Annals of the Royal College of Surgeons of England (1979) vol 6i
CHARLES TOMES LECTURE, 1978
Concepts of occlusion G C Dickson FDSRCS Orthodontic Department, Royal Portsmouth Hospital, Portsmouth
Introduction The Oxford English Dictionary defines the verb 'to occlude' as 'to stop up, close, obstruct (pores, orifice)'. In dentistry the word has the restricted meaning of closure of the teeth; the term may be qualified by the manner in which they are brought together-protrusive, retrusive, centric, etc. It is not possible to bring the teeth together without moving the mandible so the temporomandibular joint must be included in the true study of occlusion and logically it is really important to include the representation of the occlusal surfaces on the cerebral cortex, without which the complex movements of the mandible could not be coordinated.
The prosthetists' concept We owe much more of our thinking on the subject of occlusion to the early prosthetists than to any other branch of dentistry. Their approach was analytic and mathematical; they were concerned with the reproduction of the natural dentition in the form of full dentures and they measured accurately the slopes of the cusps of the teeth, the condylar pathway, the shape of the dental arch, the curves of the occlusal plane in coronal and sagittal planes, and many other parameters. One of the many 'articulators' on which dentures are constructed was named by its designerHanau-the 'Quint', because it reproduced these five features: i) The condylar guidance (or slope of the path of the mandibular condyle). 2) The incisal guidance (or slope of the lingual surfaces of the upper incisor teeth down which the lowers must slide). 3) The height and slope of the cusps of the posterior teeth. 4) The inclination of the occlusal plane (relative to the temporomandibular joints).
5) The curvature of the occlusal plane (coronally and sagittally). The complexity and ingenuity of these machines was matched only by their namesthe Grainger Gnatholator, the Gysi Trubyte, the Stansbery Tripod, and many more. These pioneers were aiming to reproduce on an artificial denture the basic elements of the natural unworn dentition. More recently Hamish Thomson1 has clarified the position by pointing out that 'the three categories of occlusion have to be seen as objectives of occlusion rather than as diagnosable features of the natural dentition' and later that 'the most desirable dentition has freedom from deflective cusp interferences which will reduce to a minimum the adaptive jaw movements which may result from cusp interferences'. The reason for Thomson's concern over deflective cusp interferences is of course the fact, clearly shown by Hankey2 and Ballard3 over 20 years ago, that occlusal interference of some form is the triggering influence in almost all forms of temporomandibular joint dysfunction. Indeed, Berry4 has recently found that a large number of his patients with this condition had had their upper incisors retracted as part of orthodontic treatment.
The orthodontic concept Edward Angle-the founder of modern orthodontics-lived at the time of the early prosthetic revolution and was a perfectionist. He therefore chose as the model to which he would aim for perfect occlusion a skull irreverently named by his students 'Old Glory'. It had 32 teeth firmly interdigitating and quite unworn
(Fig. i). With the modification that some orthodontists choose to reduce the number of teeth to 28 the same objectives still apply 6o years later. All questions in that time have been di-
Delivered at the Royal College of Surgeons of England on i6th June I978
178
G C Dickson
'Old Glory'-the skull used by EdFIG. I ward Angle to illustrate ideal occlusion.
