mandibular movements

MANDIBULAR MOVEMENTS

Dr Rohan Bhoil

CONTENTS Introduction

Methods of studying mandibular movements

Importance

Factors which regulate jaw motion

Mandibular positions

The clinical understanding

Conclusion

INTRODUCTION

Mandibular movements are

complex.

METHODS OF STUDYING MANDIBULAR MOVEMENTS

Direct Clinical Observation

Sophisticated Electronic Instrumentation :

Facial Clinometers , Roentgen Fluoroscopy , Radionucleotide Tracking; Optical Pantography; Gnathic Replicator etc

SIGNIFICANCE OF UNDERSTANDING MANDIBULAR MOVEMENTS•Developing Tooth Forms For Dental Restorations,• Understanding Occlusion, •Arranging Artificial Teeth,• Treating TMJ Disturbances,• Preserving Periodontal Health• And the designing, selection and adjustment of articulators.

FACTORS WHICH REGULATE JAW MOTION•The Neuromuscular Role

•Contacts Of Opposing Teeth

•Anatomy And Physiology Of TMJ's •The Action Of Muscles /Ligaments

NEUROMUSCULAR ROLE

Mastication is a programmed event residing in a ‘chewing centre’ located within the brain stem, probably in reticular formation of pons.

The cerebellum ensures a coordinated response from muscles during mandibular movements, by acting as a feedback control mechanism..

Loss of propioceptors, located principally in periodontal ligaments, eliminates an important source of control in positioning of mandible; for the edentulous patients.

INFLUENCE OF OPPOSING TOOTH CONTACTS

Opposing tooth surface contacts are related to• occlusal surfaces of teeth• muscles• TMJs’• and neurophysiological components.

INFLUENCE OF THE TEMPOROMANDIBULAR JOINTS

Mandibular movements except opening and closing movements at the terminal hinge position, are combinations of rotation and translation.

ROTATION:

The movement of a rigid body in which the parts move in circular paths with their centers on a fixed line called the axis of rotation. The plane of the circle in which the body moves is perpendicular to the axis of rotation.

TRANSLATION

• The motion of a body at any instant when all points within the body are moving at the same velocity and in the same direction.

TMJ

Rotation - in the lower compartment of the TMJ.The translatory or gliding movements - in the upper compartment.

• A complex joint.

• GINGLYMOARTHRODIAL JOINT.

• TMJ consists of 4 main structures:-– Condyle– Temporal bone (Squamous part) – Articular disc– Ligaments

THE TEMPORO MANDIBULAR JOINT

Articulating part of TMJ includes convex head of mandibular condyle and the convexoconcave part of the temporal bone i.e. mandibular fossa and articular tubercle.

An intrarticular disc made of fibrous tissue divides joint cavity into upper and lower compartments.

ARTICULAR DISC

SAGITTAL PLANE ANTERIOR VIEW

LIGAMENTS OF TMJ

1. Fibrous capsule

2. Lateral temporomandibular ligament

3. and two accessory ligaments i.e. sphenomandibular and the stylomandibular ligaments.

TMJ LIGAMENTS

ACCESSORY LIGAMENTS

Stylomandibular ligament limits excessive protrusive movement of mandible.

MUSCULAR INVOLVEMENT IN JAW MOTION

ROLE OF MUSCLES

There are four main muscles of masticationMasseter, Temporalis and the Medial and Lateral Pterygoids.All these except the lateral pterygoids act as ELEVATORS.

MASSETER

Functions:-

Elevation

Superficial portion: protrusion

Deep portion : stabilization against articular eminence

TEMPORALIS:-

It is a significant positioning muscle of the mandible

• It has 3 types of fibres

– Anterior fibers – or vertical.

– Middle fibers –run obliquely.

– Posterior fibers – consists of fibers that are aligned almost horizontally.

MEDIAL PTERYGOID

FUNCTION:• Elevation and protrusion.• Unilateral contraction –mediotrusive movement

LATERAL PTERYGOID

Function

Inferior Lateral Pterygoid:-• Simultaneous contraction:

depression and protrusion• Unilateral contraction: mediotrusion & movement

to opposite side.

Superior Lateral Pterygoid • During opening: remains

inactive.• Becomes active only in

conjunction with elevator muscles.

