Biomedical subjects

G A Macho

Publications and source records attributed to G A Macho.

At least 19 recordsLinked to original sources

PubMed2003

An investigation into fractured surfaces of enamel of modern human teeth: a combined SEM and computer visualisation study.

It has long been recognised that the enamel microstructure may hold important information with regards to phylogeny and masticatory biomechanics. Further, the biophysical and adaptive processes involved in enamel formation and in the creation of different microstructures are poorly understood. This lack of understanding is in part due to technical difficulties when visualising the 3D structure of enamel. Using modern visualisation techniques, models of various regions of different modern human teeth were created. Underlying these models are consistent mathematical representations of the interplay between cell-to-cell adhesion, integrity of the advancing enamel front and (potentially decreasing) constraints on the prism course from the dentino-enamel junction (DEJ) to the outer surface. Seven modern human teeth (I1, 1 lower C, 1 P4, 1 M2, 2 M2 and 1 M3) were fractured longitudinally and formed the basis for the creation of the models. For validation purposes the teeth were then fractured transversely, thus allowing quantitative comparisons between the prism pathways on the newly fractured transverse plane and the transverse pathways as predicted by the model. It was found that these predictions were fairly accurate provided that (a) the light position with respect to the model corresponds with the beam position with respect to the scanned surface and (b) the path of prisms was carefully reconstructed/extrapolated from SEM in cases where prisms were broken. Given that these predictions were based on the mechanisms governing enamel formation as applied to the model, it is suggested that such theories must be reasonable. In other words, biophysical processes, rather than complicated (genetic) positional information, suffice to create different enamel microstructures. In addition, systematic differences were found in prism deviation from their c-axis in different enamel pieces. Given the nature of these differences it is suggested that enamel formation is not only the result of biophysical processes (proximal causes), but could be due to the structures having been selected for in order to counteract masticatory stress exerted during the lifetime of the species (ultimate causes). As to whether and to what extent this may be the case is not yet clear but it is apparent that computer visualisation does have potential to quantify enamel microstructure and to address such questions. Given its non-destructive nature, computer modelling could have particular relevance for studying fragmented fossilised remains.

Dental Enamel↗

PubMed2001

Primate molar crown formation times and life history evolution revisited.

Comparative studies have convincingly demonstrated that the pattern and timing of tooth emergence are highly correlated with life-history variables and brain size. Conversely, a firm relationship between molar formation time and life-history variables has not yet been established. It seems counterintuitive that one aspect of dental development should be correlated with life-history variables, whereas the other should not. In order to shed light on this apparent discrepancy this study analyzed all data on primate molar crown formations available in the published literature in relation to life-history variables, brain size, and female body mass. Crown formation times were found to be particularly highly correlated with both female body mass and brain size. Species that depart from the overall brain/body allometry by being relatively large-bodied, e.g., Gorilla gorilla and later Theropithecus oswaldi, also have shorter molar crown formation times than expected. The reverse is not found for species that depart from the overall brain/body allometry due to their larger brains, i.e., Homo sapiens. This finding is interpreted within an evolutionary and ecological framework. Specifically, by focusing on ecological commonalities, a scenario is proposed which may allow predictions to be made about the evolutionary history of other extinct primates also. If confirmed in future studies, crown formation time may again become a powerful tool in evolutionary enquiry.

Animals↗

PubMed1999

Effects of loading on the biomechanical [correction of biochemical] behavior of molars of Homo, Pan, and Pongo.

In a previous study, we found systematic differences in the biomechanical behavior of modern human molars using finite element stress analyses (FESA), which led us to propose that molars are adapted to differently-directed loads depending on their position within the mouth (Spears and Macho [1998] Am. J. Phys. Anthropol. 106:467-482). While the FESA results thus derived have not been verified experimentally, such an interpretation seemed reasonable. To refine the model previously presented, this study assessed the effects of 1) food particle size on the biomechanical behavior of molars, and those of 2) differences in morphology, particularly enamel thickness, on stress distribution. In order to appraise the evolutionary significance of the findings, the FESA results for modern humans were subsequently compared with those obtained for molars of one individual of Pan and Pongo, respectively. Bearing in mind limitations imposed by the FESA models created and analyzed in this study, constant cleavage-type loads and cuspal tip loads at different directions were employed on all teeth: this facilitated comparisons of patterns of stress distribution across molars and species. In Pan and Homo, cleavage-type loads exerted by big food particles tended to be better dissipated anteriorly than posteriorly, although trends in Pongo were less clear-cut. Furthermore, similar to modern humans, the buccal cusps of mandibular molars appeared to be able to dissipate the loads associated with a pestle-type action, while maxillary molars were better designed to dissipate the loads which would result if they acted as mortars against which the food is crushed/ground. While increases in enamel thickness lowered the overall stress values in teeth only slightly, changes in outer morphology could have a more profound effect on these stress levels. Overall, Pan appeared to be most generalized, while Homo and Pongo showed a number of unique specializations, which are in accordance with what is currently understood about their respective masticatory apparatus and dietary niche.

