MRI-based observation of the size and morphology of temporomandibular joint articular disc and condyle in young asymptomatic adults

Abstract

Objective:

To investigate the size and morphology of the temporomandibular joint (TMJ) articular disc and condyle in young asymptomatic adults by using magnetic resonance imaging (MRI) and to provide a reference for clinical diagnosis and scientific study of temporomandibular disorders (TMD).

Methods:

Overall, 93 undergraduate volunteers without TMD were enrolled from the freshmen pool at the Qingdao University. All participants underwent MRI of the oblique sagittal and oblique coronal TMJ planes. The articular disc and condyle were subsequently measured, and their morphology was evaluated. The obtained data were then grouped and analyzed statistically. Finally, intragroup correlation coefficient (ICC) was used to evaluate the interobserver measurement reliability.

Results:

We totally received 186 TMJ imaging samples. Based on our analysis, disc’s anterior band in young asymptomatic adult females were thicker than males of the same age (p = 0.024). Moreover, the media-lateral dimensions of the condylar head of adult females were shorter than males of equal age (P＜0.001). The bilateral articular disc morphology was the same in 72.4% of subjects while the condylar morphology was the same in 63.4% of participants. Finally, using measurement reliability assessment, we demonstrated that our conclusions are reliable (ICC ≥0.7).

Conclusion:

The thickness of the anterior band of the disc and the media-lateral dimensions of the condylar head were gender-related. Additionally, the morphology of the bilateral articular disc and condyle was different among the subset of young asymptomatic adults.

Introduction

Temporomandibular joint (TMJ) is the only movable joint in the maxillofacial area, and it is composed of the temporal bone articular surface above, the condylar process below, the articular disc between, and the articular capsule and articular ligaments around it. Temporomandibular disorders (TMDs) are among the most common diseases in the oral and maxillofacial region.¹ Intriguingly, TMDs have a high prevalence among young and middle-aged females, particularly, those between the ages of 20 and 30 years.² Previous studies revealed that the size and morphology of the articular disc and condyle strongly related to TMD onset.^2,3 In particular, the size and morphology of the articular disc strictly correlate with the anterior displacement of the articular disc,⁴ whereas the size and morphology of the condyle significantly associate with osteoarthritic onset.⁵ Therefore, it is of great significance to clarify the normal structure of TMJ in young asymptomatic adults so TMD diagnosis can be made earlier and with more accuracy.

In the past, multiple TMJ studies were conducted via autopsy. However, this brings forth numerous limitations like primarily old age, inability to accurately determine TMD signs and symptoms, and alteration of the original TMJ structure via embalming. In recent years, various imaging methods are used to examine the TMJ, such as cone-beam CT (CBCT), computed tomography (CT), and magnetic resonance imaging (MRI), among which MRI is the most optimal approach due to its high resolution of soft tissue and cartilage.^6–8 Additionally, MRI has advantages of being a relatively low risk and non-ionizing radiation procedure.

In the current study, we employed MRI to closely examine the size and morphology of the TMJ articular disc and condylar, in young asymptomatic adults, to provide a reference for clinical diagnosis and scientific study of TMD.

Methods and materials

Patient selection

We initially selected 100 volunteers from the freshmen pool at the Qingdao University. The inclusion criteria for our analysis were as follows: (1) >18 years old; (2) No history of TMD and TMJ injury, and no symptoms or signs of TMD, ascertained by an experienced clinical specialist; (3) The upper row of teeth occluded perfectly with the lower row of teeth, without any history of orthodontic treatment; (4) No history of rheumatism or rheumatoid arthritis, and no history of bone and connective tissue diseases; (5) No history of prolonged widening of the jaws, such as in dental treatment, singing, etc; and (6) No contraindication to MRI, such as metal implants in vivo or claustrophobia. This study was reviewed and approved by the Ethics Committee of the Affiliated Hospital of Qingdao University, and all the volunteers provided us with signed informed consent.

