Dr. Derek Mahony
Registered Specialist in Orthodontics
BDS(Syd) MScOrth(Lon) DOrth RCS(Edin) MDOrth RCSP(Glas) MOrth RCS(Eng)
MOrth RCS(Edin) FRCD(Can) FICD FICCDE FACD FADFE GradDipDentalSleepMedicine (WA)
49 Botany Street, Randwick NSW 2031, Sydney, Australia

Aim

Edward H. Angle dominated orthodontic armamentarium, diagnosis and treatment planning for almost a half century until Charles Tweed successfully challenged his mentor’s nonextraction mantra.  The ensuing diagnostic regimen used by Tweed, however, proved to have serious limitations and clearly resulted in the extraction of too many teeth. This caused a subsequent deterioration of soft tissue appearances of patients that neither they nor their doctors liked.  This article will describe and illustrate how new expansion techniques differ qualitatively from those of Angle, and how these techniques offer patients and doctors less invasive and more comfortable therapies which do not jeopardize facial appearances.

Introduction

For the first third of this past century, orthodontics found itself dominated by one man, Edward H. Angle, with the resultant intellectual stagnation that arises from such monomaniacal control.  This recognition in no way detracts from Angle’s contributions – notably his clear and simple classification system along with the edgewise bracket.  Both of these inventions have endured for a century, and that is no mean achievement in any scientific discipline.  Nevertheless, orthodontists’ unquestioning acceptance of his limited diagnostic and treatment planning regimens hindered the advancement of this discipline more than it helped, and the last half of this past century was spent trying to overcome the stupor of the first half.

Angle’s influence continued until an apostate student of his, Charles H. Tweed,[1] had enough courage and objectivity to challenge Angle’s non-extraction scheme.  It wasn’t a tremendous leap of intellectual power.  Tweed simply and honestly recognized that when 100% of your patients relapsed, there might be something wrong with the diagnosis and/or treatment planning.

Dr. Tweed acted appropriately in the face of this challenge – quite unlike the ancient dentist who chided a young colleague who was describing his meticulous technique of endodontic filling to the monthly assembly of dentists.  The old man explained his own technique that used a simple matchstick sharpened with a pocketknife and then jammed into the canal.  When the young dentist asked if a lot of these root canal fillings didn’t subsequently fail, the older man replied, “Every damn time!”

Dr. Tweed tired of those orthodontic abscesses and, unlike his peers, sought to correct the deficiencies he saw in Angle’s philosophy.  Some would say that he overcorrected, but that said, we must pay homage to anyone who has the skill and temerity to successfully challenge a mentor and his minions.  Tweed’s success brings to mind the remark of C.S. Lewis, who said, “No genius is so fortunate as he who has the skill and ability to do well that which others have been doing poorly.”

Nevertheless, I don’t think that Tweed would have ever been able to deliver his paper describing his extraction technique had Dr. Angle still been alive.  Angles influence over the society that bore his name was too immense to permit such hubris from a young upstart.  But as Samuelson, the MIT economist, once noted: “Science progresses slowly – funeral by funeral.”  And so it was and is in orthodontics.

Nonextraction Philosophy

Aside from the edgewise bracket and the classification system, Angle’s most enduring legacy has been his belief in nonextraction therapy.  Angle had unsuccessfully experimented with premolar extractions while using his ribbon arch appliance, but he never solved the problem of paralleling the roots to prevent the extraction spaces from opening.  If he couldn’t do it, then, ergo, no one else could, and this resulted in a virulent opposition to any extractions and an insistence upon enlarging the arches to accommodate all of the teeth.

This dogma stayed dominant for several decades until Tweed advocated the extraction of premolars based on his diagnostic triangle, which was the first systematic treatment planning stratagem orthodontists had.  Tweed received corroboration simultaneously from another former Angle protégé in Australia, Raymond Begg,[2]  who had studied aborigines and concluded that nature intended for enamel to wear.  He decided that orthodontists could mimic nature by extracting teeth prior to orthodontic therapy.   The Tweed and Begg Extraction Philosophies eventually prevailed and remained uncontested for some time.

