Morphologic adaptation of temporomandibular joint after chincup therapy

Hiroshi Mimura, DDS, PhD, a and Toshio Deguchi, DDS, MSD, PhD b

Nagano, Japan

The purpose of this study was to clarify the morphologic changes of the temporomandibular joint (TMJ) after chincup application, with longitudinal TMJ cephalometric laminagraphs. The subjects were 19 prepubertal patients with true, mild skeletal Class III malocclusions with anterior crossbite. All underwent chincup therapy from the beginning of treatment. The control subjects were 16 patients with functional anterior crossbite, with normal jaw relationships. Cephalometric laminagraphs and lateral cephalograms were obtained before and after treatment. Statistical analysis of the data revealed the results as follows: (1) The chincup therapy changed the direction of growth of the mandible, especially, the ramus swing-back. (2) The chincup group showed a relatively more slender mandibular neck compared with the control group. (3) The condylar heads were bent forward after the chincup application, and the glenoid fossa was deepened and widened. The clearance between condyles and fossae was decreased by the orthopedic force of the chincup appliance. (Am J Orthod Dentofac Orthop 1996;110:541-6.)

T h e use of chincup for the prognathism has been known for about a century.1 It is well-known that the incidence of anterior crossbite and prognathism in Mongoloid people is higher than that in the white population? Therefore chincup therapy has been the usual technique for the treatment of growing patients with true, mild skeletal Class III malocclusions in Japan.

There are many reports describing the changes of the maxillofacial skeleton after chincup therapy, 3-9 with some describing the transmitted mechanical stress from this orthopedic appliance, l°-la There are also some studies that investigated the histologic changes of condylar growth after the application of orthopedic forces. ~3-16 Koski ~7 reported that the mandibular head was not a primary growth center, but the growth site was very responsive to mechanical stress. Chincup therapy applies pressure on the temporomandibular joint (TMJ), inhibiting and redirecting the growth of the condyle. However, there is no report describing the longitudinal study of the change in the TMJ itself, after chincup application.

Ricke t t s 18'19 introduced orthodontic application of the TMJ laminagraphy. Sagittal arthrotomography of the TMJ is now a routine orthodontic examination, which is usefhl in revealing TMJ structure and spatial changes.

From the Department of Orthodontics, Matsumoto Dental College. aAssociate professor. bProfessor and Chairman. Reprint requests to: Dr. Toshio Deguchi, Department of Orthodontics, Matsu- moto Dental College, 1780 Gohbara Hironka Shiojiri, Nagano 399-07, Japan. Copyright © 1996 by the American Association of Orthodontists. 0889-5406/96/$5.00 + 0 8/1/63678

The purpose of this study was to clarify the mor- phologic adaptation of the TMJ after chincup applica- tion, with longitudinal TMJ sagittal arthrotomograms.

MATERIALS AND METHODS

The subjects were 19 patients with true, mild skeletal Class Ill malocclusions with anterior crossbite (8 males and 11 females; mean age, 10 years 2 months), who were treated with chincups and 0.018-inch edgewise appliances (Table I). Clear and visible lateral cephalometric radiographs and sag- ittal arthrotomograms, both before and after treatment, were obtained. All underwent chincup therapy from the beginning of treatment, and the anterior crossbite was corrected with a modified Mershon's lingual arch appliance in the first phase of the treatment. Chincup use was started before the adoles- cent period, and continued after the pubertal period in all subjects, as confirmed by skeletal age analysis of the hand- wrist x-ray films and annual increment of body height. The durations of chincup therapy were varied (from 5 months to 5 years 11 months), but averaged 2 years 1 month.

The control subjects were 16 patients (4 males and 12 females; mean age, 9 years 8 months) with typically func- tional anterior crossbite but normal jaw size. However, the initial contact at the incisors guided the mandible forward, and therefore the ANB angles were relatively smaller than the actual size differences between maxilla and mandible. The control subjects were treated with only the modified Mershon's lingual arch appliance, to move upper incisors forward without the use of the chincup appliance.

