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Mandibular and articulator movements
MANDIBULAR Part
III.
The
AND
ARTICULATOR
Mandibular
E. J. SHANAHAN, Brooklyn, N. Y. THOMAS
MOVEMENTS
Axis Dilemma D.D.S., AND ALEXANDER
LEFF, D.D.S.
T
HERE IS WIDESPREAD BELIEF that the mandible opens and closes on an axis and that the mandibular teeth open and close into centric occlusion on arcs with centers of rotation in the regions of the condyles. This mechanical concept of a physiologic function has led to articulators that provide for the opening and closing movements of the maxillary teeth on an axis in conformity with this theory. This concept has also brought about the development of face-bows for locating axis points on each side of the face. In this study, the larger opening and closing movements of the mandible were observed in the region anterior to the lower incisor teeth. It was felt that these larger movements would either confirm or deny the presence of an axis ill the region of the condyles. If the opening and closing movements of the mandible proved to be along an arcuate path similar to a hinge articulator when viewed from the side, the evidence would favor the axis theory. From the front, a hinge articulator appears to open and close on a straight line.’ If the mandible also opened and closed on a straight line when viewed from the front, it would be further proof of the validity of the axis theory. In short, according to the mandibular axis theory, the opening and closing movements of a hinge articulator from the front or side should coincide with the normal opening and closing movements of the mandible.
METHOD
AND
MATERIALS
Photographic records were made of the normal opening and closing movements of the mandible from both the side and front. Both views were photographed simultaneously. Two small electric lights attached to a thin rod were fastened to the lower central incisors with thin wire and cold-curing acrylic resin. A small control light was attached similarly to the upper central incisors. This light was used to detect movements of the head while the recordings were bei.ng made. The lights and wires did not interfere with the movements of the mandible or inconvenience the subject (Fig. 1). Side and front view records of the movements of the lights were obtained by placing the camera at the side of th’e face and a mirror in front of the subject at an angle of 45 degrees to the camera. The two lights attached to the lower central Read
before
the
Greater
New
York
Academy
of
Prosthodontics
in
New
York,
N.
Y.
\‘01u111e
1L
K11mlxr
2
MANDIBUL.4R
AXIS
293
DILEMMA
incisors and extending forward from them were used to record two closing paths and to provide two points of reference. OHSI’RVATIONS
AND
opening and
RECORDINGS
The first test was made utilizing simple opening and closing movements of the mandible. From the side, the opening and closing paths were close together and in the form of an arc on the photographic records. However, from the front, the paths were separated and the mandible deviated to the left side, forming a cycle. From the front, the mandible did not open and close on an axis; from the side, the rotation center of the pseudo arc was not in the region of the condyle (Fig. 1). These two views of opening and closing movements were not tracings of axis movements and therefore did not support the mandibular axis theory. From the side, there was no evidence of rotation about a mandibular axis in the region of the condyle with a concomitant anterior translation. In the second test, the same patient repeated the same movements. The tracings revealed that the mandible did not follow the previous paths but deviat’ed to the opposite side and formed a compact cycle (Fig. 2). Here again, there was no evidence to support the condylar axis theory. A different subject was studied in the third test. This time, the subject opened and closed the mandible three times. The tracings indicated that the mandible deviated slightly to the right side during all three movements. There was no evidence that the mandible opened and closed on a condylar axis (Fig. 3).
Fig.
1.
Fig.
2.
Fig. l.-Side and front views of opening and closing movements. The dotted line shows a radius originating from axis A. The axis is not in the region of the condyle. The mirror vie= shows deviation of the mandible to the left during the movements. Fig. 2.-The side and front views of another opening and closing movement made by the same subject as seen in Fig. 1 show a different radius originating from axis A. The mirror view shows deviation of the mandible to the right.
294
SHANAHAN
Fig.
AND
LEFF
3.
pig. 4.
Fig.