Several years past before Holdaway[3, 4] published his articles that suggested the soft tissue as the determining feature of diagnosis.  This disputed Tweed’s narrow diagnostic regimen that focused on the mandibular incisor and totally neglected the soft tissue. Tweed’s triangle set in motion a trend that emphasized more prudence in the extraction of teeth.  Soon others added their discoveries regarding soft tissue and the maxillary incisors as main determinants of diagnosis and treatment planning.[5-7]

From the inception of this specialty, with Dr. Angle, diagnosis never had too much importance because everyone received the same nonextraction treatment with the same expansive appliance.  The marvel of it all is that the collection of orthodontic records never became important.  A few months ago an orthodontist boasted that since invoking a different treatment regimen, he was treating 98% of his patient’s nonextraction.  One was tempted to ask if he still took records because with diagnostic certainty such as that, records are clearly redundant.  Orthodontists shouldn’t waste patients’ time and money taking impressions, cephalometric X-rays or doing treatment simulations if all treatment plans are essentially the same.  One doesn’t need orthodontic records to come to such a preconceived conclusion.

Obviously, this one-size-fits-all treatment planning didn’t benefit patients a hundred years ago, and it doesn’t in our own age.  But such simplicity continues to hold enormous appeal for many orthodontists.  Orthodontists pride themselves in being scientists, and without doubt they receive good training in the scientific method; but it takes very little anecdotal information to eclipse the scientific judgment of many in the profession.  Albert Szent-Györgyi was probably more right than he knew when he said, “The brain is not an organ of thinking but an organ of survival like a claw and fang.  It is made is such a way as to make us accept as truth that which is only advantage.”

No matter how spectacularly orthodontic therapy changes, it will benefit our patients minimally if we do not have a concomitant improvement in our diagnostic and prognostic knowledge.  This remains the number one imperative for those who practice orthodontics.  Orthodontists should view any new therapy unaccompanied by equally sophisticated diagnostic knowledge suspiciously.  Patients have already received far too much orthodontic treatment and far too little diagnosis.

Instrumentation

The first attempts to correct malocclusions used simple large arch wires ligated to the malposed teeth.  Pierre Fauchard of France developed the precursor of the modern appliance – expansion arch (Figure 1).

Figure 1: Fauchard’s expansion arch

This arrangement gave only tipping control, in one dimension, and soon proved inadequate for controlling rotations.  In 1887 Edward H. Angle introduced the E arch, i.e. expansion arch that used a labial wire supported by clamp bands on the molar teeth which ligated to the other teeth (Figure 2).

Figure 2: Angle’s E Arch

Metallurgical developments by the early 20^th Century allowed clinicians to encase all of the teeth with bands and solder attachments that could control the horizontal rotations.  Angle developed a popular attachment known as the pin and tube attachment in 1911 (Figure 3), and it satisfied many of the requirements of clinicians; but this demanded unusual dexterity, patience and skill, so dental clinicians evolved to a ribbon arch bracket (Figure 4), which Angle introduced in 1916. It provided good control in two dimensions and became popular quickly.  The ribbon arch attachment also marked the first time orthodontic attachments gained the name bracket.[8]

Figure 3: Pin and tube appliance

Figure 4: Ribbon arch

When Angle launched the ribbon arch bracket, he had already started work on the edgewise bracket primarily as a supplement to his ribbon arch appliance.  Nevertheless, the edgewise bracket did not suddenly spring full-grown from Angle’s fertile mind, but slowly evolved with several iterations (Figure 5).  When Angle realized that this bracket could deliver three-dimensional control of the teeth with horizontal, one directional placement and simultaneous engagement of all the teeth, he changed the bracket several times until he achieved the #447 (Figure 6) in 1928.  It received early and enthusiastic endorsement from dental clinicians throughout the United States and eventually eclipsed other useful orthodontic appliances such as the McCoy open tube appliance, the Atkinson universal appliance and the Johnson twin wire attachment.

Figure 5: Angle’s many iterations of the edgewise bracket

Figure 6: Angle’s 447 edgewise bracket, “the latest and best in orthodontic mechanisms.”

The universal application and durability of the edgewise bracket confirmed Angle’s immodest claim that it offered the “latest and best in orthodontic mechanisms”.[9]  Innovators have added minor but practical trimmings such as rotating wings, twin brackets, different dimensions, preadjusted appliances, lingual applications, etc., but the essence has remained edgewise.  For any instrument, particularly in the health sciences, to remain virtually unchanged (and almost as useful for close to a century) approaches unbelievability.  In the automobile industry, this would be equivalent to the Model T Ford remaining as the epitome of motoring sophistication.

