Kevin B. Scally 1993
In 1958 at a Dental Conference in Christchurch, New Zealand, Dr R.G. Every, a Christchurch dentist, presented an argument that tooth grinding in human beings was in fact tooth sharpening. This idea and its clinical and biological significance were subsequently published in the Lancet [1,2]. Over the subsequent years, he championed his hypotheses and presented an extensive amount of evidence to support the original views expressed in the Lancet articles [Every 3,4,5,6,7,8]. During that period he coined the word “thegosis” to define the instinctive tooth sharpening behaviour he had discovered .
Thegosis is derived from the Greek word thego: to whet and to sharpen; also, metaphorically to excite and provoke . The phenomenon was first recorded by Homer who correctly described specific dental weapon sharpening in the wild boar:
‘Just as when hounds, and young men in their bloom, press round a boar, and he comes forth from his deep lair whetting his white tusks between crooked jaws, and round him they rush: and in the midst of all this is heard the noise (from the whetting) of his teeth, and straightway they await his assault, dread as he is – even so then round Odysseys dear to Zeus, rushed the Trojans’
Homer the Iliad, 700 BC, [in Every (6)]
Since 1958 Every’s hypotheses have been tested and increasingly independent support has been found for his original views [9,10,11,12,13,14,15,16,17,18].
During the 60s and early 70s there was some discussion of thegosis in two disciplines, palaeontology and anthropology, however, while there was some acknowledgment of his discovery , all discussion of thegosis and its significance has ceased to be cited in recent literature in these disciplines . In addition, other authors familiar with the concepts have either ignored, or in some instances, misinterpreted Every’s ideas and misquote his arguments .
In 1984, an international symposium on thegosis was held in Christchurch, to draw together scientists working in the area of thegotics and to create an environment for fruitful discussion. One of the aims of the symposium was to bring discussion of thegosis into the literature on tooth wear. This is slowly being achieved [22,23].
It is now clear that thegosis is a universal phenomenon. It is found in many animals and in each, it affords the same biological advantage: honing the teeth for efficient function either as tools, weapons, or tools and weapons .
Every has argued that in the evolving hominid there was a change in the dental weapon from a slashing canine weapon, as found in the apes to that of a short canine condition that allowed the evolution of a segmentive bite . He has listed a number of biological advantages for this change and this interpretation is consistent with evolutionary processes as currently understood: selection for biological advantage. This is in contrast to the implausible contemporary view which suspends natural selection for 2.5million years while the dental weaponless hominid continued to loose any semblance of a weapon with the gradual reduction of the snout and waited for their wits to evolve so that they could make weapons to defend themselves. Every’s argument makes sense of the paleontological record and is a more plausible account of the developing hominid that was never weaponless. Indeed the snout reduction improved the segmentive bite [4,6,7,8].
In contemporary dentistry, any dental wear is considered damaging and pathological and a great deal of dentistry is undertaken to repair this assumed damage . Yet armed with an understanding of the human dentition, a dentition that evolved where abrasive and sharpening wear are in concert, much of the so called “pathological” tooth wear conditions can be reinterpreted. Indeed, it is the normal biological situation for the teeth to be serviced and enhanced by thegotic activity. Yet in a number of identifiable situations, this can go awry and oral damage can result. [1,25]. This damage can be conveniently considered under a number of headings, classes of clinically identifiable phenomenon, including symptoms from clenching (Table 1).
When considering thegosis, a novel concept that challenges a lot of conventional dental teaching, it is helpful to consider the concepts embodied in current teaching and the action or therapy current teaching prescribes against those of the thegotic model. Accepting that teeth are honed as dental tools has a clinical yield since it makes sense of occlusal equilibration and the use of splints (Table 2). But the thegotic model stresses that humans have a viable biological weapon that is honed in response to threat. Any therapy based on the thegotic model must address both the physical and emotional factors [1,2,6]. (This comparison is further analysed in Table 3.).
In many instances current teaching and the thegotic model are complementary. In other instances, the concepts are diametrically opposite.
The thegotic model considers clenching. While clenching is a discrete activity, it is complementary to thegotic activity. While concentrating, for example, most of us lightly clench our teeth. This is usually intermittent and an appropriate response to a low level of stress. When concentrating for extended periods, and the situation is extended and stressful (eg a full day in a dental clinic with difficult patients) then the outcome is likely to be fatigued masticatory muscles and temporal muscle fullness and headache. The clenching response is appropriate since it splints the teeth together to protect them from damage when hit. Clenching can be considered the daytime complement to nocturnal thegosis.