The lateral cephalometric radiographs were obtained in a conventional manner, with a cephalostat (TEXCO T-1; Tokyo Engine Co. LTD, Tokyo, Japan). The sagittal arthrotomo- grams were obtained, with a cephalometric laminagraph (Sectograph; Quint Co. LTD, Los Angeles, Calif.) of geo-

541

542 Mimura and Deguchi American Journal of Orthodontics and Dentofacial Orthopedics November 1996

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: t ~ ®'

: S N • ® : A N D

(~) : M a x ~ t d . i b ~ x I a r p 1 a x l e

@ : G o n i a l a n g l e

@ : R a m u s a n g l e

Fig. 1. Angular measurements on lateral cephalogram.

metric distortion of 1.06 and tomographic layer of 2 mm. Before TMJ laminagraphy, all these patients underwent axial head plate projection to orient the head and to assure the visualization of the sagittal plane across the center of the joint. The slice depth and the inclination of the slice path were measured on the tracings, and the Sectograph was set to intersect the center and long axes of the condylar head in each patient? °

The six angles determined in angular analysis and five distances measured in linear analysis, with the cephalograms, are shown in Figs. 1 and 2, respectively. The ANS-PNS was measured on the Frankfort horizontal (FH) plane after the projection, and the ANS-Me on the plane perpendicular to the FH plane. The three angular and seven linear morpho- metric measurements of the TMJ, determined by using the cephalometric laminagraphs, are shown in Fig. 3.

On the TMJ tracings, the FH plane was drawn and two lines parallel to the FH plane were drawn in contact with mandibular fossa and eminence. A tangent to the posterior border of the ramus was drawn; this is the ramus plane on the sagittal arthrotomogram. From the superior contact point of the ramus plane, a line parallel to the FH plane was drawn. On this line, the median point of the superior contact point and the point intersecting the anterior outline of mandibular neck was marked; this is the median point of mandibular neck. A line parallel to the ramus plane, crossing the median point of mandibular neck, was drawn. Crossing the median point, a connecting line to the farthest point was drawn; this is the condylar head plane. The angle formed by the two planes is the condylar head angle.

The significance of difference for each of these values

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Fig. 2. Linear measurements on lateral cephalogram.

was statistically analyzed with the Student's t test (paired t test).

RESULTS

The values of maxil lofacial skeletal parameters before treatment in the chincup group and the control group were compared (Table II). Although the mean age in the chincup group was 7 months higher, the ANB angle in this group was -1 .1 °, indicating a skeletal Class III pattern, whereas that in the control group was 1.6 °, indicating a skeletal Class I pattern. There were no statistical differences between the groups in the SNA, SNB, or ANS-PNS (maxillary length). However, there was a significant difference between the groups for the Ar-Pog (mandibular length), Ar-Go (ramus length), and Pog-Go (mandibu- lar body length). The chincup group had an opened gonial angle and a forward rotated ramus angle, as a result of increased lower facial height (ANS-Me). The opened gonial angle in the chincup group compensated for the increased ramus angle, and therefore no signif- icant difference existed between the groups for the mandibular plane.

The values of the TMJ parameters before treatment in each group are summarized in Table III. The chin- cup group showed a longer condylar head length, even though there was no significant difference for fossa

American Journal of Orthodontics and Dentofacial Orthopedics Mimura and Deguchi 543 Volume 110, No. 5

Table I. Ages of subjects in the chincup group and the control group

Mean Range

Pretreatment

Group Mean

Posttreatment

I Range

Chincup use

Mean Range n m

Chincup 10 yrs. 2 mos. 7 yrs. 1 mo.-I 3 yrs. 1 mo. 15 yrs. 0 mo. 12 yrs. 7 mos.-18 yrs. 0 mo. 2 yrs. 1 too. 0 yrs. 5 mos.-5 yrs. 11 mos. 19

Control 9 yrs. 8 mos. 7 yrs. 9 mos.-12 yrs. 0 mo. 14 yrs. 1 mo. 11 yrs. 8 mos.-18 yrs. 1 mo. - - - - 16

width, fossa depth, or condylar head width. These results indicate that the chincup group had a relatively more slender mandibular head and a similar mandibu- lar fossa as compared with the control group.