Other than adding wings and doubling the bracket to make the popular twin edgewise bracket, Angle’s invention has remained basically unchanged.  Holdaway[10] suggested angulations for brackets to help set anchorage, parallel roots and artistically position teeth, while Lee[11] had built some anterior brackets with the ability to torque incisors. But it was Andrews that was to develop an appliance that would apply 1^st, 2^nd and 3^rd order movements to the teeth without making changes in the wire – hence the Straight Wire Appliance.[12]

Preadjusted orthodontic appliances have dominated the profession for the past 30 years, and the belief in them shows little sign of abating even though many have questioned the one-size-fits-all idea.[13-18]

And Back Again

The publication of Frankel’s[19] work with functional appliances illustrated significant enlargement of dental arches and reawakened an interest in nonextraction therapy.  Nevertheless, Frankel mechanics required the use of removable appliances, and that didn’t resonate well with many orthodontists or their patients.  After a brief flurry of interest in the United States, few clinicians continued to use the Frankel appliance on a regular basis.

Nevertheless, the successful use of orthopedic appliances alerted orthodontists to the possibility of increasing arch widths and arch perimeters with minimum forces.  Although mandibular canines offer significant resistance to expansion, mandibular premolars and first molars often demonstrate substantial and stable expansion. Brader[20] hinted at this with his work on the tri-focal ellipse arch form, but he didn’t follow through about how this might give wider and more accommodating arch forms.

Low-force titanium coil expanders have shown their ability to develop arches laterally,[21] and recently Damon[22] has suggested that low arch wire forces, coupled with a passive tube and a small wire-to-lumen ratio, enable teeth and their accompanying dentoalveoli to expand in all planes of space.  Damon feels that using small, low-force wires such as those of Copper Ni-Ti™ (Ormco Corporation, Orange, CA) achieves the ideal biological forces proposed long ago by several investigators.[23] [24, 25]

Self-ligating brackets that essentially form a tube developed several decades ago with the Ormco Edgelok[26] being the first, closely followed by the Speed bracket[27]. Both of these early self-ligating systems suffered from the fact that the Straight-Wire Appliance phenomenon debuted at the approximately the same time, plus a lack of appreciation for what the newer titanium wires could achieve.

Damon has persisted since 1995 with his version of a self-ligating bracket (Figure 8) and has fundamentally changed the types of arch wires and the sequence in which clinicians use them.  His experience has shown that with many patients he can often eliminate distalisation of molars, extractions (excluding those needed to reduce bimaxillary protrusions) and rapid palatal expansion.  He offers compelling clinical evidence of doing this with consistency. [22]

The Damon bracket is essentially a tube designed with the right dimensions to  foster sliding mechanics where needed and enough  play in the system for torque and rotational control using the larger cross section wires. Damon starts  cases with a large lumen arch wire slot and .014 or smaller diameter hi-technology arch wires. Starting cases with a large dimension passive arch wire slot and small diameter wires diminishes the divergence of the angles of the slots.  This lowers the applied force and binding friction. (figure 7)

Fig 7a: Binding

Fig 7b: Divergence

Figure 8: Damon 3 bracket opened and closed

The most logical questions readers could propose would be why has Damon shown successful expansion whereas Angle did not?  The quantity of expansion probably differs little, but the quality of expansion offers a quantum change.  Mollenhauer[28] has suggested as much with his appeal for light forces.  Even though Angle used a ribbon arch, (which suggests a thin, delicate wire) the actual size of the wire had the dimension of .036 x .022 inches. Ligating to this wire would overwhelm the periodontium and prevent the development of a supporting dentoalveolus.  Rather than forming new bone, the supporting dentoalveolus would simply bend and upon completion of treatment quickly return.  Astute clinicians often see this with molar distalization from headgear use and over treat such movement in order to compensate for this regressive bone bending.

Schwartz [25] stated that it takes 20 to 26 g/cm² of force to collapse the capillaries in the Periodontal Ligament. With RPEs and headgears this force sometimes exceeds 10 pounds!

Profitt [29] states that that optimal force levels for orthodontic tooth movement should be just high enough to stimulate cellular activity without completely occluding blood vessels in the  periodontal ligament.

True Biomechanics is staying in the Optimal Force Zone i.e. keeping forces below capillary blood pressure. Conventional ties (o-rings and stainless steel ligatures and spring clips) make staying in the Optimal Force Zone nearly impossible due to the increased binding and friction.

