ORIENTATION JAW RELATION

INTRODUCTION

1) The recording of jaw relation is one of the most important

phases in complete denture treatment.

2) It is a known fact that the temperomandibular joint has a

significant role during this procedure.

3) Unlike the other joints of the body, the temperomandibular joint

is peculiar in that it is lined by a fibro cartilage. The disc of the

joint divides it into 2 sinovial cavities.

In otherwords the condyle of the mandible is associated with the

petrous part of temporal bone through this joint. So it can also

be called as a Cranio-mandibular joint.

4) Maxilla is a part of the cranium and is also a fixed entity. The

maxilla becomes related to the mandible when the teeth of both

jaws come in contact. So the entire craniomaxillary complex is

articulated with a moving bone, which is the mandible.

5) The relationship of the maxilla to the temporomandibular joint is

not the same in all persons, i.e., the anatomy of maxilla and the

temperomandibular joint varies from persons to persons.

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6) The opening movement to bring the jaw from occlusal to rest

position is almost a pure hinge movement. Here the mandible

moves on an arc of a circle with a definite radius from the

temperomandibular joint. This path of the condyle is determined

by the curvature of the condylar head and the curvature of

glenoid fossa. Since the radius is not constant for all the

patients, it has to be determined for every individual patient, i.e.,

the relation of maxilla to the opening and closing axis has to be

determined.

The relationship of the maxilla to the cranium in three planes

viz: anteroposterior, lateral and vertical is called the orientation jaw

relation.

Parts of a face bow : condylar rods, fork, u-shaped frame.

There are two basic types of face bows.

a. Arbitrary face bow.

b. Kinematic or hinge bow.

The arbitrary face bow helps to locate the opening axis of the

jaws in an approximate manner.

It is only the kinematic face bow which can record the anatomic

axis in an accurate way. Here the condylar styli rest exactly on the

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condyles. When the points of the condylar styli rotate only and do not

translate, the points are on the opening axis of the jaw. The fork of a

kinematic face bow is attached to the mandible. In an edentulous

patient, since the mandibular record bases cannot be stabilized, the

kinematic face bow cannot be attached firmly. In a dentulous person

the face bow can be clutched to the mandibular teeth and the true hinge

axis can be recorded.

DEFINITIONS

Orientation Relation: They are those that orient the mandible to the

cranium in such a way that, when the mandible is kept in its most

posterior position, the mandible can rotate in a sagittal plane around an

imaginary transverse axis passing through or near the condyles.

The axis can be located when the mandible is in its most

posterior position by means of a Face bow.

Definition of Face bow

1. It is a calper like device that is used to record the relationship of

the jaws to the temporomandibular joints or the opening axis of

the jaws and to orient the casts in this same relationship to the

opening axis of the articulator.

- Boucher

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2. It is a caliper like device used to record the spatial relationship

of the maxillary arch to sum anatomic reference point or points

and then transfer this relationship to an articulator. It orients the

dental cast in the same relationship to the opening axis of the

articulator. Customarily the anatomic references are the

mandibular transverse horizontal axis and another selected

anterior point.

- The Glossary of Prosthodontic terms

6th Edition, 1994

Terminal hinge position

It is the most retruded hinge position. Movement from this

position, a condition response, is always less than median – full –

mouth opening.

The terminal hinge position is significant because it is a

learnable, repeatable and recordable position that coincides the position

of centric relation. The limits of the hinge movement in this position

have been determined to be about 12-15 degrees from maximum

intercuspation or about 19-20mm at the incisal edges. The condiles are

in a definitive position in the fossae during terminal hinge movements.

A face bow relates the maxilla to the intercondylar line. It is

commonly assumed that the opening mandibular movements occur

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about this horizontal axis. Graphic records of mandibular movements

and radiographic investigations of the temperomandibular joint have

repeatedly shown that this assumption is well founded, and that in

normal subjects and for small opening movements the horizontal axis

of rotation does infact pass through the condyles. It is true that in wider

opening movements – the axis becomes progressively displaced

downwards. This is of anatomic interest rather than prosthetic interest.

It must be emphasized that in restorative and prosthetic treatment we

are concerned with relations between teeth only when they are in

occlusion or at most slightly separated.