• Active during power stroke & when teeth held together

THE SUPRAHYOID MUSCLES

These depress the Mandible, if the hyoid bone remains fixed.Thus help when the mouth is opened wide or against resistance.

MOVEMENTS

OPENING & CLOSING

Lateral Pterygoid is the main muscle responsible for opening.

Opening of mouth is limited by the superior lamina of the articular disc.

PROTRUSION AND RETRACTION

Protrusion is brought about by simultaneous contraction of the lateral and medial pterygoids of both sidesWhileRetraction is mainly due to contraction of the posterior horizontal fibres of temporalis muscle

CHEWING

Chewing or side to side movements occur due to alternate contraction of muscles of both sides.

CERTAIN BASICS : JAW MOTION

MANDIBULAR ROTATION

In opening and closing and lateral movements.

AXES OF MANDIBULAR ROTATION

Rotation occurs around three axes: transverse, vertical and sagittal, that move constantly during normal jaw function.

MOVEMENT ABOUT A HORIZONTAL AXIS, AS SEEN IN A HINGE AXIS OPENING.

MOVEMENT OCCURS AROUND A VERTICAL AXIS DURING A LATERAL EXCURSION.

THE MANDIBLE ALSO ROTATES AROUND A SAGITTAL AXIS WHEN ONE SIDE DROPS DOWN DURING A LATERAL EXCURSION

BASIC JAW POSITIONS

Centric Occlusionit is defined as maximum intercuspation of teeth.

Centric Relationthe most posterior relation of the upper to the

lower jaw from which lateral movements can be made at a given vertical dimension. {Boucher}

• CENTRIC RELATION (GPT-8) : the maxillomandibular relationship in which the condyles

articulate with the thinnest avascular portion of their respective disks with the complex in the antero-superior position against the slopes of articular eminences. This position is independent of tooth contact. This position is clinically discernible when the mandible is directed superiorly and anteriorly.

BASIC JAW POSITIONS….

Centric relation is used to transfer position of mandible in relation to maxilla, to an articulator.

The centric occlusion is a tooth determined position, whereas the centric relation is a jaw to jaw relation determined by the condyles in the fossae.

TERMINAL HINGE AXIS

When the condyles are in their most superior position in the articular fossae and the mouth is purely rotated open, the axis around which movement occurs is called the ‘Terminal Hinge Axis’.

Condylar guidance – “Mandibular guidance generated by the condyle and articular disc traversing the contour of the glenoid fossa”

Incisal guidance – “ The influence of the contacting surfaces of the mandibular and maxillary anterior teeth during mandibular movements”

Retruded Contact Position - guided occlusal relationship occurring at the most retruded position of the condyles in the joint cavity.

MANDIBULAR MOVEMENTSMandible performs

habitual and border movementsopening and closing movementsprotrusive and lateral movements

And stopping positions along these movements centric occlusion, centric relation, protrusive border position, right and left lateral border positions and a rest position.

THE CLINICAL UNDERSTANDING OF MANDIBULAR MOVEMENT PARALLELOGRAM OF FORCES.

Factor of muscle pull related to positioning of mandible after loss of teeth.

Direction of forces is affected by occlusal vertical dimension.

Mainly two parallelograms.

In edentulous, occlusal plane is made parallel to ala-tragus line.

THE ENVELOPE OF MOTION.

Defining the limits of possible mandibular motion and certain mandibular reference positions.

Records made in sagittal and frontal planes.

Envelope of motion in the sagittal plane.

•CO, Centric occlusion;

•CR, centric relation;

•MHO, maximum hinge-opening position

• MO, point of maximum opening of the jaws

•P, most protruded position of the mandible with the teeth in contact

•Rest, postural rest position

SAGITTAL PLANE BORDER & FUNCTIONAL MOVEMENTS:-1. Posterior opening border.

2. Anterior opening border.3. Superior contact border.

4. Functional.

Posterior Opening Border Movements:-1st STAGE

• Condyles: terminal hinge position

• Pure rotational movement

2ND STAGE

• Axis of rotation shifts to bodies of rami.

1st STAGE

• Opening range: 20-25mm

• The movements are the only repeatable hinge axis movement of mandible.

2ND STAGE

• Condyles: anteriorly & inferiorly.• Mandible: posteriorly &

inferiorly.• Max opening: 40-60mm.

