Articular Eminence Morphology of American Historic and Contemporary Populations.

OBJECTIVES: This study was performed to determine the values of the articular eminence inclination (AEI), the articular eminence height (AEH), and the length of the articular eminence curved line (AEL) among American prehistoric and contemporary populations.
MATERIALS AND METHODS: The study was carried out on 120 human dry skulls divided into 4 groups: Illinois group (IP), Kentucky group (KP), African Americans group (AAP) and American Caucasians group (ACP). Each group comprised 30 human dry skulls. Measurements of AEI (by two methods), AEH and AEL were performed using computer software on 5 sagittal sections through silicone molds of the articular eminence. The obtained results were statistically analyzed at significance level of p < 0.05.
RESULTS: No statistically significant differences of AEI, AEH and AEL values were obtained among American populations (IP, KP, AAP, ACP) neither by body side, sex and age (p > 0.05). The mean AEI M1 values ranged from 31.56° to 38.72°, the mean AEI M2 values ranged from 44.14° to 58.37°, the mean AEH ranged from 5.82 to 6.86 mm, and the mean AEL values ranged from 11.57 to 12.73 mm.
CONCLUSIONS: AEI, AEH, AEL values are variables, but they did not differ significantly among American populations. Left-right differences as the result of natural body asymmetry were not statistically significant. Sex and age did not significantly affect the articular eminence morphology.

Introduction

The temporomandibular joint (TMJ) as an integrative component of the masticatory system allows a large range of mandibular movements (). The range of mandibular movements is dependent on the articular eminence morphology which is an important TMJ biomechanical element. During mandibular movements, the mandibular condyle and articular disc slide together (condyle-disc complex) along the posterior wall of the articular eminence (-).

The TMJ morphology has been investigated on primates, laboratory animals and humans correlate its morphology with biomechanics, type of consumed food, and masticatory stress (-). The articular eminence morphology undergoes remodeling throughout an individual's life (, ) and its morphology is affected more by function than by skull base characteristics or genetics (). At birth, the articular eminence is completely flat (). The full articular eminence inclination (AEI) value is reached by the age of 30 years (, , ). The AEI values usually vary from 21° to 64° () in relation to the Frankfurt horizontal plane (, ). During the articular eminence development and growth, the articular eminence height (AEH) and the length of articular eminence curved line (AEL) values also increase, reaching the average AEH value of 6 mm, and an average AEL value of 12 mm (). Furthermore, the articular eminence morphology depends on many other factors such as sex and ancestry, changes in dentition, degenerative changes of articular eminence and condyle as well as on morphological changes of the cranial base, which has been documented in the human evolutionary process (-).

The aim of this study was to establish the range of the AEI, AEH and the AEL values of skulls in four population groups from different periods of American history, and to determine differences, if existed, in the articular eminence morphology among these groups. One sample was from prehistoric Native American populations (Woodland Period Illinois (IP) 900 AD to 1500 AD), one sample from Archaic Period Kentucky (KP) (500 BC to 500 AD), and two samples of early 20th century Americans (African Americans (AAP), and American Caucasians (ACP)) from Medical School cadavers at Washington University, St. Louis, Missouri. A null hypothesis was established as follows: there is no statistically significant difference in AEI, AEH and AEL values between IP, KP, AAP and ACP groups.

Materials and methods

The study was carried out on 120 human dry skulls divided into 4 groups: Woodland Period llinois population (IP; from 900 AD to 1500 AD years); Archaic Period Kentucky population (KP; from 500 BC to 500 AD years), African Americans (AAP; 20th century St. Louis, Missouri) and American Caucasian (ACP; 20th century St. Louis, Missouri). Each group comprised 30 human dry skulls. The skulls are curated at the Department of Anthropology, Smithsonian Institution, Washington DC, USA. All selected skulls were fully preserved without observable taphonomic damage in the measured area (articular eminence, fossa articularis, orbitae, and meatus acusticus externus) or/ and without visible pathological conditions in the articular eminence and glenoid fossa.