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Parts of a Face bow:

It consists of:

i) u-shaped frame or assembly that is large enough to extend

from the region of the temperomandibular joint to a position 2-3

inches in front of the face and wide enough to avoid contact with

sides of the face.

ii) Condylar rods that contact the skin over the

temperomandibular joint.

iii) Fork which can be attached to the occlusal rim.

The fork of the face bow attaches to the face bow by means of a

locking device.

Classification of Face bow

I. Arbitrary Face bow

a) Facia type.

b) Ear piece type

II. Kinematic Face bow

Arbitrary Face bow:

They use arbitrary or approximate points on the face as posterior

reference points. The condylar rods are positioned on these

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predetermined points during the face bow transfer procedure. Arbitrary

face bows are most commonly used for complete denture patients.

This placement of condylar rods will generally locate the

opening axis within 5mm of the true center of the opening axis of the

jaws.

Facia type : Facia type of face bow utilizes approximate points on the

skin over the temperomandibular joint region as posterior reference

points. These points are located by measuring from certain anatomical

land marks on the face.

Ear piece type : It was first described by Dalbey in 1914. This type of

face bow uses the external auditory meatus as the arbitrary posterior

reference point. For this a special ear piece is required instead of a

condylar rod. The external auditory meatus is assumed to have a fixed

relationship to the hinge axis. Special condylar compensators on the

face bow or the articulator then compensates for this by positioning the

condylar inserts at a prescribed distance behind the rotational axis of

the articulator.

Kinematic Face bow:

It is used to determine and locate the exact hinge-axis points.

The fork of the kinematic face bow is attached to the mandibular

occlusal rim. Then as the patient retrudes the mandible and opens and

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closes the jaws the dentist-observes the movement of the points of the

condylar rods. Kinematic face bow is a more complex instrument

requiring the fabrication of clutches which have to be attached to the

lower jaws.

Since the face bow is used to orient the casts on the articulator in

the same relation to the opening axis of the jaws, the face-bow record

is not a maxillomandibular record. It is a record made for the

orientation of the casts to the instrument. However, the use of a

kinematic face bow can aid in recording of centric relation.

Plane of orientation

The maxillary cast in the articulator is the base line from which

all occlusal relationships start and it should be positioned in space by

identifying three points. The plane is formed by two points located

posterior to the maxillae and one point located anterior to it. Posterior

points are referred to as the posterior points of reference and the

anterior one is known as the anterior point of reference.

Posterior Points of Reference

The position of the terminal hinge axis on either side of the face

is generally taken as posterior reference points.

Prior to aligning the face bow on the face, the posterior

reference points-must be located and marked. They are located by:

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a) Arbitrary method.

b) Kinematic method.

a) Arbitrary method

Often the posterior points are located by measuring prescribed

distances from skin surface landmarks. Some of the commonly used

posterior points were shown by Beck to be near the hinge axis

clinically. He concluded that the Bergstrom point most frequently is

closest to the hinge axis. He identified the Beyron point as the next

most accurate posterior point of reference.

Studies by Weinburg show that a deviation from the hinge axis

of 5mm will result in an anteroposterior displacement error of 0.2mm

at the second molar. An error of this size is usually of no consequence

in removable prosthesis with non-rigid attachments. These prosthesis

and the mobility of supporting tissues may make a precise location of

the hinge axis an exercise with no advantage.

Following are some of the most commonly used measurements

and landmarks:

1. 1cm infront of the line from the apex of the tragus of the

ear to the outercanthus of the eye.

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2. Bergstrom point : 10mm anterior to the center of the

spherical insert for the external auditory meatus and 7mm below

the Frankfort horizontal plane.

3. Beyron’s point : 13mm anterior to the posterior margin of

the tragus of the ear on a line from the center of tragus extending

to the corner of the eye.

4. Gysi point : located 13mm in front of the most upper part

of the external auditory meatus on a line passing to the outer

canthus of the eye. This method was proposed by Gysi, Hanau,

Snow and Gilmer and is the most common point used today.

Method of palpation (Dawson’s method)

An alternative method of locating the hinge axis arbitrarily is by

a method of palpation.

From behind the patient, place the index finger tip over the joint

area and ask the patient to open wide.