rected towards the means of achieving this objective and none to the validity of the objective itself. Causes of malocclusion In an earlier paper' I outlined what I believed to be the causes of modern man's malocclusion. To summarise,
I
concluded that
malocclusion today was due mainly to two factors: firstly, the vast increase in genetic variability which normally and naturally follows any population explosion such as that which has occurred to man in the past 2000 years; secondly, it is due to overcrowding resulting from lack of tooth wear. P R Begg6 has shown by measuring erupted and unerupted teeth of Australian Aboriginals that an amount of tooth substance totalling 4.5 mm is worn off the mesial and distal surfaces of the teeth in each quadrant of the mouth by the time the third molars erupt (about 8 years of age). Extrapolating, one could say that by about 25 years of age the amount of tooth material worn away in the mouth is about equivalent to four premolar teethone in each quadrant. With this as a background it is appropriate to look briefly at animal occlusions and then at man's ancestors to see if in them a clue can be found to normality in modern occlusion. Food chains and teeth In the ocean the food chain is a long one. All plants are small, planktonic, and often unicellular. These are eaten by very small animals such as ostracods which in turn are eaten i
by other crustacea, and there may be several sizes of fish eating each other before the top of the food chain, such as the shark, is reached. It follows that the vast majority of species in the ocean are carnivorous. It is strange that evolution has not produced grazing fish in large numbers to devour the vast amount of seaweed which covers the millions of miles of shoreline. Grazing reptiles and amphibians, such as dinosaursY had early success, but vegetarian reptiles are now rare in the extreme. It may well be that, as suggested by Kermack7, the limitation of their evolutionary horizon was due to the fact that their masticatory structures, including their temporomandibular joints, were unable to cope with a herbivorous habit. As he observes, vegetable protein comes packaged in little cellulose boxes and it is necessary to have a masticatory apparatus capable of breaking open the boxes. The present herbivores are almost entirely mammals and insects. On land food chains are therefore very much shorter, as mammals have developed teeth which will grind and crush and temporomandibular joints capable of sustaining the pressures and exerting the control of the mandible needed for this operation. The carnivorous dentition As a simple carnivorous dentition that of a predatory fish such as a pike is a good example. Not only are there rows of large teeth around the upper and lower jaws, but the under surface of the vomer and the upper surface of the tongue are carpeted with small teeth. The pike seizes and holds and swallows its prey whole; it does not masticate. The teeth of more advanced carnivores such as the dog, cat, lion, etc. consist of welldeveloped canines, small incisors, and bladelike molars and premolars, each tooth often being separated from its neighbours. Flesh is torn from the prey with the canines and repeatedly stabbed with the cheek teeth, using a simple hinge movement of the mandible. It is then swallowed whole. The herbivorous dentition Of the available animals to use to illustrate a herbivorous dentition, the horse is perhaps the most useful. There are no canines; incisors are used to tear off the grass, which is then passed back to the cheek teeth (Fig. 2). Upper
Concepts of occlusion
179
FIG. 2 Upper and lower jaws of horse.
and lower cheek teeth each form a tightly packed block looking at first inspection like a very large single tooth in each quadrant. These teeth are transversely grooved but have sagittally directed folds of enamel which stand high relative to the worn dentine between them (Fig. 3). The upper teeth are almost twice as wide as the lower, and the two sides of the maxilla are so far apart that it is impossible for the animal to close upper and lower teeth of both sides at the same time. A wide lateral swing of the mandible brings the lower cheek teeth slicing across their upper opponents rather like the blades of a pair of scissors. By occluding with only a small part of the total tooth area at one time higher pressures are brought to bear than would be the case if the whole dental area met at one time. T his occlusion is not present when the teeth first erupt: the teeth of the foal have sagittally disposed cusps which must completely prevent
such movement until they have worn off (Fig.
4)-
This comparison of the horse with a carni-
vore demonstrates the difference in the techniques of biting or chewing and shows the complete lack of lateral movements and spaced
teeth of the carnivore in contrast to the wide lateral excursion and tightly packed cheek teeth of the herbivore. Another vegetarian, the gorilla, has welldeveloped canines for aggressive purposes, but these are not long enough to prevent considerable lateral movements to crush the leaves which form its diet. Its teeth are tightly packed and the cusps wear off the cheek teeth. In both horse and gorilla the buccal side of the lower teeth and palatal side of the upper teeth wear so that the occlusal surfaces slope downwards and outwards. In fact it does seem that this slope (in the coronal plane) and the possession of tight contact points is a characteristic of animals which eat fibrous foods.
