Morphologic, functional, and occlusal
characterization of mandibular lateral
displacement malocclusion
Kyoko Ishizaki,a Koichi Suzuki,a Tomofumi Mito,a Eliana Midori Tanaka,b and Sadao Satoc
Yokosuka, Japan, and Bogota´, Colombia
Introduction: Mandibular lateral displacement (MLD) is clinically characterized by deviation of the chin, facial
asymmetry, dental midline discrepancy, crossbite in the posterior region, and high prevalence of internal
derangement of the temporomandibular joint. Morphologic and functional characteristics of MLD should be
clarified to correct and prevent this malocclusion. Methods: We examined the morphologic features, occlusal
scheme, and functional behavior of MLD in 116 patients. Facial morphology was examined with posteroanterior
cephalograms, occlusion guidance on the articulator after face-bow transfer, and condylar movement with
the condylograph. Results: The superiorly inclined occlusal plane was associated with mandibular deviation in
the same direction. The posterior occlusal plane on the shifted side was significantly steeper than that on the
nonshifted side. Functional analysis of condylar movement showed a close relationship between the direction
of MLD and the direction of condylar lateral shift during opening and closing, and protrusion and retrusion. The
occlusal guidance inclination in the buccal segment of the nonshifted side was steeper than that in the shifted
side. Conclusions: The results suggested that reduced vertical height of the dentition on 1 side induced
mandibular lateral adaptation with contralateral condylar shift (asymmetry); this leads to condylar lateral shift
during functional movement. (Am J Orthod Dentofacial Orthop 2010;137:454.e1-454.e9)
aPostgraduate research fellow, Department of Craniofacial Growth and
Development Dentistry, Division of Orthodontics, Kanagawa Dental College,
Yokosuka, Japan.
bProfessor, Department of Orthodontics, Universidad Militar Nueva Granada,
Fundacio´n C.I.E.O., Bogota´, Colombia; visiting researcher, Department of
Craniofacial Growth and Development Dentistry, Division of Orthodontics,
Kanagawa Dental College, Yokosuka, Japan.
cProfessor, chairman, and research associate, Department of Craniofacial
Growth and Development Dentistry, Research Institute of Occlusion Medicine,
Research Center of Brain and Oral Science, Division of Orthodontics,
Kanagawa Dental College, Yokosuka, Japan.
Supported by grants-in-aid for open research from the Japanese Ministry of
Education, Culture, Sports, Science and Technology.
The authors report no commercial, proprietary, or financial interest in the
products or companies described in this article.
Reprint requests to: Eliana Midori Tanaka, Department of Craniofacial Growth
and Development Dentistry, Division of Orthodontics, Research Institute of OcclusionMedicine,
Research Center of Brain and Oral Science, Kanagawa Dental
College, 82 Inaoka-Cho, Yokosuka, Kanagawa 238-8580, Japan; e-mail, satos@
kdcnet.ac.jp.
Submitted, April 2009; revised and accepted, October 2009.
0889-5406/$36.00
Copyright 2010 by the American Association of Orthodontists.
doi:10.1016/j.ajodo.2009.10.031
Mandibular lateral displacement (MLD) is relatively common in patients with malocclusion. The occurrence of MLD is of particular interest, since most types of malocclusion involve some facial asymmetry. MLD is characterized by mandibular deviation to 1 side evidenced by deviation of the chin from the facial midline, crossbite in the posterior region, and dental and skeletal midline discrepancies.1-4 It was also reported that MLD patients with facial symmetry have internal derangement of the temporomandibular joint (TMJ).5-7 For the clinicians, MLD is a challenging anomaly and can sometimes be compromised in its results, because it is difficult to treat orthodontically, even with orthognathic surgery because of the asymmetry of the skeletal frame. This might be attributed to a lack of understanding of the morphologic and functional characteristics of this malocclusion. In recent craniofacial biology research, the general consensus seems to be that the adaptation of skeletal and dentoalveolar elements of the face after functional
displacement of the mandible leads to reestablishment of the structural and functional balance of the orofacial region.8 It was been suggested that the dentofacial complex is obviously adaptable to the functional demand in occlusal configuration and the change of occlusal function in growing facial bones.9,10 It was also pointed out by Petrovic and Stutzman11 that the cant of the maxillary occlusal plane is an important factor contributing
to mandibular positioning.