The most important caveat Damon offers clinicians is not to use their ordinary mechanics with his system, and I could not agree more.  When I first began to use the Damon system, I continued to use the regular sequence of arch wires and saw little advantage to these new, more expensive brackets.  Nevertheless, as I began to use the brackets according to Dr. Damon’s advice, I started seeing phenomenonal changes.  The following patient illustrates typical responses to the biomechanics offered by the Damon System:

Summary

The paradigm shift in our current thought processes is the belief that alveolar bone can be altered  and re-shaped with low clinical forces. Using low force, low friction orthodontics, the alveolar bone allows the bodily movement of teeth in all directions.

The architecture of alveolar bone appears to improve over time following low force orthodontics so clinicians should be very creative on how to maintain the appropriate biologic forces during  all phases of treatment.

Orthodontists are currently witnessing an interest in qualitatively different expansive biomechanics that offer patients the possibility of obviating the use of distalizers, rapid palatal expanders and many needless extractions.  The bracket systems that make this possible should command the utmost respect and clinicians should use them as recommended with light forces.

I am witnessing  shorter treatment in most of my Damon cases with less discomfort to my patients. The playing field seems to be leveled between adults and children. These changes I am seeing are more than enough reasons for me to question my previous force systems.

1. Tweed, C.H., The Frankfort mandibular incisor angle (FMIA) in orthodontic diagnosis, treatment planning and prognosis. Angle Orthod, 1954. 24: p. 121-169.
2. Begg, R., Begg orthodontic theory and technique. 3rd ed. 1977, Philadelphia: W.B. Saunders Co.
3. Holdaway, R.H., op. cit., A soft tissue cephalometric analysis and its use in orthodontic treatment planning, Part II. Am. J. Orthod., 1984. 85(4): p. 279-293.
4. Holdaway, R.H., A soft tissue cephalometric analysis and its use in orthodontic treatment planning, Part I. Am. J. Orthod., 1983. 84(1): p. 1-28.
5. Alvarez, A., The A Line: A New Guide for Diagnosis and Treatment Planning. J. Clin. Orthod, 2001. 35(9): p. 556-569.
6. Creekmore, T.M., Where teeth belong and how to get them there, J. Clin. Orthod, 1997. 30(9): p. 586-608.
7. Sarver, D.M., Profitt, W.R., Special Considerations in Diagnosis and Treatment Planning. 4th ed. Orthodontics – Current Principles and Techniques, ed. Tom Graber. 2005, St. Louis, MO: Elsevier Mosby. 1213.
8. Renfroe, E.W., Technique Training in Orthodontics. 1st ed. 1960, Ann Arbor, MI: Edwards Brothers Inc. 230.
9. Angle, E.H., The latest and best in orthodontic mechanism. Dental Cosmos, 1929. 71: p. 164-174, 260-270, 409-421.
10. Holdaway, R.A., Bracket angulation as applied to the edgewise appliance. Angle Orthod, 1952. 22: p. 227-236.
11. Lee, I.F.
12. Andrews, L.F., Straight Wire, the Concept and Appliance. 1989, San Diego, CA: L.A. Wells Company.
13. Andreiko, C., JCO interviews Craig Andreiko, DDS, MS, on the Elan and Orthos Systems. J. Clin. Orthod, 1994. 28(August): p. 459-472.
14. Sachdeva, R., SureSmile technology in a patient-centered orthodontic practice. J. Clin. Orthod., 2001. 35(April): p. 245-253.
15. Melsen, B.F., Giorgio, Biomechanics in Orthodontics, Libra Ortodonzia.
16. Creekmore, T.C., R., Straight Wire: The Next Generation. Am J.Orthod & Dentofacial Orthop, 1983. 104(July): p. 8-20.
17. Dellinger, E.L., A scientific assessment of the straight-wire appliance. Am J Orthod, 1978. 73: p. 290-299.
18. McGann, B.D., Individual patient (IP) appliances. World J. Orthod, 2005. 6(2): p. 189-192.
19. Frankel, R., The Frankel Appliance (The Function Corrector(Removable Orthodontic Appliances, ed. T.G. Neumann. 1977, Philadelphia, Pa: W. B. Saunders Company.
20. Brader, Dental arch form related with intraoral forces: PR=C. Am. J. Orthod., 1972. 61(June): p. 541-561.
21. Williams, M.O., White, L.W., A Rationale for Expansion. World J. Orthod, Pending Publication.
22. Damon, D.H., Treatment of the face with biocompatible orthodontics. 4th ed. Orthodontics – Current Principles and Techniques, ed. Tom Graber. 2005, St. Louis, MO: Elsevier Mosby. 1213.
23. Reitan, K., Tissue behavior during orthodontic tooth movement. Am J Orthod, 1960. 46: p. 881-900.
24. Rygh, P., Elimination of hyalinized periodontal tissues associated with orthodontic tooth movement. Scand J Dent Res, 1973. 81: p. 467-480.
25. Schwartz, A.M., Tissue changes incidental to orthodontic tooth movement. Int J Orthod Oral Surg Radiography, 1932. 18: p. 331.
26. Wildman, A.J., Lee, I.F., Hice, T.L., Lang, H.M., Strauch, E.C. Jr., The Edgelok Bracket. J Clin Orthod, 1972. 6(11): p. 613-633.
27. Hanson, H., Dr. G. Herbert Hanson on the Speed Bracket. J. Clin. Orthod., 1986. 20(3): p. 183-189.
28. Mollenhauer, B., Ultralight forces for simultaneous orthodontics and orthopedics: part III. dentofacial orthopedics. World J. Orthod, 2000. 1: p. 195-201.
29. Proffit and Fields, Contemporary Orthodontics, Second edition, Mosby 1993

