinical indicator of mandibular elevator strength as a whole, but inadequate to disclose asymmetric conditions.³⁴

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Sasaki 1989- Found that the variation of maximum bite force could be accounted for largely by muscle cross-sectional areas rather than simply by muscle lever arms Human subjects commonly show large variations in bite force produced at the first molar teeth.

To evaluate the role of muscle cross-sectional sizes and lever arms in bite-force production, they correlated these variables in 11 healthy adults. Axial and coronal images obtained by magnetic resonance were combined with conventional lateral cephalograms and dental cast data to reconstruct the craniomandibular morphology in each subject. The cross-sectional sizes of the right masseter and medial pterygoid muscles, their lever arms, and the bite-point lever arms were measured directly from these reconstructions. Physiological recordings of bite force were made in the region of the right first molar by means of a customized transducer aligned perpendicular to the functional occlusal plane. The average bite force for the sample as a whole was 189 +/- 78 N. Despite the fact that craniofacial spatial morphology may differ among subjects, jaw muscle size alone seems to explain most of the variation in bite force reported by them.³⁵

Kiliardis 1993 -The occlusal relationship, body height, finger force, maximal bite force, and bite force endurance amplitude were recorded. All bite force variables and finger force increased with age in both sexes. A positive correlation was found between the maximal bite force in the incisor region and the ratio of upper to lower facial height;

this is, subjects with a high bite force had a relatively short lower anterior height. The maximal bite force for molars and endurance amplitude were positively correlated to stature and finger force but not to facial characteristics. A longitudinal study to follow

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each individual child during growth would be of interest to evaluate the importance of muscular influence on facial growth^.36

Braun 1995 A new device for measuring and recording bilateral bite force in the molar/premolar region has been developed. Because this new device is elastic and conforms to the occlusal surfaces of the teeth, and because the sensing element is relatively comfortable, it is believed that experimental subjects are less reluctant to register true maximal forces than in earlier studies. Potential correlations of maximum bite force to gender, age, weight, body type, stature, previous history of orthodontic treatment, presence of TMJ symptoms (jaw motion limitation, clicking with pain, or joint pain), or missing teeth were studied. The mean maximum bite force as related to gender was found to be statistically significant, while the correlation coefficients for age, weight, stature, and body type were found to be low.³⁷

Braun 1996 While earlier studies have shown adult males have a greater mean bite force than females, this difference is not evident during growth and development.

Gender-related bite force difference likely develops during the post pubertal period in association with greater muscle mass development in males.³⁸

Hunt 1997 - Human clinical studies of muscle function have mainly revolved around the measurement of either occlusal force, or muscle volume and area in relation to differing facial morphologies. It is now evident that patients with a short vertical dimension are capable of producing occlusal forces far in excess of those recorded from

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patients with normal vertical facial form. Conversely, individuals with long face deformities produce very weak occlusal forces as measured either during swallowing, chewing or maximum clenching.³⁹

Ingervall 1997- Facial morphology was evaluated on profile cephalograms. In addition, the number of teeth in contact in the intercuspal position was recorded with occlusal foils. In the girls, maximum bite force was correlated with the inclination of the mandible, the size of the gonial angle, and the ratio between posterior and anterior face heights. The correlations implied a large bite force with a small mandibular inclination and gonial angle, a large posterior face height in relation to the anterior face height, and a small bite force with the opposite facial characteristics. These correlations were non-existent or weaker in boys. In both sexes, bite force was correlated with the number of occlusal contacts. Elimination of the influence of age and occlusal contact in the group of girls by the use of partial correlations reduced the correlation between bite force and facial morphology. A significant correlation with the size of the gonial angle remained, however, and the correlation with mandibular inclination was close to significance. In addition to the correlations found with facial morphology, the study clearly demonstrated the need to take gender and occlusal contacts into consideration in future studies of masticatory muscle function and strength in relation to facial morphology.⁴⁰

Raadsheer 1999 -From the jaw muscles, only the thickness of the masseter muscle correlated significantly with bite force magnitude. Bite force magnitude also correlated significantly positively with vertical and transverse facial dimensions and the

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inclination of the midface, and significantly negatively with mandibular inclination and occlusal plane inclination. The contribution of the masseter muscle to the variation in bite force magnitude was higher than that of the craniofacial factors.⁴¹

Garcia morales 2003- Independent of chronological age, larger children have larger moment arms and require less muscle activity to attain any given force. Greater hyper divergence is related to poorer mechanical advantage and lower maximum bite force.

