0003~9969/91 $3.00+ 0.00
Arch oral Biol.Vol. 36, No. I I, pp. 799-804,1991 Printedin Great Britain.All rightsrcscrvcd
Copyright0 1991PcrgamonPressplc
CHANGES IN MANDIBULAR CONDYLE MORPHOLOGY RELATED TO TOOTH WEAR IN A PREHISTORIC HUMAN POPULATION C. P. OWEN,’R. J. C. WILDING’and A. G. MORRIS’ ‘Faculty of Dent:istry, University of the Western Cape, Private Bag X12, Tygerberg 7505 and ‘Department of Anatomy, University of Cape Town Medical School, Rondebosch 7700, South Africa (Received 4 April 1991; accepted 28 May 1991) Summary-The exact relationship between tooth wear and changes in the shape of the articulating surface of the mandibular condyles is unknown. It is assumed that to some degree these changes are in response to the loads on the joint generated during chewing. Excavated skeletal remains from sites of primitive human habitation provide an opportunity to study the masticatory system of human beings in their natural context, with the features of form and function expressed to their full potential. Accurate measurements of the condylar head can also be made on such remains. A collection of skulls from an excavation site known as Oakhurst on the South African coast provided material for this study. Ckclusal wear was measured using the scale devised by Molnar; approximal wear was assessed by measuring the size of the
wear facet between the first and second molars. The width of the condyles were measured directly, and a method was devised to measure changes in the proportion of medial and lateral articulating facets. An increase in the s.ize of the lateral facet of the condyle was found to be associated with increased tooth wear. It is suggested that the direction of condylar remodelling may give an indication of the reaction at the joint to optima1 mastication. Key words: tooth wear, condyle morphology, remodelling.
IN’IRODUmON Tooth wear patterns have been used to deduce functional characteristics of the dentition (Mills, 1955; Crompton and Hiiemae, 1969; Butler, 1981). Wear of the occlusal surface is an indication of both the masticatory force and. the abrasive properties of the food, while approximal wear is more dependent on the amount of masticatory force applied to the teeth, as it is the lateral component of this force which causes wear (Wolpoff., 1971). A quantitative measure of the amount of approximal wear might, therefore, give an indication of masticatory function and of the types of food prep,aration, which may be obscured by examination of occlusal wear alone (Hinton, 1982). Changes in the shape of the condyles that were unrelated to disease were observed by Moffett et al. (1964). They could find no association between the condylar changes and age, but attributed them to functional changes in the dentition. This idea was given support by Pritchard (1972), who concluded that in order to cope better with altered forces, connective tissues would remodel to better advantage. Mongini (1972) produced histological evidence of remodelling of the condyle through replacement of the external, circutierential lamellae of bone, concluding that remodelling by apposition was most pronounced on the .medial side of the articulating surfaces. Studies of orthodontic patients have confirmed that the condyle can ada t to altered mechanical factors (Carlsson and &berg, 1979; Pullinger, Thurston and Turley, 1985).
Several investigators have suggested that flattening of the condyle was associated with marked tooth wear (Staz, 1951; Scott, 1957; Brown 1965). A later study by Seward (1976) made use of templates to measure cor&lar changes and a simple, three-category system of assessing tooth wear. In a similar study, Hylander (1979) used an improved scale of tooth wear developed by Molnar (1971) which allowed for eight categories of wear. Using Molnar’s scale, both Hylander (1979) and Hinton (1981) found that the depth of the glenoid fossa was directly related to tooth wear. None of these studies was able to confirm or refute Pritchard’s (1972) claim that remodelling of the condyle was specific to the loads generated during chewing, or substantiate the suggestion by Mongini (1972) that the medial aspect of the condyle was most a&ted in subjects with substantial chewing forces associated with tooth wear. Studies of condylar changes in patients with osteoarthritic degeneration of the temporomandibular joint suggest that dental factors are of little importance in the aetiology of the disease (Toiler, 1974; Kreutziger and Mahan, 1975). In contrast is the work of Mongini (1977, 1980), who concluded that the state of the dentition is related to reshaping of the condyles. Our purpose now was to determine whether there were any predictable relationships between forces on the dentition and the pattern of changes in condyle remodelling associated with tooth wear. It was considered advisable to study individuals in whom substantial forces generated at the teeth had been acting long enough to cause both occlusal wear and remodelling of the joint. 799
c. P. OWN et at. 1 Unworn
2 Wear facets No observable dentine
5 3 or more large dentine patches Secondary dentine: none to slight
6 Secondary dentine moderate to extensive. Entire tooth surrounded by enamel
Lsp pattern partially or completly obliterated smali dentine patches
7 Enamel worn on at least one side Extensive secondary dentine
4 3 or more small dentlne patches
8 Roots functioning in occlusal surface
Fig. 1. Ckclusal wear rating scale, after Molnar (1971).
