Twin Block Functional Therapy: Applications in Dentofacial Orthopedics William J Clark
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The Art of OrthodonticsChapter 1

 
INTRODUCTION
Orthodontics presents a philosophical challenge in that both art and science are of equal importance. A quotation of Edward Angle (1907), from the turn of the 20th century, is still pertinent today:
The study of orthodontia is indissolubly connected with that of art as related to the human face. The mouth is a most potent factor in making or marring the beauty and character of the face, and the form and beauty of the mouth largely depend on the occlusal relations of the teeth.
Our duties as orthodontists force upon us great respon­sibilities, and there is nothing which the student of orthodontia should be more keenly interested than in art generally, and especially in its relation to the human face, for each of his efforts, whether he realizes it or not, makes for beauty or ugliness; for harmony or inharmony; for perfection or deformity of the face. Hence it should be one of his life studies.
Although orthodontics has gained wide recognition by the general public, it can be argued that the term “orthodontics” is self-limiting and does not describe adequately the wider aesthetic and holistic aims of a specialty that is as concerned with harmonious facial balance as with a balanced functional occlusion.
The true art of the speciality lies in its pursuit of ideals in the arrangement and function of the dentition, but never at the expense of damaging facial aesthetics. Beauty is a precious, indefinable quality that is expressed in balanced facial proportions. Facial balance and harmony are goals of orthodontic treatment, of equal importance to a balanced functional occlusion.
 
DENTAL CHESS
Orthodontics may be thought of as the dental equivalent of chess. The analogy is appropriate in many respects. The game is played with 32 ivory pieces that are arranged symmetrically about the midline on a board in two equal and opposing armies.
The opening moves are crucial in determining the strategy of the game. From the outset, the game is won or lost depending on the strategy of development of the individual pieces.
Indeed, these opening moves can determine whether the game is eventually won or lost. It is a mistake in chess to become obsessed with the individual pieces. Rather, one must take a broader view and look at the game plan as a whole to maintain a balanced position of the pieces on the board in order to achieve mutual protection and support.
In dental chess, the board is analogous to the facial skeleton which is of fundamental importance in supporting the individual pieces. As the orthodontic chess game progresses and the dental pieces are developed, the board may become overcrowded, with pieces converging upon each other, so that even the most experienced player may at times sacrifice pieces only to realize as the game deve­lops that the gambit was miscalculated.
Only after the passage of time, on proceeding to the end game, can the success of the strategy be evaluated. Successful treatment is judged in terms of facial balance, aesthetic harmony and functional stability in the end result. One may conclude that the objectives of treatment have been achieved only when the final post-treatment balance of facial and dental harmony is observed.
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ORTHODONTICS AND DENTAL ORTHOPEDICS
An essential distinction exists between the terms “orthodontics” and “dental orthopedics.” They represent a fundamental variance in approach to the correction of dento­facial abnormalities. By definition, orthodontic treat­ment aims to correct the dental irregularity. The alternative term “dental orthopedics” was suggested by the late Sir Norman Bennett, and although this is a wider definition than “orthodontics” it still does not convey the objective of improving facial development. The broader description of “dentofacial orthopedics” conveys the concept that treatment aims to improve not only dental and orthopedic relationships in the stomatognathic system but also facial balance. The adoption of a wider definition has the advantage of extending the horizons of the profession as well as educating the public to appreciate the benefits of dentofacial therapy in more comprehensive aesthetic terms.
A fundamental question that we must address in diag­nosis is: “Does this patient require orthodontic treat­ment or orthopedic treatment, or a combination of both, and to what degree?" Alternatively, does the patient require dentofacial surgery, or to what extent can orthopedic treat­ment be considered as an alternative to surgery?
An orthodontic approach aims to correct the dental irregularity and is inappropriate in the treatment of what are essentially skeletal discrepancies. By definition orthodontics must either be combined with dentofacial orthopedics or maxillofacial surgery in the correction of significant skeletal abnormality.
If the malocclusion is primarily related to a musculo­skeletal discrepancy we should select an orthopedic approach to treatment. It is in the treatment of muscle imbalance and skeletal disproportion that functional orthopedic appliances come into their own. Functional appliances were developed to correct the aberrant muscle environment—the jaw-to-jaw relationship—and as a result restore facial balance by improving function. To achieve the best of both worlds it is necessary to combine the disciplines of fixed and functional appliance therapy.
 
