Hinge

HINGE-AXIS : A DILEMMA IN PROSTHODONTICS
INTRODUCTION
Any three dimentional object that moves in a coordinated rotational
path of motion which is a part of a circle – or ellipse has an axis of rotation.
If the path of motion of the object is a part of a circle, the axis of rotation
itself is not moving.
The figure above illustrates the transverse things. Axis of the
mandible which is the centure of its circular motion.
It the path of motion of the object is a part of an ellipse, the
transverse hinge axis itself must move clinically, the condyles would be
traslating as the patient opened the jaw.
The path of motion of either type of monomer must be angular to
the transverse flinge axis by definition whether the hinge axis moves or
not.
The flinge like action of the TMJ has been described by anatomist,
for over a hundred years. Its application to dentistry however, had to wait
for the gnathological society in 1920’s.
Prior to that, snow, gysi and others had been aware of the presence
and importance of an opening and closing axis, yet their methods were so
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crude that they concluded, that the axis was some place below the
condyles. The gnathological society developed a means of attaching a face
bow kinematic rigidly to the mandibular teeth. This permitted accurate
location of the opening and closing axis. This was the cornerstone of al
future accomplishments and is still the basis for articulation.
According to GPT 1987 term hinge axis is objectionable. It is
described as transverse horizontal axis which is : an imaginary line around
which the mandible may rotate through the sagittal plane.
DEFINITION
The hinge axis is an imaginary line connecting the centre of rotation
of one condyle to the centre of rotation of the other condyle (according to
glossary of prosthetic terms, 1997) around which the mandible may rotate
through the sagittal plane.
LOCATION
From the centers of rotation originate the vertical opening and
closing movements, as well as pure lateral movement. Any combination of
vertical and lateral movement has its centre in the same point. The center
of rotation of each condyle is constant to the condyle and hence to the
mandible the flinge axis (the imaginary) line joining these centeres then is
constant to the mandible.
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As the mandible moves in its various excersions the hinge axis
moves right along with it. The mandible is capable of executing a hinge
like closure in any position.
It permits duplication of all the areas of closure of the mandible on
an instrument and thus the cusps can be tailored to harmonize these arcs.
The flinge axis is located in the most retruded position of the
mandible – the terminal hinge position.
Terminal hinge position is used because it is learnable repeatable
and recordable.
Once it is located, we endeavor to trace the path of this center in the
order to be able to duplicate every possible combination of these two
movements which the patient will use in function.
IMPORTANCE
1. The importance of flinge axis lies in the fact that, by its determination
and transference to an articulator, it is possible to have casts of the teeth
(outh) in the exact dynamic relationship to each other as exists in the
patients head.
2. The flinge axis permits control of the vertical dimension on the
articulator.
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3. It makes it possible to duplicate all the eccentric relations and all
possible contacts of the teeth in these relations.
4. It is only by means of the flings axis (and centric relation) that the teeth
can be related accurately to the terminal flinge position i.e. CR=C.O.
AXES OF MANDIBULAR ROTATION
Rotational movements of the mandible are made around 3 axis.
a) Transverse, b) Vertical, c) Sagittal
That move constantly during the normal jaw functions.
During opening and closing movements, the mandible moves in the
sagittal plane around a transverse axis that passes through both the
condyles.
The transverse axis can be located when opening and closing
movemens occur with the mandible in its most posterior position. The
transverse axis moves with the mandible in protrusive, lateral and
lateroprotrusive movements. Thus, if the mandible is in a forward position
and opening / closing occurs, the rotation would still take place about the
same transverse axis in the lower compartment of TMJ. Since the mandible
cannot be fixed in space in the forward position, the transverse axis would
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be instanteneous for any given location and would move and tilt with the
mandible.
In a lateral excersion, the mandible rotates in a horizontal place
around vertical axis, passing through the condyles of the working side as
the condyle of the non working side moves forwards and medially. Since, it
is physiologically impossible to make a lateral mandibular movement with
no translation of the condyle on the working side, again the vertical axis is
moving and tilting along with the mandible during a lateral mandibular
movement, the condyle on the balancing side that is moving forward and
mesially also moves downwards because of the slope of the articular
eminence. This downward movement of the condyle on the balancing side
courses the mandible to rotate around a sagittal axis in the frontal plane
passing through the condyle on the balancing side. As the condyle on the
working side rotates around the vertical axis and translates, the sagittal axis
moves in a corresponding manner.
When the mandible moves into rotational lateral excursion, the
vertical axis 1. The sagittal axis, 2. Rotate and twist through the space. At
the same time, the left end of the transverse axis, 3. Moves forward and
medially.
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REVIEW OF LITERATURE
1. The flinge like action in the lower compartment of the TMJ was
described in the earliest editions of Gray’s anatomy.
2. Snow recognized the importance of the flinge axis in the mandibular
movements and developed a face bow to be used to transfer the position
of the axis to the articulator.
3. In 1921, McCollum, Sheart and Others reported discovery of the
positive method of locating this axis.