In the chincup group, Ar-Pog (mandibular length) and ANS-PNS (maxillary length) were increased after treatment. However, the ANB angle was reduced to improve the original skeletal Class HI pattern (Table II). The ramus angle was decreased, reflecting man- dibular swing back. However, no significant change in the mandibular plane was seen. In the control group, the maxilla and the mandible were also increased in size, but there was no significant change in the ramus angle after treatment (Table II). After treatment, the significant differences between the chincup and the control groups were seen in the ANB angle, Pog-Go (mandibular body length), Ar-Go (ramus length), and Ar-Pog (mandibular length). The gonial angle after the chincup application was slightly closed, although a significant difference was not observed. The chincup group and the control group showed a similar increase in the length of the mandible, about 10 mm (Table II). However, the chincup effect was to swing back the ramus, with a slight decrease of the gonial angle, resulting in a lower facial height (ANS-Me) increase (Table II).

After chincup therapy, a significant change in the TMJ was observed in the eminence to FH plane angle, condyle head angle, height of condyle, height of fossa, width of fossa, and superior and anterior condyle spaces (Table III). Summarizing, in the chincup group, the fossae were deepened and widened, mandibular heads were bent forward and elongated, and the clear- ance between the condyles and fossae was narrowed. On the cephalometric laminagraph, no significant change of the ramus inclination was observed.

In contrast, in the control group, only the height of the condyle showed a significant change after treat- ment (Table III). Comparison of the posttreatment values revealed a significant difference between the groups for the eminence to FH angle, condyle head angle, height of fossa, width of fossa, height of condyle, and anterior condyle space (Table III). These differences indicate that chincup orthopedic force

causes condylar forward bending, enlarging of the mandibular fossa or bone formation of the eminence, and narrowing of the anterior clearance between the condyle and fossa.

DISCUSSION

Although it is desirable that comparative studies of treatment groups be conducted in age and sex matched groups, in our study it was very difficult to find patients meeting these criteria, having clear cephalo- grams and laminagraphs, both before and after treat- ment. However, mean ages in the chincup group and control group were almost equal before and after treatment. All the patients were confirmed to be in the prepubertal or the pubertal growth period by the data for hand-wrist x-ray films and annual increment of the body height. Therefore the developmental age of our subjects was comparable.

Chincup use is well-known to affect the direction of growth of the mandible. 3'8 However, it is controver- sial whether chincup use inhibits mandibular growth. 7'9

Our results indicate that the mandibular length (Ar- Pog) after chincup therapy was significantly longer in the chincup group. The increase in the mandibular length was almost the same in both the chincup and the control groups. In the data of TMJ change, the signif- icant difference of the height of the condyle we observed after treatment in both groups indicates that the subjects in both groups had not yet entered the adolescent period and reflected the growth potential of the mandibular head. The increase in the height of the condyle was similar in both groups, much like the mandibular length. The difference between the two groups in the ramus angle is similar to that described previously?

The TMJ parameters indicated condylar forward bending, deepening and widening of the mandibular fossa or additional bone formation on the eminence, and narrowing of the clearance between the condyle and the fossa. The height of the condyle before treat- ment was about 1.5 mm longer in the chincup group than in the control group. However, both groups showed 1.5 mm elongation after treatment. Therefore the difference was still present after treatment. This

544 Mimura and Deguchi American Journal of Orthodontics and Dentofacial Orthopedics November 1996

®~,,

'.~ ~ (D : g n ° l i n e n e e t o FI--I ~ n g l e

". \ \ ( ~ : R .... inclination ', \ \ ~: Condyle head a n g l e

~\ \ ®:width o3 * .... '\~ \ (~ : H e i g h t o¢ Condyle

\~ ' ~ : Height of Neck

'~ ~ : W i d t h o f C o n d y l e ~ : Superior Condyle Space ~ : Anterior Condyle Space

\ Fig. 3. Measurements on cephalometric laminagraph. 1 to 3 are angular measurements and 4 to 10 are linear measurements,

measurement was the only parameter in control group that showed significant change after treatment. The condyle itself has growth potential and the linear increase at condyle was almost the same in both groups. The mandibular length (Ar-Pog) also showed a similar increase of about 10 mm in both groups. All these data suggest that chincup use does not decrease the overall mandibular growth, but does change the direction of growth, and consequently the form of the mandible effectively.