~ ~~~~~~~~~~~~~~~~~~~~~~J/ ~ ~ ~ ~:
FIG. 3 Distribution of enamel ridges of horse.
on
teeth
FIG. 4 Newly erupted teeth of foal.
i8o
G C Dickson
Man's ancestors which is identical with that described below The earliest teeth so far suspected of being in in Anglo-Saxon skulls. the line of human evolution belong to Ramapithecus, a short-muzzled creature amongst long-muzzled apes. Between this creature and the mid-Pleistocene (about two million years ago) there is a big time gap and now two lines are in evidence. The first of these contains the genus Australopithecus-literally, 'Southern ape-man'. Of these, Australopithecus boisei is characterised by having enormous cheek teeth, tight contact points, and wear which is more on the buccal side in the lower and palatal in the upper. Australopithecus africanus is fairly widely distributed in Africa, and the mandible of a child which is well preserved shows a wear pattern remarkably similar to that of some modern children, whose enamel is soft enough to be worn by a modern diet (Fig. 5). The second line contains Homo habilisthe earliest of the genus of which we have specimens with teeth. These are very much like those of modern man, but all specimens show considerable wear with loss of cusps and exposure of dentine. Originally known as Sinanthropus, the skulls discovered first at Choukoutien near Peking and later in Java and eastern Africa are now known as Homo erectus. He was prognathous, had large teeth and large dental arches, and was apparently markedly sexually dimorphic as regards size. He lived about half a million years ago-a mere ioooo generations. The large teeth are very like those of modern man, except for size, and show wear
Stone-Age man Artists' impressions usually depict Stone-Age man as stooping, hairy, and carrying a shillelagh, but in fact his cranial capacity was the same as that of modern man. With appropriate sartorial attention he would be indistinguishable from ourselves-unless one looked at his teeth, which would show far greater wear than is ever seen today. Anglo-Saxon teeth Stone-Age skulls are rare and usually poorly preserved, but the Royal College of Surgeons of England possesses an excellent collection of Anglo-Saxon skulls in the Odontological Museum and as wear is identical with that of Neolithic teeth it is convenient to illustrate it by means of these skulls. Wear on the permanent teeth is naturally greatest on the first molars, with progressively less wear towards the third molars, and in fact Miles8 has shown how the ages of skulls can be assessed from the extent of wear on these teeth. In the first stage the enamel is worn off the cusps of the first molars, buccally in the lower and palatally in the upper (Fig. 6). Subsequently the wear on the exposed dentine produces more loss of tissue than on the enamel so that where there were cusps there are now hollows, and with enamel ridges where once there were fissures. Next the enamel ridges begin to wear down, leaving only the deepest part of the fissure enamel as
Wear in the mixed dentition of Australopithecus afrncanus (left) and a modern Caucasian child (right). FIG
Concepts of occlusion
I8I
FIG. 6 Palatal wear on upper and buccal wear on lower teeth. Anglo-Saxon.
islands in the dentine. Lastly the remnants of the occlusal enamel wear away, leaving a sloping, saucer-shaped surface of dentine with a thin rim of buccal, labial, and proximal enamel which is very vulnerable to fracture at the amelodentinal junction, leading to food packing and direct dentine caries, almost the only type of caries known at this period. Diet Paleolithic man has been described as a hunter-gatherer and Neolithic peoples as farmers. Tooth wear was similar throughout the Stone Age and into Anglo-Saxon times, and at no time does it indicate that meat was a significant part of the diet. The flattening of the cusps would indeed make it extremely difficult to eat any part of an animal except the entrails unless it was very well cooked. The stone artefacts left behind show a huge preponderance of scrapers. These are flaked stones carefully shaped to form an even curve with a cutting edge which was extremely thick; in fact the surfaces forming the edge are almost at right angles to each other. Knives are extremely rare and it is my opinion that man, having been cradled in the heat of Africa and having lost most of his body hair, hunted principally to provide skins with which to insulate himself against a much colder
climate; scrapers were to prepare the skins for this purpose. It has been suggested by Jolly9 that before this stage was reached man may have had a seed-eating habit similar to that of gelada monkeys. This would well- account for Neolithic man choosing to cultivate cereal crops as soon as he became sedentary. Mills"0 has suggested that his dentition, like that of the gelada, was adapted to crushing small round objects and that this in turn gave rise to an articular eminence enabling man's molars not simply to slide over each other but to swing alternately towards and away from each other as the mandible is moved from side to side. This is borne out by the palatal wear on the upper and buccal wear on the lower teeth which produces the reversal of the curve of Monson to which I have already referred. The function of cusps It is quite reasonable to assume that, as with the horse, the function of the cusps on human teeth is primarily to guide the teeth into occlusion, and once this is accomplished they would seem to be more of a liability than an asset.