Dr. Derek Mahony
Registered Specialist in Orthodontics
BDS(Syd) MScOrth(Lon) DOrth RCS(Edin) MDOrth RCSP(Glas) MOrth RCS(Eng)
MOrth RCS(Edin) FRCD(Can) FICD FICCDE FACD FADFE GradDipDentalSleepMedicine (WA)

49 Botany Street, Randwick NSW 2031, Sydney, Australia

Introduction

The term neuromuscular occlusion has become associated with certain limited methodologies that are used to obtain a muscle-compatible occlusal relationship.  In reality, there are several different approaches that can be used to determine a “neuromuscular” maxillo-mandibular relationship, even with a fully edentulous case. Within each method, however, the common basis for all muscle-oriented approaches involves first determining the resting length of the masticatory muscles.

Historically, opening the bite has been considered hazardous and/or foolhardy by many dentists and with good reason.  Arbitrary opening of the bite, especially when accomplished strictly on an articulator, can result in a difficult, uncomfortable and unappreciative patient.  Some dentists have recommended against ever opening a bite, perhaps after an especially troublesome experience with a patient.

In spite of the risks, there are some advantages associated with opening an over-closed bite. The identification can be traced back at least 70 years to an ENT physician, Dr. J. B. Costen.^1-3  Dr. Costen discovered, perhaps quite by accident after referring many of his symptomatic, edentulous patients to a local dentist for new dentures, that many returned with their head and ear pain symptoms greatly relieved.  His publications were positively received at the time and, in fact, what we refer to today as temporomandibular disorders (TMDs) were originally referred to as “Costen’s Syndrome.”  While we know today that many TMD patients are not over-closed, over-closed patients do often exhibit some of the signs and symptoms commonly associated with TMD.  Thus, although over-closure in and of itself is not pathognomonic of TMD, it should be considered as a risk factor.

The use of the patient’s own muscles to determine the vertical dimension of occlusion was already being explored in the 1940s by people like orthodontist John R. Thompson.⁴   Sears⁵ introduced the concept of the “Pivot Appliance” in the 1950s, which was designed to open the bite enough to allow the patient’s muscles to reposition the mandible.  Following their lead, others^6-28 have subsequently evolved the current array of neuromuscular registration methods presently in use.  At the same time several studies^29-32 have demonstrated that a muscle-determined position, although similar, is not identical to centric relation. 

Common signs and symptoms of over-closure

When asked, over-closed patients often report symptoms such as frequent headaches, dull pain of the elevator muscles and pain or stiffness in their neck muscles.  Ear stuffiness, tinnitus and/or vertigo are also commonly reported.  A more subtle symptom, less often reported, is frequent gastrointestinal distress in various forms that has no clear, identifiable cause.  This may also be accompanied by a report of difficulty in chewing and/or swallowing.  An overclosed patient will usually report several, but not all, of the following symptoms.

