ANATOMY & PHYSIOLOGY OF TMJ TEMPOROMANDIBULAR JOINT The area where cranio-mandibular articulator occurs is called the temporomandibular joint. Ginglymo-arthroidal compounds synovial joints: Ginglymoid joint hinging movement in one plane. Arthroidal joint hinge + gliding movements. Compound Presence of three bones (articular disc is considered as non-ossified bone). EMBRYOLOGY The temporomandibular joint develops relatively late in embryonic life compared with the large joints of the extremities. During the seventh prenatal week the jaw joint lacks the condylar growth cartilage, joint cavities, synovial tissue and articular capsule. The two skeletal elements, mandible and temporal bone are not yet in articular contact with each other. In contrast with this in the same prenatal specimen all the major components of the elbow, hip and knee joints are present in a form and arrangements closely resembling that seen in the adult. In a week-old human embryo, Meckel’s cartilage, the cartilage bar of the first branchial area extends all the way from the chin to the base of the skull. It persists in this form, serving as a temporary strut or scaffolding against with the mandible and the base of the skull develop until the temporomandibular joint takes over this function in fetal life. 1
The articular disk is one of the first components of the joint to become organizable from its earlier appearance, in the 6week old embryo. The disk is associated with the mandibular component of the joint and seems to be a derivative of the first branchial arch. Number of investigators â€“ Kjellberg (1904), Hapman and Woolard (1938), Symmons (1952), Moffett (1957 believe the extension of the lateral pterygoid muscle posteriorly between the temporal squama and the mandibular condyle to the malleus contributes to the formation of the medial part of the articular disk. Other investigators refer to this connection as the retrodiscal ligament. Most synovial joints develop from blastemata. The temporomandibular joint, like the joints of the clavicle is formed from discontinuous blastemata separated from each other by a zone of undifferentiated mesenchyme in the embryo. As the bastemata approach each other through growth of the condyle, the intervening mesenchyme condenses in to layers of fibrous connective tissue, which form the peculiar articular tissue seen in this joint. During the twelth week the condylar growth cartilage makes its first appearance and the condyle begins to develop a hemispherical articular surface. By the thirteenth week the condyle and articular disk have moved up into contact with the temporal bone. Only when such articular contact has been made to do the joint cavities develop, the inferior space appearing first. Before the disk actually becomes compressed between the condyle and the temporal bone, the entire disk is vascularized. Blood vessels from the terminal branches of the external carotid artery and the associated veins enter the disk posteriorly and extend completely through it to anastamose with branches coming in anteriorly from the pterygoid vascular plexus. By the twenty-sixth week all the components of the temporomandibular joint are present except for the articular eminence. Meckelâ€™s cartilage still extends through 2
the Claserian fissure, but by the thirty-first week it has been transformed into the sphenomandibular ligament. The Claserian fissure is the opening between the squamous and tympanic parts of the temporal bone through which Meckel’s cartilage passes into the middle ear in the fetus. It becomes the squamo-tympanic fissure after birth. I)
Anatomy of the TMJ:
The TMJ consists of 4 main structures: a) Condyle. b) Squamous part of the temporal bone. c) The articular disc. d) Ligaments. a)
The portion of the mandible which articulates with the cranium around which movement occurs.
From the anterior view, it has medial and lateral projections called poles. Medial pole is more prominent than the lateral pole.
Media – Lateral length 15-20mm. Anterio-posterior width 8-10mm.
The actual articulating surface of the condyle extends both anteriorly and posteriorly to the most superior aspect of the condyle. The posterior articulating surface is greater than the anterior articulating surface.
A given point on each condyle has a free but relatively limited mobility along its cranial joint surface. this is called as â€œcontact movement surfaceâ€? of the condylar point. It is about 10-12mm long and 2-3mm broad.
Contact movement surface of the incisal point is slightly over 11mm deep (sagittal direction) and 20mm broad (frontal direction).
The tooth bearing part of the mandible therefore has a complete freedom of movement inside a relatively marrow but long space.
Squamus part of the temporal bone: -
Mandibular condyle articulates at the base of the cranium with the squamous portion of the temporal bone. it is referred to as the articular/glenoid fossa.
Posterior to this fossa is the squamo-tympanic fissure which runs medio-laterally.