Hankey2 and Ballard3 showed quite convincingly that the primary cause of the temporomandibular joint dysfunction syn-
182
G C Dickson
FIG. 7 Buccolingual stresses on worn and unworn first molars of modern man.
drome is mandibular deviation produced by cuspal interference; this is scarcely possible if the cusps are worn away. Berry4 has brought forward a reasoned argument that the presence of cusps is actually harmful by introducing lateral stress. He illustrates this with the stress in an anteroposterior plane, but in fact I regard the lateral (coronal) stress as being more important as the teeth do not offer each other mutual support in this plane (Fig. 7). Evolution is a slow process, and while human adaptability is remarkable, it is really expecting rather a lot to hope to show no ill effects by changing the hard, fibrous, abrasive diet of the Stone Age to one of Mars bars, Instant Whip, and fish fingers (to name but a few) in 25 generations! Medical as well as dental evidence does indicate that we have perhaps pushed our tolerance a little too far. In the light of all this evidence there does seem to be strong reason for regarding the 'ideal occlusion' of Old Glory as being a completely abnormal dentition-infantile teeth in adult jaws. This is something which could not have happened to man throughout his entire evolution until the last few hundred years.
From the point of view of mastication it is no longer necessary to have cusps as our food is so highly prepared, but from the point of view of the temporomandibular joint it matters very much; there may well be a good case for reducing cusp height and angle in the same manner (though not necessarily to the same extent) as the Anglo-Saxons. Figure 8 shows an Anglo-Saxon skull whose owner was some 40 years old, which was a fairly advanced age in those days. The teeth are worn, the cusps have gone, there is gross calculus on the labial aspects of all teeth, yet the bony support is excellent. Could this be due to the absence of lateral stress? When carrying out clinical trials and clinical investigations we make use of a few dozen or a few hundred cases. Nature has been experimenting for millions of years with billions of cases, rejecting the worst and retaining the best. Clinicians would do well to make more use of the experimental results which are to be found in the bones of our ancestors.
References I Thomson, H (I975) Occlusion. Bristol, Wright. 2 Hankey, G (1956) Proceedings of the Royal Society of Medicine, 49, 1904, I983. 3 Ballard, C F (I956) Proceedings of the Royal Society of Medicine, 49, 1904, I983. 4 Berry, D C, and Poole, F D G (I974) Journal of Oral Rehabilitation, i, i9i. 5 Dickson, G C (I969) Transactions of the British Society for the Study of Orthodontics, 55, I48. 6 Begg, P R (I954) American Journal of Orthodontics, 40, 298, 373, 462, 517. 7 Kermack, K A (I972) Proceedings of the Royal
Society of Medicine, 65, 389.
FIG. 8 Worn and calculus-covered teeth with excellent bony support in Anglo-Saxon skull.
8 Miles, A E W (1962) Proceedings of the Royal Society of Medicine, 55, 88i. 9 Jolly, C J (1970) Man, 5, 5. io Mills, J R E (1972) Proceedings of the Royal Society of Medicine, 65, 392.