As the condyle translates forward, the finger tip will drop into a

depression where the condyle was. The patient should then close. As

the condyle translates back into centric relation position, its position

can be located by finger tip.

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By asking the patient to open and close, it will be possible to

locate the axis with an average accuracy. This axis generally occurs

near the center of the depression felt the by the finger tip.

After locating the point, it should be marked.

b) Kinematic method

Most accurate method for recording the correct horizontal axis is

by a “Trial and error method” developed by McCollum in 1921 using a

kind of kinematic device.

The technique of locating the terminal hinge axis position is

essentially the same for dentulous and edentulous patients, but the

methods of attaching the clutch to the mandible are quite different.

Clutch fabrication for edentulous patient

Firmly attaching a clutch to edentulous mandible, presents a

problem:

- Make an accurate impression of the mandibular basal seat and

pour an accurate stone cast.

- On the cast make an accurate record base of self curing or

processed acrylic resin.

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- Attach compound occlusal rims firmly to the record base and

secure a specially designed bite fork to the rims, with the stem

extending forward parallel to the sagittal plane.

- Attach this assembly to the mandible with chin clamps or chin

straps.

Once the clutches have been fabricated, the following sequence

is followed:

- Place the patient in a semisupine position with the head rest

tilted slightly backward (so that patient can retrude his mandible

easily).

- Connect the mandibular clutch to face bow.

- A graph or grip paper is placed adjacent to the skin over the

temperomandibular joint region to help detect stylus movments.

- Guide the mandible to centric position and assist the patient in

making hinge openings and closings to a maximum of 10-13°.

- Check the movement of the stylus; initial movement may be arc

shaped.

- Adjust the stylus tip towards the center of the arc until the tip

rotates instead of arcing.

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- Remove the grip paper and record the point on the skin with the

help of a dye.

Precaution : Care must be taken to record only the retruded hinge axis.

The mandible is capable of producing hinge like motion at any point

along the protrusive pathway.

Anterior Point of Reference:

The selection of the anterior point of the triangular spatial plane

determines which plane in the head will become the plane of reference

when the prosthesis is being fabricated.

Reasons for selecting an Anterior point of reference:

1. 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.

2. A planned choice of an anterior reference point will allow the

dentists and auxiliaries to visualize the anterior teeth and the

occlusion in the articulator in the same frame of reference that

would be used when looking at the patient. For example, when

using the Frankfort horizontal plane as the plane of reference,

the teeth will be viewed as though the patient was standing in a

normal postural position with the eyes looking straight ahead.

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Selection:

Various anterior reference points that may be used are as

follows:

1. Orbitale:

In the skull, orbitale is the lowest point of the infraorbital

margin. On a patient it can be palpated through the overlying

tissue and skin. One orbitale and two posterior points that

determine the horizontal axis of rotation will define the axis-

orbital plane.

Orbitale and the two posterior land marks are transferred

from the patient to the articulator with the face bow. The

articulator must have an orbital indicator guide that is in the

same plane as the hinge of the articulator.

2. Orbitale minus 7mm:

The Frankfort horizontal plane passes through both poria and

one orbital point. Because porion is a skull land mark, Sicher

recommends using the midpoint of the upper border of external

acoustic meatus as the posterior cranial landmark on the patient.

Gozalez pointed out that this posterior tissue landmark on the

average lies 7mm superior to horizontal axis. The recommended

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compensation for this discrepancy is to mark the anterior point

of reference 7 mm below orbitale on the patient.

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 eye brows. The nasion

guide, or positioner of Quick Mount face bow which is designed

to be used with the whip mix articulator fits into this depression.

The cross bar (u-shaped frame) is located 23mm below the

midpoint of nasion pointer. When the face bow is positioned

anteriorly by the nasion guide, the cross bar will be in the

approximate region of orbitale. The face bow cross bar and not

the nasion guide is the actual anterior reference point locator. So

this face bow employs an approximate axis-orbital plane.

4. Alae of the nose

A part of many complete denture techniques is to make the

tentative or actual occlusal plane parallel with the horizontal

plane. This can be achieved as follows:

A line from the alae of the nose to the center of the auditory

meatus is called Camper’s line, Augsberger concluded in a

review of literature that the occlusal plane parallels this line with

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minor variations in different facial types. Knowing this, the

dentist can transfer Camper’s line from the patient to the

articulator by marking the right or left alae on the patient, setting

the anterior reference pointer to it and with the face bow

transferring the ala anteriorly and the hinge points posteriorly,

from the patient to the articulator.

