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916-786-4TMJ Advanced Dental Concepts 916-786-4TMJ |
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The Functional Orthodontist January-March 1999 The "Missing Link"
in the Origin
of Trigeminal Neuralgia: Abstract: Tic Douloureux (Trigeminal Neuralgia) has afflicted mankind for centuries, perhaps for all time. This sharp stabbing paroxysm of pain along the branches of the trigeminal nerve is described as "...one of the most painful problems that plagues mankind." Many theories about the cause of trigeminal neuralgia have been previously presented. Often these theories build on the previous foundations when new research presents itself. The complete picture still eludes researchers today. Much of the mechanism has been proposed, but researchers lacked one essential component. There has never been an answer to why these pains only occur in cranial segments and why, thankfully, TN is rare. What sets the stage for the development of TN? The unique neurophysiology of the trigeminal nerve and the accompanying ability of the Temporomandibular joints to create a sensitized neural system are the last piece of the puzzle. This central sensitization of the Trigeminal Nerve allows the development of a small cluster of neurons that act as a central trigger for the paroxysmal pain. The role of the TMJ in trigeminal neuralgia is illustrated by this case report. When the fit came, there was, to use my Lady’s own expression of it, as it
were a flash of fire all of the suddaine shot into all those parts and at everyone of
those twitches, which made her shreeke out; her mouth was constantly drawn on the right
side towards the right ear by repeated convulsive motions, which were constantly
accompanied by her cries... Tic Douloureux (Typical Trigeminal Neuralgia) is a severe, sharp paroxysmal pain following one or more branches of the trigeminal nerve. Tic douloureux primarily affects the maxillary branch of the trigeminal nerve, has pain of a severe sharp, electrical nature, and has the presence of trigger zones. These trigger zones can respond to virtually any type of input from the kiss or touch of a loved one, to a cool breeze, to washing of the face. The afflicted patients will steadfastly attempt to avoid any contact with the trigger zones, thus hoping to avoid triggering a frighteningly painful attack. Tic douloureux has been described as "one of the most painful problems that plagues mankind". Historical references to typical trigeminal neuralgia have been rioted as far back as the Romans of the first century A.D. However, depending on your source, either John Locke in 1677, or Johannes Bausch in 1672, is credited with the first accurate clinical description. The French physician, Nicolaus Andre, is credited with first recognizing this condition as a unique medical entity and for naming this condition: "tic douloureux" which translates as "unbearably painful twitch". In the 1800’s, the distinctive sensory pathways of the three branches of the trigeminal nerve were elucidated and tic douloureux was shown to be a trigeminal phenomenon. From this finding came the name "trigeminal neuralgia" (TN). The nineteenth century English physician, John Fothergill first represented tic douloureux as a disease of older patients with most victims over the age of 50. Female victims outnumber males about two to one. The patient will usually describe the pain as sharp, "like a bolt of lightning" or a "red-hot poker jammed into my face". The pain is described as being unbearable and lasting for seconds to about a minute. In addition to sudden, lancing pain along a known nerve tract, true neuralgias exhibit trigger zones. The trigeminal trigger zones are small, 2-4 mm, and are usually found near the base of the nose or in the oral cavity and gums. [top] Tic Douloureux is called "typical trigeminal neuralgia" to differentiate it from two other conditions also commonly classified as trigeminal neuralgia. Typical Trigeminal Neuralgia (TTN) is the classic tic douloureux described above. Vascular compression of the trigeminal nerve causes up to 90% of the cases of typical trigeminal neuralgia. Tumors and multiple sclerosis primarily account for the other causes. Treatment is initially pharmaceutical using anticonvulsants, such as Tegretol, Baclofen or more recently, Neurontin. However, nearly half of these patients suffer recurrence of symptoms, making surgery necessary. Surgically, intervention consists primarily of radio frequency neurolysis, alcohol or phenolic neural injections or microvascular decompression (repositioning the offending vessel). Success rates as high as 80-95% are reported for these interventions. Recurrence rates are very low for the neurosurgical