Mandibular and articulator movements

MANDl3ULAR THOMAS

AND

ARTICULATOR

E. J. SHANAHAN,

Brooklyn,

MOVEMENTS

D.D.S., AND ALEXANDER

LEFF, D.D.S.

N. Y.

PURPOSE OF THIS STUDY was to compare the opening and closing movements of the mandible with the opening and closing movements of a hingeaxis articulator.

T

HE

MANDIBULAR

MOVEMENTS

A mandibular diagnostic unit was constructed to record the movements of the mandible in as simple a manner as possible. The unit consisted of a tiny flashlight hlh which was powered by a single flashlight battery and attached to a plastic chinpiece (Fig. 1) . After the light was placed in position on the chinpiece, the entire bulb was coated with black enamel. Then a pin-point opening was made in the enamel so that a single beam of light was projected. The objective was to photograph the movements of the light from a side view during the opening and closing movements of the mandible. The plastic chinpiece and light were attached to the point of the chin by silicone and adhesive paste or tape. The patient was seated in the chair (Fig. Z), and the room was darkened. The shutter of the camera was opened. The chin light was turned on, and the patient was requested to open and close his mouth just once. The image on the film, made by the light attached to the chin, was in the form of a cycle (Fig. 3). This indicated that the mandible opened on one path and closed on another. The opening movement was slower than the closing movement, and there was some adjustment of the mandible by the musculature before it started on the closing path of the cycle. ARTICULATOR

MOVEMENTS

The opening and closing movements of the hinge-axis articulator were recorded graphically by attaching a pencil to the incisal guidepost. A pad of paper was placed in position, and the articulator was opened and closed just once (Fig. 4). Then the light was attached to the incisal guidepost of the hinge-axis artlulator. The room was darkened, the shutter of the camera was opened, the unit light was turned on, and the maxillary arm of the articulator was opened and closed just once. Presented 7, 1958. Received

before for

the Greater

publication

New

York

Academy

Jan. 2, 1959. 941

of Prosthodontics,

New

York,

N. Y., Dec.

942

SHANAHAN

AND

LEFF

The image on the film, made by the light attached to the hinge-axis articulator during the opening and closing movements, was in the form of an arc (Fig. 5). Thus, the opening and closing movements of the articulator were along the same path, and there were no variations such as were found during the corresponding mandibular movements.

Fig. 1.-A mandibular diagnostic to determine mandibular movement.

Fig. S.-The level for positioning GRAPHIC

AND

unit,

with

chin

light,

camera,

abin light is in position. The instrument in front the head before the tracings are photographed. MATHEMATIC

and

accessories,

of the

mouth

was

used

is a spirit

REPRESENTATIONS

The mandibular and articulator opening and closing movements were plotted graphically to provide a more precise representation of the differences (Fig. 6).

Volume 9 Number G

MANDIBULAR

AND

ARTICULATOR

MOVEMENTS

fN.?

The basic differences between the movements which are apparent graphically can be demonstrated also mathematically. Since the mandibular opening and closing curves represent the same type of mathematical equation, the mandibular opening curve was used in comparison with the articulator curve. The values of x and 1 at suitable intervals were noted for each curve.

Fig. 3.-The ments of the hesitations.

light image mandible in

represents the right

Fig. 4.-A axis articulator

tracing ls made of the arc ln the right sagittal plane.

the cyclic paths of sagittal plane. The

of the opening

and

the opening light spots

closing

and closing moveshow momentary

movements

of a hinge-

A third-degree equation was required to express the mandibular curve, whereas a second-degree equation was sufficient for expressing the articulator curve. Mathematically speaking, the mandibular tracing is the more complex one since it is a curve with inflection, while the articulator tracing is a simple arc of a

SHANAHAI~

AND

LEFF

and articulator circle. The differences between the mandibular closing movements are expressed mathematically as follows : Curve (Patient)

1

Curve (Articulator)

3

opening

+ cx + d + .23x - .16

1 Thirddegree equation

(x - h)” + (y - k)’ = rz xa + f + 18.94x - 6.38~ = .163

1 Seconddegree equation

y = ax’ + bx’ y = .11x8 - .40x*

and

The opening and closing mandibular tracings were not constant for the same patient and varied with each recording. The obvious consistency was that the mandibular tracings were complex curves that did not bear any resemblance to the

Fig. 5.-The image of a hinge-axis articulator

of the arc of light represents in the right sagittal plane.

both

the opening

and

closing

movements

d 6 Fig. parison. mandible,

6.-The mandibular cycle and the articulator arc tracings are plotted for comCurve 1 is the opening paUh of the mandible, curue 2 is the closing path of the and curve 3 is the opening and closing path of the articulator.

MANDIRGLAR

AND

ARTICULATOR

XIOVEMENTS

t115

articulator tracings. Functional mandibular chewing recordings made in several planes served to increase the complexity of the tracings and to enhance the aboyta findings. Further studies are in progress to determine the value of individual mandibular tracings as a diagnostic aid. CONCLUSION

The normal opening and closing movements of the mandible do not coincide with the opening and closing movements of a hinge-axis articulator. 862 CARROLL BROOKLYN

ST. 15, N. Y.

833 ST. JOHN’S BROOKLYN 15.

PLACE

N. Y.