Aswereported earlier, the cant of the occlusal plane is closely associated with various dentoskeletal frames during craniofacial growth and development.12-15 It was speculated that horizontalization or flattening of the maxillary occlusal plane induces functional forward adaptation of the mandible followed by active transformation of the TMJs. Our previous investigation3,5 indicated that the cant of the frontal occlusal plane (OP) in MLD malocclusion tilted superiorly to the side to which the mandible was shifted, often with TMJ symptoms on the side with the shifted condyle. In this
context, it must be considered that the vertical and transverse positions of the mandible associated with the vertical height of the posterior teeth (posterior vertical dimension) are important to understand MLD malocclusion, although the interrelationship among occlusal plane deviation, MLD, and functional disturbances still remains to be elucidated. These previous findings led us to reevaluate MLD patients to consider a therapeutic functional approach based on occlusal plane control by reestablishing the appropriate occlusal vertical dimension on the affected side; this is an effective treatment for the correction of the facial asymmetry due to MLD, with consequent improvement of occlusal and articular functions. In this study, the morphologic, occlusal, and functional characteristics and orthodontic approach to MLD malocclusion are discussed.
Fig 1. Measurements on posteroanterior cephalograms
in MLD subjects: OP, frontal occlusal plane; AG, frontal
mandibular plane; MA, mastoid plane; CI, condylar inclination;
ZP, zygomatic plane; Z, point at lateral border of
center of zygomatic arch; Co, condylion: most posterosuperior
point of condylar process; Ar, articulare: point
of intersection of the dorsal contour of the mandibular
condyle and the temporal bone.
MATERIAL AND METHODS
The Human Research Ethics Board at Kanagawa Dental College approved this study, and informed consent was read and signed by each participant.
Initial records of patients with nonhereditary craniomandibular asymmetry before, during, and after orthodontic treatment at the orthodontic department of Kanagawa Dental College were selected. A total of 116 patients (average age, 20.8 6 7.8 years; 35 males: age, 20.7 6 6.8; 81 females: age, 20.9 6 8.3 years) diagnosed as having MLD without pathologic conditions that affect the TMJs, including congenital disorders such as hemifacial microsomia, condylar hyperplasia, rheumatoid arthritis, and osteoarthritis, participated in this study. These patients’ facial morphology was examined
by using posteroanterior cephalograms, occlusion guidance on the articulator after face-bow transfer, and condylar movement with a condylograph.
All patients were treated or intended to be treated for their MLD malocclusion according to the treatment protocol at Kanagawa Dental College based on vertical dimension and occlusal plane control. The morphologic characteristics of MLD were studied on the cephalograms as shown in Figure 1. The midfacial reference plane to assess facial asymmetry was a line running through crista galli and anterior nasal spine. The angle between the midfacial plane and the line running through anterior nasal spine and menton was defined as the degree of MLD. A positive value indicated MLD to the left side and a negative value to the right side. The angle between the perpendicular line of the midfacial plane and the line running through the occlusal surface of the bilateral maxillary first molars was defined as the OP. The angle between the perpendicular line of the midfacial plane and the line connecting the
right and left antegonial notches was defined as the frontal mandibular plane (AG). The angle between the perpendicular line of the midfacial plane and the line connecting the right and left mastoid processes of the temporal bone was defined as the frontal mastoid plane. Positive values of OP and AG indicated that these planes inclined superiorly toward the left side. Since the condyle follows the growth of the whole ramus, to evaluate condylar deviation, the lateral ramus line, which was the tangent line from the cross point of the condylar process tracing and the tracing of the mastoid process to the lateral shape of gonial tracing, was drawn as an indicator of condylar inclination.16 Then, the central points of the oval shape of right and left zygomatic arches were connected as the zygomatic plane.