4. Sloane (1952) stated “The mandibular axis is not a theoritial
assumption, but a definity demonstrate biomechanical fact. It is the axis
upon which the mandible rotates in an opening and closing function
when comfortably, not forcefully retruded.
5. Sincher said “The flinge position or TH position is that position of the
mandible from which or in which pure hinge movements of a variably
wide range is possible.
6. According to Brekke in reference to a single IC transverse axis of
rotation theorazied that “This optimum condition does not prevail in the
mandible apparatus which is asymmetric in shape and size and has its
condyloid process joined at the symphyses with no connection directly
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at the condyles. The asumption of a single IC transverse axis is
therefore open to serious quenstions.
7. Page in 1951 suggested the existance of 2 naturally independent non
colinear axes. HC theorazied that since mandible is assymetric in shape
and size, condyles cant be in a common plane of orientation. The
assumption of single IC axis is therefore open of serious questions.
8. Tropazzono and Lazzari found that since multiple condylar hinge axis
points were located, the high degree of infalliability attributed to hinge
axis points may be seriously questioned. After further investigations,
they reached the following conclusious.
1. The presence of multiple hinge axes has been established.
2. Relaxation of the patient, during the making of THA recordings is
essential.
3. Because of the presence of multiple hinge axes points, increase or
decrease the VD on the articulator is contraindicated unless a new
intraocclusal record is made on the patient at the desired VDO.
9. Alill (1963) concluded that there is only one hinge axis. The meaning
of this statement is somewhat diluted by his further conclusion that the
accuracy of location of terminal hinge position is a matter of
interpretation.
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10.Lucia Concluded that “The centers in the TH position provide a definite
starting position relation of the mandible to the maxilla. How the teeth
are arranged when the mandible and maxilla are so related will depend
on one’s belief about CO and CR whether those two factors should be
in harmony or CD should be slightly ahead of CR by having the CR of
the mandible to the maxilla properly related on the articulator, the
dentist can develop the CO accurately according to his own
specifications”.
Controversy has arisen to the presence of a single axis, the methods
used to locate the axis, the method and validity of recording the positions
on the skin for further references and the relationship of THP to the
position of CR. These differences exist in concepts and interpretations of
findings and the findings of others and decide which are valid and
applicable for the present situration.
ERRORS IN LOCATION OF T.H.A
The pin point location of the T.H.A. is indicated only with
gnathological procedures since dynamic extra oral tracings are used to
imitate the 3-D movements of the hinge axis on the instrument. With other
articulators, particularly semi-adjustable types (Hanau), the pin point
location of THA is not necessary. It has been shown mathemetically and
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experimentally that reasonable errors in THA location (± 5 mm) produce
extremely small AP and displacement.
When the CR record is removed and the articulator is closed.
When the CR record is removed, the articulator closes on its hinge
axis, rather than the HA of the patient. The AP displacement at 2nd
molar in
approx 0-2mm.
The same reasonable error in the THA location (± 5mm) has
practically no effect on the eccentric intra occlusal record readings on the
articulators. This evidence supports that the THA can be located by
arbitrary means. Preston stated that a superior – infererior. Error in HA
location results in a larger occlusal discrepancy than an error in AP
location.
Even though, the THA may be found kinematically, there is no
clinical method of finding the vertical end sagittal axes. The horizontal
plane of the articulator actually respresents the sagittal axes of the pts at the
VDO which the jaws in CR. The vertical axes of the patient are
respresented by the posts of the articulator.
The orientation of occlusal plane is not arbitrary, because once the
maxillary cast is attached to the articulator, the relationship of the cast to
the vertical and sagittal axes of the instrument is established. The
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relationship of the maxillary cast to these axes on the articulator is different
than the relationship of the maxillae to these same axes on the patient. As a
result of this discrepancy protrusive records transfer erroneous condylar
readings. With the semi adjustable articulator, the balancing cusp inclines
are primarily effected. The magnitude of error is so small that it justifies
the use of face bow.
Thus, the location of the HA is mainly related to the accurate
transfer of the CR record rather than eccentric condylar inclination. A
reasonable error in the THA location (which ± 5 mm) results in negligible
A-P mandibular displacment when CR record is removed and the
articulator is closed. This A-P displacement can be limited further by a
thinner I.O., centric relation record.
When the CR record is obtained without a change in the VD, there
is no A-P mandibular displacment at all. The face bow mounting in this
instance is mainly concerned with the accurate transfer of the condylar
inclination. A marked change in the V.D. because of a thick CR record
causes guess A-P mandibular displacement that requires more extensive
intra oral adjustments.
Another technique is described by La Pera for location of HA. In
this technique, the patient opens his mouth as far as possible to develop a
record of the opening path. It the locating needle is in front of HA, the
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opening path of the needle will have a greater inclination than the path
during forward movement. If the locating needle is behind HA the opening
path would be less inclined than the path during forward movement. If the
needle is above or below the HA, the two lines will cross each other.
However, only when the needle is located exactly on the HA will a single
line exist for both forward and opening movement.