Immediately anterior to the fossa is a convex bony prominence called the articular eminence. Steepness of this surface dictates the pathway of the condyle when the mandible is positioned anteriorly.
The posterior roof of the mandibular fossa is quite thin, indicating that this area of temporal bone is not designed to sustain heavy loads. The articular eminences however is composed of thick dense bone and is more likely to tolerate such heavy forces.
The articular disc: -
Composed of dense fibrous connective tissue devoid of any blood vessels or nerves.
In the sagittal plane, it can be divided into 3 regions according to thickness. The central area is the thinnest, called the intermediate 4
zone. The disc becomes thicker anteriorly and posteriorly with the posterior zone, slightly thicker than the anterior zone. -
From an anterior view, the disc is more thicker medially that laterally, which results in increased space between the condyle and the fossa towards the medial aspect of the joint.
The articular disc is attached posteriorly to a region of loose connective tissue which is highly vascularised and innervated called the retrodiscal tissue.
Superiorly Superior retrodiscal lamina (elastic fibers) attaches the disc to tympanic plate – Bilaminary zone. Inferiorly Inferiorly retrodiscal lamina (collagenous fibres) attaches the inferior border of the disc to the posterior margin of the articulate surface of the condyle. -
Anteriorly, the articular disc is attached to the capsular ligaments.
Superior attachment anterior margin of the articular surface of the temporal bone. Inferior attachment anterior margin of the articular surface of the condyle. -
Articular disc divides the joint into 2 distinct cavities: i.
Superior cavity bordered by the mandibular fossa and the superior surface of the disc.
Inferior cavity bordered by the mandibular condyle and the inferior surface of the disc.
Classified as: i. Functional ligaments: Collateral ligaments. Capsular ligaments. Temporomandibular ligament. ii. Accessory ligament - Sphenomandibular ligaments. Stylo-mandibular ligaments. i.
Collateral-ligament (discal ligaments): -
Attach the medial and lateral borders of the articular disc to the poles of the condyle.
They function to instruct the movements of the disc away from the condyle.
They are responsible for the hinging movement of the TMJ. Capsular ligament:
The entire TMJ is surrounded and incompassed by the capsular ligament.
It is attached superiorly to the temporal bone along the borders of the articular surfaces of the mandibular fossa and articular eminence. Inferiorly it attaches to the neck of the condyle.
It resists any medial, lateral or inferior forces that tend to separate or dislocate the articular surfaces.
A significant function is to encompass the joint and retain the synovial fluid. 6
Temporomandibular ligament (lateral ligament) 2 parts: i)
Outer oblique portion â€“ it extends from the outer surface of the articular tubercle and zygomatic process posteroinferiorly to the outer surface of the condylar neck. It resists the excessive dropping of the condyle, limiting the extent of mouth opening. If the mouth was to open wider, the condyle would need to move downward and forward across the eminence. This change in opening movement is brought about by the tightening of the TM ligament.
Inner horizontal portion: extends from outer surface of articular tubercle and zygomatic process and attaches to the lateral pole of condyle and posterior part of articular disc. It limits the posterior movement of the condyle and disc.
Sphenomandibular ligaments: Arises from spine of sphenoid and attaches to lingual. Does not have significant limiting effects on the mandible.
Stylomandibular ligament: Across the styloid process and attaches to angle and posterior border of ramus of the mandible limits excessive protrusive movement of mandible.
Neuromuscular aspect of the masticatory system: The energy required to move the mandible and allow function of the masticatory system is provided by the muscles. The most important of these are the muscles of mastication each of which has a different function but all act in a cooperative way to effect jaw movement. However
collaborate with other muscle groups namely the suprahyoids, infrahyoids and the post vertebral muscle groups. 7
I] Masticatory Muscles: 1.
II] Suprahyoid muscles: 1. Mylohyoid. 2. Geniohyoid. 3. Stylohyoid. 4. Digastric. Their function is two fold: -
If the muscles of mastication, close the jaws and fix the mandible in its position, the suprahyoid muscles will elevate the hyoid bone and larynx which is attached to it by a membrane, which is necessary for swallowing.
If however the infrahyoids are contracted, the hyoid bone is made stable and immovable. If the suprahyoids then contract, against the secured hyoid bone the mandible will be retracted and depressed. Its movement will be down and back.