3.-The side and front views of three successive opening and closing cycles. The line is a radius originating from axis A of one of the arcs. The axis is not in the of the condyle. masticating cycles are photographed while a cracker is chewed. From the Fig. 4 .-Three front, one of the cycles deviated to the left. The other two cycles first deviated to one side, then swung over to the other. dotted region
BILATERAL
DEVIATION
It was evident from each of the front views of the three tests that the mandible may deviate to the right or to the left during the opening and closing movements. The very nature of these functions precludes the mandible from opening and closing in precise movements as on an arc or even in the same cycle. The
Volume 12 Number 2
MANDIBULAR
AXIS
DILEMMA
29.5
mandible naturally tends to move freely in the form of cycles. The cycles may be to the right or left. There is no definite cycle of movement, but all the opening and closing movements are cyclic. CHEWING
TESTS
FOR
AXIS
DETERMINATION
A subject chewed a piece of hard cracker three times to explore the possiof the presence of a mandibular axis during the mastication of food. The view tracings showed that the chewing was done in cycles and that the mandeviated to the left in one of them. The mandible deviated first to one side and swung over to the other side in the other two cycles (Fig. 4). The lines appeared to be in the form of arcs in the side view tracings. However, none of these pseudo arcs had a radius that terminated in the region of the condyle. The muscles exercise marvelous control over the mandible during opening and closing movements and mastication (Fig. 5). None of the vertical movements lend credence to the existence of a natural mandibular axis. bility front dible then
ARTIFICIAL
MANDIBULAR
AXIS
The term artificial mandibular axis designates an axis that is the result of forcing the mandible backward. This axis cannot be found during normal physiologic mandibular movements. An artificial mandibular axis can be produced in one of two ways: the patient may voluntarily retrude the mandible as far as possible during the opening and
Fig. &-The functional opening and closing cycles and lateral excursions compared with the masticating cycles (A). Note the manner in which the mandible as the bolus is crushed and moved about the mouth with the assistance of the tongue.
(B) are is moved
296
SHANAHAN
Fig.
AND
J. Pros. Den. March-April, 1962
LEFF
6.
Fig. B.--Self-induced retrusion of the mandible is artificial mandibular axis. Note the strained expression of tremulous tracings produced by this abnormal effort. The radius are represented by the dotted line. Fig. 7.-An artificial mandibular axis is produced by the opening and closing movement. This axis is similar retrusion seen in Fig. 6.
Fig. used in the face artificial pressing to the
7. an attempt to obtain an and neck muscles and the axis (A) and the artificial the chin backward during one formed by self-induced
closing movements, or the dentist can apply firm, backward pressure to the chin during the movements. The unusual strain involved in forcing the mandible backward is apparent in the photographic records of the movements of the lights during voluntary retrusion. The muscles of the face and neck are taut. The tracings from the side appear irregular. The tracings from the front also give a clear indication of the patient’s struggle to move the mandible back to its most retruded position. However, there was a definite indication that the mandible did open and close on an axis located in the region of the condyles during the retrusive movements (Fig. 6). The artificial mandibular axis was more easily attained by exerting firm pressure on the chin, thus forcing the condyles into their most retruded positions. From this position, the mandible could be opened and closed on an arc of a circle the center of which was in the region of the condyles. Tracings of these retrusive movements show the superimposition of the opening and closing movements in both the front and side views. The tracings from the front are almost vertical. This can only be the result of a true artificial mandibular axis located in the region of the condyles (Fig. 7). CONCLUSIONS
A study of the tracings of the natural opening, closing, and masticating movements of the mandible does not show the presence of a mandibular axis in the
MANDIBULAR
AXIS
DILEMMA
297
region of the condyles. An artificial mandibular axis can be produced during the opening and closing movements by forcing the chin backward. However, an artificially produced mandibular axis, jaw position, or jaw movement is not a normal physiologic movement. There was no evidence of rotation about a mandibular axis in the region of the condyles with concomitant anterior translation in these studies of the opening. closing, and masticating mov&nents. REFERENCE
1. Shanahan, T. E. J., and Leff, A.: Mandibular and Articulator Movements. II. Illusion Mandibular Tracings of Masticatory Movements, J. PROS. DEN. 12:82-85, 1962. 862 CARROLL ST. BROOKLYN 15, N. Y. 10 PLAZA BROOKLYN
ST.