The conventional explanations for craniomandibular disorders increasingly cite stress as the major etiological factor. However, no clear mechanism is given relating the ‘stress ‘to the symptoms of gross anatomical trauma that are evident (internal joint derangement for example). The thegotic model provides a link between the two prevailing schools of thought: the equilibration school that argues the dysfunction is due to occlusal factors and the psychological school that argues the symptoms are due to stress (Table 4). The thegotic model details a mechanism that explains how some individuals can unconsciously cause physical damage and the psychological mechanism that would provoke the activities of thegosis and clenching.
To accept the thegotic model requires a complete change in understanding about how human teeth work and dental tools and weapons, and how they are sharpened. It demonstrates that the anatomy, physiology, and the instinctive behaviour are not separate but integrated. This shift in paradigm is necessary to fully take advantage of the therapeutic opportunities of the thegotic model.
The advantage of the thegotic model is the confidence that a clinician can have when evaluating thegotic trauma. The oral signs and symptoms are compelling evidence for a biologically appropriate but unconscious behaviour. For example, if erupting wisdom teeth limit the full thegotic excursion – the anterior tooth honing pathway – then removal of wisdom teeth is justified. If the unworn dentition’s cusps are the primary element for tooth or deeper structure trauma, then a contrived occlusal scheme in plastic is warranted: a thegotic splint. Conventional pharmacological and physical measures are also used. But, more importantly, a confidence about the behaviour or psychological factors involved make discussion about this easier to initiate and it gives the dentist an opportunity to involve our psychologist colleagues in the joint management of some patients.
In my experience, when there is no conscious history of trauma, most of the macro-trauma to the temporomandibular joint is explained by traumatic consequences of normal nocturnal thegotic activity in most patients presenting with cranio-mandibular disorders .
When we examine the full facet matching pathways of our patients and ourselves the full potential for trauma can be understood. When articulators are modified to reproduce the full thegotic excursion so that facet analysis of dental casts can be undertaken, the traumatic tooth relationships can be analysed. When this is done many of Dr Every’s claims can be verified.
When the three dimensional relationships of other anatomical structures during thegotic excursions are understood persuasive evidence for macro-trauma to deeper structures can be clearly demonstrated. When the variations in these structures (both normal and pathological) are considered a wide range of head and neck symptoms make sense.
When the resistance to the paradigm shift is overcome the claims Dr Every was making in 1960 can begin to come to fruition in both clinical practice and in the gold mine of research opportunities they offer.
APPENDIX 1 THEGOSIS:
GREEK thego, to whet, sharpen; also metaphorically, to excite, provoke.
A phylogenetically derived behaviour that sharpens (whets, hones) a tooth by grinding it violently against another; the action occurs concomitantly with a sharpening (in the metaphorical sense) of the appropriate emotion, that is, a sharpening of the physiological determinants that not only prepare (thegose) the tooth but prepare the animal to grasp, attack, defend, incise or masticate.
The morphological adjustment of the tooth and the concomitant physiological excitation of the emotion are an integral reaction to a situation of stress. The behaviour, moreover, occurs discretely from that of ingesting or of grasping. The inherent advantage is the effective preparation for survival by:
Every R.G. A New Terminology for Mammalian Teeth: founded on the Phenomenon of Thegosis. Pegasus Press, 1972.
ORAL SIGNS AND SYMPTOMS: THE THEGOTIC MODEL
Physiological events leading to symptoms of fatigue:
1. Prolonged clenching: masticatory muscle fatigue, TMJ symptoms from meniscus compression.
2. A spate of thegotic activity without thegotic trauma but where the activity is beyond the fitness level of the gnathological musculature and articulation. Symptoms of masticatory muscle fatigue or joint tenderness.
3. Trauma to cusps on individual teeth when the mandible moves from centric occlusion to the canine edge-to-edge position during a thegotic excursion.
4. Trauma that occurs to teeth during the thegotic stroke from the canine edge-to-edge position to the limit of the thegotic stroke.
5. Trauma to other anatomical structures towards and at the limit of the thegotic stroke:
6.The traumatic psychological event(s) that provoke a clenching or thegotic response are often central to the cranio-mandibular symptoms both fatigue and thegotic injury. These can be conscious and accessible to counselling, and unconscious and may be accessible to other psychological therapies.
