why and how to locate hinge axis

1. HINGE AXIS

2. Contents
 Introduction
 Description of hinge Axis
 Defnition
 Mandibular movements
 Hinge axis and centric relation
 Loction of Hinge axis “Why and how
 TMJ vs Various type of articulator”
 Error during hinge axis location and Prevention
 Contoversies

3. Introduction
The TMJ is described as a diarthrodial ginglymus joint.
The joint cavity is divided into upper and lower parts by an intra
articular disc
 The upper articular surface is formed by the following parts of the
 (a) temporal bone
 (b) articular eminence anterior part of mandibular fossa
 The inferior articular surface is formed by the head of the mandible
 The articular surfaces are covered with fibrocartilage.
It is capable of two types of basic movements;
 sliding (diarthrodial) and
 hinge(ginglymus). .
.
The upper compartment Permit gliding movements and the lower
rotatory as well as rotatory movements.

4. Description of the hinge axis
Any three dimensional object that moves in a coordinated rotational
path of motion which is a part of a circle or ellipse has a axis of
rotation (hinge axis).
If the path of the motion of the object is a
part of a circle, the axis of
rotation itself is not moving.
If the path of motion of the object is a part of ellipse, the hinge axis
itself must move.
Clinically the condyles would be translating as the patient opened
the jaw

5. Defnition
 The hinge axis is an imaginary line around which
the condyles can rotate without translation
 Hinge axis can be obtained by gometry, clinical
and anatomical method
 Hinge axis point = posterior reference points
two points, located one on each side of the face in the area
of the transverse horizontal axis, which together with an
anterior reference point, establish the horizontal reference
plane.

8. Horizontal Axis of Rotation
.
Pure hinge movement motion around the horizontal axis during opening
and closing motion of mandible within limit
The limits of hinge movement in this position have been determined to be
about 12 degrees to 15 degrees from maximum intercuspation or about 20
to 25 mm at the incisal edges.

9.  When the condyles are in their most superior position in the
articular fossa and the mouth is purely rotated open, the axis
around which movement occurs is called the terminal hinge axis
 Terminal hinge position is the most retruded hinge position and it
is significant because it is a learnable, repeatable and recordable
position that coincides with the position of centric relation

10. Frontal (Vertical) Axis of
Rotation
Mandibular movement around the frontal axis occurs when one
condyle moves anteriorly out of the terminal hinge position while
the vertical axis of the opposite condyle remains in the terminal
hinge position ]

11. Sagittal Axis of Rotation
Mandibular movement around the sagittal axis occurs when one condyle moves
inferiorly while
the other remains in the terminal hinge position
It does occur in conjunction with other movements, however, when the orbiting condyle
moves downward and forward across the articular eminence.

12. SAGITTAL PLANE BORDER
movement
 Mandibular motion viewed in the sagittal plane
can be seen to have four distinct movement
components
 1. Posterior opening border
 2. Anterior opening border
 3. Superior contact border
 4. Functional

13. Posterior opening border
This pure rotational
opening can occur until
the anterior teeth are
some 20 to 25 mm
apart.
2 nd Stage:-
As the condyle translates the axis of
rotation of the mandible shifts into the
bodies of rami likely to be the area of
attachment of sphenomandibular ligament,
resulting in the second stage of the
posterior opening border movement
1st stage
ROTATIONAL MOVEMENT OF THE
MANDIBLE WITH THE CONDYLES
IN THE TERMINAL HINGE POSITION

14.  Anterior Opening Border Movements; The posterior
movement of the condyle from the maximally open position to
maximally protruded position produces eccentricity in the anterior
border movement. Therefore, it is not a pure hinge movement.
 Superior Contact Border Movements It depends on:- Amount
of variation between centric relation and maximum
intercuspation. The steepness of the cuspal inclines of the
posterior teeth The slide from CR to maximum intercuspation,

16. Geometry method
 The transverse hinge axis can be located geometrically by
erecting perpendiculars that bisect two or more secants of the
circular path. The transverse hinge axis is always perpendicular
to the arc of rotation.