MATERIALS AND METHODS
A collection of 20 skulls provided the material for the study. The skeletal remains had been retrieved from various cave sites in the area of Oakhurst, on the southern coast of the Cape Province, South Africa. They were assumed to have been prehistoric hunter-gatherers who were partly or wholly coastal dwellers. The specimens were estimated by carbon dating to be 6000-9000 yr old.
measurements, either because of ante- or postmortem loss of teeth, or because of fractured enamel (one specimen had approximal caries). The presence of a helicoidal occlusal plane was determined by observing the direction of the occlusal surfaces of the mandibular molars. If that surface faced outwards the specimen was categorized as having a helicoidal occlusal plane. Condylar measurements
Age assessment The ages at death of adult specimens were estimated from age changes in the postcrqnial skeleton. Specimens were grouped into age categories of 25,30, 35 and 45 yr. The sample also included three children whose age was estimated by analysing the stages of tooth eruption. Tooth wear measurements Three observers independently measured the occlusal wear on all the adult incisor and molar teeth using the scale described by Molnar (1971) (Text Fig. 1). Although the wear scale does not provide continuous data it was decided to calculate, for each specimen, a mean value for the wear of incisors, and first and second molars. Approximal wear was assessed by measuring the width of the approximal wear facet between the first and second permanent molars (Plate Fig. 2). The average of three measurements for each facet was recorded. To allow for the influence of tooth size on this measurement, the length and breadth of the occlusal surface of the second molar was also measured and used to weight the approximal wear dimension. Not all specimens could be used for these
Not all specimens had both condyles intact: in all 36 condyles were used, 17 left condyles and 19 right. As no suitable method for making reliable measurements of condylar morphology could be found in the literature, the following method was devised. An acrylic tray was made so that an impression could be taken of the condyles of the various mandibles with the bones placed in approximately the same orientation (Plate Fig. 3). A putty-type silicone (Optosil, Bayer Dental, Fed. Rep. Germany) was used for the impression; after curing, the impression was marked, removed from the tray and put aside. This allowed the same tray to be used for all impressions. After completing all the impressions, the silicone was replaced accurately in the tray, the rest of which was filled with dental gypsum (yellow stone), including the impression. The resultant models therefore had a convenient base which when held upright was at right angles to the posterior border of the ramus and condyle. Dots were then placed on the condyles, one each over the medial and lateral poles, being defined as the points of maximum curvature in all directions. The models were then held against an orthodontic model trimmer, and ground to create a
Plate 1 Fig. 2. Approximal wear was assessed by measuring the width of the approximal facet. Fig. 3. Impressions were made of each condyle using a putty-type silicone in an acrylic tray.
Condyle morphology and tooth wear
Plate 1
802
c. P. OWEN
section passing through the medial and lateral poles. This ground section gave a posterior viewpoint of the articulating surface, and when placed on a photocopying machine, the copy provided an outline of the section. In order to record the dimensions of this outline, three points were selected: the points of maximum convexity of the medial and lateral ‘poles’ of the outlines, and the point of maximum convexity of the height of the curvature of the condyle-the ‘gable’. Lines were constructed through these points (Text Fig. 4) to enable measurements of the width (W) and the medial facet portion (M). Lines were drawn by three observers and the mean of each measurement was calculated. The ratio of the medial facet to the width was calculated and described as the medial: width ratio (M : W), in order to identify whether the lateral or the medial facet was the larger. For each specimen the mean of the left and right measurements was calculated. RESULTS
Age of specimens
The age at death for the adult specimens ranged from 25 to 45 yr, the median was 30 yr (Table 1). Two of the children were estimated to be 4 yr old and the third, 6yr old. Occlusal wear
The three observers evaluated the degree of wear with complete agreement. The mean wear for incisors was 5.7 with a mode of 8. The mean wear for the first molars was 4.8 (mode 5) and for the second molars, 3.7 (mode 3). The age estimate had a significant positive correlation with occlusal wear of the first permanent molars (p = 0.9; p < 0.001) but not with wear of the incisor and second molar. A helicoidal plane of occlusion was found in 13 of the 20 specimens. Approximal wear
The mean width of the approximal facet between the first and second molar was 5.7 mm, with a range from 3.7 to 7.7 mm. The mean width of the approximal facet per specimen correlated significantly with age (p = 0.6, p < 0.02) and the extent of occlusal wear of incisors (p = 0.7, p < 0.01) and second molars (p < 0.01).
et of.