38, N. Y.
of
III.
The
AND
ARTICULATOR
Mandibular
E. J. SHANAHAN, Brooklyn, N. Y. THOMAS
MOVEMENTS
Axis Dilemma D.D.S., AND ALEXANDER
LEFF, D.D.S.
T
HERE IS WIDESPREAD BELIEF that the mandible opens and closes on an axis and that the mandibular teeth open and close into centric occlusion on arcs with centers of rotation in the regions of the condyles. This mechanical concept of a physiologic function has led to articulators that provide for the opening and closing movements of the maxillary teeth on an axis in conformity with this theory. This concept has also brought about the development of face-bows for locating axis points on each side of the face. In this study, the larger opening and closing movements of the mandible were observed in the region anterior to the lower incisor teeth. It was felt that these larger movements would either confirm or deny the presence of an axis ill the region of the condyles. If the opening and closing movements of the mandible proved to be along an arcuate path similar to a hinge articulator when viewed from the side, the evidence would favor the axis theory. From the front, a hinge articulator appears to open and close on a straight line.’ If the mandible also opened and closed on a straight line when viewed from the front, it would be further proof of the validity of the axis theory. In short, according to the mandibular axis theory, the opening and closing movements of a hinge articulator from the front or side should coincide with the normal opening and closing movements of the mandible.
METHOD
AND
MATERIALS
Photographic records were made of the normal opening and closing movements of the mandible from both the side and front. Both views were photographed simultaneously. Two small electric lights attached to a thin rod were fastened to the lower central incisors with thin wire and cold-curing acrylic resin. A small control light was attached similarly to the upper central incisors. This light was used to detect movements of the head while the recordings were bei.ng made. The lights and wires did not interfere with the movements of the mandible or inconvenience the subject (Fig. 1). Side and front view records of the movements of the lights were obtained by placing the camera at the side of th’e face and a mirror in front of the subject at an angle of 45 degrees to the camera. The two lights attached to the lower central Read
before
the
Greater
New
York
Academy
of
Prosthodontics
in
New
York,
N.
Y.
\‘01u111e
1L
K11mlxr
2
MANDIBUL.4R
AXIS
293
DILEMMA
incisors and extending forward from them were used to record two closing paths and to provide two points of reference. OHSI’RVATIONS
AND
opening and
RECORDINGS
The first test was made utilizing simple opening and closing movements of the mandible. From the side, the opening and closing paths were close together and in the form of an arc on the photographic records. However, from the front, the paths were separated and the mandible deviated to the left side, forming a cycle. From the front, the mandible did not open and close on an axis; from the side, the rotation center of the pseudo arc was not in the region of the condyle (Fig. 1). These two views of opening and closing movements were not tracings of axis movements and therefore did not support the mandibular axis theory. From the side, there was no evidence of rotation about a mandibular axis in the region of the condyle with a concomitant anterior translation. In the second test, the same patient repeated the same movements. The tracings revealed that the mandible did not follow the previous paths but deviat’ed to the opposite side and formed a compact cycle (Fig. 2). Here again, there was no evidence to support the condylar axis theory. A different subject was studied in the third test. This time, the subject opened and closed the mandible three times. The tracings indicated that the mandible deviated slightly to the right side during all three movements. There was no evidence that the mandible opened and closed on a condylar axis (Fig. 3).
Fig.
1.
Fig.
2.
Fig. l.-Side and front views of opening and closing movements. The dotted line shows a radius originating from axis A. The axis is not in the region of the condyle. The mirror vie= shows deviation of the mandible to the left during the movements. Fig. 2.-The side and front views of another opening and closing movement made by the same subject as seen in Fig. 1 show a different radius originating from axis A. The mirror view shows deviation of the mandible to the right.
294
SHANAHAN
Fig.
AND
LEFF
3.
pig. 4.
Fig.