III] Infrahyoid muscles: 1.
They mainly function to depress the hyoid bone and along with the suprahyoid muscles stabilize the hyoid bone during function. IV] The post vertebral muscles are also important in chewing and maintaining functional balance. They form a continuous chain from the base of the skull to the base of the spine. They are anti-gravity muscles which sustain functional posture in relation to chewing. Masseter
Elevation Protraction Extreme lateral movements
Principal positioner. Elevator Unilateral contraction leads to lateral movement on side
Elevation Lateral positioning of mandible Simple protraction Sub activity dividing protraction and opening
Protrusion Lateral movement (LP + MP + M + T)
BLOOD SUPPLY Generally, all blood vessels in the vicinity of a joint contribute to its supply. Thus joints are usually excellent sites for the development of a collateral circulation. Its blood supply is derived from articular branches of the numerous arteries that make up the terminal field of the external carotid artery. LYMPHATIC DRAINAGE: The lymphatic drainage of the joint has been described briefly by Tanasesco (1912) who found lymph channels on each surface of the joint. These channels are most prominent on the lateral and posterior surfaces. The lymphatic vessels on the lateral surface drain into the preauricular and parotid nodes. On the posterior surface, six or seven channels converge on the external carotid artery, fuse in to large trunks, across the digastric muscle, and enter the submandibular nodes. NERVE SUPPLY: Auriculo temporal nerve. Massetric nerve laterally. Mechanics of mandibular movements: Mandibular movements occur as a complex series of inter related 3dimensional rotational and translational activities. It is determined by combined and simultaneous activities of both TMJâ€™s. Two types of movements occur in the TMJ: (i)
Rotational movements: Rotation is the movement of a body about aits axis. Rotation occurs
when the mouth opens and closes around a fixed point or axis within the condyles. Rotation occurs in the inferior cavity of the joint between the superior surface of the condyle and inferior surface of the articular disc. Rotation of the mandible can occur in 3 reference planes around a point called the axis. They are: a)
Horizontal axis of rotation: -
Mandibular movement around a horizontal axis of rotation is an opening and closing motion. It is referred to as hinge movement and the horizontal axis around which it occurs is referred to as hinge axis.
The hinge movement is the only example of mandibular activity in which a pure rotational movement occurs.
When the condyles are in the most superior position in the articular fossa, and the mouth is purely rotated open, axis around which movement occurs is the “terminal hinge axis” – to study in detail.
b) Frontal (vertical axis of rotation): Mandibular movement around the frontal axis occurs when one condyle moves anteriorly out of the terminal hinge position with the vertical axis of the opposite condyle remaining in the terminal hinge position.
c) Sagittal axis: Mandibular movement around the sagittal axis occurs when one condyle moves inferiorly while other remains in the terminal hinge position. II)
Translational movements: It can be defined as a movement in which every point of the growing
defect has simultaneously the same velocity and direction. Translation occurs within the superior joint cavity between the superior surface of the articular disc and inferior surface of the articular fossa. Border movements: When the mandible moves through the outer range of motion, reproducible desirable limits result, which are called as border movements. Border movements can be described in 3 reference planes: (i)
Sagittal plane border and functional movements:
In the sagittal plane, it has 4 distinct components: a) Posterior opening border.
Determined by ligaments and morphology of T.M.J.
b) Anterior opening border. c) Superior contact border.ďƒ Occlusal and incisal surfaces d) Functional ďƒ neuromuscular system
Posterior opening border movements: Occurs as two stage hinging movements. In the first stage, the condyles
are stabilized in their most superior positions in the articular fossae. The most superior condyle position from which hinge axis movement occur is called anterior relation. (Retruded contact position, terminal hinge axis or ligamentous position). In anterior relation, mandible can be rotated around the horizontal axis to a distance of only 20-25mm as measured between the incisal edges of the maxillary and mandibular teeth. At this point of opening, the TMJ ligament tightens, after which, continued opening results in an anterior and inferior translation of the condyles. This is the 2ns stage of the posterior opening border movements. As the condyles translate, axis of rotation of the mandible shifts into the bodies of the rami, most likely in the area of attachment of the sphenomandibular ligament. Maximum opening is reached when the capsular ligament will prevent further movement of the condyles. This opening is in the range of 40-60mm measured between the incisal edges of the Mx and Md teeth. b)
Anterior opening border movement: When the mandible is maximally opened, closure accompanied by
contraction of the inferior lateral pterygoids (which keep the condyles positioned anteriorly) will generate the anterior opening border movement. Since the maximum protrusive position is determined in part by the stylomandibular ligaments, as closure occurs, tightening of the ligament produces a posterior movement of the condyles. 13
Condylar position is most anterior in the maximally open but no the maximally protruded position. c)
Superior contact border movements: Throughout this entire movement, tooth contact is present. It depends
Amount of variation between centric relation and maximum intercuspation positions.