THE ORAL SIGNS, THEIR INTERPRETATION, AND THE PRESCRIBED ACTION
CONVENTIONAL TEACHING AND THE THEGOTIC MODEL
|1. Evidence of tooth grinding activity||BRUXISM (pathological)||Build up lost structure to juvenile form with onlays or crowns.|
|2. Evidence of wide lateral jaw movements||PARAFUNCTION (pathological)||Ask patient not to do it.|
|3. Evidence of tooth grinding activity from centric occlusion to the anatomical limit. A path that is oblique to incision and diametrically opposite to the masticatory stroke||THEGOSIS (normal). This activity sharpens the teeth as tools||Advise the patient that it is normal and do nothing,
If there are signs and symptoms of trauma then reconcile the mismatch of the wearing teeth by occlusal adjustment to allow atraumatic thegosis.
Explain and demonstrate the tooth sharpening actions to the patient.
If damage is a consequence of anatomical variation or abnormality, then address this by contriving to make the activity possible so that thegotic damage is eliminated or minimised. For example a thegotic occlusal equilibration, a therapeutic occlusal onlay, oral surgery to remove wisdom teeth.
Ideally, a range of therapies should be integrated; physical therapy (occlusal adjustment, a therapeutic thegotic occlusal onlay, heat, etc.) pharmacological therapy (anti inflammatories, analgesics) and behavioural therapy, (counselling and psychotherapy)
|THEGOSIS (normal). This activity sharpens the anterior teeth as a dental weapon as well as dental tools.||As for 3. But if their is a persistence of thegotic trauma, especially from attempts to hone the anterior teeth it is appropriate to ask the behaviour question: “Why do you persist?”. “Of what are you fearful and fighting?” Management involves the integration of both physical and psychological therapy.
If the thegotic trauma has caused irreversible damage to the TMJ or associated structures the symptoms may persist and surgical intervention may be necessary.
|THEGOSIS(normal but extreme) In response to a life threatening event, either real or metaphorical.||It is often necessary to elicit the help of a psychologist early in the management so that the acute psychological crisis can be dealt with and the physical symptom management accompanies this.|
In the thegotic model, the concepts of resilience, duration, frequency, fitness, and intensity are relevant. From clinical observation that with irregularities in the thegotic pathways and anatomical interference to a full thegotic stroke, not all patients develop overt symptoms. It seems that the intensity of the thegotic activity and clenching are important in the production of signs and symptoms.
CURRENT TEACHING AND CURRENT THERAPUTIC PRESCRIPTION
CONTRASTED WITH THEGOTICS CONCEPTS AND ITS THERAPUTIC PRESCRIPTION
This table lists sets out the concept or belief system matched with the clinical action and contrasts current teaching with teaching derived from thegotic concepts.
Current Teaching Concepts
Clenching Bruxism Canine Protection Group Function Parafunction
|CLENCHING (Current Teaching)|
|Clenching is a habit and pathological. Prolonged clenching is a habit and pathological||Counsel patient that they do it in the hope they will stop|
|Contrasts with the Thegotic Concept|
|Clenching is an instinctive behaviour to protect the dental system (tool & weapon) by splinting it together in anticipation of injury.
Prolonged clenching, while instinctively provoked can produce symptoms but it is psychologically motivated.
|Look for biological reason for clenching. For example, a chronic state of fright or flight or a flight/fright ambivalence will produce prolonged clenching.
This may require assessment and treatment by a trained counsellor or psychologist.
|BRUXISM (Current Teaching)|
|Tooth grinding wear is bruxism and it is Pathological.||Restore worn teeth to the ideal eruptive morphology. Restore the ‘bruxed’ pieces, eg: worn maxillary canine tips.|
|Contrasts with the Thegotic Concept|
|Most tooth-grinding wear is thegosis and appropriate.||Accept, monitor, adjust the occlusion to the patient’s comfort and requirements (smoothing sharp edges and thegotic interferences for example) imagining how the dentition would look if thegosis and abrasion were harmonious.|
|CANINE PROTECTION(Current Teaching)|
|Interferences from centric to the canine edge-to-edge position (“canine protected” occlusion).||Adjust occlusion so that posterior teeth are discluded by the canines to the canine edge-to-edge position.”canine disclusion”|
|Contrasts with the Thegotic Concept|
|Without abrasion, individual molar and premolar cusps can be subjected to the full thegotic load. In the juvenile dentition the canine is often subjected to the full thegotic load. Until the canine is worn it can constitute a major impediment to the development of the segmentive bite||Adjust the occlusion to distribute the thegotic forces on to as many teeth as possible.|
|GROUP FUNCTION (Current Teaching)|
|The canine premolar and molar cusps should equally share the contact load to the canine edge-to-edge position – “group function”.||Adjust the occlusion so that this is achieved. Adjust the occlusion so that the posterior teeth are discluded by the canine and premolars.|
|Contrasts with the Thegotic Concept|
|The canine premolar and molar cusps should share contact load from centric occlusion to the canine edge-to-edge position on both the contra-lateral and ipsi-lateral side thus honing the posterior teeth.||Adjust the occlusion so that the thegotic load is shared. If the posterior teeth are heavily restored adjust the occlusion to a “canine protected” occlusion.
