A Rebuttal by Kevin B. Scally; December 1999
Is there such a phenomenon and what is the evidence for it?
Both observation and experimentation have established the phenomenon in three phyla: Echinodermata, Arthropoda, and Chordata (and that includes mammals and our genus) (Scally 1973, Scally et al 1974, Every 1960). So it is difficult to dismiss thegosis when it is clearly multiphyletic. A phenomenon that has evolved independently in at least three major phyla over the last 600 million years clearly has a significant biological advantage. Every’s conclusion: the behavior sharpens teeth (and tooth-like structures). He coined the word thegosis (Greek thego: to whet, to sharpen, also metaphorically to excite, provoke) to describe the phenomenon (Every 1970).
What is its role in the maintenance of dental tools?
The behavior maintains the teeth and tooth-like structures as dental tools. The efficiency of these incisive and masticatory structures in many animals (both vertebrate and invertebrate) is dependent on thegotic servicing. The behavior can be considered the fourth dimension of tooth anatomy: a behavior that controls tooth shape after eruption. It provided a mechanism to control the shaping and sharpening of the wearing tooth where abrasion from mastication, incisive and other functions are usually random. (Every 1960, Every 1970, Every 1965, Every Kuhne 1970, Every Kuhne 1971, Every 1972, Every 1973, Every 1975, Every et al 1998).
What is its role in the maintenance of specific dental weapons?
In some mammals there are specific instances where teeth have evolved as specific dental weapons. For example:
In agonistic situations the anterior teeth of other dentitions, that are primarily dental tools, can function secondarily as dental weapons: the incisors of rodents (for example rats or mice), lagomorphs (rabbits, hares etc), and caviomorphs (for example, guinea pigs).
Is the phenomenon evident in our species?
When dealing with our own species, several challenges arise when proposing an innate behavior to sharpen teeth is still present in Homo sapiens. Murray and Sanson make that challenge clear. The specific characteristics set out for thegosis-facets by Every (Every 1972, Every 1973) are apparent in our genus. The major difficulty here is that what a thegotitian would note as thegosis other colleagues would diagnose as bruxism (see later).
Does the behaviour enhance our teeth as dental tools?
In contemporary Homo sapiens the best evidence is in the anterior dentition. Lateral excursions with the teeth in contact (currently considered parafunction), do produce bevelled edges on the molar, premolar canine and incisor teeth. Besides defining the occlusal anatomy of the molar and premolar teeth, thegotic action also defines the leading lingual enamel blade in the mandibular incisors and the reciprocal labial enamel blades in the maxillary incisors (Every 1973). These blades advantage the teeth during incisive action. Elsewhere this has been elaborated in some detail (Scally 1995). When this alternative argument for incisive, canine, premolar, and molar function is understood it will be easy to see and appreciate how thegosis advantages human teeth.
Similar beveled edges, produced by thegosis, can be seen on the lingual leading blades of the mandibular molars and the buccal blades of the maxillary molars. This analysis and interpretation of the human dentition is based on Every’s analysis of the posterior dentition in prosimians, where he shows the evolution of the characteristic molars and premolars have maintained sharpness and blade length while elaborating the crushing advantage of these teeth (Every 1973).
Does the behaviour enhance our teeth as dental weapons?
The most contentious application of thegosis is the analysis of the evolution of the anterior dentition in the human. The preoccupation with this seemingly outrageous idea has deflected much sound observation and experimental evidence for this thegosis (Wallace 1975, Shandle et al 1938, Young 1998). It is difficult to “prove” this argument, rather, the conclusion comes from the analysis Every made of various dentitions where specific dental tools and weapons evolved. (Every 1972, Every 1973, Every 1975)
With regard to our own species, current anthropological theories have the evolving hominid loosing their specific maxillary canine dental weapon and becoming biologically defenseless. In contrast, Every argues that there was a change in the weapon system from that of a sexually dimorphic male canine weapon to that of a segmentive biting weapon where all the anterior teeth are sharpened. This weapon is viable in both sexes.