Steepness of cuspal inclines of posterior teeth.
Amount of vertical and horizontal overlap of anterior teeth.
Lingual morphology of maxillary anterior teeth.
The general inter arch relationships of the teeth.
The initial contact in terminal hinge closure (centric relation occurs between the mesial inclines of a maxillary tooth and the distal inclines of the mandibular tooth.
When muscular force is applied to the mandible, a supero-anterior movement or shift will occur until the intercuspal position is reached. This slide is present in 10% of the population and is approx 1.25mm Âą1mm.
When the mandible is protruded from maximum intercuspation contact between the incisal edges of the mandibular anterior tooth and lingual inclines of the maxillary anterior teeth results in an antero-inferior movement of the mandible.
This continues until the maxillary and the mandibular teeth are in an edge to edge relationship, at which time a horizontal pathway is followed. 14
As the incisal edges of the mandibular teeth pass beyond the incisal edges of the maxillary teeth, the mandible moves in a superior direction, until the posterior teeth contact.
The occlusal surfaces of the posterior teeth, then dictate the remaining pathway of the maximum protrusive movement which joins with the most superior position of the anterior opening border movement.
Functional movements: -
They usually take place between the border movement and therefore considered free movements.
Most functional activities require maximum intercuspation and therefore typically begin at and below the intercuspal position.
When the mandible is at rest it is found to be located approx-24mm below the intercuspal position. This position has been called cervical rest position. This position is variable.
The myostatic reflex is active at this position, and the teeth can be quickly and effectively brought together for immediate function.
Postural effects on functional movements: a)
Head position is erect: Postural position of the mandible is 2-4mm below the intercuspal position (elevator muscles contact ďƒ mandible goes directly to ICP).
postural position of the mandible will be altered to a slightly retruded position. This change is related to the stretching and elongation of various tissues that are attached to and support the jaws (elevation
muscles contact â€“ path of closure is slightly posterior to path of closure in erect position). c)
Head positioned 30Â° downward (alert feeding position). If the elevator muscles contact with the head in this position, the path of closure will be slightly anterior to that in upright position. (Elevator muscles contract â€“ path of closure is slightly anterior to path of closure in erect position).
Horizontal plane border movements: -
Gothic arch tracer is used to record mandibular movements in the horizontal plane.
Consists of recording plate attached to the maxillary teeth and a recording stylus attached to the mandibular teeth. As the mandible moves, stylus generates a line on the recording plate that coincides with this movement.
The mandibular movements when viewed in a horizontal plane are rhomboid-shaped and has 4 distinct component movements plus a functional movement.
Left lateral border.
Continued left lateral border with protrusion.
Right lateral border.
Continued right lateral border with protrusion.
a) Left lateral border:
With the condyles in CR position, contraction of the right inferior lateral pterygoid, will cause the right condyle to move anteriorly and medially. If the left inferior lateral pterygoid stays relaxed the left condyle will remain situated in CR and the result will be a left lateral border movements. b) Continued left lateral border movements with protrusion with the mandible in the left lateral border position contraction of the left inferior lateral pterygoid muscle along with continued contraction of the right inferior lateral pterygoid muscle will cause the left condyle to move anteriorly and to the right. This causes a shift in the mandibular midline back to coincide with the midline of the face. c) Right lateral border movement: Once the left border movements have been recorded the mandible is returned to CR and the right lateral border movements are recorded. -
Contraction of the left inferior lateral pterygoid muscle will cause the left condyle to move anteriorly and medially.