Animals↗

PubMed1999

Did knuckle walking evolve twice?

Although African great apes share a similar quadrupedal locomotor behaviour, there are marked differences in hand morphology and size between the species. Hence, whilst all three species (two genera) of African ape frequently knuckle walk as adults, debate remains as to whether this behaviour is derived from a common ancestor or whether it evolved in parallel in chimpanzees and gorillas. This exploratory morphometric study of the sub-adult and adult wrist of these two genera aims to contribute to this debate. A total of twenty-seven dimensions of the lunate, triquetral, hamate and capitate of sub-adult and adult Pan troglodytes and Gorilla gorilla were analysed in order to determine whether carpal dimensions are generally ontogenetically scaled, and whether differences in growth trajectories, or length of growth, and adult morphologies can be explained by behavioural differences between the two species. Only 56% of all dimensions studied were ontogenetically scaled in sub-adults and some of these dimensions exhibit differing adult proportions between the two species. In general, the dimensions analysed fell into two categories: Pan and Gorilla either follow the same growth trajectories (Pattern A) or the Pan reduced major axis (RMA) regressions were significantly transposed above those of Gorilla (Pattern B). Additionally, it was found that Gorilla carpals appear to cease growing relatively earlier than those of Pan. While a small number of differences, notably those of the lunate, can be accounted for by differences in behaviour between the species, the majority of differences indicate heterochronic modifications of development during evolution, which correspond to kinematic differences in knuckle walking between the African great apes. In light of morphological, behavioural and ecological data currently available it is parsimonious to suggest that knuckle walking has evolved in parallel in the two lineages.

Animals↗

PubMed1998

Biomechanical behaviour of modern human molars: implications for interpreting the fossil record.

Finite-element models of 29 intact molars were created and subjected to cleavage-type loads in order to assess differences in the biomechanical behaviour of molars. A simulated food particle, which was one-third the size of the intercuspal distance and had the properties of a Mezzettia seed, was pushed onto the occlusal basin of these models at various angles, resulting in either both or one particular cusp being preferentially loaded. In all cases, the maximum tensile stresses occurred in enamel at the intercuspal fissure. With regard to first maxillary molars, supporting (functional) and guiding (nonfunctional) cusps apparently dissipate loads equally well, whereas, in second and third maxillary molars, the guiding cusps are better designed to resist loads. Overall, lingual cusps of maxillary posterior molars dissipate loads poorly. Conversely, loads exerted toward supporting cusps of mandibular molars are consistently well dissipated, regardless of position along the tooth row. Because the directions of loads to which these teeth are best adapted change along the tooth row, it seems reasonable to suggest that these may correlate with the well-documented structural and functional orofacial complex. This study indicates that the biomechanical behaviour of molars and the orofacial skeleton are likely to have undergone complementary directional changes during evolution. Consequently, caution must be exercised in making inferences about dietary adaptations of extinct species on the basis of isolated teeth or fragmentary gnathic remains without proper regard of the orofacial skeleton as a whole.

Anthropology, Physical↗

PubMed1994

Enamel thickness and the helicoidal occlusal plane.

In the present study 38 unworn maxillary molars (M1 = 16, M2 = 12, M3 = 10) of modern humans from a Slavic necropolis were sectioned through the mesial cusps in a plane perpendicular to the cervical margin of the crown. Five slightly worn M1s and one slightly worn M3 were also used thus increasing the total sample to 44, but measurements made on the worn areas were coded as missing values. Seven measurements of enamel thickness as well as the heights of the protocone and the paracone dentine horns were recorded in order to analyze whether changes in these dimensions in anteroposterior direction can be related to the helicoidal occlusal plane. Uni- and multivariate analyses revealed that the distribution of enamel thickness within and between maxillary molars corresponds to a helicoidal occlusal wear pattern. Enamel thickness along the occlusal basin increases from anterior to posterior, which may lead to rapid development of a reverse curve of Monson in first molars when compared to posterior teeth. However, although these overall differences together with the serial, especially delayed eruption pattern of human molars, contribute to the marked expression of the helicoidal occlusal plane in Homo, differences in enamel patterning between molars indicate that a helicoidal plane is a structural feature of the orofacial skeleton. In contrast to first upper molars, second and third molars show absolutely and relatively thicker enamel under the Phase I wear facet of the paracone, i.e., the lingual slope of the paracone, than under the Phase II facet of the protocone, i.e., the buccal slope of that cusp. These proportional differences are most pronounced in M3, as evidenced by uni- and multivariate statistics. It thus appears that the pattern of enamel thickness distribution from M1 to M3 follows a trend towards providing additional tooth material in areas that are under greater functional demands, that is, corresponding to a lingual slope of wear anteriorly and to a flat or even buccal one posteriorly. In addition, the heights of the dentine horns in anteroposterior direction change in a way that lends support to the hypothesis that the axial inclination of teeth could be one of the most important factors for the development of the helicoidal occlusal plane. Finally, the changes in morphology and enamel thickness distribution from first to third upper molars found in this study suggest that molars could be "specialized" in their function, i.e., from performing proportionally more shearing anteriorly to increased crushing and grinding activities posteriorly.

