Material and methods

Female subjects were recruited from patients who consented to bilateral MRI of their TMJs. All subjects had a primary complaint of malocclusion, and routine lateral cephalograms were taken in natural head position with an Asahi CX-90SP II (Asahi Roentgen, Kyoto, Japan). Natural head position was determined by having the subjects look straight into a mirror in a standing position. A chain plumb line was suspended in front of the cassette to indicate a true vertical line. The MRI images were taken to evaluate TMJ status mainly because of TMJ symptoms including TMJ sounds, pain, masticatory muscle tenderness, limited mandibular movement, and locking. Exclusion criteria were (1) age less than 17 years, (2) any systemic disease, (3) history of orthodontic treatment, (4) history of facial cosmetic or orthognathic surgery, (5) history of trauma involving the TMJ, (6) juvenile rheumatoid arthritis, (7) history of TMJ treatment, (8) airway obstruction, (9) oral habits, (10) TMJ disc displacement of a greater severity on the unilateral side, and (11) partial TMJ disc displacement or TMJ disc displacement with partial reduction. This research protocol was approved by the institutional review board of the Seoul National University Dental Hospital (CRI11040).

Radiologists with MRI experience with the TMJ interpreted the images blinded to the clinical information. According to disc position, TMJ disc status was divided into 3 categories as follows.

• 1

  Normal disc position. In the closed-mouth position, the intermediate zone of the disc was interposed between the condyle and the posterior slope of the articular eminence, with the anterior and posterior bands equally spaced on either side of the condylar load point.

• 2

  Disc displacement with reduction. The disc was anteriorly displaced relative to the posterior slope of the articular eminence and the head of the condyle in the closed-mouth position, but the disc was reduced on mouth opening.

• 3

  Disc displacement without reduction. The disc was anteriorly displaced relative to the posterior slope of the articular eminence and the head of the condyle, and the disc was not reduced on mouth opening.

The position and shape of the articular disc of the TMJ were carefully evaluated according to the classification criteria. We excluded patients with a unilaterally different disc displacement status because the possible skeletal morphologies associated with unilateral disc displacement would be obscured by averaging of the right and left landmarks used to determine their location, and unilaterally different disc displacement status may asymmetrically influence craniocervical posture or hyoid bone position, which is difficult to measure in lateral cephalometric analysis. From the originally selected patients, only those with bilateral normal disc status (BN), bilateral disc displacement with reduction (DDR), and bilateral disc displacement without reduction (DDNR) were included in this study.

One investigator (S-J.A.), who was blinded to the clinical information and the disc position, traced all lateral cephalograms. Eighteen landmarks were recorded on each radiograph using a digitizer with a desktop computer, and 25 variables were calculated from these landmarks: 9 variables for craniocervical posture, 7 for hyoid bone position, and 9 for craniofacial morphology (4 for vertical and 5 for sagittal craniofacial morphologies). The positions and definitions of the landmarks are shown in Figure 1 , and the locations of the reference planes are shown in Figure 2 . Measurements for craniocervical posture, hyoid bone position, and craniofacial morphology are shown in Figures 3, 4 , and 5 , respectively.

Landmarks used in this study: 1 , nasion; 2 , sella; 3 , orbitale; 4 , porion; 5 , basion; 6 , anterior nasal spine; 7 , posterior nasal spine; 8 , Point A; 9 , Point B; 10 , pogonion; 11 , menton; 12 , gonion; 13 , RGn (most protrusive point of retrognathion); 14 , hyoidale (Hy, most superior and anterior point on the body of the hyoid bone); 15 , cv2tg (tangent point of the superoposterior extremity of the second cervical vertebra); 16 , cv2ip (most posteroinferior point on the second cervical vertebra); 17 , cv3ia (most anteroinferior point on the third cervical vertebra); 18 , cv4ip (most posteroinferior point on the fourth cervical vertebra).

Craniocervical reference planes used in this study: 1 , nasion-sella line (NSL, plane through nasion and sella); 2 , true horizontal plane (HOR, true horizontal plane passing through sella); 3 , Frankfort horizontal plane (FH, plane through porion and orbitale); 4 , nasal line (NL, line through the posterior nasal spine and anterior nasal spine); 5 , mandibular plane (MP, line through gonion and menton); 6 , cervical vertebrae tangent (CVT, line through cv2tg and cv4ip); 7 , odontoid process tangent (OPT, line through cv2tg and cv2ip).