If the right inferior lateral pterygoid muscle stays relaxed, the right condyle will remain situated in the CR position. The resultant movement will be right lateral border movement.
d) Continued right lateral border movement with protrusion contraction of the right inferior lateral pterygoid muscle along with the continued contraction of the left inferior lateral pterygoid muscle will cause the right condyle to move anteriorly and to the left. -
Since the left condyle is already in its maximum anterior position, the movement of the right condyle to its maximum anterior position will cause the shift in the mandibular midline back to coincide with the midline of the face.
e) Functional movements: 17
As in the sagittal plane, functional movements in the horizontal plane most often near the intercuspal position. During chewing, range of jaw movements begins same distance away from MICP but as food is broken down into smaller particles, jaw action moves closer and closer to ICP. III)
Frontal (vertical) border and functional movements: When mandibular motion is viewed in the frontal plane, a should like
pattern can be seen that has 4 distinct movement components: a)
Left lateral superior border.
Left lateral opening border.
Right lateral superior border.
Right lateral opening border.
The movement in the plane has not been traditionally traced, on understanding of them is useful in revisualizing mandibular activity 3dimensionally. a) Left lateral superior movement: When the mandible in maximum intercuspation a lateral movement is made to the left. A recording device will disclose an inferiorly concave path being generated. The precise mixture of this path is primarily determined by the morphology and inter arch relationships of the maxillary and mandibular teeth that are un contact during this movement. Of secondary influences are the condyle-disc-fossa relationships and morphology of the working or rotating side TMJ. 18
b) Left lateral opening border: From the maximum left, lateral superior border position on opening movement produces a laterally convex path. As maximum opening is approached, the ligaments lighter and produce a medially directed movement that causes a shift in the mandibular midline to coincide with the midline of the face. c) Right lateral superior border movements: Once the left frontal border movements are recorded the mandible is returned to maximum intercuspation from this position a lateral movement is made to the right that is similar to left lateral superior border movements. d) Right lateral opening border movements: From the right lateral superior border position on opening movement produces a laterally convex path similar to that of the left lateral opening border movements. e) Functional movements: As in other planes, functional movements in the frontal plane begin and end and the intercuspal position. During chewing, the mandible drops directly inferiorly until the desired opening, is achieved. It then shifts to the side the bolus is placed and rises up. As it approaches maximum intercuspation, the bolus is broken down between the opposing teeth. It the final mm of closure the mandible quickly shifts back to the ICP. Envelope of motion (given by Posselt): By combining mandibular border movements in all 3 planes a 3-D envelope of motion is produced. This represents maximum range of movement of the mandible. 19
The superior surface of the envelope is determined by tooth contacts. Other borders are primarily determined by ligaments and joint anatomy.
REFERENCE POINTS POSTERIOR POINTS OF REFERENCE The arbitrary method is an accepted technique for locating the mandibular hinge axis. Although many studies have compared various arbitrary hinge axis points with kinematic location, there is no consensus as to which arbitrary point most closely and consistently lies on or near the kinematic axis.
Various arbitrary hinge axis points: Name
12mm anterior to posterior border of tragus and 5mm inferior to line extending from the superior border of tragus to OCE.
12mm anterior to center of TAM and 2mm Frankfort plane. 12mm anterior to center of EAM and 2mm inferior to portioncanthus line.
According to the design of their ear-bow in anteroposterior direction at anterior wall of EAM and in superior-inferior direction approximately at level of most prominent point of 21
posterior border of tragus. Prothero
On line from superior margin of EAM to OCE interecting with line 13 edge of EAM according to Rachey condyle marker.
12mm anterior to most prominent point of posterior border of
tragus on line from it to OCE.
13mm anterior to posterior margin of tragus online from the center of tragus to OCE.
13mm anterior to anterior margin of EAM on line from superior margin of EAM to OCE.
10mm anterior to center of spherical insert of his face-bow and 7mm below Frankfort plane.