The maxillary length showed a similar increase after treatment in both groups. This may indicate that correction of anterior crossbite accelerated forward growth of the maxilla in both groups.

In the chincup group, significant change after treat- ment was observed for the superior and anterior condyle space. However, only the values for the ante- rior condyle space showed a significant difference in the comparison between the two groups. The anterior condylar space might have been narrowed because of the forward bending of the condyle.

Orthopedic chincup force is directed from the chin to the condyle posterosuperiorly. Mechanical stress is concentrated in the weakest part of the skeleton, i.e., the slender mandibular neck that is frequently broken in a maxillofacial traumatic injury. 21 This site is specu- lated to be most responsive to mandibular orthopedic force. Our results clearly indicate that the condylar head in the chincup group was bent forward, whereas the condylar head angle in the control group was not markedly changed after treatment. Part of this, of

course, is guided growth of the condyle. de Alba et al. 10 visualized the direction of applica-

tion of orthopedic force by the chincup, by using a three-dimensional photoelastic technique. Their result illustrated chincup stress emanated through the man- dibular body, the angle, and retromolar triangle of the mandible, radiating in a posterosuperior fashion and concentrating at the neck of the condyle. We obtained similar results by affixing a thin three-dimensional strain gauge to a young human dry skull, and applying a chincup force. Compressive stress was always con- centrated at the anterior border of the mandibular neck, and tensional stress was observed at the posterior aspect of the mandibular neck, under the specific condition, when the chincup force was directed toward the condyle, with the teeth slightly apart. In the upper craniofacial structures, de Alva et al. reported that stress was observed at the pterygoid plate, due to the action of the simulative coil spring for the external pterygoid muscle and the posterior surface of the glenoid fossa. We found that the posterior surface of the glenoid fossa received maximum pressure. Al- though trajectory and magnitude of stress are affected by many conditions, such as teeth contacted, chincup force direction, and silicone rubber disk insertion, the maximum compressive force is invariably concentrated at the anterior condylar neck.

Kanematsu 16 investigated histologically the effect of chincup force in Macaca irus, and revealed that chincup application inhibited the bone deposition on the condylar neck and stimulated that on the posterior

American Journal of Orthodontics and Dentofacial Orthopedics Mimura and Deguchi 545 Volume 110, No. 5

"fable II. Comparison of maxillofacial skeletal parameters before and after treatment

Chincup group

(1) Pretreatment (2) Posttreatment

SNA 80.5 +- 3.8 81.3 ± 4.2

SNB 81.6 ± 3.9 81.7 ± 3.8 ANB -1.1 ±2.0 -0.3 _+2.2

Mandibular plane angle 30.2 ± 4.1 28.9 ± 5.4

Gonial angle 129.9 ± 5.5 126,6 ± 7.5

Ranms angle 10.1 ± 3.9 7.5 _+ 3.6 Ar-Pog 110.0 --_ 6.5 119.7 ± 7.0

M-Go 43.7 ± 3.4 48.9 ± 5.8

Pog-Go 76.6 ± 5.0 83.7 ± 5.9

ANS-PNS (FH) 48.9 ± 2.3 51.6 ± 2.1

ANS-Me (FH) 66.1 ± 4.9 73.1 ± 5.8

ControIgro~

(3)P~tma~ent (4)Posn~a~ent

81.7±3.7 82.8_+3.3

80.2±4.4 80.4±3.7 1.6±2,9 2,4±1.4

29.8±4.1 28.1±4,7

125.3±4.3 123.8±4.9

6.0±4.6 5.8±4.2 102.2±3.7 112.2±6.1

41.0±2,6 45.7±3,3

72.9±3,0 79.5±5.1

47.7±2,3 51.8±2.6

62.0±2,6 68.1±4.3

Significance

Ivs3 lvs2 3vs4 2vs4 m

* * * #

* * * * *

*Significant at the 0.05 level (P < 0.05). **Significant at the 0,01 level (P < 0,01).