2. MRI examination

MRI examination and analysis were performed using the Siemens’ Magnetom Prisma v.3.0 MR MRI system (Siemens, Erlangen, Germany), where 60 channel head coils were used, instead of the traditional surface coils, to obtain the overall spatial information of TMJ muscle and soft tissue structure. All volunteers were placed supine, and their heads were scanned. The positioning line was aligned to the external ear canal. To capture axial images, the TMJ oblique sagittal and oblique coronal planes were imaged at the closed oral position. Next, the oblique sagittal images were adjusted perpendicular to the long condyle head axis (mediolateral axis), while the coronal plane was adjusted parallel to the long condyle head axis. TMJ oblique sagittal proton density-weighted (OSAG-PDW-FSE) and oblique coronal T₁-weighted images (OCOR-T1WI-FSE) were subsequently obtained.

OSAG-PDW-FSE used parallel acquisition technology, with excitation times of three, TR 2070ms, TE 28 ms, FOV 120 × 120 mm, matrix 192 × 144, and plane resolution of 0.6 × 0.6. Alternately, OCOR-T1WI-FSE used parallel acquisition technology, with excitation times of three, TR 550 ms, TE 6.8 ms, FOV 120× 120 mm, matrix 192 × 144, and plane resolution of 0.6 × 0.6. A total of 16 images were captured for each of the two scanning sequences. Each image layer had a thickness of 2 mm and the interval between layers was 10%.

Image observation

Three images were chosen from the inner slices, with the largest sectional condyle area, for further measurement and the average value was obtained. These images were independently observed by three dental imaging specialists. Any obscure MRIs, due to volunteer movement, were eliminated from analysis. Intragroup correlation coefficients (ICCs) were used to evaluate the reliability of the three observers.

Articular disc and condylar sizes were measured using the Centricity Dicom Viewer software (accuracy 0.01 mm) (GE Healthcare, Chicago, USA). The articular disc thickness Ta, TB, TC⁹ (Figure 1a) and length AB + BC¹⁰ (Figure 1b), as well as anteroposterior (Figure 1c) and media-lateral dimensions (Figure 1d) of the condylar head were measured. Of note, whenever L1 and L2 were more than 30° oblique from the horizontal plane, the horizontal distance between the tangents were measured to denote L1 and L2.¹¹

Figure 1.

The size of the articular disc and condyle. (a): The thickness of anterior band (Ta), intermediate zone (Tb), and posterior band (Tc) of articular disc (Ta, Tb, and Tc represent the lines connecting the edge tangent lines of the anterior band, intermediate zone and posterior band). (b): length of the articular disc (AB + BC) (AC is the leading edge of the anterior band and posterior edge of the posterior band of articular disc, and B is the midpoint of Tc. If anterior and posterior band cannot be accurately distinguished, AC was measured). (c): The anteroposterior dimensions of the condylar head (L1) (y1 and y2 represent the vertical lines passing through the sides of the condyle, and L1 represents the line connecting the intersection of the two perpendicular lines and the condyle). (d): The medio-lateral dimensions of the condylar head (L2) (y1y2 represents the vertical line passing through both sides of the condyle, L2 represents the line connecting the intersection point of the two perpendicular lines and the condyle).

Evaluation of the articular disc and condylar morphology in the oblique sagittal plane: The articular disc morphology was divided into six categories: biconcave, thickening posterior, thickening anterior, biplanar, biconvex, and folded (Figure 2).^12–14 Similarly, the condyle morphology was divided into three categories, namely, round, flattened, and beak-like (Figure 3).^15,16

Figure 2.

Morphology of the articular disc. (a) Biconcave; (b) Thickening posterior; (c) Thickening anterior; (d) Biplanar; (e) Biconvex; (f) Folded.

Figure 3.

Morphology of the condyle. (a) Round; (b) Flattened; (c) Beak-like.

Statistical analysis

IBM SPSS Statistics for Windows (v.20.0; IBM, Armonk, NY) was employed for all data analyses. Quantitative data are expressed as mean ± standard deviation (mean ± SD), while qualitative data are expressed as frequency and percentage. One-way analysis of variance and t test were used to compare differences between indicated groups. Finally, P＜0.05 was considered statistically significant.

Results

MRIs from seven volunteers were blurred due to movement during shooting. Hence, they were eliminated from the final analysis. We ultimately analyzed a total of 186 joints of 93 volunteers (36 males and 57 females), ranging in age from 18.1 to 21.3 years for males (mean 19.3 years) and 18.1 to 22.2 years for females (mean 19.4 years).