The debate will continue but before dismissing his hypothesis, it is necessary to review the full suit of features involved in this change. They include the change in the tempromandibular joint, changes in the enamel and dentine thickness of the incisors and canines, and the development of the lower body of the external pterygoid; to name just a few (Every 1970). It is not appropriate to develop this here but just to alert the reader to the literature and that the hypothesis is not implausible and makes sound evolutionary sense. From it new models for the etiology of craniomandibular disorders and their management can be suggested (Scally 1986, Scally Beaven 1999).
Do teeth touch during mastication?
The point made by Every is that to produce facets from tooth-to-tooth contact considerable force is involved. His comment about tooth-to-tooth contact during mastication and swallowing is about the forcefulness of the contact not so much as to whether or not it occurs.
The sound produced does not necessarily indicate thegosis.
Murray and Sanson suggest that the sound produced is independent of the dentition but the anatomy of the specific weapon and whetstone systems, suggests that the relationship of enamel to dentine’s primary advantage is an edge holding ability (the canine weapon/whetstone complexes in the pig and baboon for example) (Every 1970, Every 1972). The secondary function of the anatomical and physiological arrangement is that the sound produced acts as an auditory signal of tooth sharpening. In man-made tools these structural relationships of hard and flexible composite tools are designed for just such a purpose. The noise produced during tool honing is a secondary phenomenon.
One wonders if an animal species has evolved a specific sound producing tooth-grinding system that is independent of a specific weapon but is specifically used in agonistic situations. In the recorded situations where the sound of tooth sharpening is cited, the animals involved have agonistic sound production advantage from the action. In a number of species without a specific dental weapon the sound produced in agonistic situations signals the threat, but the sound is secondarily derived from a dentition that phylogenetically could be used as an effective weapon.
How well have Murray and Sanson reported Every’s thegosis hypothesis and the evidence for it?
One would expect that a critical review of the phenomenon of thegosis would include all relevant references. Four papers, three of which are critical to the debate were not cited (Every 1973, Mills 1976, Scally 1980, Craddock and Johnson 1961). Many of the criticisms leveled at the concept of thegosis by Murray and Sanson are dealt within them.<>
Murray and Sanson focus on the lack of published evidence for the phenomenon. It is true that Every’s early publications were brief (Every 1965, Every Kuhne 1970, Every Kuhne 1971, Every 1972,). They were the synthesis of a large body of unpublished evidence for the phenomenon. However, the account of the significance of extreme mandibular movements discussed by Craddock and Johnson in the NZDA gave support to Every’s clinical findings (Craddock Johnson 1961). Mills concludes the “attrition” (Mills’ synonym for thegosis) is a common phenomenon in mammals and that it is a necessary behavior for the maintenance of teeth (Mills 1976). However, subsequent papers by Every present substantial evidence for thegosis and detail many of the claims made in his early papers. They also demonstrated the application of the terminology Every devised to analyze morphological features of teeth. In two of the papers Every presents substantial evidence for the behavior and discussed many of the subtitles of the thegosis-facets for a number of mammalian species (Every 1973, Every 1975).
One paper by Scally gives a general account of thegosis in a range of vertebrate and invertebrate species and are a summary of Every’s own arguments (Scally 1982). Scally also presents compelling evidence for prenatal thegosis in the guinea pig, and some analysis of the thegosis facets in this species (Scally 1980). A further paper by Archer et al. shows how the terminology derived from thegosis can be usefully used to analyze dental evolution and phylogenetic relationships (Archer et al.). It is difficult to dismiss the phenomenon when confronted with this evidence and I would suggest that Murray and Sanson extend their bibliographical search and read the papers.
Their review serves to give an account of Every’s ideas and it highlights some of the contradictions in the thegotic literature but it goes astray in four main areas;
In response to Every’s statement
“Most mammals sharpen their teeth. This ensures that the teeth are effective tools and weapons.” (Every 1960)
Murray and Sanson contend that
“Except for a very few perhaps special cases, there is no evidence presented that most mammals sharpen their teeth and it is inappropriate for these papers to be cited as evidence for tooth sharpening in mammals.” (Every 1960, Every 1965).