3.-The side and front views of three successive opening and closing cycles. The line is a radius originating from axis A of one of the arcs. The axis is not in the of the condyle. masticating cycles are photographed while a cracker is chewed. From the Fig. 4 .-Three front, one of the cycles deviated to the left. The other two cycles first deviated to one side, then swung over to the other. dotted region
BILATERAL
DEVIATION
It was evident from each of the front views of the three tests that the mandible may deviate to the right or to the left during the opening and closing movements. The very nature of these functions precludes the mandible from opening and closing in precise movements as on an arc or even in the same cycle. The
Volume 12 Number 2
MANDIBULAR
AXIS
DILEMMA
29.5
mandible naturally tends to move freely in the form of cycles. The cycles may be to the right or left. There is no definite cycle of movement, but all the opening and closing movements are cyclic. CHEWING
TESTS
FOR
AXIS
DETERMINATION
A subject chewed a piece of hard cracker three times to explore the possiof the presence of a mandibular axis during the mastication of food. The view tracings showed that the chewing was done in cycles and that the mandeviated to the left in one of them. The mandible deviated first to one side and swung over to the other side in the other two cycles (Fig. 4). The lines appeared to be in the form of arcs in the side view tracings. However, none of these pseudo arcs had a radius that terminated in the region of the condyle. The muscles exercise marvelous control over the mandible during opening and closing movements and mastication (Fig. 5). None of the vertical movements lend credence to the existence of a natural mandibular axis. bility front dible then
ARTIFICIAL
MANDIBULAR
AXIS
The term artificial mandibular axis designates an axis that is the result of forcing the mandible backward. This axis cannot be found during normal physiologic mandibular movements. An artificial mandibular axis can be produced in one of two ways: the patient may voluntarily retrude the mandible as far as possible during the opening and
Fig. &-The functional opening and closing cycles and lateral excursions compared with the masticating cycles (A). Note the manner in which the mandible as the bolus is crushed and moved about the mouth with the assistance of the tongue.
(B) are is moved
296
SHANAHAN
Fig.
AND
J. Pros. Den. March-April, 1962
LEFF
6.
Fig. B.--Self-induced retrusion of the mandible is artificial mandibular axis. Note the strained expression of tremulous tracings produced by this abnormal effort. The radius are represented by the dotted line. Fig. 7.-An artificial mandibular axis is produced by the opening and closing movement. This axis is similar retrusion seen in Fig. 6.
Fig. used in the face artificial pressing to the
7. an attempt to obtain an and neck muscles and the axis (A) and the artificial the chin backward during one formed by self-induced
closing movements, or the dentist can apply firm, backward pressure to the chin during the movements. The unusual strain involved in forcing the mandible backward is apparent in the photographic records of the movements of the lights during voluntary retrusion. The muscles of the face and neck are taut. The tracings from the side appear irregular. The tracings from the front also give a clear indication of the patient’s struggle to move the mandible back to its most retruded position. However, there was a definite indication that the mandible did open and close on an axis located in the region of the condyles during the retrusive movements (Fig. 6). The artificial mandibular axis was more easily attained by exerting firm pressure on the chin, thus forcing the condyles into their most retruded positions. From this position, the mandible could be opened and closed on an arc of a circle the center of which was in the region of the condyles. Tracings of these retrusive movements show the superimposition of the opening and closing movements in both the front and side views. The tracings from the front are almost vertical. This can only be the result of a true artificial mandibular axis located in the region of the condyles (Fig. 7). CONCLUSIONS
A study of the tracings of the natural opening, closing, and masticating movements of the mandible does not show the presence of a mandibular axis in the
MANDIBULAR
AXIS
DILEMMA
297
region of the condyles. An artificial mandibular axis can be produced during the opening and closing movements by forcing the chin backward. However, an artificially produced mandibular axis, jaw position, or jaw movement is not a normal physiologic movement. There was no evidence of rotation about a mandibular axis in the region of the condyles with concomitant anterior translation in these studies of the opening. closing, and masticating mov&nents. REFERENCE
1. Shanahan, T. E. J., and Leff, A.: Mandibular and Articulator Movements. II. Illusion Mandibular Tracings of Masticatory Movements, J. PROS. DEN. 12:82-85, 1962. 862 CARROLL ST. BROOKLYN 15, N. Y. 10 PLAZA BROOKLYN
ST.
38, N. Y.
of