THE PHILOSOPHICAL DIVIDE
In each succeeding generation the clinical approach to treatment is determined by the background of scien­tific research. The growth processes of the maxillofacial complex that control the response to treatment are of special significance. Since the beginning of the 20th century, the pendulum of scientific opinion has swung back and forth in the evaluation of the “form and function” philosophy in relation to the implementation of orthodontic and orthopedic treatment.
At the turn of the last century, a division occurred in the evolution of orthodontic technique that split treatment philosophy into the separate disciplines of fixed and functional appliance therapy.
The two schools of thought had a common origin in the “form and function” philosophy as a basis to establish treatment objectives. The general goal was to correct arch-to-arch relationships, as defined by Angle, while at the same time improving the skeletal relationships through the stimulation and guidance of adaptive remodeling of bone to support those corrected dental relationships.
This philosophical divide in treatment approach can be related to geographical factors as well as to differences in socioeconomic development between the USA and Europe. In his efforts at developing the foundations of modern US fixed appliance technique, Angle attempted to accommodate a full complement of teeth in every case, irrespective of the degree of crowding or lack of available underlying bony support. The following generation of orthodontists subsequently rejected Angle’s “form and function” philosophy as a basis for fixed appliance therapy, and discarded the functional concept of growth in favor of a concept of genetic control that dismissed the potential of environmental factors to influence growth. One dogmatic philosophy was replaced by another.
Provided skeletal development is within the range of normal, fixed appliances are ideally suited to detailing the occlusion by precise three-dimensional (3D) control of tooth movement. Fixed appliances are designed specifically to apply the optimum forces to move teeth, but they are less effective in the treatment of major muscle function imbalances or their companion jaw-to-jaw skeletal discrepancies.
 
THE GENETIC PARADIGM
In the development of orthodontic technique the concept of genetic control of the pattern of maxillofacial development was based on serial growth studies that came about as a byproduct of the development of the cephalostat by Broadbent (1948).
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These studies formed the basis for an entire philo­sophical approach to orthodontic treatment, where the existing skeletal framework was accepted as genetically predetermined and therefore not subject to environ­mental factors.
In the literature, there is scant evidence of significant growth changes showing increased mandibular growth as a result of an orthodontic as opposed to an orthopedic approach to therapy. Other studies did confirm that auxiliary orthopedic forces restricted downward and forward maxillary growth. As a result, maxillary dental retraction became commonly accepted as a reliable method of correcting Class II malocclusion overjet problems.
However, a strict interpretation of the genetic paradigm is called into question increasingly by current research and is no longer the only valid basis for the practice of orthodontics combined with dentofacial ortho­pedics. The present findings of modern research into bone growth represent a philosophical review that once again recognizes the potential of improving the existing growth pattern by altering the muscle environment and/or functional environment of the developing dentition in an orthopedic approach to treatment.
 
TREATMENT CONCEPTS
A fundamental difference in approach exists between orthodontic and orthopedic schools of thought in relation to treatment philosophy and the management of malocclusion.
In the evolution of orthodontic technique, multiband fixed appliances were developed for treatment in the permanent dentition. It was customary to delay treatment until the permanent canines and premolars had erupted, at a stage when the malocclusion was already fully deve­loped. The concept of treatment was to retract the upper arch using the perimeter of the orthodontically corrected, albeit retruded, lower arch as a template on which to rebuild the occlusion.
However, the majority of Class II malocclusions present a laterally contracted maxilla that is often related correctly to the cranial base but is associated with an underdeveloped mandible. The fundamental skeletal problem is not correctly addressed by an approach which is designed to retract a normal maxilla to match a deficient mandible.
A skeletal mandibular deficiency is well-established at an early stage of dental and facial development. The orthopedic approach to treatment endeavors to correct the skeletal relationship before the malocclusion is fully expressed in the permanent dentition. Early diagnosis and interceptive treatment aims to restore normal function and thereby enable the permanent teeth to erupt into correct occlusal and incisal relationships.
The concept of functional therapy is to expand and develop the upper arch to improve archform and to use the maxilla as a template against which to reposition the retrusive mandible in a correct relationship to the normal maxilla. The functional orthopedic approach addresses the skeletal problem of a retrusive mandible, and the malocclusion is controlled at an earlier stage of development. Class III malocclusion is also identified by early diagnosis and may often respond to an interceptive approach to treatment which aims to reduce the skeletal discrepancy and restore normal function in order to promote normal growth and development.
 
ORTHODONTIC FORCE
Fixed appliances are designed to apply light orthodontic forces that move individual teeth. Schwarz (1932) defined the optimum orthodontic force as 28 g/cm2 of root surface. By applying light forces with archwires and elastic traction, fixed appliances do not specifically stim­ulate mandibular growth during treatment.
A bracket or “small handle” is attached to individual teeth. Pressure is then applied to those teeth by ligating light wires to the brackets. The resulting forces applied through the teeth to the supporting alveolar bone must remain within the level of physiological tolerance of the periodontal membrane to avoid damage to the individual teeth and/or their sockets of alveolar bone.
Smith & Storey (1952), investigating optimum force levels in the edgewise appliance, found that 150 g was the optimum force for moving canines, compared to 300 g for molars.
Allowance must be made, however, for frictional forces within the bracket slots themselves, in the region of 125–250 g, which must be overcome to move teeth along archwires.
 