Attaching the bite Fork

Various methods are used. The method also varies for the

complete denture patient and the dentate patient.

For a complete denture patient, the following methods may be

used:

Method I

- Low fusing base plate wax is softened and rolled in a horse shoe

shape.

- The prongs of the fork are embedded in the wax and the margins

are folded over; thickness of wax is approximately 6mm.

- The prongs with the attached wax are placed between the

occlusal rims in the patient’s mouth with the midline of the fork

coinciding with midline of occlusal rim. Stem of the fork must

be parallel to the sagittal plane.

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- Instruct the patient to close his jaws, to imbed the occlusal rims

into the soft wax.

- The relation of maxilla to mandible is not important at this stage

except to place favourable pressure to stabilize the maxillary

record base.

Following are the advantages of this method:

1. Patient is in a comfortable position with the jaws closed and

arms at rest.

2. Maxillary record base is accurately seated and not subjected to

tipping.

Method II

The bite fork of the Hanau face bow is designed to be inserted

into the maxillary occlusal rim.

- The bite fork is heated over flame and is inserted into facial

surface of maxillary rim parallel to occlusal rim.

- Rod extension should parallel the patient’s midsagittal plane.

- The maxillary rim along with face bow assembly is then held in

position by the patient using both thumbs with light pressure.

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Steps in Recording

- Seat the patient in a comfortable position in the dental chair with

the backrest extending slightly below the scapula. Patient’s head

should be in an upright position with the head rest supporting

the occiput.

- Locate the axis points by methods described previously. Record

points on the skin or on adhesive tape placed in the area.

- Contour the maxillary occlusal rim, establish the occlusal plane.

- Apply a thin layer of petroleum jelly to the occlusal rims to

facilitate separation of bite fork from the wax.

- Reduce mandibular occlusal rim to allow adequate interocclusal

distance for attached fork and wax.

- Adjust the condylar rods to the face for centering the bow by

placing the ends over the condyle points so that the ends lightly

touch the skin or tape. Secure either the right or left condyle rod;

lock and remove the bow from face.

- When the infraorbital notch is used as the anterior point of

reference, the pointer should be placed in the clamp provided for

it on the bow. Palpate the infraorbital notch and mark it with a

skin marker. Place the pointer tip over the mark and secure the

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clamp to the pointer. Remove the assembly form the face and

allow the wax to set hard before removing the bite fork and face

bow record from the occlusal rims.

Face Bow Mounting

1. Set the sliding condylar rods symmetrically on both the sides

until the bow gently springs over the articulator condylar shaft.

2. Raise or lower the face bow to adjust for the vertical position

until the occlusal plane anteriorly is on a level with the groove

marked around the incisal pin. If an orbital pointer is used,

adjust the pointer to touch the pointer plate attached to the

maxillary member of the articulator.

3. Soak the maxillary cast in water for atleast 5 minutes to insure

adhesion of plaster to stone.

4. Secure the incisal guide pin with its top flush with the top of

maxillary member of the articulator. Open the maxillary member

of the articulator and apply a creamy mix of dental plaster to the

top of the articulator until the incisal guide pin is stopped on the

guide table and the mounting plate is embedded in the plater.

5. Carefully remove the excess plaster. Allow the plaster to harden

before removing the face bow assembly. The maxillary cast is

related to the opening axis in the articulator in the same

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anteroposterior and horizontal position as the maxillae in the

skull are related to the opening axis in the temperomandibular

joint.

From here further records can be made starting with a tentative

center relation record.

Discussion of significance of orientation jaw relation

Since efficient dentures are constructed with and without the use

of a face bow, we may admit at the outset that a face bow is not

essential to a good prosthetic restoration, in the sense that a matrix

band is essential to the insertion of a good Class II filling.

To say that a face bow locates the apex of the Bonwill triangle is

not an incorrect answer to the question what the face bow does? But

this merely prompts another question : what is Bonwill’s triangle and

why is its location important?