ANTERIOR POINTS OF REFERENCE Three points in space determine the position of the maxillary cast in an articulator the dentist is most frequently concerned with selecting the posterior two of the three reference points. The selections of the anterior point of the triangular spatial plane determine which plane of the head will become the plane of reference when the prosthesis is being fabricated. The dentist can ignore but cannot avoid the selection of an anterior point. The act of affixing a maxillary cast to an articulator relates the cast to the articulators hinge axis, to the anterior guidance, and to the mean plane of the articulator. The act achieves greater importance by the use of a constant third point of reference. When three points are used the position can be repeated, so that different maxillary casts of the same patient can be positioned in the articulator in the same relative position to the end controlling guidances. SELECTION OF AN ANTERIOR REFERENCE POINT. In selecting the reference plane, the dentist should have knowledge of the following anterior points and the rationale for selection of each. 1. Orbitale. In the skull, orbitale is the lowest point of the infraorbital rim. On a patient it can be palpated through the overlying tissues and the skin One orbitale and two posterior points that determine the horizontal axis of rotation will define the axis-orbital plane. Relating the maxillae to this plane will slightly lower the maxillary cast anteriorly from the position that would be established if the Frankfort horizontal plane were used. Practically, the axisorbital plane is used because of the ease of locating the marking and because the concept is easy to teach and understand.
2. Orbitale minus 7mm. The Frankfort horizontal plane passes through both poria and one orbital point. Because potion is a skull landmark-, Sicher recommends using the midpoint of the upper border of the external auditory meatus as the posterior cranial Iandmark on a patient. Bergstrom's arcon articulator automatically compensates for this error by placing the orbital index 7mm higher than the condylar horizontal axis.
3. Nasion minus 23mm. According to Sicher, another skull landmark, the Nasion can be approximately located in the head as the deepest part of the midline depression just below the level of eyebrows. The Nasion guide, or positioner, of the Quick-mount face-bow, which is designed to be used with. the WhipMix articulator, fits 'into this depression.
4. Incisal edge plus articulator midpoint to articulator axis-horizontal plane distance. Guichet has emphasized that a logical position for the casts in the articulator would be one which would position the plane of the articulator. A deviation from this objective may position casts high or low relative to the instrument's upper and lower arms. The effects of these high or low positions may be inaccurate occlusal relationships due to dimensional changes in the artificial stone or plaster used for cast-mounting purpose In accordance with this concept, the distance from the articulator's midhorizontal plane to the articulator's axis-horizontal plane is measured. This same distance is measured above the existing or planned incisal edges on the patient, and its uppermost point is marked as the anterior point of reference on the face. The inner canthus is used because it is an accessible, unchanging landmark on the head. With this technique the face-bow transfer will carry the predetermined posterior points of reference and this anterior point of reference to the articulator's axishorizontal plane. 5. Alae of the nose. A part of many complete denture techniques is to make the tentative or the actual occlusal plane parallel with the horizontal plane. A line from the ala of the nose to the center of the auditory meatus describes the Camper's line. Augsburger concluded, in a review that the occlusal plane parallels this line with minor variations in different facial forms.
Other intraoral landmarks, esthetics, consideration for the residual ridges, and tongue and cheek guidance factors may alter the final occlusal plane. Also when relating the maxillary case in space to a horizontal reference plane, the relating planes are usually thought of and being viewed from the lateral aspect. When viewed from the frontal aspect, there are reference lines as well. The hinge line,-- the interpupillary line, and a transverse line across the occlusal surfaces are three common frontal-view reference lines. The later two are observed in the articulator. Generally these three lines are not parallel. This is caused by posterior hinge reference points that are not equidistant from the eye pupas. An occlusal plane that is parallel to the interpupillaly line will be pleasing to the eye of the viewer. It cannot be guaranteed that an occlusal plane parallel to the hinge will have the same pleasing appearance.
VARIOUS SCHOOLS OF THOUGHT From early experiments there have evolved four main schools of thought regarding the horizontal axis. They are as follows. Group I. Absolute location of the axis. There are those who believe that there is a definite transverse axis and it should be located as accurately as possible. McCollum, Stuart &Lucia believe that the hinge axis is a component of every masticatory movement and cannot be disregarded. The investigators who endorsed this concept have established a repeatable point of reorientation from which the above information and relationships may be obtained. Group II: Arbitrary location of the axis. The second group includes those who believe that the arbitrary location is not worth the added effect. Craddock for one stated that the search for the axis, in addition to being troublesome, is- of no more than academic interest. Though, this group believed in location of the axis. Group III Nonbelievers in the transverse axis locations. Then, there is a third group who believes it is impossible to locate the terminal hinge position with accuracy. Lauritzen and Watford confirm this, and Kurth and Feinstein, using an articulator and a range of 2mm. That could be considered a point of rotation or non-movement. The opening and closing movement was limited to approximately 10 to 11 degrees. Borgh and Posselt could not record the axis on a modified Hanau H articulator without errors. The errors amounted -to 1 to 5mm. At a 10 to 15 degree opening.