17. Hinge axis and centrix relation
Mandibular musculature of edentulous patient loses its precise
guiding signals (loss of periodontal propioceptors) in the closing
movements
of the jaw.
A new and precise pattern of proprioceptive stimuli is established
by teaching the patient to move the mandible to most posterior
position.
This new proprioceptive stimuli established through the TMJ then
guides the mandible into a repeatable border position .This
biologic phenomena is carried out when a record of centric
relations is made.
It is assumed that anteroposterior relation of mandible to
maxilla at terminal hinge position is same as centric relation

18. Why we need to locate hinge axis
 Hinge axis stable reference or anatomic structure which can
transfer to articulator
 It provide to increase and decrease vertical dimension without
changing centric relation
 To determie centric occlusion is harmonious to centric relation
 Centric relation interocclusal records at an increased vertical
Dimension
the distances from the centers of rotation of the condyles to any
given cusp are exactly the same in the patient's mouth as on the
articulator
 Restorations developed at an increased vertical dimension

19.  The terminal hinge axis plus one other anterior point
serves to locate the maxillae in space.
 Records the static starting point for functional
mandibular movements
 It is a starting point of lateral movements.
 Allows the transfer of opening axis of the articulation
so that occlusion would be on the same arc of closure
as the lower jaw.
 The interocclusal centric relation record is made in
terminal hinge position and is used to orient the
mandibular cast to the maxillary cast.
 Hinge axis method of orienting casts in an articulator
permits control of the vertical dimension on the
articulation

20. How to locate hinge axis
 True hinge axis by hinge axis locator using Kinematic Face-bow
 Arbitrary method -Using anatomical landmark
 Facia Type (With orbital indicator)
 Ear Piece Type (With nasal relator)
 Hanau face-to-face (spring bow)
 Slidematic (Denar)
 Twirl bow
 Whipmix
 Other –palpale method, geometry method

21. the hinge axis locator,
A device which is attached to the maxillary and
mandibular teeth and extends extraorally
posteriorly to the condylar regions.
A grid attached to the maxillary teeth is located in
the general area of the condyle.
A stylus attached to the mandibular teeth is
positioned over the grid. The mandible is then
arched in a hinge axis movement, and the stylus
is adjusted until it does not move from its location
but merely rotates about a point.
When the adjustment is completed, the stylus is
positioned directly over the exact hinge axis of
the condyle.
This area is marked by placing a dot on the surface

23. “trial and error” method for hinge axis
 The most accurate way to determine the hinge axis is by
the “trial and error” method developed by McCollum and
Stuart in 1921.
 A device with horizontal arms extending to the region of
the ears is fixed to the mandibular teeth.
 A grid is placed under the pin at the end of the arm, just
anterior to the tragus of the ear.
 The mandible is manipulated so that the condyles are in
the optimum position in the mandibular fossae with the
articular discs properly interposed, from which it is guided
to open and close 10 mm. As it does, the pin will trace an
arc
 The arm is adjusted in small increments to move it up,
down, forward, or back, until the pin simply rotates without
tracing an arc.
 This is the location of the hinge axis, which is marked with
ink on the patient’s face

24. An arcing movement of the stylus
on the side arm (A) indicates that it
is not located over the THA.
The side arm is adjusted so the
stylus will rotate without moving
during opening and closing (B).
This indicates that it has been
positioned over the THA.
.

25. Location of hinge axis by average
anatomic
measurements
 Arbitrary Hinge Axis: the Location of hinge axsis
can be approximated when arbitrary type of face
bow is used.
 The two posterior points of reference that form
the hinge axis are located by measuring
prescribed distances from skin surface
landmarks.

26. Arbitrary Hinge Axis
Bergstrom’s point - A point 11mm anterior to the centre of a
spherical
insert for the auditory meatus and 7 mm below the Frankfort
horizontal plane.
Beyron’s point - A point 13 mm anterior to the posterior margin of
tragus
of the ear on a line from centre of the tragus to the outer canthus of
eye.
Gysi point -10mm anterior to posterior margin of tragus on a line
from
centre of tragus to outer canthus of eye.
Teteruck and Lundeens point- 13mm anterior of tragus on line

28. hinge axis based on an anatomical
average is accepted ???
 This technique should provide enough accuracy for the
restoration of most mouths, if the occlusal vertical dimension is
not to be altered to any significant extent.
 An error of 5.0 mm in the location of the THA will produce a
negligible antero-posterior mandibular displacement of
approximately 0.2 mm
 Caliper-style ear facebows possess a relatively high degree of
accuracy, with 75% of the axes located by it falling within 6 mm
of the true hinge axis.