The equivalent triangle theory states that the distance between

the condyles and between each condyle and incisor point is 10cm. This

being so, the use of a simple pair of calipers in the laboratory is all that

is necessary to mount the casts always at the same average distance

from the intercondylar line.

But the 10cm value is just an average some patients have

dimensions larger or smaller than the average.

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Effect of cast orientation on occlusion

Proper alignment of the occlusal plane with relation to the end

controlling factors is of great importance for the cusp inclines and cusp

pathways of a prosthetic reconstruction. The effect of various cast

positions on cuspal inclines and pathways is demonstrated in the

figure.

Failure to use the face bow can lead to errors in occlusion of the

denture. It is true that the errors may be small if the error in orientation

of cast is small. Likewise, the errors produced by failure to use the

face-bow would be negligible if all the interocclusal records were made

precisely at the occlusal vertical relation and if zero degree teeth were

used. However, if cusp teeth are used or if interocclusal records are

made with the teeth out of contact so that the vertical separation of cast

or dentures must be reduced on the articulator, the face bow record is

essential. The face bow transfer allows a more accurate arc of closure

on the articulator when the interocclusal records are removed and the

articulator is closed.

Zuckerman (1982) has stated that:

1. Superior/ inferior errors in location of hinge axis produces

greater errors in occlusion than equivalent errors in posterior /

anterior location.

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2. Magnitude of error is directly proportional to the magnitude of

error in location of hinge axis.

3. magnitude increases as vertical dimension at which centric

relation is recorded, increases.

Consequences of these errors:

When the CD patient performs the function of chewing with

these errors in the dentures, it may lead to:

a. Instability of the dentures which causes early resorption of the

residual ridge.

b. The path of closure of the mandible is altered and the biological

soundness of the temperomandibular joint is affected. The

symptoms manifest as pain, crepitus and subluxation of the

joints. The symptoms are features of MPDS.

Some dentists consider that the use of a face bow is not

necessary under the following conditions:

1. When monoplane teeth are arranged on a plane in occlusal

balance and the mandible is in most retruded position.

2. When no altrations of the occluding surfaces of teeth are

required to necessitate changes in vertical dimension of

occlusion originally recorded.

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3. No interocclusal check records that would be at a different

vertical dimension from that in the original interocclusal record.

4. When articulators that are not designed to accept a face

bow transfer are used in denture procedures.

When these conditions are analyzed, the following arguments

may be put forth:

1. It is questionable if one occlusal form of posterior tooth is

indicated for all edentulous patients.

2. Changes do occur in the vertical dimension of occlusion as a

result of waxing, flasking, processing and mounting procedures.

Resorption of the bone and the changes in soft tissues that form

the basal seat for dentures alter the vertical dimension of

occlusion.

3. Dentists use interocclusal check records to verify articulator

mountings.

4. Occluding surfaces are altered to correct for changes in the

vertical dimension of occlusion.

5. There is no scientific proof that the errors when the face bow is

not used are within the acceptable physiologic range in all

individuals.

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6. When an articulator with rotational centers that can be adjusted

to conform to rotational centers of mandibular movements is

used.

References

1. Evaluation of use of face bow in CD occlusion, vol. 39, 1978; 5-

14.

2. Hinge axis of mndible. J.P.D., 1 : 327, 1951.

3. Hinge axis registration, experiments on the articulator. J.P.D.,

7 : 35, 1958.

4. Hinge axis location on a experimental basis. J.P.D., 11 : 1059,

1961.

5. Transverse hinge axis, rod or imaginary. J.P.D., 9 : 775, 1959.

6. Geometric significance of transverse axis. J.P.D., 10 : 631, 1960.

7. Evaluation of face bow. J.P.D., 2 : 633, 1952.

8. Variations in location of arbitrary and true hinge axis points.

J.P.D., 11 : 224, 1961.

9. Study of arbitrary center and kinematic center of rotation for

face bow mountings. J.P.D., 7 : 162, 1957.

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10.Location of terminal hinge axis by intra oral means. J.P.D., 23 :

11, 1970.

11.Face bow its significance and application. J.P.D., 3 : 618, 1953.

12.Clinical evaluation of Arlin concept of Articulation. 9 : 409,

1959.

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