Sex was identified by biological metric and non-metric traits (pelvic shape, articular joint size, cranioscopic features) on the Illinois and Kentucky archaeological samples, and for the Terry Collection from cadavers at Washington University Medical School anatomy laboratory. The reported age (age at death) was available for AAP and ACP skulls. Because there were only two samples with known age, the skulls were grouped into three age groups: first group – individuals who were 30 years of age or younger; second group - individuals from 31 to 45 years of age; third group – individuals who were 46 years of age or older.

The investigated TMJ parameters were: the articular eminence inclination (AEI), the articular eminence height (AEH) and the length of articular eminence curved line (AEL) in the sagittal plane. The AEI was measured in degrees by two methods. Angle (Figure 1) between the line connecting the most superior point of the glenoid fossa and the most inferior point of the articular eminence with the Frankfurt horizontal plane presented method 1 (M1); while the angle (Figure 2) between the best fitting line to the posterior wall of the articular eminence and the Frankfurt horizontal plane presented method 2 (M2) [Figure 1 and Figure 2 here]. The AEH was measured as vertical distance (mm) between the most superior and the most inferior point of the articular eminence, while the AEL (mm) was measured as length of curved line between the most superior point of the glenoid fossa and the most inferior point of the articular eminence (Figure 3) [Figure 3 here]. The AEI, AEH and AEL were measured on five digitalized sagittally positioned sections through the left and right side silicone impressions of the articular eminence and glenoid fossa. Finally, the measured AEI, AEH and AEL values were presented as means of the values measured on all five sections through silicone impressions of articular eminence and glenoid fossa.

Figure 1

Measurement of articular eminence inclination (AEI) by method M1.

Figure 2

Measurement of articular eminence inclination (AEI) by method M2.

Figure 3

Measurement of articular eminence height (AEH) and length of articular eminence curved line (AEL).

Silicone impressions (Optosil, Heraeus, Hanau, Germany) of left and right side articular eminence were made for each skull. First, the axio-quick face bow (SAM Präzisionstechnik GmbH, Munich, Germany) was mounted on each skull with ear sets inserted into the meatus acusticus externus and the nasion extension placed on nasion anatomical point in order to achieve parallelism of the face bow with the skull’s Frankfurt horizontal plane. In order to achieve a parallelism of the silicone impression base with the Frankfurt horizontal plane, a specially designed device was used. When this device was placed on the face bow, its horizontal plate was parallel to the Frankfurt horizontal plane. After application of the impression material in the area of articular eminence and glenoid fossa, the horizontal plate of specially designed device was slightly pressed over the impression material in order to form a silicone impression base parallel to the Frankfurt horizontal plane. After the silicone impression material was cured, the impressions of the articular eminence and glenoid fossa were removed from the skulls. The silicone impressions were sliced (in anteroposterior direction (sagittal plane)) from lateral to medial side into five sections (between each section was a distance of 4 mm) using common egg slicer (Westmark Gmbh, Lennestadt-Elspe, Germany). First - the most lateral section was made through the most lateral top of the articular eminence (articular tubercle). Each section was placed on the graph paper on the horizontal surface of the camera holder (Hama, Manheim, Germany) and digitalized by Olympus C-770 camera (Olympus, Tokyo, Japan) with a distance of 35 cm. The AEI M1, AEI M2, AEH and AEL were measured using the computer software VistaMetrix (Skillcrest LLC, Tucson, USA).

Data analysis was performed using SPSS 15.0 statistical software (SPSS Inc., Chicago, IL, USA) and the results were obtained with descriptive statistics, the independent-sample Student’s t-test, one way ANOVA, and the Pearson correlation. The results were considered significant at the p < 0.05 level.

Results

Distribution according to sex: the IP group - 12 (40%) males and 18 (60%) females, the KP group - 20 (66.7%) males and 10 (33.3%) females, the AAP group - 15 (50%) males and 15 (50%) females, ACP group - 15 (50%) males and 15 (50%) females.