Table 1. The data for each specimen, ranked according to age cohorts Wear Age Helicoid (yr) plane 4 4 6 25 25 25 30 30 30 30 30 30 35 35 35 35 35 45 45 45
N N N-Y Y N Y Y Y N N Y Y Y Y Y Y Y N Y
I
Ml
M2
Condyles APP (mm)
____ ____ 5 3 3 6 6 7 3 6 6 8 8 7 7 5 8 0 8
3.0 4.0 3.0 5.0 3.5 4.5 3.5 5.0 5.0 7.0 5.2 5.5 5.2 6.7 5.6 5.5
3; 2.0 2.0 4.0 6.0 3.5 2.6 3.0 4.0 6.0 3.0 5.0 6.0 3.0 5.3 2.5 2.7
5.5 3.8 4.3 5.2 6.2 5.2 5.8 5.9 7.1 7.1 5.5 6.6 5.2 6.6
Width (mm) 12.2 13.8 14.4 20.5 19.4 16.9 20.9 18.5 19.1 17.9 16.9 16.3 16.4 16.3 18.4 18.9 17.9 18.3 15.7 18.0
M: W 0.63 0.69 0.57 0.43 0.51 0.53 0.45 0.34 0.37 0.52 0.52 0.22 0.43 0.43 0.44 0.44 0.46 0.57 0.57 0.58
Wear values are according to Molnar’s (1971) scale for incisors (I), molars (Ml and M2). Approximal wear (APP) is given as the average width of the interproximal facet. The value M: W is the ratio of the medial facet to condylar width.
The width of the approximal facet for all specimens was stratified according to the level of tooth wear. The mean facet width in each category of wear was calculated and plotted against tooth wear (Text Fig. 5). Condylar dimensions
The mean width of the condyles was 18.0 mm (SD 1.4 mm). The mean M : W ratio was 0.45, indicating that the average medial facet portion of the condyle was approximately equal to the lateral facet length. A weak but significant negative correlation (p = - 0.5; p < 0.05) was found between the M : W ratio and the wear of the second molar. Thus the greater the wear on the second molar, the smaller the M: W ratio and the more pronounced the lateral facet length of the condyle. A pronounced lateral condylar facet was also associated with the presence of a helicoidal plane of occlusion (X-value = 5.6; p < 0.05). No significant association was found between the M : W ratio of the condyle and approximal wear. DISCUSSION
Fig. 4. An outline of the cross-section of the head of the condyle. The length of the medial facet “M” and the total width “w” were used to derive the medial:width ratio.
Age had a significant correlation with wear on the first molars, but not on the premolars or second molars. Incisal wear was advanced (Molnar level 8) in several specimens; this correlated not with first molar wear but with second molar wear. In some specimens the incisors showed signs of lingual and palatal wear consistent with repetitive tugging at fibrous food held between the teeth. It is possible that such habits had a greater influence on incisal wear than age alone.
Condyle morphology and tooth wear The difference between wear on the 8rst and second molars gives an indication of the wear which took place during the 6 or 7 yr before the second molar erupts; it therefore gives an indication of the rate of occlusal wear. The diKerence between the mode values for wear was 2 categories. This compares with the difference between wear on tlrst and second molars reported by Hinton (1982). Hinton (1982) represented the degree of approxima1 wear by plotting the mean facet width against occlusal wear categories. He found that approximal wear was greatest in the oldest of the three samples studied (the archaic sample) and assumed that such wear was consistent with a diet requiring large forces for mastication. He concluded that the archaic sample was primarily d hunter-gatherers in contrast to the food cultivators of his Mississippi sample. The approximal wear of the individuals in our study and the estimated date of the Oakhurst site match Hinton’s archaic sample (Text Fig. 5). This suggests that they were coastal-dwelling hunter-gatherers.
