THE ZONE OF NEUTRALITY
Where do the teeth and muscles want to be?
Clayton A. Chan, DDS
The masticatory system is happiest when every thing including the teeth (occlusion), the masticatory muscles and TM joints function within the zone of neutrality. This is the physiologic rest position where the mandible is supported by an appropriate passive resting length of depressors and extensor muscles in all directions vertically, laterally, and antero-posteriorly . It is a zone where the muscles of mastication are physiologically relaxed and the temporomandibular joints are normalized for optimal occlusal stability, function and comfort. This zone of neutrality includes the physiologic range of mandibular motion in which each individual can maximally function within optimal parameters of muscle rest, isometric muscle activity and mandibular movement. It is a zone that can be measured and quantified using surface electromyography and mandibular scanning instrumentation. It is from this identified zone that the clinician is able to compare normal from abnormal. Within this zone of neutrality the neuromuscular clinician is able to identify a starting point of treatment to restore and establish an optimal occlusion.
When dental occlusion is not synchronized along the same path of neuromuscular closure as well as the excursive chewing and swallowing paths of mandibular movement within this physiologic zone, proprioceptive noxious stimuli can be triggered to the central nervous system telling the muscles, the teeth as well as the surrounding periodontium that something is out of balance. Thus, an avoidance pattern in mandibular movement and masticatory muscle incoordination can develop.
MASTICATORY MUSCLES AND OCCLUSAL INTERFERENCES
When the masticatory muscles try to bring the teeth together and there are interfering inclines within the closing and opening paths of mandibular movement, the muscles of mastication may become further irritated resulting in muscle tenderness and muscle pain. Muscles that are irritated can affect the "quality" of mandibular movement, function and posture. Clinicians identify these aberrant movements of the mandible as either bradykinesia (abnormal slowness of movement) or dyskinesia (the impairment of the power of voluntary movement) resulting in fragmentary, disruptive or incomplete movements. Once triggered, the masticatory muscles try to protect the occlusion with the various cusps and inclines which further the cascading effect of teeth sensitivities, soreness in the bone around the teeth, further wear and tear on dentition, masseter soreness/ achy feelings, temporal headaches, neck and shoulder aches, tenderness at the base of the skull and neck (sub occipital region), pressure behind the eyes, ear congestion feelings in the ears, just to name a few of the resulting occlusal symptoms which are all part of craniomandibular disorders. These aberrant mandibular motions can be clearly visualized and recorded with mandibular jaw tracking instrumentation. Muscular pathologies that are formerly unseen through common diagnostic techniques can now giving insightful diagnostic perspectives to muscular dysfunction.
UNDERLYING FACTORS AFFECTING THE NEUTRAL ZONE
It is more often than clinically realized, that patient's will unknowingly place their tongue between their teeth as a means to prevent the teeth from actually resting together during the wake hours of the day since this compromised tongue posture brings a temporary feeling of comfort to the neuromuscularly compromised patient. A lack of proper vertical dimension of occlusion, inadequate volume in the oral cavity for tongue space, constricted/ narrow dental arches, accentuated curve of spee (bicuspid drop-off), an aberrant tongue swallowing pattern, are just a few factors that can contribute to why a patient might want to keep their teeth apart for the sake of masticatory muscle comfort. In essence, the tongue between the teeth is nature's way to compensate for the deficient vertical by shimming or splinting the mal-aligned bite.
As neuromuscular dentist we have found that the mandible wants to be postured typically forward of CR and forward of the habitual CO, confirmed with computerized mandibular scanning (CMS). It is also found that the mandible usually wants to physiologically be positioned inferior of the habitual CO confirmed with resting SEMG amplitude of muscle activity. We often and casually say "down and forward" but in reality (as seen on tomographic analysis) it is the condyles that we have often observed as coming down and forward within the glenoid fossa along the articular eminence. The mandibular incisal edges, once the jaw is physiologically relaxed (after TENS-Myomonitor), positions posteriorly of CO in relation to straight vertical, but inferior from the habitual CO (seen on computerized mandibular scanning). That means that the lower incisors rarely are forward or anterior of the maxillary incisors.
MUSCLE PHYSIOLOGY - Biological Laws that Govern
Physiologically when the musculature becomes hypertonic, lactic acid increases causing the burning sensation of pain due to the decrease in oxygen. The hypertonicity of muscle are sufficient to occlude the capillary beds to reduce the metabolic turnover. Muscles then cry out wanting to find relaxation, desiring to move to an aerobic state rather than an anaerobic state. There is a transfer of energy with a decrease in ATP, Ca2+ and a decrease in available contractile proteins, the actin and myosin filaments, which reduces the optimal number of cross-bridging to take place with the muscle fibers. Muscle tonus then is less then optimal, the patient wants to keep the teeth apart to try to maintain or re-establish a position where it feels right, that is a position where the muscles are getting more oxygen, more ATP, and an increase in Ca2+, thus feeling more comfortable.