procedures, but these do have a significant morbidity rate. Atypical Trigeminal Neuralgia (ATN) is a constant pain of varying intensity involving any of the three divisions of the trigeminal system. The pain tends to be dull and aching in character with the trigger zones increasing the intensity and often setting off the lightning bolts of TTN. Though there is no confirmed primary cause, ATN is often seen following surgical procedures and facial trauma. ATN is typically treated with anti-inflammatory injections of the trigger areas combined with anti-convulsant drugs. Failure of conservative treatment leaves only the surgical acts mentioned for TTN. Atypical Facial Pain (AFP) is a rare and mysterious affliction. AFP is characterized by a long suffering patient with pain sites that are inconsistent with anatomy(such as pain crossing the midline), an absence of a definitive cause of the pain, and a high incidence of a history of psychiatric problems. These patients often get worse after surgery and must be handled with caution. Patients suffering from AFP should have psychiatric counseling in conjunction with any therapy. In the turmoil created by emotional and psychiatric symptoms, underlying organic problems are frequently missed. Atypical Facial Pain has also been called "undiagnosed facial pain" for there are often undiscovered physical sources of the pain. Treatment requires a thorough history and meticulous clinical exam. Often these patients have triggers at dental extraction sites. These trigger sites must be tested by anesthetic injection for Neuralgia-Inducing Cavitational Osteonecrosis (NICO, Ratner Bone Cavities). These areas of bone necrosis appear as normal bone on radiographs. NICO lesions have predictable pain referral patterns. While these three clinical variations are unique phenomena, the common characteristic of trigeminal neuralgias (and virtually all chronic pains) is a sensitization of the neural pathways involved in the transmission, interpretation and response to the noxious neural input. These pathways are hyper-sensitized and enlarge in response to a continuous noxious afferent input. Interruption of the temporal summation of afferent impulses is the basis of pharmacological treatment of tic douloureux. Anti-convulsive drugs, like Tegretol and Baclofen, raise the excitatory threshold in the central nervous system and change the responsiveness of the hyperactive sensory circuits. This is accomplished by depressing synaptic transmission and by inhibiting polysynaptic reflex activity in the trigeminal nucleus. Surgical interventions are based on either destroying a portion of the trigeminal nerve root or by eliminating the vascular compression of the trigeminal nerve. After successful treatment, these neural pathways may cool down, but they do not necessarily disappear. These neural pathways can lie dormant and unused until a triggering episode revives them. This phenomenon is seen in the following case report. A seemingly minor traumatic occlusal event sends enough noxious afferent neural input into the central nervous system to "heat up" a dormant sensory feedback pathway. [top] Patient History:
[top]
Discussion: At first glance, it would appear that the original Typical Trigeminal Neuralgia (pain-free periods punctuated by the tic) that the patient suffered had returned as Atypical Trigeminal Neuralgia (dull, aching pain punctuated by the tic). The more likely scenario is that the patient was suffering from reflexive muscle splinting and muscle spasm pain secondary to what Dr. Terry Spahl calls "neuromuscular reflexive displacement of the mandible causing superior posterior displacement of the condyle (NRDM/SPDC)". The patient experienced facial pain again after years of relief, as the dormant central nervous system (CNS) circuitry reactivated and the patient again suffered the painful attacks. To understand what happened with this patient, we must examine what happened in the CNS. The CNS constantly receives vast amounts of afferent messages from the synovial joints about how the body is functioning and how the body is positioned. Synovial joints are designed by nature to have a definitive range of motion. The periphery of the synovial TMJoint contains numerous nociceptive and proprioceptive nerve fibers. These are especially numerous in the ligaments supporting the joint. The receptors in the ligaments tell the brain where in space the joint is at all times, and are especially important if the joint is being forced into a pathologic, painful position. These painful positions with their nociceptive impulses occur when the joint is taken to or near the extreme peripheral range of motion. Also, chronic micro-trauma, as with bite collapse, can elongate and damage the TMJoint ligaments. As the lower anterior teeth collapse up the inclined planes on the lingual of the upper central incisors, the condyles are displaced onto the retrodiscal tissue with its highly innervated vascular complex. All these forces combine to send constant streams of noxious input to the central nervous system. The pain circuitry becomes ingrained and wound-up (or hyper-sensitized). Receptive field enlargement can occur through arborization of neural tissues. The newly formed nerves are all nociceptive in nature for they are all small unmyelinated free c-fibers. These neural-plastic changes open the door to numerous pathologic possibilities. [top] Theories of Trigeminal Neuralgia:
Rappaport and Devor have proposed an excellent theory for the mechanism of TN. They list 14 essential features that must be explained by any theory for the mechanism of the TNs. They are:
(1). Neural triggering: How does the stimulus event provoke a sensory response that outlasts itself, and indeed may continue to build after the stimulus has ended.? A lingering response following a stimulus suggests an independent, self- sustaining process. The CNS has two mechanisms of this type: rhythmic circuits and intrinsic autorhythmicity. Rhythmic circuits, also called reverberating circuits, (Image 7) combinations of inhibitory and excitatory synapses (Image 8) group together produce neural discharges that can continue indefinitely. Autorhythmicity means that individual neurons can sustain independent firing patterns in the absence of excitatory stimuli. Both of these mechanisms have been demonstrated in the central nervous system. [top] (2)
(2).Neural Amplification: How does a localized, non-noxious (trigger) stimulus evoke a sensory response that: (a) spreads to cover a large contiguous tissue area, and (b) is intensely noxious? This response is more intense than normally expected from the stimulus. Neural circuits containing a large number of mutually excitatory synapses are inherently prone to explosive paroxysmal responses due to the positive feedback. These circuits require a damping effect by inhibitory interneurons to balance the proclivity toward explosive neural responses. Rappaport and Devor state: "In the presence of small (depolarizing) stimuli, autorhythmic neurons are silent. However, as the stimulus strength increases, a threshold is reached at which firing suddenly jumps from zero to quite high frequencies. If many such neurons were at rest just below their rhythmic firing threshold, weak excitatory coupling among them could cause the entire population to explode into activity even if only a few were triggered." (3). The Stop Mechanism: What process prevents an attack from continuing indefinitely, and causes the triggering mechanism to become temporarily refractory? While the automatic, reverberating firing of neural circuits could potentially continue indefinitely, they generally fire only for a limited duration. The "stop mechanism" can be either extrinsic, such as a descending inhibitory signal from the higher brain levels or intrinsic. Intrinsic stop mechanisms include depletion of essential substances, such as ATP or a neurotransmitter. Another method would be by activation of specific inhibitory suppressive membrane channels. Typically, these channels are membrane openings that allow the movement of either Calcium or Potassium ions to enter the cell, thus changing the surface polarity and inhibiting the circuit.
[top] The Role of the Distalized Condyles: Chronic irritation of the peripheral trigeminal system by vascular compression and posteriorized condyles not only fires up the central nervous system but also depletes the inhibitory mechanisms in the trigeminal nucleus. The chronic noxious input combined with decreased inhibitory action allows semi-permanent hyperactive sensory circuits to build up. In the nervous system, if only a few synapses fire at a low level, they might not be enough to make the receptor neuron fire. However, if these few synapses continuously discharge low level signals and these signals are repeated at a fast enough rate, the signals can have a cumulative effect and create an input great enough to cause the neuron to fire. The excitatory state necessary to trigger a TN attack builds up over time like deposits in a bank. When the trigeminal system becomes sufficiently charged, any contact with the trigger zone can be enough to discharge the neural network. These periodic discharges in the subnucleus caudalis of the trigeminal spinal nucleus are like an explosion in the neural circuitry. This is an example of temporal summation of neuronal input reaching the excitatory threshold. This mechanism explains the suddenness, intensity and brevity of the attacks.