The distance between the zygomatic arches and the cross point of the zygomatic plane and the condylar inclination was measured. The distance between right and left was subtracted to indicate condylar deviation, with a positive value indicating deviation to the right side and vise versa. It is easily recognized by correlation figures showing negative values for MLD right and positive values for MLD left.
The mounted maxillary casts were transferred to a 3-dimensional (3D) digitizer (Gamma GmbH, Klosterneuburg, Austria). The tooth guidance surfaces
were designed for protrusion and laterotrusion. The start and end points of the occlusal guidance were called functional points Fl and F2 (Fig 2). F1 and F2 were located on the mesial and distal marginal ridges of the maxillary central incisors and the mesial marginal ridges of the maxillary lateral incisors, canines, premolars, and first molars. The F1 points were created in maximum intercuspation with mandibular incisal edges, cusps of the canines, and buccal cusps of the premolars and the first molars in contact and are the starting points for eccentric movements.
The most eccentric contact right before disocclusion is F2. It is located next to the incisal edge on the lingual surface of the maxillary front teeth. The F2 points in the premolars and molars was close to the cusp tip, usually not coinciding with it. A line connecting Fl and F2 showed an angulation to the axis-orbital plane referenced by anatomic face-bow transfer by using the anatomic hinge axis points and the left incisura infraorbitalis for mounting the maxillary casts in an articulator (SAMPra¨zisionstechnik, Munich, Germany). A 3D digitizer recorded the coordinates of both functional points after a mobile pin was placed on the respective spot. Calibration of the digitizer preceded every recording. The digitizer had a measuring accuracy of 0.01 mm,
and each point was recorded 3 times. The corresponding inclinations of the maxillary second molars were measured for study purposes. A mobile measuring pin was manually moved to Fl and F2, and the digitizer recorded the coordinates of these points with reference to the axis-orbital plane. The inclinations of the guidance were then calculated from the coordinates, providing the average of the 3 measurements with reference to the axis-orbital plane.
Occlusal planes were also calculated by connecting the inclinations of the F1 points with reference to the axis-orbital plane. Three occlusal planes were measured. The conventional occlusal plane was the line connecting F1 of the central incisor and first molar, the anterior occlusal plane was that of the central incisor and first premolar, and the posterior occlusal plane was that of the second premolar and second molar. The 3D condylar paths of the patients were recorded during opening and closing movements of the mandible and analyzed with a computerized condylography (Cadiax, Gamma Dental, Klosterneuburg, Austria) system. The condylography system consisted of a cranial attachment and a mandibular face-bow. A functional clutch was fixed to the labial surface of the mandibular dentition, and mandibular movements were confirmed to have no interference. The mandibular face-bow was then connected to the clutch. Two-dimensional condylar movement in the sagittal plane was recorded on digitizers, which were attached to the cranial face-bow and placed over the TMJ region bilaterally. Lateral deviation of the condyle during mandibular functional movement was detected by the stylus attached to the mandibular face-bow. After positioning the stylus on the hinge axis, condylar movements in both TMJs were recorded 3-dimensionally from a reference position established by using unforced chin-point guidance.17
To investigate the lateral shift of the condyle during opening movement (DY shift), the condylar paths in the transverse plane were observed on the condylographic tracing. As shown in Figure 3, there were several types of DY shift that indicated medial (inward) and lateral (outward) deviation.
The DY shift was seen in different conditions ofmandibular functional movement, which was evaluated by condylographic axis movement. They were classified into 3 groups according to mandibular lateral translation (MLT): (1)MLTwith closed lock (closed lockDY),which
Fig 2. Functional points and occlusal guidance in the
dentition. Functional points F1 and F2 are located on
the mesial and distal marginal ridges of the incisors
and the mesial marginal ridges of the maxillary lateral incisors,
canines, premolars, and first molars. The F1s are
created while in maximum intercuspation and are the
starting points for eccentric movements. The F2s are
the most eccentric contacts right before disocclusion.