CHARLES TOMES LECTURE, 1978
Concepts of occlusion G C Dickson FDSRCS Orthodontic Department, Royal Portsmouth Hospital, Portsmouth
Introduction The Oxford English Dictionary defines the verb 'to occlude' as 'to stop up, close, obstruct (pores, orifice)'. In dentistry the word has the restricted meaning of closure of the teeth; the term may be qualified by the manner in which they are brought together-protrusive, retrusive, centric, etc. It is not possible to bring the teeth together without moving the mandible so the temporomandibular joint must be included in the true study of occlusion and logically it is really important to include the representation of the occlusal surfaces on the cerebral cortex, without which the complex movements of the mandible could not be coordinated.
The prosthetists' concept We owe much more of our thinking on the subject of occlusion to the early prosthetists than to any other branch of dentistry. Their approach was analytic and mathematical; they were concerned with the reproduction of the natural dentition in the form of full dentures and they measured accurately the slopes of the cusps of the teeth, the condylar pathway, the shape of the dental arch, the curves of the occlusal plane in coronal and sagittal planes, and many other parameters. One of the many 'articulators' on which dentures are constructed was named by its designerHanau-the 'Quint', because it reproduced these five features: i) The condylar guidance (or slope of the path of the mandibular condyle). 2) The incisal guidance (or slope of the lingual surfaces of the upper incisor teeth down which the lowers must slide). 3) The height and slope of the cusps of the posterior teeth. 4) The inclination of the occlusal plane (relative to the temporomandibular joints).
5) The curvature of the occlusal plane (coronally and sagittally). The complexity and ingenuity of these machines was matched only by their namesthe Grainger Gnatholator, the Gysi Trubyte, the Stansbery Tripod, and many more. These pioneers were aiming to reproduce on an artificial denture the basic elements of the natural unworn dentition. More recently Hamish Thomson1 has clarified the position by pointing out that 'the three categories of occlusion have to be seen as objectives of occlusion rather than as diagnosable features of the natural dentition' and later that 'the most desirable dentition has freedom from deflective cusp interferences which will reduce to a minimum the adaptive jaw movements which may result from cusp interferences'. The reason for Thomson's concern over deflective cusp interferences is of course the fact, clearly shown by Hankey2 and Ballard3 over 20 years ago, that occlusal interference of some form is the triggering influence in almost all forms of temporomandibular joint dysfunction. Indeed, Berry4 has recently found that a large number of his patients with this condition had had their upper incisors retracted as part of orthodontic treatment.
The orthodontic concept Edward Angle-the founder of modern orthodontics-lived at the time of the early prosthetic revolution and was a perfectionist. He therefore chose as the model to which he would aim for perfect occlusion a skull irreverently named by his students 'Old Glory'. It had 32 teeth firmly interdigitating and quite unworn
(Fig. i). With the modification that some orthodontists choose to reduce the number of teeth to 28 the same objectives still apply 6o years later. All questions in that time have been di-
Delivered at the Royal College of Surgeons of England on i6th June I978
178
G C Dickson
'Old Glory'-the skull used by EdFIG. I ward Angle to illustrate ideal occlusion.