Variables of craniocervical posture (all are angular measurements): 1 , true horizontal plane to cervical vertebrae tangent angle (HOR/CVT); 2 , Frankfort horizontal plane to cervical vertebrae tangent angle (FH/CVT); 3 , nasal line to cervical vertebrae tangent angle (NL/CVT); 4 , mandibular plane to cervical vertebrae tangent angle (MP/CVT); 5 , true horizontal plane to odontoid process tangent angle (HOR/OPT); 6 , Frankfort horizontal plane to odontoid process tangent angle (FH/OPT); 7 , nasal line to odontoid process tangent angle (NL/OPT); 8 , mandibular plane to odontoid process tangent angle (MP/OPT); 9 , the cervical curvature, downward-opening angle between odontoid process tangent and cervical vertebrae tangent (OPT/CVT, positive when the cv4ip is located on the left side of odontoid process tangent).

Variables of the hyoid bone position (all are linear measurements except for Go/Hy/Me): 1 , linear distance between the hyoidale and basion (Hy-Ba); 2 , perpendicular distance between the hyoidale to nasion-sella line (Hy to NSL); 3 , perpendicular distance between the hyoidale to nasal line (Hy to NL); 4 , linear distance between the hyoidale and RGn (Hy-RGn); 5 , linear distance between the hyoidale and cv3ia (Hy-cv3ia); 6 , perpendicular distance between the hyoidale and cv3ia-RGn plane (Hy to cv3ia-RGn, positive when the Hy is located below the cv3ip-RGn plane); 7 , hyoid angle, angle of Go-Hy-Me (Go/Hy/Me, the angle is larger when the hyoidale is located above the mandibular plane).

Variables of craniofacial morphology: 1 , Frankfort horizontal plane to mandibular plane angle (FMA); 2 , anterior facial height (AFH, linear distance between nasion and menton); 3 , posterior facial height (PFH, linear distance between sella and gonion); 4 , ANB angle; 5 , SNA angle; 6 , SNB angle; 7 , N perpendicular to Point A (ANP); 8 , N perpendicular to pogonion (PNP); 9 , facial height ratio (FHR, ratio of posterior facial height [ 3 ] to anterior facial height [ 2 ]).

Lateral cephalograms of 20 randomly selected subjects were measured again to test the magnitude of measurement errors. The intraclass correlation coefficients for the reliability of tracing, landmark identification, and analytic measurements were greater than 0.98.

Descriptive statistics were calculated for all variables. The differences in the cephalometric variables for craniocervical posture, hyoid bone position, and craniofacial morphology with respect to the TMJ disc displacement status (BN, DDR, and DDNR) were tested with 1-way analysis of variance. Scheffé multiple comparisons were performed at a significance level of 0.05 to analyze between-group relationships. To investigate the correlations between craniofacial morphology and craniocervical posture or hyoid bone position, Pearson correlation coefficients were calculated.

Results

A total of 170 female subjects were included in this study ( Table I ). Their age range was 17.0 to 50.8 years (mean age, 24.5 ± 5.7 years). There were no significant differences in age distribution among the 3 study groups (data not shown).

Table II presents the differences in craniocervical posture, hyoid bone position, and craniofacial morphology with respect to TMJ disc displacement status (BN, DDR, and DDNR). Significant differences were found in craniocervical posture between the BN and DDNR groups ( Table II ). Subjects with DDNR had larger angles between the craniofacial reference planes and the cervical vertebrae (FH/CVT, NL/CVT, FH/OPT, and NL/OPT) than did the subjects with BN, indicating that subjects with DDNR had extended craniocervical posture compared with those with BN. Although the subjects with DDR demonstrated intermediate values, there was no significant difference in craniocervical posture between the BN and DDR groups, or between the DDR and DDNR groups. Angles between the cervical vertebrae and the true horizontal plane (HOR/CVT and HOR/OPT) or the mandibular plane (MP/CVT and MP/OPT) were not significantly different among the 3 groups. Cervical curvature (OPT/CVT) also did not vary significantly among the different TMJ disc displacement groups.