greatest on the balancing-side condyle. The power stroke of mastication and the slide into the position of maximum intercuspation are accompanied by the greatest forces (Ahlgren and Owall, 1970; Gibbs, 1975). If the chewing cycle is a grinding and not a chopping stroke, the balancing condyle would be forced laterally against the articular eminence. The forces which generate the wear pattern of a helicoidal occlusal curve would be mirrored in the facet direction of the condyles. Our tindings show that substantial masticatory forces and prolonged chewing were necessary to process the foods eaten. This was coniirmed by the presence of large approximal facets associated with extensive occlusal wear. Although the sample was relatively small, there was evidence of an association between specific changes in the mandibular condyles and changes in the dentition. In a sequel to this study we are investigating the relationship between tooth wear and changes in the form of the glenoid fossa. Acknowledgements-This
Condylar changes
Several studies have shown a relationship between changes in the occlusion (either wear or tooth loss) and changes in the morphology of the condyle (Seward, 1976; Mongini, 1977; Richards and Brown, 1981). However, them is insufficient evidence to reveal even a qualitative relationship between forces at the teeth and the direction of the reaction at the joint, We found that a dominant lateral facet on the articular surface of the condyle was associated with increase in occlusal wear. According to Pritchard (1972), remodelling of connective tissue occurs to provide a more favourable distribution of stress so as to avoid tissue damage. Hence the increase in the size of the lateral facet with wear suggests that the forces on the joint are predominantly lateral during optimum chewing functbon. There is some support for this notion from other sources. In a study by Hansson et al. (1977), thinning out of the disc was found mostly on the lateral side of the condyle in adult specimens. Mongini (1975) found that the condyles had slanting medial and lateral slopes in specimens that had uniform tooth wear. The increase in the: size of the lateral facet may be explained by considering the conditions under which there is the greatest load on the condyle. It has been shown (Hylander, 1975) that the reaction forces are
3.51 2
803
I 3
I 5
I 4
M2 occlusal
I 6
wear
Fig. 5. Facet width striatified according to wear levels. The Oakhurst specimens were comparable with Hinton’s (1982) archaic sample; both these samples showed greater approximal wear than the more recent Mississippi sample (Hinton, 1982).
study was prompted, and encouraged by, Professor J. W. Osborne, Department of Oral Biology, University of Alberta, Edmonton, whose invaluable advice was crucial. Professor C. J. Thomas, now of the Faculty of Dentistry of the University of Sydney, kindly acted as one of the observers.
REFERENCES Ahlgren J. and Owall B. (1970) Muscular activity and chewing force: a polygraphic study of human mandibular movements. Archs oral Eiol. 15,271-280. Brown T. (1965) Physiology of the mandibular articulation. AIM. dent. J. 10, 126131. Butler P. (1981) Dentition in function. In Dental Anatomy and Embryology (Ed. Osbom J. W.), Chap. 10. Blackwell, London. Carlsson G. E. and &erg T. (1979) Remodelling of the temporomandibular joint. In Temporomandibular Joint, Function and Dysfunction (Eds Zarb G. A. and Carlsson G. E.), pp. 159-170. Munksgaard, Copenhagen. Crompton A. W. and Hiiemae K. M. (1969) How mammalian molar teeth work. Dzkcouery 5, 23-34. Gibbs C. G. (1975) Electromyographic activity during the motionless period during chewing. J. prosthet. Dent. 34, 35-40. Hansson T., &erg T., Carlsson G. E. and Kopp S. (1977): Thickness of the soft tissue layers and the articular disc in the temporomandibular joint. Acra odont. stand. 35, 77-83.
Hinton R. J. (1981) Form and function in the temporomandibular joint. In Craniofacial Biology (Ed. Carlson D. S.), Monograph No. 10,. Craniofacia? Growth Series, DD. 37-60. Center for Human Growth and Develooment. _ University of Michigan. Hinton R. J. (1982) Differences in interproximal and occlusal tooth wear among prehistoric Tennessee Indians: implications for masticatory function. Am. J. Phys. Anthropol. 57, 103-l 15. Hylander W. L. (1975) The human mandible: lever or link? Am. J. Phys. Anthropol. 43, 227-242.
Hylander W. L. (1979) An experimental analysis of temporomandibular joint reaction force in macaques. Am. J. Phys. Anthropol. 51, 433456.
Kreutxiger K. L. and Mahan P. E. (1975) Temporomandibular degenerative joint disease. Oral Surg. 40, 165-182. Mills J. R. E. (1955) Ideal dental occlusion in the primates. Trans. Br. Sot. Study Orthod. 1955, 56-71.