Fig 3. Different types of MLT visualized on condylographic tracings during opening and closing
movements: A, MLT with severely limited condylar movement (closed lock DY); B, MLT with excessive
condylar rotation in maximum opening position (overrotation DY); C, MLT without limitation and
overrotation (DY MLT) occurred in relatively close position to the reference position.
Table I. Occlusal plane differences betwen sides of dentitions with MLD
showed lateral translation with severely limited condylar movement during opening and closing movements; (2) MLTwith overrotation (overrotation DY), which showed lateral translation with excess condylar rotation in maximum opening position; and (3) MLT without closed lock and overrotation, which showed lateral translation (DY MLT) relatively close to the reference.
Statistical analysis
Statistical analyses were performed with the SPSS program for Windows (version 15, SPSS, Chicago, Ill). The mean values and standard deviations for all
measured variables of occlusal plane inclination, occlusal guidance inclination, and condylar movements (open/close, protrusion/retrusion, and mediotrusion/ medioretrusion) were calculated for each subject. In addition to standard descriptive statistical calculations, the Student andWelch t tests were used to detect differences of outcome measurements between the 2 groups: shifted and nonshifted sides in MLD (Tables I-V), and the
statistical significance levels were established at P \0.05 and P \0.01. Percent distribution of the 4 patterns of lateral translation including the
relationship between MLD and 6Y-shift directions was also calculated (Tables VI, VII). The Pearson product-moment correlation coefficients were calculated to determine different associations between variables related to MLD malocclusion (OP, AG, and condylar deviation). The statistical significance of correlation was set at P \0.01 (Table VIII).
Data were analyzed in the total sample, and no classification was made according to sex, because there was no significant difference in the variables between the sexes.
Table II. Occlusal guidance differences betwen sides of dentitions with MLD
RESULTS
Analysis of posteroanterior cephalograms of MLD patients showed a high correlation between the OP inclination and the MLD. A superiorly inclined occlusal plane was associated with MLD in the same direction, suggesting that the side of the dentition with less vertical height induced mandibular adaptation with a lateral shift to the side. Inclination of the AG also indicated significant correlation with MLD and a correlation coefficient of 0.607. In addition to these, analysis of the relationship of the OP and AG showed a high correlation with a correlation coefficient of 0.595, indicating that the occusal plane inclination was strongly related to a 3D shift of the mandible toward the side with less vertical dimension (Table VIII).
Condylar shift and the AG inclination, and condylar shift and MLD were also significantly correlated with relatively low correlation coefficients: 0.227 and
0.129, respectively (Table VIII). These findings suggested that condylar deviation in MLD was toward the opposite direction from the MLD side, and the mandible was not simply laterally shifted but also rotated in a 3D manner.
Onthe articulator-mounted cast, sagittal inclination of the occlusal planes and the occlusal guidances (F1-F2) of each tooth were measured; the results are shown in Tables I and II. There were no significant differences in the OP inclination between the shifted and nonshifted sides with respect to conventional OP (central incisor-first molar) and anterior OP (central incisor-first premolar). However, the posterior OPs (second premolar-second
molar and first molar-second molar) on the shifted side were significantly steeper than those on the nonshifted side. These results coincided well with those obtained for the OP inclination, which showed an OP superiorly inclined to the shifted side.
Measurement of occlusal guidance showed that, although there were no significant differences between the shifted and nonshifted sides in the incisors, buccal segments from the canine through the first molar had significantly steeper inclinations on the nonshifted side. Measurement results of quantity of condylar movement, sagittal condylar inclination at several distances (3, 5, and 10 mm of translation); transversal condylar inclination (same as the Bennett angle), and difference of start and end points of condylar movement are shown in Tables III, IV, and V. Measurement of condylar movement during opening and closing, protrusion and retrusion, and mediotrusion showed that the shifted side of the condyle moved significantly more than the nonshifted side. The sagittal condylar inclination in the nonshifted side tended to have a steeper inclination than that in the nonshifted side (Table III).