Age at death was known only for AAP and ACP skulls. The individuals in AAP and ACP groups were divided into 3 age groups: 30 years of age or younger - 10 (33.3%) for the AAP, and 6 (20%) for the ACP skulls; 31 to 45 years of age – 10 (33.3%) for the AAP, and 14 (46.7%) for the ACP skulls; and 46 years of age or older – 10 (33.3%) for AAP skulls, and 10 (33.3%) for the ACP skulls.

Mean AEI M1 values for all five sections through the silicone impressions of articular eminence are presented in Table 1.; mean AEI M2 values for all five sections through the silicone impressions of articular eminence are presented in Table 2.; mean AEH values for all five sections through the silicone impressions of articular eminence are presented in Table 3.; mean AEL values for all five sections through the silicone impressions of articular eminence are presented in Table 4. [Table 1, 2, 3, 4 here]

Table 1

AEI M1 mean values (of all five sections through silicone impressions) for each group according to the body side and sex.

AEI M1(°)		N	Min	Max	X	SD	MEN			WOMEN
							N	X	SD	N	X	SD
Illinois	R	30	21.32	40.28	31.561	4.75	12	32.74	2.79	18	31.18	5.58
	L	30	25.14	42.78	36.621	4.36	12	37.74	3.55	18	35.88	4.77
Kentucky	R	30	23.96	43.52	34.36	4.59	20	33.87	5.12	10	35.35	3.33
	L	30	20.66	44.34	34.54	5.75	20	33.94	6.11	10	35.74	5.04
AfricansAmericans	R	30	22.16	45.36	36.48	5.68	15	37.83	6.43	15	35.13	4.63
	L	30	24.72	45.84	36.42	5.65	15	37.79	5.55	15	35.04	5.60
AmericanCaucasians	R	30	23.96	47.64	36.26	6.42	15	38.68a	5.73	15	33.84a	6.33
	L	30	26.64	49.46	38.72	5.57	15	40.59	5.09	15	36.85	5.56

AEI M1 – articular eminence inclination measured by method 1; N – number of skulls; Min – minimal value; Max – maximal value; X – mean value; SD – standard deviation; R – right; L – left; 1 – statistically significant difference between R and L samples, p < 0.05; a - statistically significant difference according to the sex, p < 0.05.

Table 2

AEI M2 mean values (of all five sections through silicone impressions) for each group according to the body side and sex.

AEI M2(°)		N	Min	Max	X	SD	MEN			WOMEN
							N	X	SD	N	X	SD
Illinois	R	30	30.74	55.60	44.141	6.84	12	43.41	4.50	18	44.63	8.13
	L	30	37.98	63.46	52.671	6.62	12	54.04	5.70	18	51.76	7.18
Kentucky	R	30	31.42	60.16	46.44	7.72	20	44.99	8.17	10	49.36	6.09
	L	30	28.16	68.54	50.25	10.12	20	47.87	10.19	10	55.00	8.56
African Americans	R	30	28.16	68.26	50.24	9.77	15	53.20	10.77	15	47.28	7.94
	L	30	33.60	74.82	52.50	10.29	15	56.38a	10.04	15	48.62a	9.29
American Caucasians	R	30	31.18	73.42	52.85	12.06	15	56.98	11.06	15	48.72	11.93
	L	30	33.44	79.62	58.37	10.37	15	61.11	9.08	15	55.63	11.15

AEI M2 – articular eminence inclination measured by method 2; N – number of skulls; Min – minimal value; Max – maximal value; X – mean value; SD – standard deviation; R – right; L – left; 1 – statistically significant difference between R and L samples, p < 0.05; a - statistically significant difference according to the sex, p < 0.05.

Table 3

AEH mean values (of all five sections through silicone impressions) for each group according to the body side and sex.