Adjust the occlusion so that this is achieved. Conceptually this is equivalent to a “balanced” occlusion but includes the concept that contra- lateral thegosis enhances contra-lateral blade edges and that it redefines the maxillary palatal cusps and the mandibular buccal cusps via oblique and transverse thegosis.
|PARAFUNCTION (Current Teaching)|
|All jaw positions beyond the canine edge-to-edge position are parafunctional. (ct)||Treat this habit like nail biting. Explain that this is abnormal and counsel the patient to stop it.|
|Contrasts with the Thegotic Concept|
|A specific class of jaw activity beyond the canine edge-to-edge position has a functional role in sharpening the anterior teeth. This is innately programmed and not amenable to behavioural modification|
|a) Dental tool sharpening:||Adjust the occlusion to allow this to occur so that teeth are not traumatised. Sometimes this is not possible without major tooth reduction. It can be achieved with a plastic overlay, worn at night during a treatment period. The onlay contrives an atraumatic environment by mimicking the worn condition|
|b) Dental weapon sharpening:||Adjust the occlusion as outlined for dental tool sharpening. The occlusal onlay is designed to allow the jaw to move freely to its limit without trauma to oral or deeper structures.
Often it is necessary to consider ways to adjust the anatomical constraints or limitations or inhibitions to the full thegotic stroke. This may require removal of wisdom teeth, for example.
Consider the psychological reasons that provoke the behaviour. This may require assessment by a trained counsellor or psychologist.
The key point is to find what stops honing the anterior teeth. The thegotic model argues that these teeth are honed to service the biological weapon. A corollary is that if anterior teeth honing is frustrated by an anatomical variation or abnormality the activity will continue, inspite of the trauma it causes. The day time consequence of which is pain and dysfunction
HOW THEGOTICS INTEGRATES CONTEMPORARY MODELS FOR THE ETIOLOGY OF CRANIO-MANDIBULAR DISORDERS
|FUNCTIONAL MODEL||PSYCHOLOGICAL MODEL|
|Discrepancies between centric occlusion and centric relation produce micro-trauma to the joint and muscle splinting. This explains the patient’s symptoms||Occlusion has no place in the genesis of symptoms. Craniomandibular disorders are largely stress induced.|
|Interferences from centric occlusion to canine edge-to-edge position and their avoidance set up muscle symptoms (myositis) and joint symptoms (arthralgia).||Occlusion is largely irrelevant|
THE THEGOTIC MODEL
The thegotic model integrates these two disparate and competing contemporary models for the etiology of cranio-mandibular disorders (TMJ Pain Dysfunction Syndrome, Myofascial Pain Dysfunction Syndrome).
Interferences are considered relevant since they inhibit free thegotic excursions. These excursions are from centric occlusion to the canine edge-to-edge position and beyond to the anatomical limit. The first phase of the excursion sharpen the posterior teeth. The second phase sharpens the anterior teeth. Sharpening the posterior and anterior teeth is an appropriate biological response to a variety of stressors. But there are additional interferences from anatomical relationships. For example, when thegotic excursions are considered wisdom teeth can be viewed as ‘space occupying lesions’ since they can inhibit anterior tooth honing by limiting the lateral excursions necessary to sharpen the anterior teeth.
Other anatomical variations become preconditions for thegotic trauma: long styloid processes, variations in the location of the foramen spinosum, for example. (For detail of this see Every ).
The symptoms are extensive and varied:
1. Tooth damage: pulpitis, mobility
2. TMJ damage: meniscus damage and derangement, ligament tearing, straining and spraining
5. Headache: tension, cluster and migraine