Every’s early papers were a published synthesis of the phenomenon based on evidence he had collated. Subsequent papers (see below) filled out the evidence for the phenomenon. It took several years to have the Lancet papers published and the correspondence between the editor and Dr Every runs to hundreds of pages. In view of the historical significance of these papers it is entirely appropriate that these references be used since they establish primacy for the discovery of the phenomenon and the hypothesis suggested by it. Some of these have been developed in subsequent publications. As he suggested in his 1960 paper (Every 1960), the behaviour is innate and in many animals it is a necessary component of tooth form and function. That is not to say that in mammals every genus of every order is an obligative tooth-sharpening (thegotic) group. There are many species of mammal that have abandoned the behaviour as well as teeth. The dentition of each genus needs to be analyzed on a ‘case by case’ basis and so does the tooth-sharpening behaviour. What is clear however is that the behaviour was a fundamental feature of early mammalian evolution (Every 1972, Every 1973).
How well have other authors reported Every’s ideas?
There continues to be confusion and misquoting of Every’s work, especially with regard to the direction of the thegotic action. Walker and Teaford are largely responsible for this confusion by insisting that Every postulated the thegotic action was always opposite to masticatory action (Teaford Walker 1983, Teaford Walker 1983). This is simply not true: to quote:
“In marked contrast to any action involved in tooth function, masticatory or otherwise, thegotic action involves forceful grinding of one tooth against another. Masticatory (or other action) and thegotic action, therefore, occur on separate occasions. Although the horizontal component of the thegotic action may occur in the same direction as mastication or other action, in many mammals it is discretely variant, even diametrically opposite” (Every 1975).
That Teaford and Walker use the guinea pig prenatal observations of tooth wear to support their straw man seems to have gone unchallenged, and their observations and conclusions are in contradiction to Scally where he shows the that the thegotic stroke in the posterior dentition in the guinea pig from in utero evidence is of varying stroke length and direction (Scally 1980). This evidence partially supports Every’s view of thegotic stroke direction in this species. Every also presents in some detail the variety of thegotic stroke directions found in mammalian dentitions elsewhere (Every 1975).
Continuing experimental work and clinical investigations that have been undertaken to sort out the detail of the thegotic hypothesis on a ‘species by species’ basis continues to uncover subtleties for their behavior. This is to be expected. But the experimental work has not shown that the behavior is exceptional or that it does not occur.
The inappropriate use of stressed animals to define thegosis-facets.
It is true that the initial evidence for thegosis was variously gathered and arguments can be put forward for the unusual “stressful” circumstance for the “evidence”. What Murray and Sanson seem to overlook is the necessity for clear-cut examples for the thegosis-facet. In the case of intra uterine thegosis in the guinea pig there is a special circumstance where there is no possibility of abrasive wear and the facets can only be from forceful tooth-to-tooth contact in this animal with a hypsodont dentition. The guinea pig is an obligate thegoser. The teeth can not function without the behavior. Indeed, there is evidence for thegotic sculpting in this animal where a special class of jaw movements are first used to shape the erupting blanks into teeth (Scally and Knox 1987). This is true for many hypsodont dentitions and they are the easiest dentitions in which to study the phenomenon (Every et al. 1998).
Other experimental work and analysis of the sheep dentition in a recent paper illustrates how hypsodont teeth are advantaged by a specific tooth-to-tooth contact wear at a time other than mastication (Every et al. 1998). In other dentitions the behavior has various expressions and it is true that in those dentitions it is easiest to demonstrate the behavior where stress can be induced. However, say from a random clean kill of sheep or rabbits, it is easy enough to demonstrate thegosis facets that are identical to the experimentally produced ‘stressed’ facets.
Murray and Sanson argue Every’s hypotheses are circuitous. They do not seem to appreciate that when new phenomena are discovered, existing terminologies are often inadequate to describe it, and new terms, derived from the insights of the discovery, are required to develop the discovery (Every 1973). What is more, often the new discovery highlights the confusion in the accepted definition and application of existing terms. This is certainly the case for bruxism, attrition, and parafunction.
Thegosis has been strictly defined and specific criteria have to be met before it can be applied as a descriptive term to the worn ‘tooth’ surface. Often, as in the case of humans, there is a specific muscle responsible for the movement (the lower body of the external pterygoid). The orientation of this muscle is oblique to incision and diametrically opposite to that of mastication. It is more parsimonious to conclude that the primary function of the muscle to sharpen the teeth rather than to argue the primary function is in jaw opening, an action oblique to the orientation. What is more, the thegosis-facets define blade edges. Thegotic activity defines those edges. The very surfaces we are taught to describe as ground flat and blunt are the ground backs to the blades or defined crushing surfaces when analyzed via thegotic methodology.