1. Frequent headaches with no identifiable cause
2. Ear stuffiness with no indication of ear pathology
3. Difficulty in chewing tough foods
4. Difficulty or discomfort in swallowing
5. Frequent gastrointestinal distress
6. Vertigo
7. Tinnitus
8. Persistent dull pain in masticatory elevator muscles
9. Neck pain or stiffness
10. Possible increased wear of incisor teeth

Under examination, a number of signs indicating over-closure may appear.  These include; 1) a measured freeway space greater than 3 mm, 2) EMG or visual identification of a tongue-thrust swallow, 3) the appearance of less than fully erupted molars, 4) a deep curve of Spee, 5) one or more posterior edentulous spaces, 6) lingually tipped mandibular molars, 7) EMG identification of elevator muscle hyperactivity at rest of more than 2.0 microvolts average (or 2.2 microvolts RMS), 8) worn and shortened teeth (there is no scientific evidence that human teeth “grow out” in response to wear in the way that elephant’s teeth do), 9) horizontal skin creasing and saliva weeping at the corners of the mouth, 10) a so-called “Shimbashi” measurement (in centric occlusion) of less than 16 mm from the cemento-enamel junction of the maxillary central incisor to the cemento-enamel junction of its opposing mandibular tooth and 11) long-term chronic internal derangement of the TM Joint(s).  However, patients rarely seek dental treatment for any of these objective signs.  Instead, they are more likely to seek rehabilitative treatment for headache, jaw-ache, ear-ache, difficulty in chewing/swallowing or for purely esthetic reasons.  In other cases they are unaware of their condition, apparently due to their excellent adaptability.  In the over-closed patient the “reason” for treatment, either cosmetic or functional, is often dependent more on his/her individual adaptability than on the dental conditions present.  While some signs simply indicate the “progress of the destruction” that a pathological maxillo-mandibular relationship fosters, other signs may indicate a successful adaptation.

1. Freeway space > 3 mm [if pain level is low, it is an adaptation, otherwise it is not]
2. Tongue thrust swallow [if full arch tongue thrust, usually a successful compensation]
3. The appearance of less than fully erupted molars [tongue inhibition of natural eruption]
4. A deep curve of Spee [often associated with one or more missing molars or a deep anterior overbite with retroclined upper incisors]
5. One or more posterior edentulous spaces [leads to deep curve of Spee]
6. Lingually tipped posterior teeth [tongue thrust during swallow, restricted maxillary arch]
7. Hyperactivity of elevator muscles at “rest.” [an adaptation, successful if no elevator muscle pain]
8. Worn/short teeth, abfractions (ground off) [not a successful adaptation]
9. Skin creasing at corners of mouth [may appear as aesthetic problem only, not an adaptation]
10. Saliva weeping at corners of mouth [an esthetic and functional problem, not an adaptation]
11. CEJ (cemento-enamel junction) to CEJ in C.O. < 16 mm. [less than the normal adaptive range]
12. Internal derangement(s) of the TMJ [if no degeneration, may be a successful adaptation]

Maxillo-mandibular bite relationships

Centric occlusion (CO = habitual)

The maxillo-mandibular position of maximum intercuspation is most often the dental treatment position, primarily by default.  This is of necessity whenever single tooth preparations or small restorations are involved, since they must fit within the patients existing occlusal scheme.  It is only at times of major reconstructive, orthodontic and/or surgical treatments that the option of opening a bite or establishing a new maxillo-mandibular relation may present itself.  However, many clinicians still prefer to “play it safe” and retain the existing habitual (CO) maxillo-mandibular relationship, even during major rehabilitative procedures.  By definition, the use of centric occlusion as a treatment position excludes re-establishing a proper vertical dimension in an over-closed patient.  However, if the patients condition is actively deteriorating this may not be a safe option at all, as the continued physiologic breakdown may lead to failed dentistry and/or a flair up of craniofacial pain.

Centric relation (CR)

The concept of centric relation has a very long history and was originally devised, at least in part, to accommodate the use of articulators during prosthodontic treatment.  Although we now know that the jaw doesn’t function like a hinge, originally it was convenient to make that assumption when using articulators to make prostheses.  Today, one clear difference between centric relation procedures and strictly muscle-oriented methodologies is the priority given by CR methods to evaluating the function of the temporomandibular joints.  Typically, centric relation operators give first priority to establishing stable joint function, while muscle-oriented (neuromuscular) approaches tend to focus almost exclusively on muscle comfort.