rected towards the means of achieving this objective and none to the validity of the objective itself. Causes of malocclusion In an earlier paper' I outlined what I believed to be the causes of modern man's malocclusion. To summarise,
I
concluded that
malocclusion today was due mainly to two factors: firstly, the vast increase in genetic variability which normally and naturally follows any population explosion such as that which has occurred to man in the past 2000 years; secondly, it is due to overcrowding resulting from lack of tooth wear. P R Begg6 has shown by measuring erupted and unerupted teeth of Australian Aboriginals that an amount of tooth substance totalling 4.5 mm is worn off the mesial and distal surfaces of the teeth in each quadrant of the mouth by the time the third molars erupt (about 8 years of age). Extrapolating, one could say that by about 25 years of age the amount of tooth material worn away in the mouth is about equivalent to four premolar teethone in each quadrant. With this as a background it is appropriate to look briefly at animal occlusions and then at man's ancestors to see if in them a clue can be found to normality in modern occlusion. Food chains and teeth In the ocean the food chain is a long one. All plants are small, planktonic, and often unicellular. These are eaten by very small animals such as ostracods which in turn are eaten i
by other crustacea, and there may be several sizes of fish eating each other before the top of the food chain, such as the shark, is reached. It follows that the vast majority of species in the ocean are carnivorous. It is strange that evolution has not produced grazing fish in large numbers to devour the vast amount of seaweed which covers the millions of miles of shoreline. Grazing reptiles and amphibians, such as dinosaursY had early success, but vegetarian reptiles are now rare in the extreme. It may well be that, as suggested by Kermack7, the limitation of their evolutionary horizon was due to the fact that their masticatory structures, including their temporomandibular joints, were unable to cope with a herbivorous habit. As he observes, vegetable protein comes packaged in little cellulose boxes and it is necessary to have a masticatory apparatus capable of breaking open the boxes. The present herbivores are almost entirely mammals and insects. On land food chains are therefore very much shorter, as mammals have developed teeth which will grind and crush and temporomandibular joints capable of sustaining the pressures and exerting the control of the mandible needed for this operation. The carnivorous dentition As a simple carnivorous dentition that of a predatory fish such as a pike is a good example. Not only are there rows of large teeth around the upper and lower jaws, but the under surface of the vomer and the upper surface of the tongue are carpeted with small teeth. The pike seizes and holds and swallows its prey whole; it does not masticate. The teeth of more advanced carnivores such as the dog, cat, lion, etc. consist of welldeveloped canines, small incisors, and bladelike molars and premolars, each tooth often being separated from its neighbours. Flesh is torn from the prey with the canines and repeatedly stabbed with the cheek teeth, using a simple hinge movement of the mandible. It is then swallowed whole. The herbivorous dentition Of the available animals to use to illustrate a herbivorous dentition, the horse is perhaps the most useful. There are no canines; incisors are used to tear off the grass, which is then passed back to the cheek teeth (Fig. 2). Upper
Concepts of occlusion
179
FIG. 2 Upper and lower jaws of horse.
and lower cheek teeth each form a tightly packed block looking at first inspection like a very large single tooth in each quadrant. These teeth are transversely grooved but have sagittally directed folds of enamel which stand high relative to the worn dentine between them (Fig. 3). The upper teeth are almost twice as wide as the lower, and the two sides of the maxilla are so far apart that it is impossible for the animal to close upper and lower teeth of both sides at the same time. A wide lateral swing of the mandible brings the lower cheek teeth slicing across their upper opponents rather like the blades of a pair of scissors. By occluding with only a small part of the total tooth area at one time higher pressures are brought to bear than would be the case if the whole dental area met at one time. T his occlusion is not present when the teeth first erupt: the teeth of the foal have sagittally disposed cusps which must completely prevent
such movement until they have worn off (Fig.
4)-
This comparison of the horse with a carni-
vore demonstrates the difference in the techniques of biting or chewing and shows the complete lack of lateral movements and spaced
teeth of the carnivore in contrast to the wide lateral excursion and tightly packed cheek teeth of the herbivore. Another vegetarian, the gorilla, has welldeveloped canines for aggressive purposes, but these are not long enough to prevent considerable lateral movements to crush the leaves which form its diet. Its teeth are tightly packed and the cusps wear off the cheek teeth. In both horse and gorilla the buccal side of the lower teeth and palatal side of the upper teeth wear so that the occlusal surfaces slope downwards and outwards. In fact it does seem that this slope (in the coronal plane) and the possession of tight contact points is a characteristic of animals which eat fibrous foods.