29. Step wise method
 1. Recording hinge axis points
 2. Transfer to the articulator
 3. Mounting of upper casts and
 4. Mounting of lower casts with centric record

30. Patient vs articulator
Semi-adjustable articulator -locating arbitrary or average condylar hinge
axis
the centers of rotation of the condyles and any given cusp will not
be the same in the mouth as on the articulator.
Therefore the hinge axis opening and closing pathway of the
cusps will not be exactly the same

31. Fully adjustable articulator
 When the patient's hinge axis (PHA) is transferred to coincide
with the hinge axis of the articulator (AHA), the arcs of closure
for the patient and the articulator are identical. Therefore an
interocclusal record at any degree of opening (X, Y, and Z) will
provide an arc of closure to the desired occlusal position

32. Semi-adjustable or non adjustable without
increasing vertical height
 When the exact hinge axis is not located a difference will
exist between PHA and AHA.
 The AHA is inferior and anterior to the PHA. -opening and
closing pathways (arcs) are different the difference in these two
pathways has no clinical significance because there are no
occlusal contacts during the opening and closing movements.
The important feature is that both closing pathways return to the
mandible to the desired occlusal position

33. Semi-adjustable or non adjustable
with increasing vertical height
 The PHA and the AHA are not the same. when an interocclusal
record taken at an increased vertical dimension (X) is used in
mounting the cast, the mandibular teeth are at a proper distance
from the PHA but not the AHA.
 A significant discrepancy can exist if the arbitrary hinge axis is
used to mount the maxillary cast and an interocclusal record at
an increased vertical dimension is used to mount the mandibular
cast.

34. Because the closure arcs for the patient and the articulator are not
identical, when the interocclusal record is removed the cast will
arc closed on a different pathway, resulting in a different occlusal
contact position from that seen in the patient's mouth
Need fully adjustable articulator
Antero-posterior adjustment from CR to maximum Intercuspation
Bucco-lingual cusp adjustment
Selective grining

36. Variables affecting hinge axis
location
Patient variables affecting the T.H.A.
• locations Condyle
• Asymmetry
• Inability to locate a true hinge axis
• Myospasm or joint pathosis
• Emotional conditions of patient

37. ERRORS IN RECORDING HINGE
AXIS
 Movement of the skin over the condyle during registration.
 Tipping of Base will invalidate the eventual recording.
 The angle of opening movement is small about 10-12 degrees
and thus the arc of movement of the stylus is small.

38. Method to prevent error
 Patients are positioned erect in the chair with no headrest
support while this point is marked.
 The graph paper is carefully removed.
 The mandible is placed in the terminal hinge relation.
 The locator needle is retracted from its proximity to the
skin.
 Marking material is placed upon the tip of the needle, and
the needle is carefully brought into contact with the relaxed
skin. Thus an accurate marking is obtained.

39. Controversies
The different schools of thought regarding the transverse hinge axis
has led to the evolving of four main groups:
Group I - absolute location of axis;
Group II - arbitrary location of axis;
Group III – non-believers in transverse axis location;
Group IV - split axis rotation.

40.  Group I - Absolute location of axis: these are those who believe
that there is a definite transverse axis and it should be located as
accurately as possible.
 The critics of this theory claim that most of the articulators are
designed on the assumption that the transverse hinge axis is an
imaginary line connecting a point in the centre of one condyle
with a point in the centre of the opposite condyle.
Group II - Arbitrary location of axis: the second group favors the
arbitrary location of axis and subscribes to the view that
determination of the true hinge axis is not essential when one
looks at the effort required to find it. Hinge axis may be
determined by simple palpation or by following the convention of
measuring a distance of about 10 mm anteriorly along a line
drawn from the upper free margin of the tragus to the corner of
the eye

41. Group III. This group believes that it is impossible to locate the
transverse hinge axis with accuracy
The critics of this group claim that the main motion is pure rotation
plus slight translatory movements, the composite of which adds
up to a common centre of rotation.
this performance is repeatable, it becomes reliable point of
orientation.
 Group IV - Split axis rotation.
 Group IV includes those who follow the transograph theory. They
believe in the ‘Split axis’ with which each condyle rotates
independently of the other.

42.  As the mandible is not bilaterally symmetrical and
the terminal hinge position mark on one side of
the face is usually a little higher than it is on the
other, it would follow that there cannot be a
common axis.
 There must be two axis parallel to each other with
both axes at right angle to the opening and
closing movements of the mandible.

43. Transographic concept
 Transographic concept is based on the asymmetry of the
condyles.
 asymmetry – anatomicaly
 - orientation of some reference point or deviation
during openimg or closing
Condylar asymmetry. The
opening and closing
motion is at an angle to
the vertical
plane of the face, but it is
still perpendicular to the
transverse hinge axis

44. References
 Shillingberg Fundamental of fixed partial
denture 4th edition
 Okeson Management of temporomandibular
Disorder and occlusion
 Singh S, Rehan S, Palaskar J, Mittal S.
Hinge axis location clinical use and
conversation
 LAWRENCE A. WEINBERG, A.B., D.D.S
.THE TRANSVERSE HINGE AXIS: REAL OR
IMAGINARY J. Pros. Den.1 959.