Beck has proposed another reason for doubting the validity of the hinge axis location. He claims that there can be many compensating movements of the condyle other than pure rotation, and that these compensating movements are movements of translation and side shift that are integrated with the movement of rotation. He concludes that the opening and closing hinge movement of the mandible, together with its fragmentary movements, cannot be repeated by the opening and closing movements of an articulator, which is about one axis only. Therefore, an arbitrary terminal hinge position would or could be just as accurate as one located with a kinematic face-bow. Group IV Split axis rotation These are believers of the Transographic theory. They believe in the "split axis" with which each condyle rotates independently of the other. Slavens states "by definition, an axis is always a line, never a point. Again, by definition, an axis is invariably perpendicular to the path or plane of rotation It controls. That means that the transverse axis of each joint is a line and both of these are perpendicular to the same plane of opening and closing rotation. The significance of the fact that these two transverse axes are never symmetrically positioned in the same head becomes 'inescapable. Being perpendicular to the same plane of rotation, they are parallel to each other even though asymmetrically positioned and, by definition parallel lines never meet". SINGLE AXIS OR MULTIPLE About 1950, Dr. William Bransted, Dr. Raymond Gravy and D r. Robert Okey conducted an experiment, which should the presence of a single axis. Dr. Arne Lauritzen., working with a study group in 1957, repeated the same experiment and arrived at the same conclusion. 28
In 1959 the committee of the greater New York academy of prosthodontics repeated this experiment and concluded that there was only one transverse axis through both condyles. Later Lucia also conducted extensive experiments and concluded the presence of a single axis. McCollum And Stuart stated that only when a single THA exits, can the CR registrations be made at an increased VD of occlusion.. The Transographic concepts postulated the existence of 2 materially independent, non-collinear axes. Trapozanno & Lazzari support this theory. Later Wienberg 7' conducted experiments to support this Transographic theory. They concluded that multiple axes exist and their presence opens the Field for interesting conjecture. Aull discussed the impossibility of the presence of a split HA, or of a different HA for each condyle acting simultaneously. Harry page in his experimentation in 1979 also supported the above views. They don't defy the basic concept but view the rotational movement as occurring in a manner different from that which is commonly thought of as occurring in the type of movement. It is a tangential type of hinge movement occurring between a movable extension and a fixed surface (condyle and glenoid fossa).
THE HINGE AXIS AND CENTRIC RELATION To secure a centric inter-occlusal record,, we attempt to "freeze" the terminal hinge closure at a convenient vertical opening. Without the hinge axis, we would be unable to secure an accurate centric inter-occlusal record because to obtain such a record the recording medium must not be penetrated by the teeth or the occlusion rims. In order to -avoid penetration (atleast in dentulous cases), we must obtain our centric interocclusal record in an open relationship, and if we were not in the same arcs of closure, our efforts would be useless. It is impossible to check a centric inter-occlusal record without an axis mounting.
LOCATION OF THE HINGE AXIS Different methods have been used to locate and transfer the hinge axis to the articulator. The first actual kinematic location was evolved through the California Gnathologic society under the leadership of McCollum and credit for the idea of the mechanical location of an axis was given to Harlan. The first location employed a modified Snow face-bow and consumed as much as 8 hours. 30
In 1957 Posselt analyze the transverse hinge movement by geometric construction from profile roentgenograms, axis points recorded by means of kinematic face-bow and checked by profile roentgenograms and also by Gnathothesiometric measurements. The Gnathothesiometer a device which was proved useful for the measurement of the position of the mandible. This apparatus allows measurements (at three points) in the three main planes on freely movable casts of the lower law.
In 1970 Knapp developed a measuring device using 6 potentiometers as sensors.
Long in 1970 used a intraoral device to locate the transverse hinge axis and he used a Buhnergraph to verify the records. He encountered errors in either location or its transfer to the articulator, which he corrected by moving the recording shaft to the Buhnergraph and until the records made by it coincides.