[top] In the case of Mrs. H., the insertion of the partial upper denture with the distal driving interferences caused large amounts of strong nociceptive input to be fed into the CNS. The resurgence of a constant barrage of strong afferent signals into the CNS awoke the "sleeping" hyperactive sensory circuit with its paroxysmal discharges. Removing the source of the nociceptive afferent input (the distalizing interferences of the partial denture) allowed the muscles to relax and ease the ligamentous strain. This eliminated the noxious afferent stream and allowed the sensory inhibiting systems of the CNS to calm the neural circuits. Thankfully, the patient is happy, painfree and relieved of the fear that the tic douloureux had returned permanently. This case illustrates how alterations once wrought in the "plastic" fabric of the brain may lie dormant awaiting the appropriate triggering mechanism to breathe renewed life into them. Conclusion In the September 1998 issue of The Functional Orthodontist, Dr. Terry Spahl discussed how current thinking among headache experts is moving toward the unitary concept. The unitary concept states that all non-neoplastic headaches, from the simple muscle tension headaches to the serious debilitating migraine-type headaches, are merely portions of a spectrum with a common underlying etiology. As Dr. Spahl states, the common factor is the presence of "progressively posteriorly displaced condyles at full occlusion and the damage to the tissues of the bilaminar zone they cause..." It is time that we consider that the trigeminal neuralgias may also be part of this continuum. Bowsher suggests that the peripheral origin are supported by the fact that "there is immediate relief of pain and restitution of other sensory functions following procedures such as radiofrequency thermocoagulation and microvascular decompression, and the immediate return of nerve function in the decompressed proximal nerve root". We must consider that vascular compression and other demyelinating diseases perhaps also have, as an additional underlying cause for these neuralgias, the posteriorized condyles sending streams of noxious afferent impulses into the trigeminal system. As these impulses combine over time with the afferent input of vascular compression, they set up neural feedback loops in the trigeminal system. The TRG Ignition Hypothesis supports the concept that temporal summation of sub-threshold synaptic input causes the explosive periodic discharge of this feedback loop in the trigeminal system, accounting for the paroxysmal episodes of excruciating pain of tic douloureux. This combination of the TAG Ignition Hypothesis and Central Sensitization from the TMJoints is a theory that will require more research, but the facts fit the concept. The vascular compression of the trigeminal nerve can be present from the cessation of cranial growth during the teenage years. However, the neuralgia does not show up until the later years of life. Why is there a delay in onset? These later years are when bite collapse and loss of vertical dimension of occlusion typically occurs. Certainly, the NRDM/SPDG accompanying the bite collapse could be the initiating factor that starts the painful episodes. In addition, it can take years of central sensitization to cause the neuroplastic changes and to wear down the intrinsic descending inhibitory mechanisms. If this is the case, then as the current population keeps their dentition intact more than previous generations did, we should see a decrease in occurrence of this painful scourge of mankind. Further, restoring the vertical dimension of occlusion may be a non-surgical, non-pharmaceutical alternative for these patients. Therefore, doctors treating these orofacial pain syndromes must consider the TMJoint condition as an important and integral part of their evaluation and treatment False facts are highly injurious to the progress of science, for they often endure long; but false views, if supported by some evidence, do little harm, for everyone takes a salutory pleasure in proving their falseness; and when this is done, one path toward error is closed and the road to truth is often at the same time opened. – Charles Darwin The Descent of Man [top] References Bowsher, D: "Trigeminal Neuralgia: An Anatomically Oriented View" Clinical Anatomy, Vol. 10, 1997, pp. 409-415 Burchiel KJ: "Trigeminal Neuropathic Pain", Acta Neurochirurgica, Vol. 58, 1993, pp. 145-149 Clemente CD: Anatomy, A Regional Atlas of the Human Body. Philadelphia: Lea and Febiger, 1975 Dalessio DJ and Silberstein SD: Wolff’s Headache and Other Head Pain 6th ed. Oxford: Oxford University Press, 1993 Goldstein JA: Betrayal by the Brain. New York: Haworth Medical Press, 1996 Guyton AC and Hall JE: Textbook of Medical Physiology, 9th ed. Philadelphia: W.B. Saunders Company, 1996 Kanner R: Pain Management Secrets. Philadelphia: Hanley and Belfus, Inc, 1997 Lee KH: "Facial Pain: Trigeminal Neuralgia", Annals of Academy of Medicine, Singapore, Vol. 22, 1993, pp. 193-196 Levitan IB and Kaczmarek LK: The Neuron. Oxford: Oxford University Press, 1997 Moller AR: The Cranial Nerve Vascular Compression Syndrome: II. A Review of Patho-physiology", Acta Neurochirurgica Vol. 113, 1991, pp. 24-30 Morris DB: The Culture of Pain. Berkeley: University of California Press, 1991 Nurmikko T, Wells C, Bowsher, D: "Pain and Allodynia in post-herpetic neuralgia: Bole of somatic and sympathetic