Adaptation, Physiological↗

PubMed1994

Variation in enamel thickness and cusp area within human maxillary molars and its bearing on scaling techniques used for studies of enamel thickness between species.

Thirty-nine unworn maxillary molars (M1 = 18, M2 = 12, M3 = 9), representing 37 individuals of a Slavic population, were sectioned through the mesial cusps in a plane perpendicular to the cervical margin of the crown. Whether the range of variation in enamel cross-sectional area between individuals corresponds to the range of variation found between teeth within a species was investigated. Total tooth crown area, as determined by planimetry, was used as a scaling factor on the basis that a biologically meaningful scaling procedure should take into account both enamel thickness and total tooth crown area, rather than the size of the dentine-enamel junction. Uni- and multivariate statistics revealed that the enamel cross-sectional area of second and third molars is absolutely and relatively larger than it is in first upper molars and that it scales close to isometry with tooth crown area. Conversely, first maxillary molars, with their relatively and absolutely thinner enamel, exhibit positive allometry between enamel and tooth crown area. These patterns were retained even when differences in overall tooth size were adjusted for. Hence, the notion that there exists a 'species-specific' amount of enamel over the tooth crown must be reconsidered, and one should refrain from pooling different tooth types for taxonomic purposes. At least two scaling factors are needed to describe human maxillary molars alone. However, this increase in enamel cross-sectional area from anterior to posterior is not unexpected if one bears in mind that posterior teeth occupy a more advantageous position relative to the chewing muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Austria↗

PubMed1993

Histological reconstruction of dental development and age at death of a juvenile Paranthropus robustus specimen, SK 63, from Swartkrans, South Africa.

There has been disagreement about whether the earliest hominids grew in a similar manner to great apes or modern humans. This has important biological implications, since it may have been inappropriate to apply modern human developmental standards to early hominids. The aim of the present study was to combine data from replicas of tooth surfaces, computed tomographic (CT) scans, and radiographs with data from a histological section of the canine crown, in order to provide a complete description of tooth crown and tooth root development in a single early hominid specimen (SK 63). Although partially destructive in nature, we have been able to determine the most reliable data yet for aspects of dental development in an important juvenile early hominid specimen. Appositional enamel formation time in the permanent right canine was estimated at between 305 and 418 days, imbricational enamel formation time at 819 days, and total crown formation time at between 3.18 and 3.48 years. The most likely age at death was estimated at around 4 years with a range of ages calculated between 3.18 and 4.23 years based on differences in timing of initial mineralization of the canine and differences in appositional enamel formation times. Crown formation times of the lower central and lateral incisors were estimated between 2.35-2.68 years and 2.57-2.91 years, respectively. Crown formation time of the first permanent molar was estimated at 2.4 years. Wear facets on the first permanent molars indicate that gingival emergence had occurred sometime prior to death, between 3 and 4 years of age. Estimates of root extension rates in the first permanent molars and in the permanent incisors are fast, and either within or above ranges of rates estimated for modern great apes. While we recognize that data for one individual may not be representative of data for a whole population of early hominids, the data for age at death, for age of M1 emergence, and for root extension rates presented here accord with those known for modern great apes and fall beyond the known ranges for modern humans.

Age Determination by Teeth↗

PubMed1993

Linear measurements of cortical bone and dental enamel by computed tomography: applications and problems.