804
C. P. OWENet of.
Moffett B. C., Johnson L. C., McCabe J. B. and Askew H. C. (1964) Articuhu remodelling in the adult human temporomandibular joint. Am. J. Anar. 115, 119-142. Mohrar S. (1971) Human tooth wear, tooth function and cultural variability. Am. J. Phys. Anthropol. 34, 175-190. Mongini F. (1972) Remodelhng of the mandibular condyle in the adult and its relationship to the condition of the dental arches. Acta anat. 82, 437-453. Mongini F. (1975) Dental abrasion as a factor in remodelling of the mandibular condyle. Acta Anat. 92, 292-300. Mongini F. (1977) Anatomic and clinical evaluation of the relationship between the temporomandibular joint and occlusion. J. prosthet. Dem. 38, 539-551. Mongini F. (1980) Condylar remodelling after occlusal therapy. J. prosthet. Dent. 43, 568-577. Osbom J. W. (1982) Helicoidal plane of dental occlusion. Am. J. Phys. Anthropol. 57, 273-281. Pritchard J. J. (1972) The central trigger mechanism induced by mechanical forces which causes responses of mes-
enchymal cells in genera) and bone apposition and resorption in particular. Acta Morph. NeerL-&and. 10, 63-69.
Pullinger A., Thurston M. and Turley P. (1985) Condylar adaptation to unilateral posterior crossbite in adults. J. dent. Res. 64, 260, Abstract No. 851. Richards L. C. and Brown T. (1981) Dental attrition and degenerative arthritis of the temporomandibular joint. J. oral Rehab. 8, 293-307. Scott J. H. (1957) Muscle growth and function in relation to skeletal morphology. Am. J. Phys. Anthropol. 15, 197-234. Seward F. S. (1976) Tooth attrition and the temporomandibular joint. Angfe Orthod. 46, 162-170. Staz J. (1951) The treatment of disturbances of the temporomandibular articulation. J. dent. Ass. S. Afr. 6, 314-334. Toller P. A. (1974) Temporomandibular arthropathy. Proc. R. Sot. Med. 67, 153-159. Wolpoff M. H. (1971) Interstitial wear. Am. J. Phys. Anthropol. 141, 235-249.
Arch oral Biol.Vol. 36, No. I I, pp. 799-804,1991 Printedin Great Britain.All rightsrcscrvcd
Copyright0 1991PcrgamonPressplc
CHANGES IN MANDIBULAR CONDYLE MORPHOLOGY RELATED TO TOOTH WEAR IN A PREHISTORIC HUMAN POPULATION C. P. OWEN,’R. J. C. WILDING’and A. G. MORRIS’ ‘Faculty of Dent:istry, University of the Western Cape, Private Bag X12, Tygerberg 7505 and ‘Department of Anatomy, University of Cape Town Medical School, Rondebosch 7700, South Africa (Received 4 April 1991; accepted 28 May 1991) Summary-The exact relationship between tooth wear and changes in the shape of the articulating surface of the mandibular condyles is unknown. It is assumed that to some degree these changes are in response to the loads on the joint generated during chewing. Excavated skeletal remains from sites of primitive human habitation provide an opportunity to study the masticatory system of human beings in their natural context, with the features of form and function expressed to their full potential. Accurate measurements of the condylar head can also be made on such remains. A collection of skulls from an excavation site known as Oakhurst on the South African coast provided material for this study. Ckclusal wear was measured using the scale devised by Molnar; approximal wear was assessed by measuring the size of the
wear facet between the first and second molars. The width of the condyles were measured directly, and a method was devised to measure changes in the proportion of medial and lateral articulating facets. An increase in the s.ize of the lateral facet of the condyle was found to be associated with increased tooth wear. It is suggested that the direction of condylar remodelling may give an indication of the reaction at the joint to optima1 mastication. Key words: tooth wear, condyle morphology, remodelling.