The shifted side of the condyle during symmetric condylar movement (open/close, protrusion/retrusion) had a tendency to move outward; this showed negative values of transversal condylar inclination and was significantly different from the nonshifted side (Tables III and IV).
During mediotrusive movement, sagittal condylar inclination in the nonshifted side had the same tendency as opening and closing, and protrusion and retrusion movements, and transversal condylar inclination in the shifted side of the condyle showed significantly smaller values relative to the other side (Table V). The start and end difference had a larger value in the shifted side than in nonshifted side, suggesting that the joint on the shifted side was looser. There were 4 types of lateral translation behavior in opening and closing movements of the condyle: no DY shift, DY shift with limited movement (\8 mm), DY
Table III. Differences in values of condylar movement
(opening/closing) between sides of dentitions with
MLD
Table IV. Differences in values of condylar movement
(protrusion/retrusion) betwen the sides of dentitions
with MLD
Table V. Differences in values of condylar movement
(mediotrusion/medioretrusion) betwen the sides of dentitions
with MLD
shift with excess movement, and DY shift with normal quantity. Distribution of these patterns is shown in Table VI. Approximately 90% of MLD patients had DY shift. Excluding the DY shift with limited and excess movements, because these groups indicated the possibility of closed lock and loosening of TMJ, DY shift with normal quantity was 81.6%, whereas the no DY shift was only 18.4%. These results suggested that the DY shift is 1 functional indicator for MLD. The results regarding the relationship between the direction of MLD and the direction of DY shift indicated that 77.6% of the subjects coincided in both directions, suggesting that MLD was caused by not only mandibular shift, but also condylar shift (Table VII).
Table VI. Comparison of occurrence 6Y, joint derangement, and overrotation in MLD
Table VII. Coincidence of directions of MLD and 6Y
Discussion
We demonstrated that MLD is not due to simple mandibular lateral shift, but, rather, the mandible was 3-dimensionally rotated along with condylar displacement to the contralateral side. Therefore, it was speculated that decreasing or increasing the dental vertical height on 1 side because of multiple factors (bad postural habits such as 1-sided mastication, posterior discrepancy resulting in difference in eruption between both sides, and difference in restorative material height between both sides leading to occlusal interferences or external trauma history), resulting in inclination of the OP, can be a potential risk factor for dysfunctional lateral shift of the mandible to the side of less vertical dimension. 18 Contralateral displacement of the condyle
causes DY shift during symmetrical condylar movement such as opening and closing, probably because the displaced condyle compromises the integrity and synchronism of the condyle-disc assembly. Also, the deranged condition of the displaced condyle achieves a normal relationship in terms of functional movement and results in DY shift of the condyle (Fig 3). In our previous results regardingMLD and temporomandibular disorders, 65% of the MLD group had TMJ discomfort, and 84.6% of the symptoms were present in the shifted side of the TMJ, but the prevalence of TMJ symptoms in nonshifted side was only 23.1%.5 Also, our previous studies with MRI and condylograph showed that the disc-condyle relation in the deranged
joint results in different types of DY shift, with a close relationship between the direction of DY shift and direction of disc displacement.6,19 Disc displacement in the anteromedial, anterolateral, lateral, or medial directions caused DY shift to the direction that leads to a normal relationship of the disc-condyle assembly. It is not known how MLD is related to internal derangement of TMJ. As shown in this study, the mandible rotates to the side of the lower vertical dimension accompanied by a shift of the condyle to the contralateral side. Therefore, the shifted side of the condyle is more compressed in the glenoid fossa during mastication and parafunction, whereas the nonshifted side is in a distractive unloaded situation (Fig 4). It can be speculated
that the compression side of the TMJ is more susceptible to develop internal derangement. The occlusal plane is an important element in positioning
and adapting the mandible. Continuous horizontalization of the sagittal occlusal plane during the growing processes induces forward adaptation of the