This paper explores the potential of high-resolution computed tomography (CT) as a morphometric tool in paleoanthropology. The accuracy of linear measurements of enamel thickness and cortical bone thickness taken from CT scans is evaluated by making comparison with measurements taken directly from physical sections. The measurements of cortical bone are taken on extant and fossil specimens with and without attached matrix, and the dental specimens studied include a sample of 12 extant human molars. Local CT numbers (representing X-ray attenuation) are used to determine the exact position of the boundaries of a structure. Using this technique most studied dimensions, including four of human molar enamel thickness, could be obtained from CT scans with a maximum error range of +/- 0.1 mm. The limitations of the method are discussed with special reference to problems associated with highly mineralized fossils.

Animals↗

PubMed1993

Enamel thickness of human maxillary molars reconsidered.

Forty-four modern human maxillary molars (M1 = 21, M2 = 12, and M3 = 11) were sectioned through the mesial cusps in a plane perpendicular to the cervical margin of the crown. Eight measurements of enamel thickness as well as bucco-lingual (BL) and mesio-distal (MD) diameters were recorded for each tooth in order to investigate differences in these dimensions between tooth categories. Uni- and multi-variate analyses revealed first maxillary molars to have generally thinner enamel than second or third upper molars, especially with regard to the occlusal basin. Furthermore, the decrease of MD diameters from anterior to posterior is greater than that of BL diameters. Principal Component Analysis using enamel thickness measurements resulted in complete separation of first molars, while second and third maxillary molars showed a certain amount of overlap. This finding casts doubt on using an overall measure of "molar enamel thickness" derived from mixed samples of molars for taxonomic purposes. There appears to be a relationship between bite force and enamel thickness such that posterior molars, where masticatory forces are stronger, have thicker enamel than anterior teeth. It is suggested that the gradient of enamel thickness between (and within) teeth in extant and extinct species may thus provide further information about relative wear resistance as well as the biomechanical constraints of the orofacial skeleton.

Adolescent↗

PubMed1993

Computed tomography and intercuspal angulation of maxillary molars of Plio-Pleistocene hominids from Sterkfontein, Swartkrans and Kromdraai (South Africa): an exploratory study.

The non-destructive technique of Computed Tomography (CT) is used in an exploratory study to investigate variability in angles measured at the crown surface and the dentino-enamel junction of maxillary molars of Plio-Pleistocene hominids from Swartkrans, Sterkfontein and Kromdraai. Discriminant analysis of six angles used together successfully separates 'gracile' and 'robust' australopithecines, although individual angles do not. This study indicates that differences between teeth of Australopithecus africanus and A. robustus are not fully accounted for by gross differences in enamel thickness.

Animals↗

PubMed1992

Reduction of maxillary molars in Homo sapiens sapiens: a different perspective.

Crown and cusp areas, and buccolingual and mesiodistal diameters of maxillary molars of complete upper tooth rows (30 males, 30 females) were analysed in order to quantify changes in size and shape from the first to the third molar. Uni- and multivariate analyses revealed the mesial cusps, in particular the protocone (mesiolingual cusp), to be more stable than the other cusps. Although there is a gradient in size from the first to third molar, shape changes were found to be marked. Overall, the findings are in keeping with the field theory and the hypotheses of environmental constraints on later developing teeth. However, not all of the results could be entirely explained by these concepts. Functional aspects seem to account for the relative stability of the protocone and the buccolingual crown diameter. It appears that this functional complex is relatively stable despite the overall reduction of tooth size, which is probably secondary to processes occurring in the jaws and the cranium. This finding may have implications for studies on tooth reduction between populations of different time periods.

Analysis of Variance↗

PubMed1992

Computed tomography and enamel thickness of maxillary molars of Plio-Pleistocene hominids from Sterkfontein, Swartkrans, and Kromdraai (South Africa): An exploratory study.

This paper is one in a series which explores the possibility of using the non-destructive CT technique to identify patterns in tooth enamel distribution and structure of hominid molars from Plio-Pleistocene sites in South Africa, notably Swartkrans, Sterkfontein, and Kromdraai. Whereas previous investigators have emphasised gross differences in absolute and relative or average enamel thickness between hominid taxa, the present study highlights differences in enamel thickness over functionally significant regions of the crown. Differences in the distribution of enamel in A. robustus, A. africanus, and Homo sp. are identified through the use of bivariate and multivariate analyses, and are interpreted in terms of dietary regimes.

Animals↗

PubMed1992

Relationship between size of distal accessory tubercles and hypocones in permanent maxillary molar crowns of southern Africans.