IN’IRODUmON Tooth wear patterns have been used to deduce functional characteristics of the dentition (Mills, 1955; Crompton and Hiiemae, 1969; Butler, 1981). Wear of the occlusal surface is an indication of both the masticatory force and. the abrasive properties of the food, while approximal wear is more dependent on the amount of masticatory force applied to the teeth, as it is the lateral component of this force which causes wear (Wolpoff., 1971). A quantitative measure of the amount of approximal wear might, therefore, give an indication of masticatory function and of the types of food prep,aration, which may be obscured by examination of occlusal wear alone (Hinton, 1982). Changes in the shape of the condyles that were unrelated to disease were observed by Moffett et al. (1964). They could find no association between the condylar changes and age, but attributed them to functional changes in the dentition. This idea was given support by Pritchard (1972), who concluded that in order to cope better with altered forces, connective tissues would remodel to better advantage. Mongini (1972) produced histological evidence of remodelling of the condyle through replacement of the external, circutierential lamellae of bone, concluding that remodelling by apposition was most pronounced on the .medial side of the articulating surfaces. Studies of orthodontic patients have confirmed that the condyle can ada t to altered mechanical factors (Carlsson and &berg, 1979; Pullinger, Thurston and Turley, 1985).
Several investigators have suggested that flattening of the condyle was associated with marked tooth wear (Staz, 1951; Scott, 1957; Brown 1965). A later study by Seward (1976) made use of templates to measure cor&lar changes and a simple, three-category system of assessing tooth wear. In a similar study, Hylander (1979) used an improved scale of tooth wear developed by Molnar (1971) which allowed for eight categories of wear. Using Molnar’s scale, both Hylander (1979) and Hinton (1981) found that the depth of the glenoid fossa was directly related to tooth wear. None of these studies was able to confirm or refute Pritchard’s (1972) claim that remodelling of the condyle was specific to the loads generated during chewing, or substantiate the suggestion by Mongini (1972) that the medial aspect of the condyle was most a&ted in subjects with substantial chewing forces associated with tooth wear. Studies of condylar changes in patients with osteoarthritic degeneration of the temporomandibular joint suggest that dental factors are of little importance in the aetiology of the disease (Toiler, 1974; Kreutziger and Mahan, 1975). In contrast is the work of Mongini (1977, 1980), who concluded that the state of the dentition is related to reshaping of the condyles. Our purpose now was to determine whether there were any predictable relationships between forces on the dentition and the pattern of changes in condyle remodelling associated with tooth wear. It was considered advisable to study individuals in whom substantial forces generated at the teeth had been acting long enough to cause both occlusal wear and remodelling of the joint. 799
c. P. OWN et at. 1 Unworn
2 Wear facets No observable dentine
5 3 or more large dentine patches Secondary dentine: none to slight
6 Secondary dentine moderate to extensive. Entire tooth surrounded by enamel
Lsp pattern partially or completly obliterated smali dentine patches
7 Enamel worn on at least one side Extensive secondary dentine
4 3 or more small dentlne patches
8 Roots functioning in occlusal surface
Fig. 1. Ckclusal wear rating scale, after Molnar (1971).
MATERIALS AND METHODS
A collection of 20 skulls provided the material for the study. The skeletal remains had been retrieved from various cave sites in the area of Oakhurst, on the southern coast of the Cape Province, South Africa. They were assumed to have been prehistoric hunter-gatherers who were partly or wholly coastal dwellers. The specimens were estimated by carbon dating to be 6000-9000 yr old.
measurements, either because of ante- or postmortem loss of teeth, or because of fractured enamel (one specimen had approximal caries). The presence of a helicoidal occlusal plane was determined by observing the direction of the occlusal surfaces of the mandibular molars. If that surface faced outwards the specimen was categorized as having a helicoidal occlusal plane. Condylar measurements
Age assessment The ages at death of adult specimens were estimated from age changes in the postcrqnial skeleton. Specimens were grouped into age categories of 25,30, 35 and 45 yr. The sample also included three children whose age was estimated by analysing the stages of tooth eruption. Tooth wear measurements Three observers independently measured the occlusal wear on all the adult incisor and molar teeth using the scale described by Molnar (1971) (Text Fig. 1). Although the wear scale does not provide continuous data it was decided to calculate, for each specimen, a mean value for the wear of incisors, and first and second molars. Approximal wear was assessed by measuring the width of the approximal wear facet between the first and second permanent molars (Plate Fig. 2). The average of three measurements for each facet was recorded. To allow for the influence of tooth size on this measurement, the length and breadth of the occlusal surface of the second molar was also measured and used to weight the approximal wear dimension. Not all specimens could be used for these
Not all specimens had both condyles intact: in all 36 condyles were used, 17 left condyles and 19 right. As no suitable method for making reliable measurements of condylar morphology could be found in the literature, the following method was devised. An acrylic tray was made so that an impression could be taken of the condyles of the various mandibles with the bones placed in approximately the same orientation (Plate Fig. 3). A putty-type silicone (Optosil, Bayer Dental, Fed. Rep. Germany) was used for the impression; after curing, the impression was marked, removed from the tray and put aside. This allowed the same tray to be used for all impressions. After completing all the impressions, the silicone was replaced accurately in the tray, the rest of which was filled with dental gypsum (yellow stone), including the impression. The resultant models therefore had a convenient base which when held upright was at right angles to the posterior border of the ramus and condyle. Dots were then placed on the condyles, one each over the medial and lateral poles, being defined as the points of maximum curvature in all directions. The models were then held against an orthodontic model trimmer, and ground to create a
Plate 1 Fig. 2. Approximal wear was assessed by measuring the width of the approximal facet. Fig. 3. Impressions were made of each condyle using a putty-type silicone in an acrylic tray.