The data support the relationships between bite force, muscle strength and morphology in children, similar to those reported for adults. The relationships for children may be confounded by the measures used to quantify muscle function. First, potential fear, discomfort and pain may make it more difficult to obtain maximum bite forces in young children than in adults. Second, bite force by itself is not adequate to evaluate muscle strength because bite force is strongly influenced by the amount of voluntary effort, which may be less than maximal effort. True muscle strength depends upon muscle size, muscle recruitment, and the length of the muscle moment arms. Therefore, the relationship between EMG and bite force, as well as the mechanical advantage of the jaw muscles, should be determined when assessing jaw muscle strength.⁴²

Sonneson 2005 In both genders, bite force was associated with dental development in terms of erupted teeth. Only in boys was there a clear correlation between bite force and craniofacial morphology. The vertical jaw relationship and the number of teeth present were the most important factors for the magnitude of bite force in boys. In girls, the most important factor was the number of teeth present, a gender difference which may be due to different growth intensity in the studied age group. Bite force did not

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vary with Angle classification, and no correlation between bite force and head posture was found.⁴³

Bakke 2006 The maximum bite force increases with the number of teeth present. The number of occlusal tooth contacts is an important determinant for the maximally attainable bite force, explaining about 10% to 20% of the variation. The association between maximum bite force and the amount of occlusal contact is closest in the posterior region, and as a consequence, loss of molar support results in reduction of force. In contrast, malocclusions defined solely on the basis of molar and canine relationships have less influence on the level of bite force.⁴⁴

Usui 2007- The results of this study showed a significant correlation between maximum occlusal force and the mandibular plane angle in most of the age groups. The significant negative correlation between the mandibular plane angle and maximum occlusal force indicates that long-face individuals whose mandibles tend to rotate in a clockwise direction have a small occlusal force. If muscle growth is interrupted during the growth process and the muscle contraction force remains weak even in adulthood, the countenance results in a long-face configuration. Maximum occlusal force would increase progressively with age up to 20s, after termination of the growth of mandible, which follows that of maxilla. From these results, we assumed that the mandible completed growth following the maxilla, and 6–7 years after this completion maximum occlusal force attained the largest value. The routine examination of occlusal force might be useful for orthodontic treatment and stability, including muscular anchorage, extrusive mechanics and retention in each patient.⁴⁵

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Varga 2010 MVBF in subjects with normal complete dentitions is significantly related to age and gender, being in general higher in males and older subjects. Gender differences were significant only in the 18-year-old age group. Males showed significant increase in bite force between 15 and 18 years of age. In subjects with a neutral occlusion, MVBF could be best predicted using multiple regression analysis by age and gender. BMI, morphological occlusion, and jaw function in subjects with a normal occlusion had a low contribution to prediction of MVBF values.⁴⁶

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MATERIALS AND METHODS

This study was carried out using 90 untreated samples who reported to Department of Orthodontics and Dentofacial Orthopaedics, Tamilnadu Government Dental College and Hospital. They were divided into three groups based on the inclusion criteria.

Inclusion criteria:

1. Healthy individuals and not under any routine medication 2. Female patients

3. Age 14-20

4. Asymptomatic TMJ 5. Normal Maxilla Based on

Steiner’s analysis SNA 82±2⁰ and McNamara analysis N ⊥A 0-1mm 6. Retrognathic Mandible based on

SNB <80±2⁰ N ⊥ pog <-2mm

7. Prognathic mandible based on SNB> 80±2⁰

N ⊥ pog>5mm 8. Overbite- 2-4mm

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Exclusion criteria:

1. Facial muscle pain, limited range of motion, pain upon movement 2. History of orthodontic treatment

3. Facial asymmetry

4. Clinically and cephalometrically high and low mandibular plane angle.

FMA – 25-28^o

GROUP I:

30 female patients with class I malocclusion with normal maxilla and normal mandible who fulfil the inclusion criteria with maximum overjet 2-4mm.

GROUP II:

30 female patients with class II malocclusion with normal maxilla and retrognathic mandible who fulfil the inclusion criteria.

GROUP III:

30 female patients with class III malocclusion with normal maxilla and prognathic mandible who fulfil the inclusion criteria.

NULL HYPOTHESIS

There is no change in the muscle forces amongst the three groups of malocclusion.

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MATERIALS

 Cases were selected based on lateral cephalogram

Pre-treatment standardized lateral cephalogram were taken using the ORTHOPHOX machine with exposure values set at 64kvp and 12mA with exposure time of 0.5 seconds using CARESTREAM DRYVIEW LASER IMAGING lateral head film. To indicate true vertical on the film a wire mounted in front of the cassette. The subjects were oriented in the natural head position (NHP).

 0.003inch acetate paper

 Geometric box

 X-ray viewer

 Force level is detected with the help of a force sensor [fig 2] Tekscan FLEXIFORCE A502-

 square force sensor

 sensing area 2” x 2”,

 2 pin male connectors,

 Force sensor length of 81.3mm

 standard force 0-222N

 sensor thickness- 0.203mm

 substrate- polyester

 pin spacing- 2.54mm

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 The extraoral device consists of immovable head frame and movable spring loaded chin support which is constructed with iron rods[Fig 3].

 Force sensor connected to external circuit which is programmed to give the force values in Newton [Fig 4]

 The unit derived power supply from either laptop/ power bank. In this study we used laptop.

 The external circuit gives values qualitatively and quantitatively.

Fig 2: Force sensor

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Fig 3: Instruments needed for estimation of jaw opening forces.