The relationships between total occlusal crown area and the basal cusp areas of the accessory distal tubercle (C5) and the hypocone were analysed in molars of complete upper tooth rows of a southern African sample. While non-parametric tests between hypocone and C5 sizes did not yield statistically significant results, analyses of metrical data revealed a trend towards compensatory interaction between both cusps. Contrary to previous reports the size of C5 was found to be independent of total crown area. As enamel is thickest on cusp tips there may be a functional reason for the frequency of occurrence of the distal accessory tubercle. The pattern of distribution of the hypocone and C5 within a tooth row may be related to the occlusal wear pattern in Homo, which exhibits a marked lingual slope of wear in anterior molars and a horizontal or even buccal slope in posterior teeth.

Black People↗

PubMed1991

Anthropological evaluation of left-right differences in the femur of southern African populations.

Bilateral asymmetries were tested on a series of anthropological measurements recorded from 566 femora of 161 males and 122 females of southern African populations. Uni- and multivariate statistics revealed that there is a high degree of side differences apparent in the morphological features of the femur, and close to 75% of all femora could be correctly assigned to their respective sides. While most dimensions were greater on the left side, anteroposterior diameter of the proximal shaft, bicondylar width and collo-diaphyseal angle yielded significantly higher mean values on the right side. Anteroposterior diameter of the proximal shaft loaded highest in both canonical discriminant analyses. This result indicates that, overall, the left leg is stronger and is more suited to withstand axial forces and mediolateral bending. Thus, the findings seem to further support the hypothesis that most people, regardless of handedness, use their left leg for weight-bearing, whereas the right leg is more likely to be used for other tasks.

Africa, Southern↗

PubMed1990

Is sexual dimorphism in the femur a "population specific phenomenon"?

The patterns of sexual dimorphism as well as the differences in amount between the populations were studied on a sample of 162 male and 159 female left femora, which were classified as Zulu, Sotho, Xosa and South Africans of European extraction. Multivariate analyses revealed that even adjacent African tribes exhibit a different pattern of sexual dimorphism, but there were similarities between Zulu and European femora. Furthermore, relative size differences, i.e. shape, discriminated more clearly between the sexes than did absolute size. Bicondylar width yielded a statistically significant higher degree of sexual dimorphism in Europeans when compared to African populations. This finding was interpreted in terms of the biomechanical demands on the femur under different living conditions. On the other hand, sexual dimorphism of femoral length did not differ among the populations. This was unexpected since femoral length correlates highly with stature, which was reported to show a lesser degree of sexual dimorphism in Africans than in Europeans. Detailed analyses of the results of the present study led to suggest that different living conditions may affect bones in complex ways of which linear growth is only one aspect.

Africa↗

PubMed1989

Descriptive morphological features of the nose--an assessment of their importance for plastic reconstruction.

On a series of lateral radiographs of 154 males and 199 females from Vienna, Austria, 3 qualitative morphological features of the external nose were recorded. Their relationship to craniometric dimensions, age, and the contour of the nasal bones was tested using Wilks' stepwise discriminant analyses. All tests gave significant results at the 1% probability level, but, overall, correct assignment of the features to their categories did not exceed 61%. Although height dimensions were used for discrimination in females, the prominence of the nasal bones and age appeared to be the most important discriminating variables in both sexes. Analyses further revealed that the nasal bridge and the direction of the nasal septum are highly correlated, whereas the tip of the nose seemed to be independent; this may reflect the dependence of the latter on exogenous influences, especially nutrition. It was demonstrated that the profile line of the external nose does not follow its underlying bony structures, that is, the profile line of the nasal bones. Thus, the present study showed that obtaining soft tissue thicknesses alone is not sufficient for successful facial reconstruction, but that a more holistic approach should be used to elucidate the relationships between soft tissue cover and the underlying hyaline and bony structures.

Adult↗

PubMed1986

An appraisal of plastic reconstruction of the external nose.

The association between the morphometric dimensions of the external nose, including the thickness of the soft tissues, and various craniometric measurements has been examined on a sample of lateral radiographs of 154 males and 199 females from Vienna, Austria. In addition, the influence of age on morphometric dimensions and the soft tissue cover was tested. Multiple stepwise regression analysis revealed that nasal height and nasal length are best predicted by the dimensions of the skull, whereas nasal depth and the thickness of the soft tissues is greatly influenced by age. In males, over 50% of the variance of nasal height and nasal length were accounted for by the height of the bony nose and the prominence of the ossa nasalia, and in females it was about 40%. Although the thickness of the soft tissue is dependent on exogenous factors, such as nutrition, mimicry, and so forth, the dimensions of the bony nose apparently also influence the skin depths to some extent. High and prominent noses were found to have a thinner layer of soft tissue over the nasal bones than short and less prominent ones. It thus seems as if there is a tendency of the soft tissue cover to adjust to disharmonies of the bony profile.

Age Factors↗