Condyle morphology and tooth wear
Plate 1
802
c. P. OWEN
section passing through the medial and lateral poles. This ground section gave a posterior viewpoint of the articulating surface, and when placed on a photocopying machine, the copy provided an outline of the section. In order to record the dimensions of this outline, three points were selected: the points of maximum convexity of the medial and lateral ‘poles’ of the outlines, and the point of maximum convexity of the height of the curvature of the condyle-the ‘gable’. Lines were constructed through these points (Text Fig. 4) to enable measurements of the width (W) and the medial facet portion (M). Lines were drawn by three observers and the mean of each measurement was calculated. The ratio of the medial facet to the width was calculated and described as the medial: width ratio (M : W), in order to identify whether the lateral or the medial facet was the larger. For each specimen the mean of the left and right measurements was calculated. RESULTS
Age of specimens
The age at death for the adult specimens ranged from 25 to 45 yr, the median was 30 yr (Table 1). Two of the children were estimated to be 4 yr old and the third, 6yr old. Occlusal wear
The three observers evaluated the degree of wear with complete agreement. The mean wear for incisors was 5.7 with a mode of 8. The mean wear for the first molars was 4.8 (mode 5) and for the second molars, 3.7 (mode 3). The age estimate had a significant positive correlation with occlusal wear of the first permanent molars (p = 0.9; p < 0.001) but not with wear of the incisor and second molar. A helicoidal plane of occlusion was found in 13 of the 20 specimens. Approximal wear
The mean width of the approximal facet between the first and second molar was 5.7 mm, with a range from 3.7 to 7.7 mm. The mean width of the approximal facet per specimen correlated significantly with age (p = 0.6, p < 0.02) and the extent of occlusal wear of incisors (p = 0.7, p < 0.01) and second molars (p < 0.01).
et of.
Table 1. The data for each specimen, ranked according to age cohorts Wear Age Helicoid (yr) plane 4 4 6 25 25 25 30 30 30 30 30 30 35 35 35 35 35 45 45 45
N N N-Y Y N Y Y Y N N Y Y Y Y Y Y Y N Y
I
Ml
M2
Condyles APP (mm)
____ ____ 5 3 3 6 6 7 3 6 6 8 8 7 7 5 8 0 8
3.0 4.0 3.0 5.0 3.5 4.5 3.5 5.0 5.0 7.0 5.2 5.5 5.2 6.7 5.6 5.5
3; 2.0 2.0 4.0 6.0 3.5 2.6 3.0 4.0 6.0 3.0 5.0 6.0 3.0 5.3 2.5 2.7
5.5 3.8 4.3 5.2 6.2 5.2 5.8 5.9 7.1 7.1 5.5 6.6 5.2 6.6
Width (mm) 12.2 13.8 14.4 20.5 19.4 16.9 20.9 18.5 19.1 17.9 16.9 16.3 16.4 16.3 18.4 18.9 17.9 18.3 15.7 18.0
M: W 0.63 0.69 0.57 0.43 0.51 0.53 0.45 0.34 0.37 0.52 0.52 0.22 0.43 0.43 0.44 0.44 0.46 0.57 0.57 0.58
Wear values are according to Molnar’s (1971) scale for incisors (I), molars (Ml and M2). Approximal wear (APP) is given as the average width of the interproximal facet. The value M: W is the ratio of the medial facet to condylar width.