Thickness and length of the articular disc

The articular disc thicknesses of the anterior band (Ta), intermediate zone (Tb), and posterior band (Tc) were 2.25 ± 0.45 mm, 1.02 ± 0.25 mm, and 2.15 ± 0.64 mm, respectively. Moreover, the length of the articular disc was 9.15 ± 1.10 mm (Table 1). The anterior band of the disc in female young asymptomatic adults (2.31 ± 0.44 mm) were thicker than that of male adults of the same age (2.15 ± 0.44 mm) (p = 0.024).

Table 1.

The measures analyzed included the thickness and length of articular disc, the anterior-posterior and media-lateral dimensions of the condylar head. The thickness of anterior band disc (P＜0.05) and the media-lateral dimensions of the condylar head (P＜0.01) were gender-related.

	Female（n = 114）				Male (n = 72)				Total (n = 186)
	Min	Max	Mean	SD	Min	Max	Mean	SD	Min	Max	Mean	SD
Articular disc
Anterior band (Ta)	0.93	3.18	2.31	0.44	1.13	3.29	2.15	0.44	0.93	3.29	2.25	0.45
Intermediate zone (Tb)	0.57	1.86	1.05	1.24	0.57	1.73	0.98	0.25	0.57	1.86	1.02	0.25
Posterior band (Tc)	0.87	3.00	2.19	0.65	0.86	3.31	2.09	0.64	0.86	3.31	2.15	0.64
Articular disc length (AB + BC)	5.61	11.99	9.07	1.03	5.87	13.92	9.27	1.20	5.61	13.92	9.15	1.10
Condyle
Antero posterior dimensions (L1)	6.10	10.78	8.47	0.85	6.16	12.23	8.52	1.29	6.10	12.23	8.49	1.04
Medio-lateral dimensions (L2)	14.02	22.17	18.4	1.49	15.58	22.77	19.66	1.41	14.02	22.77	18.89	1.58

The anteroposterior and media-lateral dimensions of the condylar head

The anteroposterior and media-lateral dimensions of the condylar head measured in the young asymptomatic adults were 8.49 ± 1.04 mm and 18.89 ± 1.58 mm, respectively (Table 1). The media-lateral dimensions of the condylar head showed significant gender correlation (P＜0.001).

Morphology of the articular disc and condyle

Most of the articular discs morphology in young asymptomatic adults were biconcave (61.8%), followed by thickening posterior (15.6%), thickening anterior (15.1%), and biplanar (7.5%) (Figure 4), and there were no biconvex or folded discs. Among them, the proportion of biconcave articular disc morphology was less in females (58.7%) than males (66.7%). The proportion of the remaining three forms was larger in females than males, especially the thickening posterior type (female 17.6% and male 12.5%). Moreover, there were 67 young asymptomatic adults (72.4%), who had the same articular disc morphology on bilateral TMJ (Table 2).

Figure 4.

Schematic diagram of articular disc morphology and distribution rates. (a) Biconcave; (b) Thickening posterior; (c) Thickening anterior; (d) Biplanar; (e) Proportion of the articular disc morphology.

Table 2.

Morphology of bilateral articular disc in young asymptomatic adults

Disc morphology	Biconcave	Thickening posterior	Thickening anterior	Biplanar	Biconvex	Folded
Biconcave	53 (56.99%)	6 (6.45%)	12 (12.9%)	1 (1.07%)	0	0
Thickening posterior		6 (6.45%)	0	1 (1.07%)	0	0
Thickening anterior			5 (5.38%)	6 (6.45%)	0	0
Biplanar				3 (3.23%)	0	0
Biconvex					0	0
Folded						0

Most of the condylar morphology in young asymptomatic adults were round (59.1%), while others were flattened (26.9%) and beak-like (14.0%) (Figure 5). The proportion of round condyle in females (57.9%) was smaller than that of males (61.1%), and the proportion of flattened condyle in females (28.1%) was larger in females than in males (25.0%). Furthermore, 59 young asymptomatic adults (63.44%) had the same condylar morphology on bilateral TMJ (Table 3).

Figure 5.

Schematic diagram of the condylar morphology and distribution rates. (a) Round; (b) Flattened; (c) Beak-like; (d) Proportion of the condylar morphology.

Table 3.