Anterior Opening Border Movements:-

• Generated when closure accompanied by contraction of inferior lateral pterygoid.

• Not a pure hinge movement due to eccentricity.

Superior Contact Border Movements• Throughout this entire border movement tooth contact

is present. • It depends on:-– Amount of variation between centric relation and

maximum intercuspation.– The steepness of the cuspal inclines of the

posterior teeth.– Amount of vertical and horizontal overlap of

anterior teeth– Lingual morphology of maxillary anterior teeth.– General inter arch relationships of the teeth.

Envelope of motion in the frontal plane•CO, Centric occlusion

•MO, point of maximum opening of the jaws

• Rest, postural rest position.

Left lateral superior movement

Left lateral opening movement

Right lateral superior border movement

Right lateral opening border movement

• Maximum movements-

Opening : 50-60 mmLateral : 10 to 12 mmProtrusive : 8 to 11 mmAnd retrusive range is

about 1 mm.

Functional Movements:-

• Occur during functional activity of mandible.• Free movements: take place within border movements.• Occur chiefly around centric.

Functional Movements:-• During chewing, the mandible

drops directly inferiorly until the desired opening is achieved.

• It then shifts to the side on which bolus is placed and rises up.

• As it approaches maximum intercuspation, bolus is broken down between the opposing teeth.

• In the final closure, the mandible quickly shifts back to the intercuspal position.

EFFECT OF POSTURE

Envelope of MotionGives reference positions from where fundamental movements of

mandible occur-– Helps in making vertical & horizontal jaw relation records

• CR– MHO :represents Posterior terminal hinge movement, which is used to locate transverse hinge axis for mounting of casts on articulator.

– The rest position is a guide to re-establishing the proper vertical dimension of occlusion.

• Multiple restorations and complete dentures are so constructed that their occlusion is in harmony with

centric relation, because mastication in dentulous occurs at CO but in edentulous at CR.

• Except at the occlusal contact position, where envelope of movement is controlled by teeth, all other contours are controlled by muscles, joints and soft tissue forces.

Envelope of Motion

The gothic arch or arrow point tracing.It is a graphic registration of lateral border movements on a horizontal plane, which results in an angular tracing.

The direction of lateral movements is actively determined by the lateral pterygoid muscle on nonworking side and by the deep capsular ligaments of condyle on working side.

WORKING & NONWORKING SIDE

The side toward which the mandible moves in a lateral excursion is called Working side

And

The Nonworking side is that side of mandible that moves toward median line in a lateral excursion

Left lateral border movement

Continued left lateral border movement with protrusion

Right lateral border movements Condyles in CR

Contraction of left inferior

lateral pterygoid

Left condyle moves

anteriorly,medially and

Inferiorly

Continued right lateral movement with protrusion

Contraction of rt. lateral pterygoid & continued contraction left inferior pterygoid

Right condyles moves anteriorly and to left

Mandibular midline coincides with midline of face

BENNETT SHIFT

Described by Dr. Norman Bennett in 1908 , it is the direct lateral side shift that occurs simultaneously with a lateral excursion

The primary cause :is the contraction of lateral pterygoid muscle because its origin is located medially to its insertion.

When mandible shifts to the side, its movement occurs in two segments-

an immediate side shift in which the major direction of movement is mediolateral

and a progressive side shift, which begins thereafter and continues with the major direction of movement being anterior.

Physiological rest position• Established by muscles and gravity.• Is actively determined.• It is a ‘range of posture’• Usually 2-4mm below maximum intercuspation

position, but can be upto 10 mm.• Remains relatively stable for reasonable periods of

time.• Affected by short and long term intra oral and

general health factors and by position of head.

Conclusion

Knowledge of Jaw Movements is Essential for Successful Treatment of Patients. It is imperative to learn as much possible about jaw movement, in order to reproduce those aspects of its motion, considered necessary for proper functioning of the occlusion, either natural or artificial.

References

• Complete denture prosthodontics – 3rd edition, by John J Sharry

• Prosthodontic Treatment for Edentulous Patients – by Zarb and Bolender, 12th edition.

• An evaluation of mandibular border movements: Their character and significance - Harry C. Lundeen

Thank you !!!