ORTHOPEDIC FORCE
Orthopedic force levels are not confined by the level of tolerance in the periodontal membrane but rather by the much broader tolerance of the orofacial musculature. An orthopedic approach to treatment is not designed to move the teeth, but rather to change the jaw position and thereby correct the relationship of the mandible to the maxilla.
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The forces of occlusion applied to opposing teeth in mastication are in the range of 400–500 g and these forces are transmitted through the teeth to the supporting bone. Occlusal forces form a major proprioceptive stimulus to growth whereby the internal and external structure of supporting bone is remodeled to meet the needs of occlusal function. This is effected by reorganization of the alveolar trabecular system and by periosteal and endochondral apposition.
Considering the anteroposterior forces applied when the mandible is displaced forward in the presence of a Class II skeletal relationship, the investigations of Graf (1961, 1975) and Witt & Komposch (1971) have shown that for 1 mm of anterior displacement the forces of the stretched retractor muscles amount to approximately 100 g. A construction bite of 5–10 mm will therefore transmit considerable forces to the dentition through the functional receptors.
Orthopedic forces would exceed the level of tolerance of the periodontal tissues if applied to individual teeth. However, these forces are spread evenly in the dental arches by appliances that are not designed to move individual teeth, but to displace the entire mandible and promote adaptation within the muscles of mastication. The muscles are the prime movers in growth, and bony remodeling is related to the functional requirements of muscle activity. The goal of functional appliances is to elicit a proprioceptive response in the stretch receptors of the orofacial muscles and ligaments and as a secondary response to influence the pattern of bone growth corres­pondingly to support a new functional environment for the developing dentition.
 
DENTOFACIAL ORTHOPEDICS
In contrast to the philosophical change that has accompanied the evolution of fixed appliance therapy, the form and function concept steadfastly remains the basic concept of functional therapy. The functional matrix theory of Moss (1968) supports the premise that function modifies anatomy. By definition, the purpose of dentofacial orthopedics is to modify the pattern of facial growth and the underlying bone structure of the face. The objective is to promote harmonious facial growth by changing the functional muscle environment around the developing dentition. The principle of functional therapy is to reposition a retrusive mandible to a forward position by constructing aan ppliance that effects a protrusive bite when the appliance is placed in the mouth. The mechanics are reversed to correct a retrusive maxilla, but the principle remains the same.
Functional appliances are designed to enhance forward mandibular growth in the treatment of distal occlusion by encouraging a functional displacement of the mandibular condyles downwards and forwards in the glenoid fossae. This is balanced by an upward and backward pull in the muscles supporting the mandible. Adaptive remodelling may occur on both articular surfaces of the temporomandibular joint to improve the position of the mandible relative to the maxilla.
In correction of mandibular retrusion the mandible is held in a protrusive position by occlusal contact on the functional appliance. In this case a large “handle” is atta­ched to as many teeth as possible in both dental arches. The object of a functional appliance is not to move the individual teeth, but to displace the lower jaw downwards and forwards and to increase the intermaxillary space in the anteroposterior and vertical dimensions. Reposi­tioning the mandible stimulates a positive proprioceptive response in the muscles of mastication. The purpose is to encourage adaptive skeletal growth by maintaining the mandible in a corrected forward position for a sufficient period of time to allow adaptive skeletal changes to occur in response to functional stimulus.
Dentofacial orthopedics, therefore, represents a posi­tive approach to the treatment of craniofacial imbalance by addressing the underlying cause of the malocclusion, in an effort to maximize the natural potential for corrective growth.
REFERENCES
  1. Angle EH, (1907). Treatment of Malocclusion of the Teeth. 7th edition, SS White Dental Manufacturing Co,  Philadelphia.
  1. Broadbent BH, (1931). In Practical Orthodontics. 7th edition, ed GH Anderson, CV Mosby, St. Louis, p.208.
  1. Graf H, (1961). In Tecknik und Handhabung der Functionsregler. ed Frankel R, Berlin.
  1. Graf H, (1975). Occlusal forces during function. In National Cave Management Symposium Albuquerque, Proceedings of Symposium. Ann Arbor, University of Michigan. 
  1. Moss ML, (1968). The primacy of functional matrices in profacial growth. Dental Practitioner and Dental Record. 19:65–73.
  1. Schwarz AM, (1932). Tissue changes incidental to orthodontics. Australian Orthodontic Journal, 18:331–52.
  1. Smith R, Storey E, (1952). 5The importance of forces in orthodontics. Australian Dental Journal. 56:291–304.
  1. Witt E, Komposch G, (1971). Intermaxillare Kraftwirkung bimaxillarer gerate. Gerate Fortschr Kieferorhop. 32:345–52.
FURTHER READING
  1. Moyers RE, (1988). Force systems and tissue responses in orthodontics and facial orthopedics. In Handbook of Orthodontics, Year Book, Chicago.
  1. Sinclair PM, (1991). The clinical application of orthopedic forces: current capabilities and limitations. In Bone Biodynamics in Orthodontic and Orthopedic Treatment, Craniofacial Growth Series, eds Carlson DS and Goldstein SA, Ann Arbor, University of Michigan,  pp. 351–88.
  1. Witt E, (1966). Investigations into orthodontic forces of different appliances. Transactions European Orthodontic Society. 391–408.
  1. Witt E, (1973). Muscular physiological investigations into the effect of bimaxillary appliances. Transactions European Orthodontic Society.6 448–50.