Table III. Comparison of the parameters before and after treatment

Chincup group Control group

Pretreatment I (2) Posttreatment (3) Pretreatment (4) Posttreatment (1) I

Eminence to FH plane angle 34.6 + 5.2 37.3 ± 6.0 32.6 -+ 5.4 34.2 + 6.3

Ramus inclination 83.8 + 5.5 84.8 ± 4.9 84.3 ± 4.6 83.4 + 5.0

Condyle head angle 168.0 _+ 8.4 156.3 +- 9.3 168.2 ± 10.7 163.9 + 9.3

Height of fossa 7.7 + 1.2 8.7 +- 1.4 6.8 ± 2.5 7.5 ± 2.7 Width of fossa 18.6 ± 2,2 19.9 ± 1.9 17.5 ± 3.0 18.2 + 2,9

Height of condyle 7.9 +- 2.0 9.4 ± 2.4 6.4 ± 1.1 8.0 ± 1.9

Height of neck 11.5 + 3.4 12.1 ± 3.2 11.6 ± 2.8 11.7 ± 2.5

Width of condyle 8.1 + 1.0 8.1 ± t.0 8.1 ± 1.1 8.4_+ 1.2

Superior condyle space 2.4 ± 0.7 2.1 ___ 0.6 2.4 ± 0.6 2.4 + 0.7

Anterior condyle space 1.9 + 0.6 1.5 ± 0.7 2.0 ± 0.6 2.1 ± 0.9

1 vs3

Significance

l v s 2 I 3vs4 2vs4

*Significant at the 0.05 level (P < 0.05). **Significant at the 0.01 level (P < 0.01).

border of the ramus, consequently reducing the gonial angle. He also observed bone resorption on the roof of the fossa and posterior surface of the condyle and bone deposition on the anterior surface of the condyle. He did not describe the forward bending of the condyle. However, the human condyle is more slender than that in the monkey, and the remodeling described by Kane- matsu is reasonable for the forward bending.

Bjork 22'23 indicated that there was a strong associa- tion between the facial rotation and the condylar growth, that a posterior upward growing condyle cre- ated a backward rotation of the mandible, with a long face, and a forward-upward growing condyle caused mandibular forward rotation. Our results indicated that the chincup application rotated the mandible backward with gonial angle closing and with the condyle bent forward. However, the forward bent condyle was the result of the concentration of external stress, not au- tonomous growth. Chincup therapy increases the lower facial height, with a forward bent condyle, which is idiosyncratic of these skeletal patterns.

Sugawara et al. 7 showed that the skeletal profile was greatly improved during the chincup therapy, but such changes were often not maintained thereafter. A forward directed condylar head that has growing po- tential after the chincup therapy may drift the mandible forward according to Bjork's results. Therefore it is desirable that the chincup therapy should be continued until the mandibular growth is complete.

The deepened fossa and heightened eminence in- duced by chincup orthopedic force resulted in a steeper sagittal condylar path during jaw opening, protrusive and lateral movements. From the kinesiologic point of view, jaw movement is regulated by the TMJ and tooth contacts, i.e., posterior guidance and anterior guidance. Chincup use increases the inclination of sagittal condy- lar paths. Therefore, if the anterior guidance is insuf- ficient, the posterior teeth cause cuspal interference during jaw movements. Furthermore, forward bending of the condyle and the heightened eminence are the conditions under which the articular disk is susceptible to reduction with microtrauma of the TMJ. 24

5 4 6 Mimura and Deguchi American JournalofOrthodontics and Dentofacial Orthopedics November 1996

Establishment of static and dynamic occlusion is an orthodontic goal. To provide sufficient anterior guid- ance, the overbite must be deep enough. To prevent relapse and stabilize the occlusion, the overbite after treatment must be deep in patients with Class III malocclusion especially. This is desirable not only for the prevention of relapse but also for the prevention of premature contact of the posterior teeth.

CONCLUSION

The skeletal and TMJ structural changes resulting from chincup application were investigated by analysis of cepha- lograms and cephalometric laminagraphs and compared with those in control subjects. It was found that the chincup changed the direction of growth of the mandible, and a definite ramus swing-back was seen.

The mandibular neck in the chincup group was relatively more slender than that in the control group. The condylar heads were bent forward after the chincup application, the glenoid fossa was deepened and widened, and the clearance between the condyle and fossa was decreased by the ortho- pedic chincup force.

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