The width of the approximal facet for all specimens was stratified according to the level of tooth wear. The mean facet width in each category of wear was calculated and plotted against tooth wear (Text Fig. 5). Condylar dimensions
The mean width of the condyles was 18.0 mm (SD 1.4 mm). The mean M : W ratio was 0.45, indicating that the average medial facet portion of the condyle was approximately equal to the lateral facet length. A weak but significant negative correlation (p = - 0.5; p < 0.05) was found between the M : W ratio and the wear of the second molar. Thus the greater the wear on the second molar, the smaller the M: W ratio and the more pronounced the lateral facet length of the condyle. A pronounced lateral condylar facet was also associated with the presence of a helicoidal plane of occlusion (X-value = 5.6; p < 0.05). No significant association was found between the M : W ratio of the condyle and approximal wear. DISCUSSION
Fig. 4. An outline of the cross-section of the head of the condyle. The length of the medial facet “M” and the total width “w” were used to derive the medial:width ratio.
Age had a significant correlation with wear on the first molars, but not on the premolars or second molars. Incisal wear was advanced (Molnar level 8) in several specimens; this correlated not with first molar wear but with second molar wear. In some specimens the incisors showed signs of lingual and palatal wear consistent with repetitive tugging at fibrous food held between the teeth. It is possible that such habits had a greater influence on incisal wear than age alone.
Condyle morphology and tooth wear The difference between wear on the 8rst and second molars gives an indication of the wear which took place during the 6 or 7 yr before the second molar erupts; it therefore gives an indication of the rate of occlusal wear. The diKerence between the mode values for wear was 2 categories. This compares with the difference between wear on tlrst and second molars reported by Hinton (1982). Hinton (1982) represented the degree of approxima1 wear by plotting the mean facet width against occlusal wear categories. He found that approximal wear was greatest in the oldest of the three samples studied (the archaic sample) and assumed that such wear was consistent with a diet requiring large forces for mastication. He concluded that the archaic sample was primarily d hunter-gatherers in contrast to the food cultivators of his Mississippi sample. The approximal wear of the individuals in our study and the estimated date of the Oakhurst site match Hinton’s archaic sample (Text Fig. 5). This suggests that they were coastal-dwelling hunter-gatherers.
greatest on the balancing-side condyle. The power stroke of mastication and the slide into the position of maximum intercuspation are accompanied by the greatest forces (Ahlgren and Owall, 1970; Gibbs, 1975). If the chewing cycle is a grinding and not a chopping stroke, the balancing condyle would be forced laterally against the articular eminence. The forces which generate the wear pattern of a helicoidal occlusal curve would be mirrored in the facet direction of the condyles. Our tindings show that substantial masticatory forces and prolonged chewing were necessary to process the foods eaten. This was coniirmed by the presence of large approximal facets associated with extensive occlusal wear. Although the sample was relatively small, there was evidence of an association between specific changes in the mandibular condyles and changes in the dentition. In a sequel to this study we are investigating the relationship between tooth wear and changes in the form of the glenoid fossa. Acknowledgements-This
Condylar changes
Several studies have shown a relationship between changes in the occlusion (either wear or tooth loss) and changes in the morphology of the condyle (Seward, 1976; Mongini, 1977; Richards and Brown, 1981). However, them is insufficient evidence to reveal even a qualitative relationship between forces at the teeth and the direction of the reaction at the joint, We found that a dominant lateral facet on the articular surface of the condyle was associated with increase in occlusal wear. According to Pritchard (1972), remodelling of connective tissue occurs to provide a more favourable distribution of stress so as to avoid tissue damage. Hence the increase in the size of the lateral facet with wear suggests that the forces on the joint are predominantly lateral during optimum chewing functbon. There is some support for this notion from other sources. In a study by Hansson et al. (1977), thinning out of the disc was found mostly on the lateral side of the condyle in adult specimens. Mongini (1975) found that the condyles had slanting medial and lateral slopes in specimens that had uniform tooth wear. The increase in the: size of the lateral facet may be explained by considering the conditions under which there is the greatest load on the condyle. It has been shown (Hylander, 1975) that the reaction forces are
3.51 2
803
I 3
I 5
I 4
M2 occlusal
I 6
wear
Fig. 5. Facet width striatified according to wear levels. The Oakhurst specimens were comparable with Hinton’s (1982) archaic sample; both these samples showed greater approximal wear than the more recent Mississippi sample (Hinton, 1982).
study was prompted, and encouraged by, Professor J. W. Osborne, Department of Oral Biology, University of Alberta, Edmonton, whose invaluable advice was crucial. Professor C. J. Thomas, now of the Faculty of Dentistry of the University of Sydney, kindly acted as one of the observers.
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