Muscle-related centric (MC)

Fig. 1.  BioJVA* testing for normal TM joints

In general, muscle-oriented approaches consider joint position and/or stability secondary to muscle function.  In the extreme, it is simply assumed that creating “happy muscles” will automatically provide good or at least adequate joint function.  In a more practical view, both joint function and muscle function are seriously evaluated and, when indicated, a compromise is sought to provide both joint and muscle compatibility.  This represents an approach that bridges the gap between strict CR and rigid MC approaches.  Consequently, a variety of methods have evolved to capture and establish a muscle-related centric position, while maintaining favorable joint function.

The requirements of proper neuromuscular occlusion (NMO)

Fig. 2.   Bio-TENS,* a ULF-TENS unit used for muscle relaxation

The first step in all approaches to NMO requires inducing relaxation in the masticatory musculature, however, there is no rational excuse for not evaluating TM joint function prior to beginning the process.  This can be accomplished quickly and easily with Joint Vibration Analysis (JVA see figure 1.), or with more expensive and invasive imaging such as MRI.  Muscle relaxation can be aided by Ultra-Low Frequency TENS (ULF-TENS, see Figure 2.), an Aqualizer, soft music or any other technique that reduces the resting hyperactivity of the masticatory muscles.  Surface electromyography (see figure 3.) is useful for making a quantitative determination whether relaxation has occurred or whether resting muscle hyperactivity still exists.  Needles and/or fine wire electrodes not only make relaxation less likely, they record a more localized signal that is less representative of overall muscle activity.  However, needle EMG electrodes are required when one is seeking to differentiate a myopathy from a neuropathy.  Using the relaxed rest position of the mandible, with respect to the maxilla as a reference, a clinician can select a vertical dimension that allows adequate freeway space, yet avoids over-closing the bite.  There are several methods currently used for selecting the treatment vertical.  Each has its own rationale and advantages, but all of them benefit from objective diagnostic aids to ensure the best compromise between optimum joint, muscle, and tooth function.

 Fig. 3.  BioEMG II* for monitoring rest position and testing muscle function against the new bite

 Several muscle-oriented bite registration techniques

The wax swallow bite registration

A physiologic, muscle-oriented, vertical dimension can be obtained by means of the swallowing reflex technique originally proposed by the late Dr. Willie May.  Currently, the wax swallow bite technique, developed by James Carlson, is a simple, direct close approximation of a muscle-related bite registration.  Small pillars of soft wax are placed on the first molars, then the patient is instructed to swallow several times.  Subsequently, fast-curing impression material is injected around the arch to firmly establish the maxillo-mandibular relationship.  Since humans swallow thousands of times per day, it has been proposed that the swallow position should be compatible with the musculature.  This technique is recommended only after verification of good TM joint function with Joint Vibration Analysis or MRI.

The ULF-TENS bite registration                                                                          

Ultra-low Frequency TENS, originally conceived by Bernard Jankelson, is often used to relax the masticatory muscles.  It can also be used to determine a bite registration position, sometimes referred to as myo-centric.  After a patient has been “pulsed” for relaxation, usually for about 40 minutes, bite registration material (a quick-cure acrylic) is placed over the mandibular occlusal surfaces and the ULF-TENS is re-applied to “close” the mandible about 1 – 2 mm above the rest position.  During this procedure the vertical dimension is usually monitored with a mechanic’s inside calipers between marks on the chin and nose.  There is a definite “technique sensitivity” to this procedure such that different operators tend to produce slightly to greatly different results.  However, once the skill is developed, an operator may produce good consistency.  These classic TENS bites ignored the TM joints function in the past, but this should no longer be the case. A final outcome with healthy TMJ’s and muscles is our goal today.

The phonetic bite registration

As with the previously described muscle-oriented methods, this one begins with muscle relaxation.  Then the patient is instructed to speak specific sounds while the anterior teeth are observed by the clinician.  Based on the positions assumed by the teeth with specific phonetics, the clinician recognizes the vertical and antero-posterior requirements and records the position, typically also with impression material.  Admittedly, this technique requires subjective clinical judgment and the development of a skill without any objective support.