If it was argued with a carpenter that the ground back of a chisel was the functional agent he would chuckle. That blunt flatly ground surface defines a sharp edge, an edge that needs constant maintenance (via grinding), at a time other than when it is used. Teeth are similar. Evolutionary processes have evolved, time and again, teeth and analogous structures, that require servicing to maintain their efficient function. This genetically programmed behaviour, the fourth dimension of tooth morphology has specific tell-tale signs, spelt out by Dr Every . These features can be repeated experimentally. The published evidence is sufficient to support this view.
The interpretation of worn dentitions in contemporary patients and the difficulties with the terms bruxism and parafunction
Bruxism is such a diffuse and imprecise term that covers a plethora of clinical signs and symptoms that it gives each dentist and author an opportunity to create their own definition. In a recent text, the authors discuss thegosis and seem to be comfortable with the concept but use it as a synonym for bruxism (Kaidonis et al. 1998). As I have pointed out, within the bruxism paradigm there is a discrete class of activity that can be defined as thegosis but thegosis is not a synonym for bruxism (Scally 1991a).
While dentists look at worn teeth as flat and blunt, and consider wear as damaging, they are caught in a closed conceptual loop: Teeth should not wear; if they are worn it is pathological. The remedy – restore the tooth to its eruptive, unworn morphology.
The profession is disadvantaged by being taught that tooth-to-tooth contact wear on the anterior dentition, and the wide lateral excursions responsible for it are parafunctional (beyond function). That is, the very jaw movements responsible for sharpening the anterior teeth are considered pathological.
Dismissing the idea that there is a specific class of tooth-to-tooth contact wear that has a biological advantage (to sharpen the teeth), confidence in the diagnosis that bruxism (all tooth grinding) is an illness and parafunction is a maladaptive and pathological behaviour, dentists will continue to prescribe restorative and other treatment plans that are not supported by the biological and evolutionary model.
The antithesis to the ‘pathological’ view is that there is a specific class of tooth-to-tooth contact wear that is ubiquitous throughout the animal kingdom that sharpens teeth and tooth-like structures. To date it has been established in three phyla:
To argue that the identical behaviour, no matter what you call it, is maladaptive, atavistic, or regressive in humans potentially exposes the dental profession (a profession that claims to be built on a scientific foundation) to severe criticism and disdain.
Should the clinical signs of the phenomenon be considered an illness in humans as is taught?
I would argue no, but that is not to say that that in some individuals it is possible to have a pathological expression of the behavior (Scally 1991a). But until we have diagnostic tests to tease out all tooth wear processes: thegosis, abrasion, and erosion (endogenous and exogenous) I defy any clinician to confidently make a diagnosis of pathological thegosis. And bruxism is not pathological thegosis as suggested by Murray and Sanson.
Does the behavior have a part to play in the etiology of craniomandibular disorders?
Apart from the challenge to the idea that “bruxism” and “parafunction” are illnesses, the concept that teeth are sharpened provides an opportunity to examine the traumatic possibilities to the dentition and other oral structures during thegotic activity. It is important to stress that the behavior is normal but in some individuals it can have traumatic consequences in the contemporary, relatively unworn, dentition. When the simulated thegotic excursion is examined from centric occlusion to the conscious limit of the stroke, a number of potentially traumatic relationships can be demonstrated and traumatic relationships to deeper structures can be imagined. (Every 1960 Every 1965) (Scally Beaven 1999).
Elsewhere I have set out the various classes of trauma that can be explained by thegotic activity (Scally 1995). There I have also pictorially presented an analysis of how the human dentition works based on a thegotic analysis. This material in also available on the Internet (www.8.co.nz/thegotics.htm). This material sets out in some detail the application of thegotics to the hominid dentition and its clinical application (Scally 1995).
Finally, a profession that claims to underpin its teaching and practice on conjectural knowledge, could be examining the claims made by Dr Every and others. It could include assessing his evidence and designing experiments to test his hypothesises and applying these insights to the clinical practice of dentistry as I have done.