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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


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.


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.


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


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.


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


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.


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


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 2 nd 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


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 Âą 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


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


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


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


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.


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


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.


Hinge/ dental implant courses by Indian dental academy