Fig 4: External circuit which gives force values.

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METHODOLOGY

 An adjustable extra-oral device consisting of a rigid skull head frame and chin support connected to a force sensor, developed specifically for this study, was fitted to each participant.

 The participants were having the measuring device placed on the head in such a way that the skull cap was opposite the chin cap with the jaw in centric occlusion, and the measuring device was tightened to the point where there is the beginning of discomfort.

 The chin part has the force sensors in it which when connected to the circuit gave the output in mV.

 Participants pertaining to group A, B and C were instructed to sit comfortably on a chair while maintaining a straight back and looking straight ahead to keep the head as vertical as possible. This would give us the standardisation of obtaining the values in Natural head position.

 The device was connected to the patient in such a way that head frame was seated properly in patient’s head and chin support in patients’ chin without any pressure.

 The patients were asked to sit erect with the Frankfort plane parallel to the floor so as to maintain the natural head position.

 The device derived its power supply from laptop

 The patient was asked to open and close the mouth for 10 times and the values was recorded by the force sensors in the chin support excluding the first and last measurements. The mean of other measurements noted was recorded. This gave the patients’ force levels. This gives quantitative assessment.

 Qualitative assessment is done by three lights - RED for high

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YELLOW for medium GREEN for low

The recorded values were compared and correlated.

 GROUP II – FIG 7 and 8

 GROUP III- FIG 5 and 6

ANALYSIS OF DATA

 The data were tabulated in excel sheet. Statistical analysis was done using Statistical Package for the Social Sciences computer software (SPSS version 20.0) The data assessed for normal distribution using Kolmogorov-smirnov test.

The data was found to be non-normal data.

 Data was subjected to Kruskal-Wallis test to compare the force in the 3 groups:

group A class II with normal maxilla, group B class III with normal maxilla and control group class I with normal maxilla.

 For intergroup variations, Mann-Whitney test was used.

 Type of sampling: Non-random sampling

 Type of study: Cross-sectional study.

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Fig 5: Class II patient with the device in rest position

Fig 6: Class II patient with the device in open mouth position

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Fig 7: class IIII patient with the device in rest position

Fig 8: Class III patient with the device in open mouth position

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LOW FORCE- GREEN

Fig 9

MODERATE FORCE- YELLOW Fig 10

HIGH FORCE- RED

Fig 11

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RESULTS

The mean standard deviation for the three groups is given in Table 1. These results were derived by the SPSS software version 20.0.

To test the data for normality, we used Kolmogrov-smirnov test. The p value is <0.05 which implies that the data is non-normal- Table 1

TABLE 1

One-Sample Kolmogorov-Smirnov Test

Force level

N 90

Normal Parameters

Mean 13.8024

Std. Deviation 7.02892

Most Extreme Differences

Absolute .244 Positive .244 Negative -.151

Kolmogorov-Smirnov Z 2.315

Asymp. Sig. (2-tailed) .000

a. Test distribution is Normal.

b. Calculated from data.

Since the data is non- normal, we used non parametric Kruskal-wallis test to identify the significance given in TABLE -2

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Table 2: KRUSKAL-WALLIS TEST

Ranks

Class N Mean Rank

Force level

Class I 30 28.17 Class II 30 33.98 Class III 30 74.35 Total 90

Test Statistics^a,b

Force level Chi-Square 55.649

df 2

Asymp. Sig. .000 a. Kruskal Wallis Test b. Grouping Variable:

Class

Inference: There is statistical significance between the three groups.

To identify the intergroup difference, Mann-Whitney test is used- TABLE 3,4 and 5

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Table 3: MANN-WHITNEY TEST FOR CLASS I AND CLASS II

Ranks

Class N Mean Rank Sum of Ranks

Force level

Class I 30 27.80 834.00

Class II 30 33.20 996.00

Total 60

Test Statistics^a

Force level Mann-Whitney U 369.000

Wilcoxon W 834.000

Z -1.198

Asymp. Sig. (2- tailed)

.231

a. Grouping Variable: Class

INFERENCE: The p value>0.05, indicates that the is no significance between Class I and class II

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Table 4: MANN-WHITNEY TEST FOR CLASS I AND CLASS III

Ranks

Force level

Class I 30 15.87 476.00

Class III 30 45.13 1354.00 Total 60

Wilcoxon W 476.000

Z -6.492

.000

INFERENCE: Force levels between class I and class III is statistically significant p<0.005

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Table 5: MANN-WHITNEY TEST FOR CLASS II AND CLASS III

Ranks

Force level

Class II 30 16.28 488.50

Class III 30 44.72 1341.50 Total 60

Wilcoxon W 488.500

Z -6.307

.000

INFERENCE: The force levels between class II and class III is statistically significant p<0.005

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Bar diagram depicting force difference between three groups – Bar 1

INFERENCE: There was a significantly high force levels of the depressor muscles in group III compared to group I and group II.

0 5 10 15 20 25

Class I

Class II

Class III

9.08 10.13

21.91