The EMG bite registration

To enhance the precision with which one can determine the optimum muscle-related position, some practitioners recommend monitoring the activity of the masseter, temporalis and anterior digastric muscles electromyographically.  Since the electrical muscle output levels involved are just a few microvolts, this measurement requires a high common mode noise rejection amplifier.  After relaxation has been verified electromyographically, the patient is instructed to open very gradually until the digastrics show a slight increase in activity (e.g. 0.5 microvolts average).  This establishes the limit to which opening the bite is permissible and is typically used as a position for constructing removable orthodontic appliances.  Similar tests are done for closing or repositioning the bite antero-posteriorly while monitoring the elevator muscles.  The concept is to find the superior, inferior, anterior and posterior limits of muscle resting.  Then the new bite position is selected within these limits.  The exact relation chosen may be dependent on many factors, such as clinical findings and the clinician’s best judgment.  With this technique it is also possible to evaluate functional activity of the musculature with the bite registration in place to further evaluate the appropriateness of the new maxillo-mandibular relation.

Fig. 4. JT-3D* Jaw Tracker used together with EMG to monitor a bite registration

The instrument monitored bite registration

To maximize the precision with which one can determine the bite registration position, clinicians can actively monitor the position of the mandible using a magnetic jaw tracker while simultaneously recording EMG activity.  After the muscles are relaxed, a recording is made of the movement from rest to centric occlusion, light tapping in CO and protrusive guidance.  Next, the registration position is selected and targeted on the computer screen.  The treatment position chosen can reflect all of the information available regarding the patient’s current condition.  Finally, the registration material is placed in the mouth and the patient is instructed to close into it while the position of the mandible and the muscle activities are monitored on the computer screen.  (Figure 5).  This allows the clinician to immediately see the three dimensional relationship between the old centric occlusal position and the new bite position.  The saved recording can be recalled later and utilized to evaluate an appliance, provisional restorations or the prosthesis at try-in.

Figure 5.  The position of the bite registration and the levels of muscle activity, are simultaneously visible in this combined EMG and jaw tracking recording.  The vertical dimension is increased      2.5 millimeters, the freeway space is reduced from 4.1 mm to 1.6 mm.

Figure 5.  The position of the bite registration and the levels of muscle activity, are simultaneously visible in this combined EMG and jaw tracking recording. The vertical dimension is increased 2.5 millimeters, the freeway space is reduced from 4.1 mm to 1.6 mm.

Figure 6. Example of a patient with an overclosed vertical dimension, due to previous loss of teeth, that has been successfully treated using Neuromuscular Principles.

Predicting a patient’s response to correcting overclosure

The question is often asked, “How quickly will a patient adapt to a new bite registration?”  Even though the object is to “correct” a mal-relationship of the mandible to the maxilla, the patient’s current relationship still has familiarity.  The new relationship, no matter how “perfectly” established, will seem strange to the patient at first.  There are many factors that influence a patient’s adaptation to a new maxillo-mandibular relation.  It is possible to estimate a patient’s response by considering the following factors:

1. The age of the patient [younger = more adaptive, older = less adaptive]
2. The amount of the change [a big change is more difficult to adapt to than a small change]
3. The duration of the overclosed condition [a long-standing condition will be more difficult to “de-program” than one of short duration]
4. The quality of bilateral TM joint function [good joint function makes adaptation easier]
5. An overclosed bite, due to developmental abnormalities (if caught early) can be corrected easily and with rapid adaptation by the patient [children are much more adaptive]
6. Overclosure resulting from parafunction typically coincides with a strong, healthy musculature. Strong, healthy muscles make adaptation easier, but require a treatment plan to protect the restored occlusion from destructive parafunctional forces.
7. An overclosed bite due to caries, loss of teeth, etc. without evidence of parafunction, typically coincides with a weak musculature, making adaptation difficult. This is very often the case with complete removable prosthetics.

Summary

Overclosure is a common condition among patients seeking restorative and/or orthodontic rehabilitation.  By evaluating the patient for common signs and symptoms associated with overclosure, one can determine the need for re-establishing a physiologic vertical dimension.  Opening of the bite can be accomplished in a number of ways by following specific guidelines.  The use of objective diagnostic aids are extremely helpful by allowing the clinician to optimize TMJ and craniofacial muscle function at the new VDO.  The correction of the vertical dimension during a rehabilitative procedure should result in enhanced comfort and improved function in the finished case.

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