~ ~~~~~~~~~~~~~~~~~~~~~~J/ ~ ~ ~ ~:
FIG. 3 Distribution of enamel ridges of horse.
on
teeth
FIG. 4 Newly erupted teeth of foal.
i8o
G C Dickson
Man's ancestors which is identical with that described below The earliest teeth so far suspected of being in in Anglo-Saxon skulls. the line of human evolution belong to Ramapithecus, a short-muzzled creature amongst long-muzzled apes. Between this creature and the mid-Pleistocene (about two million years ago) there is a big time gap and now two lines are in evidence. The first of these contains the genus Australopithecus-literally, 'Southern ape-man'. Of these, Australopithecus boisei is characterised by having enormous cheek teeth, tight contact points, and wear which is more on the buccal side in the lower and palatal in the upper. Australopithecus africanus is fairly widely distributed in Africa, and the mandible of a child which is well preserved shows a wear pattern remarkably similar to that of some modern children, whose enamel is soft enough to be worn by a modern diet (Fig. 5). The second line contains Homo habilisthe earliest of the genus of which we have specimens with teeth. These are very much like those of modern man, but all specimens show considerable wear with loss of cusps and exposure of dentine. Originally known as Sinanthropus, the skulls discovered first at Choukoutien near Peking and later in Java and eastern Africa are now known as Homo erectus. He was prognathous, had large teeth and large dental arches, and was apparently markedly sexually dimorphic as regards size. He lived about half a million years ago-a mere ioooo generations. The large teeth are very like those of modern man, except for size, and show wear
Stone-Age man Artists' impressions usually depict Stone-Age man as stooping, hairy, and carrying a shillelagh, but in fact his cranial capacity was the same as that of modern man. With appropriate sartorial attention he would be indistinguishable from ourselves-unless one looked at his teeth, which would show far greater wear than is ever seen today. Anglo-Saxon teeth Stone-Age skulls are rare and usually poorly preserved, but the Royal College of Surgeons of England possesses an excellent collection of Anglo-Saxon skulls in the Odontological Museum and as wear is identical with that of Neolithic teeth it is convenient to illustrate it by means of these skulls. Wear on the permanent teeth is naturally greatest on the first molars, with progressively less wear towards the third molars, and in fact Miles8 has shown how the ages of skulls can be assessed from the extent of wear on these teeth. In the first stage the enamel is worn off the cusps of the first molars, buccally in the lower and palatally in the upper (Fig. 6). Subsequently the wear on the exposed dentine produces more loss of tissue than on the enamel so that where there were cusps there are now hollows, and with enamel ridges where once there were fissures. Next the enamel ridges begin to wear down, leaving only the deepest part of the fissure enamel as
Wear in the mixed dentition of Australopithecus afrncanus (left) and a modern Caucasian child (right). FIG
Concepts of occlusion
I8I
FIG. 6 Palatal wear on upper and buccal wear on lower teeth. Anglo-Saxon.
islands in the dentine. Lastly the remnants of the occlusal enamel wear away, leaving a sloping, saucer-shaped surface of dentine with a thin rim of buccal, labial, and proximal enamel which is very vulnerable to fracture at the amelodentinal junction, leading to food packing and direct dentine caries, almost the only type of caries known at this period. Diet Paleolithic man has been described as a hunter-gatherer and Neolithic peoples as farmers. Tooth wear was similar throughout the Stone Age and into Anglo-Saxon times, and at no time does it indicate that meat was a significant part of the diet. The flattening of the cusps would indeed make it extremely difficult to eat any part of an animal except the entrails unless it was very well cooked. The stone artefacts left behind show a huge preponderance of scrapers. These are flaked stones carefully shaped to form an even curve with a cutting edge which was extremely thick; in fact the surfaces forming the edge are almost at right angles to each other. Knives are extremely rare and it is my opinion that man, having been cradled in the heat of Africa and having lost most of his body hair, hunted principally to provide skins with which to insulate himself against a much colder
climate; scrapers were to prepare the skins for this purpose. It has been suggested by Jolly9 that before this stage was reached man may have had a seed-eating habit similar to that of gelada monkeys. This would well- account for Neolithic man choosing to cultivate cereal crops as soon as he became sedentary. Mills"0 has suggested that his dentition, like that of the gelada, was adapted to crushing small round objects and that this in turn gave rise to an articular eminence enabling man's molars not simply to slide over each other but to swing alternately towards and away from each other as the mandible is moved from side to side. This is borne out by the palatal wear on the upper and buccal wear on the lower teeth which produces the reversal of the curve of Monson to which I have already referred. The function of cusps It is quite reasonable to assume that, as with the horse, the function of the cusps on human teeth is primarily to guide the teeth into occlusion, and once this is accomplished they would seem to be more of a liability than an asset.