Systems", Acta Neurology Scandin., Vol. 84,1991,pp.146-152 Pagni CA: "The origin of tic douloureux: a unified view", Journal of Neurosurgical Sciences, Vol. 37, No. 4, pp. 185-194 Rappaport ZH and Bevor M: "Trigeminal Neuralgia: the role of the self-sustaining discharge in the trigeminal ganglion." Pain, Vol. 56, 1994, pp. 127-138 Shankland WE: "Craniomandibular Pain Syndromes that Mimic Temporomandibular Joint Disorders", Annals of the Academy of Medicine, Singapore, Vol. 24, No. 1, January, 1995, pp. 83-112 Shankland WE: TMJ: Its Many Faces 2nd ed. Columbus: Anadem Publishing, Inc, 1998 Spahl TJ: "The FACT Appliance in Clinical Application: A Case Report", The Functional Orthodontist. Vol. 15, No. 3, July-September, 1998, pp. 22-34 Spahl TJ and Witzig JW: The Clinical Management of Basic Maxillofacial Orthopedic Appliances, Vol. III, Temporomandibular Joint. St. Louis: Mosby-Year Book, 1991 Thompson RF: The Brain: A Neuroscience Primer, 2nd ed. New York: W.H. Freeman and Co., 1993 [top] |
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The patient was a 73-year old female who presented at my office
desiring a new removable partial upper denture. The patient was in good health with no
contributing medical history. During the examination, as I approached her mouth, she shied
away and told me that she had "tic" and to be careful of touching her face. I
questioned Mrs. H. about her experiences with head and facial pain. She had suffered from
mild to moderate headaches most of her adult life. When she was about fifty-eight, she
developed tic douloureux. She would have sporadic, lancing, burning facial pain in the
area served by the left maxillary nerve. It could be set off by the slightest provocation
and without any warning. She was treated with moderate success with medication for eight
to nine years. The medications stopped working and she finally had surgery in
approximately 1993. Mrs. H. had undergone alcohol injection of the trigeminal nerve root
and had been pain – free since the operation. The pain started to return
approximately two months after a partial denture was made at another office. 
She was on Baclofen without success at eliminating the painful
attacks. She described the pain as a dull aching in the muscles of mastication especially
the left masseter and the left temporalis. The tic developed about 3 week’s after the
head pain started. The sharp lancing pains were similar in nature to those she had had
before. She stated: "They aren’t quite as painful as I had before." Mrs. H.
was contemplating another alcohol block on the advice of her physician. Mrs. H. had the
following remaining teeth: Uppers #2, 6, 8, 9, 11, and 12; Lowers #19-29. Mrs. H. is
suffering from posterior bite collapse with a loss of vertical dimension. She had a 5 mm
anterior overbite and 3 mm of overjet. Tooth # 11 completely overlapped #22. She confirmed
that she had not always had such a deep bite, "it just got that way with time".
Further evidence of this collapse is the presence of "nesting" sites for the
lower anterior teeth worn into the lingual surfaces of #8 and #9 in an unconscious attempt
to anteriorize the mandible. (Images 1-5) 
Mrs. H. does not wear a lower partial. The patient had broken her
old upper partial approximately nine months earlier and had a new partial made at another
dental office. She complained that the new partial "was not comfortable to wear"
and wanted to have another partial made. The patient was very reluctant to be seen in
public with two front teeth missing. Therefore, she wore the partial upper denture even
though it was uncomfortable and the bite felt off. Examination of her existing upper
partial denture revealed that the patient was occluding on the framework of the partial
denture. The lower teeth were occluding on the rests on the upper right and left canines
(#6 and #11). This created a distalizing force on her mandible, especially on the left
side. The denture teeth were flat-plane and provided no resistance to the posteriorization
of the mandible. A new upper partial denture was constructed using cusped denture teeth.
Judicious coronoplasty was performed to allow a slight anterior positioning of the
mandible and some lateral freedom. The patient was left to occlude on her natural teeth
and solid occlusal stops on the partial denture teeth. The headaches and painful muscles
disappeared within 48 hours of delivery of the new denture. There have been no painful
tics since the new partial denture was delivered. The trigger zones are not active and the
patient states that she can function normally. 
The theories for the
neuropatho-physiology of trigeminal neuralgia are based on the concepts of trigeminal
nerve root demyelination and temporal summation of afferent impulses. The trigeminal nerve
root is surrounded by many arteries. The superior cerebellar artery lies adjacent to the
root of the trigeminal nerve as it enters the pons. (Image 6) This
artery contacts the portion of the trigeminal nerve root where the peripherally produced
myelin meets the centrally produced myelin. Chronic mechanical irritation of the nerve
causes a loss of the myelin in the upper portion of the nerve root. The loss of the myelin
sheaths allows ephatic conduction (cross-talk) between nerves. However, this compression
damage to the trigeminal root alone is not enough to cause TN for this same vascular
condition has been found in 50% of asymptomatic autopsy specimens. 