mandible by anterior rotation, consequently establishing a Class III skeletal frame, whereas a steep occlusal plane induces Class II skeletal problems. As shown in this study, the OP in MLD was inclined superiorly toward the shifted side; this resulted in less vertical height of the shifted side than of the contralateral side (Table I). Therefore, the mandible rotated in a 3D way accompanied by tilting of the AG to the same direction as shown in Figure 4. Strong correlation between the OP and AG inclinations suggests that, in the development of MLD, vertical dimension differences between the right and left sides cause lateral shifting of the mandible followed by functional asymmetry. In addition to these, the condyles were shifted to the contralateral side (Fig 4). From these observations, it can be postulated that this kind of mandibular rotation might cause strong compression in the shifted side of the condyle and secondarily cause internal derangement of the TMJ and osteoarthritic changes in the condyle. However, in some cases, it was suggested
that various pathologic conditions that affect the TMJ can manifest as facial asymmetries, including congenital disorders such as hemifacial microsomia,
condylar hyperplasia, internal derangements, rheumatoid arthritis, and osteoarthritis causing advanced joint degeneration that can cause shortening of the condyle with subsequent skeletal asymmetry.20 Schmid and Mongini21 also postulated that craniomandibular structural asymmetry can be congenital or hereditary, or can be acquired from traumatic infections. During growth, quantitative and qualitative alterations of the functional loads applied to the bones might modify their developmental pattern and lead to asymmetry. Although the etiology of skeletal asymmetry is not well understood and has been associated with crossbites and occlusal interferences, occlusal alterations can lead tomandibular displacement inmaximumintercuspidation and, consequently, to apparent asymmetry.22 Therefore a distinction can be drawn among structural asymmetries,
displacement asymmetries, and mixed types.21 Occlusal alterations such as interferences or differences in the vertical height of the dentition with lateral shifting of the mandible appear to be the approach from an occlusal point of view to restore occlusal and articulation function. Some case reports of MLD have shown that patients can be successfully treated based on these considerations by orthodontic treatment without surgical intervention.12,23
The resu lts from occlusal guidance measurement showed that the nonshifted side of the occlusal guidance inclination in the buccal segment was steeper than in the shifted side (Table II). This is probably 1 reason for MLD, because too steep an inclination on 1 side of the buccal teeth interferes with the mandibular adaptation to that side and induces unilateral chewing at the flat guidance side (contralateral side). Therefore, based on these observations, 2 possible mechanisms were suggested for explaining the development of MLD. First, the difference in vertical height of the dentition on both sides creates an occlusal fulcrum on the posterior molar of the higher side. In this situation, biting forces lead to a distractive load against the TMJ
on the opposite side.24,25 It was suggested that the tilting OP followed by mandibular displacement including condylar displacement can lead to internal derangement of the TMJ or asymmetrical condylar growth. Second, a difference in occlusal guidance between the 2 sides causes a slight mandibular shift with a consequent unilateral chewing habit. This generates more occlusal load on the chewing side and prevents eruption of teeth on the chewing side, tilts the OP superiorly to the same side, and compresses the condyle against the glenoid fossa on the chewing side.
Table VIII. Associations tested
Fig 4. Schematic drawing of development of MLD. The
OP in MLD is inclined superiorly on the shifted side, indicating
that the vertical height of the shifted side is less
than that of the contralateral side. The mandible rotated
3-dimensionally, accompanied by tilting of the AG in the
same direction. The condyles shifted to the contralateral
side. This kind of mandibular rotation can cause strong
compression on the shifted side of the condyle and
also internal derangement of the TMJs and osteoarthritic
changes of the condyle. OP, Frontal occlusal plane; AG,
frontal mandibular plane.
Conclusion
Our results suggest that MLD is a problem that involves many factors such as occlusal vertical dimension, occlusal plane inclination, internal derangement of TMJs, functional DY shift of the condyle during function, occlusal guidance, and facial asymmetry. All these characterizations, together with the clinical implications, should be considered in the treatment protocol for MLD malocclusion and addressed for differential control of the occlusal plane in both sides. Also, an early approach toMLDproblems is a priority, and theDYshift of the condyle during function is a useful indicator to detect MLD in growing children.
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