AEH(mm)		N	Min	Max	X	SD	MEN			WOMEN
							N	X	SD	N	X	SD
Illinois	R	30	4.10	7.59	5.82	1.06	12	6.24	0.90	18	5.54	1.09
	L	30	4.15	9.62	6.32	1.23	12	6.98a	1.28	18	5.88a	1.01
Kentucky	R	30	4.16	8.01	5.98	0.92	20	6.10	0.94	10	5.75	0.87
	L	30	3.47	7.89	5.85	1.10	20	5.91	1.18	10	5.72	0.99
African Americans	R	30	4.07	8.78	6.72	1.19	15	7.07	1.37	15	6.38	0.89
	L	30	4.17	9.11	6.60	1.21	15	7.00	1.33	15	6.20	0.96
American Caucasians	R	30	4.33	9.44	6.60	1.35	15	7.19b	1.33	15	6.00b	1.13
	L	30	5.26	9.82	6.86	1.21	15	7.43c	1.27	15	6.29c	0.86

AEH – articular eminence height; N – number of skulls; Min – minimal value; Max – maximal value; X – mean value; SD – standard deviation; R – right; L – left; a,b,c - statistically significant difference according to the sex, p < 0.05.

Table 4

AEL mean values (of all five sections through silicone impressions) for each group according to the body side and sex.

AEL(mm)		N	Min	Max	X	SD	MEN			WOMEN
							N	X	SD	N	X	SD
Illinois	R	30	9.96	14.82	11.99	1.44	12	12.59	1.63	18	11.59	1.17
	L	30	9.69	16.18	11.55	1.45	12	12.37a	1.75	18	11.01a	0.91
Kentucky	R	30	9.95	13.54	12.091	0.84	20	12.41b	0.61	10	11.43b	0.89
	L	30	9.44	12.92	11.571	1.03	20	11.80	0.91	10	11.10	1.16
African Americans	R	30	9.89	14.90	12.66	1.30	15	12.93	1.26	15	12.39	1.33
	L	30	10.51	15.26	12.48	1.23	15	13.02c	1.25	15	11.94c	0.97
American Caucasians	R	30	10.05	15.50	12.73	1.29	15	12.96	1.45	15	12.50	1.10
	L	30	10.72	15.02	12.52	1.30	15	12.99d	1.42	15	12.05d	1.00

AEL – articular eminence curved line length; N – number of skulls; Min – minimal value; Max – maximal value; X – mean value; SD – standard deviation; R – right; L – left; 1 – statistically significant difference between R and L samples, p < 0.05; a,b,c,d - statistically significant difference according to the sex, p < 0.05.

Sex differences in the AEI, AEH, and AEL values were observed, but in most of cases were not statistically significant (p > 0.05), values presented in Tables 1-4.

An analysis of the age groups showed no statistically significant differences (p > 0.05) in the AEI, AEH and AEL values for the AAP and ACP populations.

According to the Pearson correlation, the coefficient between AEI M1 and AEI M2; AEI M1 and AEH; AEI M2 and AEH, as well as between AEH and AEL values, all had statistically significant (p < 0.05) and positive correlation (high, with the Pearson correlation coefficient from 0.632 to 0.962) regardless of the population. The correlation between the AEI M1 / AEI M2 and AEL values was positive (low or medium, with the Pearson correlation coefficient from 0.116 to 0.485) but not statistically significant (p > 0.05) regardless of the population.