They summarize the 14 points
into three physiologic questions: 
.
Using the mechanisms described above
as their guides, Rappaport and Devor have elucidated a theory that answers 13 of the 14
points about Trigeminal Neuralgia. They call this the "TRG (Trigeminal Boot
Ganglion) Ignition Hypothesis". (Image 9)
Summarizing, a stimulus from a trigger zone sets off a small area of intense neural
activity within the trigeminal root ganglion (TRG) that spreads to neighboring neurons.
This short (seconds to minutes) period of autorhythmic neural firing (the tic) is followed
by an intrinsic suppressive ionic conductance. The intrinsic suppression causes a
hyperpolarization of the ignition focus. This stops the paroxysm and establishes a
refractory period. The authors actually leave two unanswered questions: Why is
"...paroxysmal neuralgia unique to cranial segments..." and "Why is
trigeminal neuralgia so rare?" The answer to the first question is that the massive
trigeminal nerve subserves a unique joint and plays a unique role in the central nervous
system. Nowhere else in the body can a single joint(s), the temporomandibular, have such a
profound effect on the neural tissues in the CNS. The TMJoints provide a central
sensitizing mechanism that can set the stage for TN to develop. Rappaport and Devor
"...suppose that variance in the degree of excitability of DRG (Dorsal Root Ganglion)
neurons varies from individual to individual" by chance. They state that
"Individuals predisposed to trigeminal neuralgia are those near the high end of the
range who are also unfortunate enough to have a dorsal root lesion in a location that
supports the formation of a TRG ignition focus." I propose that random chance and
"misfortune" are not the deciding factors in the development of TN. The
"missing link" of trigeminal neuralgia is the role of the pathologic
temporomandibular joints in the central (nervous system) sensitization that allows the
other factors to, coincidentally, become active. The summation of all 13 of the confirmed
points of the TBC Ignition Hypothesis with the Central Sensitization from the
TMJoints allows all 14 of the points to be answered. 
Many older patients have distalized
condyles and also have nerve root compression, yet few develop Trigeminal Neuralgia. TN
patients have a unique set of anatomic and physiologic circumstances working against them.
These patients have had their neural system "heated up" for so long, that the
A-beta mechanoreceptor fibers in the TMJoint ligaments have become nociceptive in nature.
There are three types of nerve fibers that predominate in the TMJoint. These are the large
myelinated A-beta and A-delta fibers, and the small unmyelinated C fibers. A-beta fibers
are primarily pressure receptors, found in the ligamentous tissues. A-delta and G fibers
can be mechanoreceptors, nociceptors or thermoreceptors. Research shows that morphine does
not stop the pain of TN. Since morphine acts in the substantia gelatinosa (Image
10) of the brain stem and this is the terminal site of the G-fibers, we can
conclude that they are not directly causing the TN. Since the pain of TN is essentially
allodynia (pain set off by non-noxious stimuli), we can deduce that it is the A-beta
fibers that are the root cause of the TN. We can make this deduction because of the
research of Nurmikko, et al. They showed that when A-delta and C fibers, but not A-beta
fibers, are blocked with local anesthetic, the pain from allodynia still occurs. Yet when
the A-beta fibers alone are blocked, the pain disappears. Nurmikko and his colleagues
further conclude that mechanical allodynia (such as TN), is caused, in part, by the
activation of rapidly adapting peripheral A-beta fibers and is suppressed by the slow
adapting peripheral mechanoreceptor fibers. Further, Bowsher reports that while the pain
from TN shoots into the gums, it is not felt in the teeth them- selves, from which the
A-beta fibers are absent. Elongation and constant strain on the ligamentous tissues of the
TMJoint cause chronic excitation of the A-beta fibers. This gradual, long-term excitation
can cause a neuroplastic conversion of the A-beta fibers from solely mechanoreceptors to
mechano/nociceptive fibers. This adds the allodynic overlay to the already "heated
up" trigeminal system caused by the distalized condyles.