Morphology of bilateral condyle in young asymptomatic adults

Condylar morphology	Round	Flattened	Beak-like
Round	42 (45.16%)	21 (22.58%)	6 (6.45%)
Flattened		11 (11.83%)	7 (7.53%)
Beak–like			6 (6.45%)

Interobserver reliability assessment

According to the benchmarks described by Fleiss,¹⁷ the interobserver reliability assessment of this study was excellent and in accordance with previous reports. Similar to prior publications, we reported that the anteroposterior and media-lateral dimensions of the condylar head (ICC_average = 0.8), as well as the thickness and length of the articular disc (ICC_average = 0.7) were significantly different between males and females.

Discussion

In the current study, the TMJ structure was determined by measuring the size and morphology of the articular disc and condylar in young asymptomatic adults. Multiple reports suggested that TMD incidence is significantly related to structural changes within TMJ. Hence, the results of this study are of great clinical significance and may assist in TMD diagnosis.

Yang et al¹⁷ identified that the thickness of the anterior band and the intermediate zone of the articular disc in TMD patients are significantly thicker than in normal controls. This data suggests that TMD induces adaptive changes in the thickness of the anterior band of the disc. In this study, we demonstrated that the thickness of the anterior band of the disc is significantly correlated with gender. Moreover, the thickness of the female disc is significantly greater than in males. Cai et al¹⁰ demonstrated that the articular disc was forward shifted and shortened, with limited mouth opening ability, in TMD patients with articular disc displacement. The present study revealed that the articular discs in female asymptomatic adults are shorter than in males, perhaps due to anatomical developmental difference. However, it is necessary to investigate whether the thicker anterior band and shorter articular disc in females contribute to the higher TMD incidence in this gender.

Elisabetta et al¹⁸ observed that the anteroposterior and media-lateral dimensions of the condylar head were related to the positioning of articular disc. Several scholars^11,19,20 found that the condyles in the so-called articular disc displacement are dramatically smaller. In this study, the media-lateral dimensions of the condylar heads of female asymptomatic adults were significantly shorter than in males. It is yet unknown whether the smaller condyle of female asymptomatic adults puts them at a greater risk for articular disc displacement, relative to males, or whether the smaller condyle of female asymptomatic adults occur in response to ontogenetic factors.

Severe articular disc deformity is markedly associated with TMD occurrence, and the thickening of the posterior band disc is one of the characteristics of early and middle Anterior Disc Displacement with Reduction (ADDWR).²¹ In a retrospective study, Hu et al¹² revealed that the main morphological changes of the articular disc in ADDWR patients were posterior band thickening or mild folding, among which the V-folding morphology was more likely to cause displacement of the articular disc. The results of this study showed that the majority of the articular disc of asymptomatic adults were biconcave (61.8%), and no folded and biconvex morphology were present. In addition, the percentages of the articular disc morphology related with thickening posterior, thickening anterior, and biplanar were higher in females than in males. Combined with the results of the Taskaya-Yilmaz²¹ and Hu’s¹² studies, it is suggested that the probability of the anterior disc displacement in female asymptomatic adults is higher than in male asymptomatic adults, and it is correlated with the articular disc morphology.

As condylar morphology is closely related to mandibular movement and masticatory muscular function, it is suggested that the condylar morphology may be different in TMD patients,²² and the flattened and beak-like condylar morphology are significantly correlated with osteoarthropathy occurrence.^15,16 In this study, the condylar morphology of most asymptomatic adults were round (59.1%), whereas the flattened and beak-like condylar morphology accounted for 40.9% of analyzed volunteers. The percentage of flattened condyle in female asymptomatic adults (28.1%) was higher than that in male asymptomatic adults (25.0%).

Ultimately, this study demonstrated that the bilateral articular disc and condyle morphologies were not identical in young asymptomatic adults. The morphology of the bilateral articular disc was same in 72.4% of young asymptomatic adults, and the morphology of the bilateral condyle was same in 63.44% of young asymptomatic adults. So, the question remains, what is the reason for the discrepancy of the bilateral articular disc and condyle? Is it physiological or pathological? Is it related to TMD occurrence? These important questions would need to be further explored in the near future.

Conclusion

This study determined the size and morphology of the articular disc and condylar in young asymptomatic adults. The thickness of the anterior band of the disc and the media-lateral dimensions of the condylar head were gender-related, and the morphology of the bilateral articular disc and condyle in young asymptomatic adults was varied.