Discussion

It has been a well-known fact that the shape and size of craniofacial structures including TMJ is affected by both genetic and environmental factors (-, ). In this context, the aim of this study was to determine the AEI, AEH and AEL values on four ancestrally different American populations along with comparison of measured values between these populations. The AEI, AEH and AEL values presented in this study are means of measured values on all five sections through silicone impressions of the articular eminence. The variability of the articular eminence morphology in anteroposterior and mediolateral direction leads to different AEI, AEH and AEL values from point to point. Considering the fact that the five sections (the sections being 4 mm apart across the joint) encompasses the whole mediolateral dimension of the articular eminence, the mean values of measured parameters served as a representative result of the measurements. The populations used in this study originated from different geographical locations and time periods. People from the IP sample lived between 900 AD to 1500 AD years alongside the Mississippi River, whereas people from the KP sample lived in the period from 500 BC to 500 AD years alongside the Green River (). Both populations were usually engaged in fishing as well as in hunting and gathering, but the IP groups were also practicing indigenous agriculture (, ). The AAP and ACP skulls presented an American contemporary population from the Robert J. Terry Anatomical Collection, thus representing the people living in or around St Louis, Missouri in the first half of the 20th century (). Due to the shift from foraging / hunting-gathering with harder and tougher food to food production by cooking, providing softer and less tough food (), it could be assumed that the AEI, AEH and AEL values differ between the prehistoric IP and KP groups, and that there should be a noticeable difference from the prehistoric groups to the contemporary AAP and ACP groups. However, the results obtained by the present study have shown small and statistically non-significant differences in the AEI, AEH and AEL values between the investigated populations. But the range of the AEI, AEH and AEL values was wide. Mean AEI M1 values ranged from 31.56° to 38.72°, and the mean AEI M2 values ranged from 44.14° to 58.37°, regardless of the population group. The AEI values in adults are usually in range from 21° to 64° (), which corresponds to the values in the present study measured by both, the M1 and M2 method. But, it is often difficult to compare the obtained results with the results from other studies because the authors use different methods and materials for studying the TMJ morphology (, , , -). The theory of TMJ change, which is dependent on the type and preparation of the foods in relation with masticatory forces, is not supported by the results obtained in the present study. However, the present study is a scientific contribution to the previously unexplored data of the AEI, AEH and AEL values measured on human skull samples of American prehistoric (IP, KP) and contemporary (AAP and ACP) populations.

The mean AEH values measured in this study were in the range from 5.82 mm to 6.86 mm without any statistically significant differences between prehistoric and contemporary American populations. The mean AEH values from this study are similar to the AEH values measured in other studies although they were not performed on archaeological material (-). There is no data about the AEL values found in the literature, but according to the results obtained in this study, the mean AEH values were in range from 11.55 mm to 12.73 mm without statistically significant differences between American populations. Koppe et al. () measured the sagittal length of temporal articular surface between the postglenoid process and preglenoid plane on human skulls. The sagittal length measured by Koppe et al. () was more extended in the anteroposterior direction than the AEL measured in the present study with consequently higher value, which amounted to as high as 26 mm. The results obtained in the present study also indicate a positive correlation between variables describing the articular eminence morphology. In other words, a higher AEI value follows a higher AEH value, and a higher AEH value results in greater AEL value.

The AEI values were measured by two methods. Both angles (method M1 and M2) represent the measure of AEI () although their definitions, and consequently obtained results, are not the same. The AEI values measured by method M2 were statistically significantly higher than AEI values measured by method M1. Similar results were also reported by Katsavrias () and Chaurasia et al. () with significantly higher AEI values obtained by method M2.

The asymmetry between the left and right side of the face and craniofacial structures (also of the whole body) is a common phenomenon and is usually developed gradually throughout life (). According to the results obtained in this study, the mean AEI, AEH and AEL values differ between the left and the right TMJ, with difference up to 11° for the AEI values, up to 1 mm for the AEH and AEL values, usually without any statistical significance. According to the available literature data (, , , ) and the results of this study, the left-right TMJ differences are a result of natural body asymmetry and possibly different masticatory forces in the left and right TMJ due to the predominant usage of one side during mastication.

The AEI, AEH and AEL values vary between male and female samples with higher values among males but mostly without statistical significance. A systematic literature review has shown that the influence of sex on TMJ morphology is variable. Wu et al. (2012) and Zabarović et al. () found sexual dimorphism with statistically significantly higher AEI values in males than in females. Meng et al. () concluded that there was no statistically significant difference in AEI and AEH values between males and females.

Conclusion

The AEI, AEH and AEL values are variable with a wide range of measured values (especially for AEI), but without statistically significant differences between the IP, KP, AAP and ACP groups. It has been previously reported that lifestyle and type of food (masticatory forces) have significant effects on the articular eminence morphology in TMJ morphological change in human evolution. However, the results of the present study neither support the reported conclusions nor confirm the micro evolutionary changes in the TMJ due to the changes in food types or food preparation over the span of the 2,500 years between these groups. This study also found that the body side and sex did not significantly affect the AEI, AEH and AEL values.