Vertical tracings that are as long as possible arch made during
opening movements. A line is drawn parallel to these tracings on which the
desired axis should be located. Then tracings of both forward and opening
movements are made, each time farther posteriorly, until both each other
become a single line. The desired HA is located at that point.
The accuracy in locating the HA at least 5 times greater than with
other methods can be obtained by using movements of the Mb.
MOVING T.H.A.
Moving THA can be located during untrained opening and closing
movements.
Several flinge axis locations are illustrated on the condylar path. At
the incisors, the arc of rotation corresponding to each T.H.A. location is
seen. When the points on each arc of rotation at the incisors are joined, the
line represents the path o mandibular openings. The character of the
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mandibular rotational path, measured at the precisors, is dependent on the
linear movement of the THA itself in relation on the incisors rotational
movements. The ratio of these 2 factors alters the character of madibular
rotation.
Clinically, no purpose is served by recording the path of the moving
THA (functional movement) out first locating the starting or terminal hinge
position. During functional movements, the pointer on the mandible clutch
assembly would merely scribe on arc, that would be useless for locating the
terminal position. Therefore, the trained mandibular flinge movement is
used to locate the moving THA.
HINGE AXIS AND MANDIBULAR MOTION PATTERN
The pattern of movement of lower incisors during opening and
closing with minimum protrution is shown in the figure.
Two distinct curvatures are seen from the lateral aspect. The first
rotational path is located upto 35mm of opening measured at the incisors
which results from a moving THA located through or near the condylar.
As the condyles go past the articular eminence (> 35mm) the pattern
changes. This new pattern of motion indicates that the THA of rotation
shifts inferiorly along the ramus of the mandible. The THA shift after 35
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mm of opening should not be confused with the transographic concept of 2
independent transverse axis for all ranges of motion.
The shifting of the axial center of rotational away from the condyle
(with more than 35 mm opening) indication that the condylar anatomy
itself is not the only factor in the mandibular movements.
HINGE AXIS AND CENTIC RELATION
The act of securing a centric intra occlusal record is related to the
terminal flinge closure. We attempt to “freeze” the terminal hinge closure
at a convenient vertical opening, but were it not for the flinge axis, it would
not be possible to secure an accurate centric intra occlusal record.
During location of terminal flinge axis, mandible is guided
posteriorly unitl TM ligaments are taut which the condyles to the swing
upwards into contact with the anterior slopes of their respective fossae. The
TM ligaments cannot be stretched under normal condition and since the
bony structure of fossa and condyles donot seem to change readily, flinge
axis position is repeatable final point is “ it is impossible to cheek a centric
intra occlusal record without an axis mounting”.
THA – SINGLE OR SPLIT
The proponants of gnathology say that there is 1 THA common to
both the condyles which can be so accurately located so as to justify their
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point tattoring. The proponents of transographics claim that each condyle
has a different THA and that transographs is the only instrument that
duplicate this.
PAGE first suggested that there are 2 non interesting THA
controlling rotation in the sagittal plane. The reasoned that since the
condyles are assymetric, theyb cannot lie in the common plane of
orientation. Since the condyles are non spherical, it is evident according to
mechanics that they should have line centers. Page’s hypothesis is not only
mechanically found but is supported by radiographic studies of condylar
positions reporting its asymetry.
FLANK from his studies stated that symmetrically laced condyle of
the articulator cannot simulate flinge radii from the transverse axes to a
midpoint on the symphysis are lingual and that the condyles are not
symmetrically oriented in a common plane.
RANGER in speaking of the location of the HA says that “In
locating this axis, one side were rotating while other was both rotating the
rotating side would be working on the true axis while the point on the other
side woule be one a false axis or combination axis.
This would lead to a commonly held fallacy of 2 transverse axis.
He further status that a rigid body cannot remove around 2 axes
simultaneously according to geometry.
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Granye’s explanation of the HA concept is as follows:
In pure vertical motion, the condyle revolves about a horizontal
axis. In pure vertical motion, it revolves about a vertical axis. In any
intermediate rotation, it revolves about an axis at right angles to the plane
of rotation. All these axes meet at a point on the condyle which he called te
mysterious center of rotation. When these points in 2 condyles are
connected by an imaginary line, it forms the tinge axis.
CLINICAL IMPORTANCE OF THA
1. The trained flinge movement is used only to locate the starting point of
mandibular opening and not the condylar path itself.
2. The location of THA is mainly related to the accurate transfer of CR
record rather than eccentric condylar inclinations.
3. The T.H.A plus one other anterior point serves to locate the maxillae in
space and to record the static starting point for functional mandibular
movements.
4. The recording and reproduction of the opening axis of the mandible
enable a given occlusal relation to be reproduced on the anticulator at
any height or vertical relation out the necessity of making or new I.O –
CR record at new V.D.O. This change in VDO is important in Cd
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constriction because sometimes the VD is varied to meet esthetics and
functional requirements.
5. The flinge axis expression a relation of border movements which
involve or include the limits if all physiological movements, thus could
be reproduced in artificial teeth arrangement.
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Hinge

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