Hankey2 and Ballard3 showed quite convincingly that the primary cause of the temporomandibular joint dysfunction syn-
182
G C Dickson
FIG. 7 Buccolingual stresses on worn and unworn first molars of modern man.
drome is mandibular deviation produced by cuspal interference; this is scarcely possible if the cusps are worn away. Berry4 has brought forward a reasoned argument that the presence of cusps is actually harmful by introducing lateral stress. He illustrates this with the stress in an anteroposterior plane, but in fact I regard the lateral (coronal) stress as being more important as the teeth do not offer each other mutual support in this plane (Fig. 7). Evolution is a slow process, and while human adaptability is remarkable, it is really expecting rather a lot to hope to show no ill effects by changing the hard, fibrous, abrasive diet of the Stone Age to one of Mars bars, Instant Whip, and fish fingers (to name but a few) in 25 generations! Medical as well as dental evidence does indicate that we have perhaps pushed our tolerance a little too far. In the light of all this evidence there does seem to be strong reason for regarding the 'ideal occlusion' of Old Glory as being a completely abnormal dentition-infantile teeth in adult jaws. This is something which could not have happened to man throughout his entire evolution until the last few hundred years.
From the point of view of mastication it is no longer necessary to have cusps as our food is so highly prepared, but from the point of view of the temporomandibular joint it matters very much; there may well be a good case for reducing cusp height and angle in the same manner (though not necessarily to the same extent) as the Anglo-Saxons. Figure 8 shows an Anglo-Saxon skull whose owner was some 40 years old, which was a fairly advanced age in those days. The teeth are worn, the cusps have gone, there is gross calculus on the labial aspects of all teeth, yet the bony support is excellent. Could this be due to the absence of lateral stress? When carrying out clinical trials and clinical investigations we make use of a few dozen or a few hundred cases. Nature has been experimenting for millions of years with billions of cases, rejecting the worst and retaining the best. Clinicians would do well to make more use of the experimental results which are to be found in the bones of our ancestors.
References I Thomson, H (I975) Occlusion. Bristol, Wright. 2 Hankey, G (1956) Proceedings of the Royal Society of Medicine, 49, 1904, I983. 3 Ballard, C F (I956) Proceedings of the Royal Society of Medicine, 49, 1904, I983. 4 Berry, D C, and Poole, F D G (I974) Journal of Oral Rehabilitation, i, i9i. 5 Dickson, G C (I969) Transactions of the British Society for the Study of Orthodontics, 55, I48. 6 Begg, P R (I954) American Journal of Orthodontics, 40, 298, 373, 462, 517. 7 Kermack, K A (I972) Proceedings of the Royal
Society of Medicine, 65, 389.
FIG. 8 Worn and calculus-covered teeth with excellent bony support in Anglo-Saxon skull.
8 Miles, A E W (1962) Proceedings of the Royal Society of Medicine, 55, 88i. 9 Jolly, C J (1970) Man, 5, 5. io Mills, J R E (1972) Proceedings of the Royal Society of Medicine, 65, 392.