The computer graphics simulation was developed in 1978 to display the effects of mandibular movement parameters of both the maxillary and mandibular teeth in the occlusal plane, which can be graphically observed in the horizontal plane. In 1979 Pantographic tracing using Denar Pantograph was used by Jackson to record the mandibular movement. Beard and Clayton in 1981 devised a modified hinge axis locator which was very similar to that used by Trapozzano and Lazzari but has multiple styli. The results obtained by this were supporting the single axis theory.
Hobo et al in 1983 developed a new electronic measuring device capable of measuring 6 degrees of freedom with an accuracy Âą0.06mm. 32
In the year 1988 Getz" et al used a double recording styli (at a distance of 2-4 inches from estimated axis area) to identify the true axis of rotation.
In 1992 the terminal hinge axis was located on the individuals using the Axiotron, a computerized Axiograph by Kinderknecht et al. In the later years there have been very few experiments, which used any newer techniques.
TECHNIQUE FOR LOCATING THE HINGE AXIS The location and transference of the hinge axis are not very difficult procedures, but they must be carefully carried out with great care because they form the foundation for many other procedures. A reference plane or clutch is cemented to the lower teeth with Truplastic. Graphlined flags are placed on the side of the face over the condyle areas to eliminate any skin movement distraction. These flags may be attached to the maxillae by means of a crossbar and a maxillary clutch, or they may be held in place by a head frame or other contrivance. A crossbar is attached to the lower reference plate or clutch. Adjustable side arms are placed on the lower. cross bar with the styli in the vicinity of the condyles. The patient must now be instructed *in the hinge type of movement. The pivoting part of the compass is on the center of rotation in the patient's condyle. The stylus point is the tracing part of the compass. If we get the tracing point exactly on the pivoting point, there will be no arcing on the tracing point. As we-approach the center, the arcs will become smaller and little more arcing is required to magnify the arc. If there is any arcing we continue adjusting until it disappears completely. The axis center must -be located on each. side. What we are locating is the hinge action on the 34
side of the face. It is a point on the hinge axis and not the actual center of rotation. The actual center is approximately 10 or 11 mm medial to this location.
POSSIBLE NEED FOR BITE PLANE THERAPY The patient naturally opens downward and forward combination of rotation and translation. We must separate the rotation from the -translation so that we can locate the center of vertical opening. In addition,, this opening and closing must be accomplished in the terminal hinge position, for here we can get repeated concentric arcs that will permit us to locate their center. If a patient has difficulty in executing a pure hinge movement, it may be. necessary to train him in this abnormal 'opening and closing movement. Training can be accomplished by using the jig.
MARKING THE AXIS LOCATION ON THE PATIENT When we are satisfied that we have located these points on the axis the marking medium, Suchn as an indelible pencil, is rubbed on the end of the stylus. We make sure the patient is in the terminal hinge position and then have him move his head out of the headrest, making sure that he does not also move out of the terminal hinge position. The stylus is gently pushed against his face to transfer the point to the skin. These marks are made permanent by using a special needle and a little pink marking dye -sulfide of mercury.
Gordon has presented an alternate technique for recording the located axis point by non-tattoing method. SELECTION OF A FACE-BOW From a purely theoretical point of view an ordinary face-bow such as a Snow or Hanau can be used to locate the hinge axis. To attempt to use either one of them in actual practice, however is impossibility. It is far easier and more accurate to use a fully adjustable face-bow (i.e. one with arms that can be independently adjusted by means of micrometer screws) for both the axis locations and the transfers. By means of a face-bow transfer and the mounting frame the upper cast can be properly mounted to the axis of the patient. There are two types of face bow, the kinematic and arbitrary. The kinematic is used to locate the true terminal hinge axis. The arbitrary facebow is the one generally used in the construction of complete dentures and is based on average computations of an axis opening of the jaw. It is simple to use and relatively accurate. Arbitrary axis for Hanau face bow When using a Hanau face bow, a Richey condylar marker is used to scribed an arc about 13 mm anterior to the external auditory meatus. 37
Arbitrary axis for Whip - Mix face bow Locating the arbitrary axis is not necessary when using the Whip-Mix articulator. The insertion of plastic ear pieces into the external auditory meatus automatically locates the face bow in the proper position.