Teeth are typically worn, mal-aligned with accentuated overbites and over-jets further compromising head, neck and mandibular posture. The problem as I stated before is that the occlusion or mandibular occlusal plane may not be positioned in a manner that supports that optimal physiologic state (outside the neutral zone), thus the warring between muscles, joints and occlusion. The natural desire to put the teeth together, yet not quite feeling right proprioceptively, triggers the chain of events of noxious pain signals. Muscles get sore and achy, teeth become temperature and sweet sensitive, leading to severe tooth pain, headaches develop and we clinicians have the job of dealing with these problems. These are some of the contributing factors that do not allow the masticatory system to stay within the physiologic zone of neutrality to meet the needs of muscle comfort for the patient.
CORONOPLASTY - Freedom of Entry and Stability
The human dentition is characterized by tapered arches, spheroidal surfaces, grooves, pointed cusps with their receiving fossae. Interocclusal space (freeway space) is also included in the scheme of the masticatory muscles when at rest. To further stabilize the neuromuscular system, coronoplasty or a neuromuscular equilibration may therapeutically be administered by reshaping the coronal surfaces of the teeth to eliminate noxious proprioceptive input into this delicate neuromuscular occlusal system. The objective of coronoplasty is to clear away interferences that deflect the terminal portion of the neuromuscular trajectory away from full closure and into a stable myocentric occlusion.
The groove, spheroid and point principles as taught in the coronoplasty technique addresses what the nervous system does not well tolerate: deflections or prematurities that require avoidance accommodations, or worse, noxious tooth contacts that cannot be avoided. Neurologically it is the removal of noxious afferent sensory input.
Coronoplasty technique, as conceptualized by the pioneering neuromuscular clinicians, do not specifically address the mechanics of the occlusal stop. It only acknowledges that the occlusal stop be a minimal area, mitigating potential for deflection. This centric stop occurs with intensity at the moment of initial tooth contact that equals all other terminal centric contacts. In natural dentition we find every variation of our idealized mechanical models (tripodized, single point contact, marginal ridge or fossa).
Functional cusp tips sometimes make contact in the fossae, sometimes on marginal ridges and even on opposing slopes. We often observe these contacts as a single point; sometimes it is tripoded or wedged because of the opposing anatomy. Nature doesn't think in mechanical terms. Whatever the scheme of the micro-contact, the purpose is to achieve terminal stability of the mandible to the cranium. This can be achieved if all contacts occur precisely at the same time without torque, unequal intensity or deflection. The key words to satisfy the underlying physiologic requirements are FREEDOM OF ENTRY during mandibular closure and STABILITY at a terminal contact position along the isotonic path of mandibular closure (neuromuscular trajectory). As in nature, more than one scheme can meet these dynamic requirements.
This is why our profession should go back to the classroom and relearn the basic biologic laws that govern physiology. Many have gotten into petty arguments about "single" or "tripod", "balanced", "cuspid rise", or "anterior guidance" never once discussing the underlying biologic principles we neuromuscular clinicians are trying to satisfy. We wonder why dentists are considered "tooth mechanics" by our medical colleagues. As a profession we should look into the mirror to answer that question.
CONCLUSION - Stay in the Zone
Any occlusal scheme for micro-contact force distribution that meets the physiologic requirements is acceptable. In summary, the coronoplasty protocol is designed to assure freedom of entry without deflection along the isotonic neuromuscular path of mandibular closure. What remains after that must fulfill the requirements of terminal stability, whether it is single or tripod contact. Once the occlusion, with the elimination of all the deflective interferences, have been removed and brought to the zone of neutrality, the chewing and swallowing process can continue unhindered. Muscles at an optimal position in all three dimensions can bring the teeth together 2000-3000 times a day without triggering the cascading of medical and dental symptoms that are related to the numerous musculoskeletal occlusal signs and symptoms that have been mentioned. Functioning within a zone of neutrality (a physiologic rest position) where the mandible wants to posture and function to support a proper bite is the key to bringing harmony to the complete neuromuscular skeletal occlusal system.
Today the use of diagnostic "eyes" such as the electromyography (EMG) and computerized mandibular scanning (CMS) have made it possible to identify the zone of neutrality that is unique to each and every patient being treated. Initial deflecting contacts that were not previously detectable in the finalizing stages of treatment can now be identified implementing these precise techniques. The precise diagnostic and treatment technique for coronoplasty is necessary to translate the EMG and CMS data into a clinical result. The precision of electronic diagnosis requires commensurate precision in application.
Dr. Chan invites you to join him and Dr. Robert Jankelson to Occlusion III (Coronoplasty) to further understand these biological laws and techniques that govern muscle physiology and the 21st century way we treat neuromuscular occlusion at LVI.
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