Among the variables for hyoid bone position, only measurements related to the mandible were significantly influenced by TMJ disc displacement status. Subjects with DDNR had a decreased hyoid angle (Go/Hy/Me) compared with those with BN or DDR (BN = DDR > DDNR). In addition, the hyoidale to the most protrusive point of retrognathion distance (Hy-RGn) decreased as TMJ disc displacement status increased in severity from BN to DDNR (BN > DDR > DDNR). However, distances between craniocervical landmarks or reference planes and the hyoid bone (Hy-Ba, Hy to NSL, Hy to NL, Hy-cv3ia, and Hy to cv3ia-RGn) did not show significant differences according to TMJ disc displacement status ( Table II ).

As previously reported, subjects with TMJ disc displacement have a retrognathic mandible with a hyperdivergent skeletal pattern ( Table II ). Our study showed that increased ANB, and decreased SNB and N perpendicular to pogonion (PNP), are specific sagittal craniofacial morphologies in subjects with TMJ disc displacement. In addition, these skeletal characteristics became more severe as TMJ disc displacement progressed from BN to DDNR. However, variables representing maxillary position (SNA and N perpendicular to point A [ANP]) were not significantly different among the 3 groups. Subjects with DDNR had a hyperdivergent skeletal pattern: eg, increased Frankfort-mandibular plane angle (FMA), decreased posterior facial height (PFH), and decreased facial height ratio (FHR) compared with those with BN or DDR. In contrast to sagittal craniofacial morphology, vertical craniofacial morphology did not vary significantly between the BN and DDR groups ( Table II ).

Correlations between craniofacial morphology and craniocervical posture or hyoid bone position are presented in Table III . Generally, craniocervical posture (FH/CVT, NL/CVT, FH/OPT, and NL/OPT) was significantly correlated with variables representing sagittal (ANB, SNB, and PNP) and vertical (FMA and FHR) craniofacial morphologies, and subjects with extended craniocervical posture had a retrognathic mandible with a hyperdivergent skeletal pattern. However, cervical curvature (OPT/CVT) was not significantly correlated with craniofacial morphology.

Table III

Correlations between craniofacial morphology and craniocervical posture or hyoid bone position

Variable	Correlation
	FMA	FHR	AFH	PFH	ANB	SNA	SNB	ANP	PNP
Craniocervical posture
HOR/CVT (°)	0.241 †	NS	0.196 ∗	NS	0.240 †	−0.169*	−0.332 †	−0.153 ∗	−0.334 †
FH/CVT (°)	0.381 †	−0.247 †	0.270 †	NS	0.399 †	−0.205 †	−0.499 †	−0.316 †	−0.591 †
NL/CVT (°)	0.256 †	−0.192 ∗	0.262 †	NS	0.315 †	−0.248 †	−0.454 †	−0.193 ∗	−0.416 †
MP/CVT (°)	−0.547 †	0.556 †	NS	0.512 †	NS	NS	NS	NS	NS
HOR/OPT (°)	0.258 †	−0.154 ∗	0.153 ∗	NS	0.242 †	−0.168 ∗	−0.333 †	NS	−0.325 †
FH/OPT (°)	0.396 †	−0.256 †	0.234 †	NS	0.399 †	−0.207 †	−0.501 †	−0.290 †	−0.578 †
NL/OPT (°)	0.283 †	−0.208 †	0.231 †	NS	0.326 †	−0.253 †	−0.467 †	−0.178 ∗	−0.422 †
MP/OPT (°)	−0.498 †	0.519 †	NS	0.468 †	NS	NS	NS	NS	NS
OPT/CVT (°)	NS	NS	NS	NS	NS	NS	NS	NS	NS
Hyoid bone position
Hy-Ba (mm)	NS	0.152 ∗	0.247 †	0.281 †	−0.162 ∗	0.162 ∗	0.257 †	NS	0.174 ∗
Hy to NSL (mm)	NS	NS	0.341 †	0.289 †	NS	0.281 †	NS	NS	NS
Hy to NL (mm)	0.237 †	NS	0.247 †	NS	0.236 †	NS	−0.161 ∗	NS	−0.267 †
Hy-RGN (mm)	−0.519 †	0.388 †	NS	0.396 †	−0.584 †	NS	0.421 †	NS	0.562 †
Hy-cv3ia (mm)	−0.181 ∗	0.191 ∗	0.165 ∗	0.278 †	NS	NS	NS	NS	NS
Hy to cv3ia-RGn (mm)	NS	NS	NS	NS	0.174 ∗	NS	NS	NS	NS
Go/Hy/Me (°)	−0.385 †	0.358 †	0.168 ∗	0.447 †	−0.324 †	NS	0.339 †	0.151 ∗	0.367 †

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