Mandibular Movement II RSD 811: Session 15 Lina Sharab, DDS,MS.,MSc.

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Special thanks to Dr. Selwitz and Dr. Haubenreich, Dr.Casky for compiling this material

Review Topics TMJ Anatomy Centric Relation Protrusive and Lateral Movements Mutually Protected Occlusion The Articulator Occlusal Interferences

TMJ ANATOMY Review and Elaboration

The Temporomandibular Joint All occlusal analysis starts at the temporomandibular joints (TMJs). The first requirement for successful occlusal treatment is stable, comfortable TMJs. Sagittal View

Definition of Occlusal Analysis A study of the relationships of the occlusal surfaces of opposing teeth, including the effect these relationships have on related structures. Also called bite analysis. The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reservedHoughton Mifflin Company

The Temporomandibular Joint Through understanding of what is normal, healthy TMJ functions is required to be able to diagnose what is abnormal. Sagittal View

Where is the Axis of Rotation? The medial poles of the condyles are the only rotation points that would permit a fixed axis of rotation b/c the condyles are not parallel to the horizontal axis.

Midmost, Uppermost Position The condyle-disk assemblies are braced at the midmost, uppermost position by compression of the medial third of the assembly against the medial apex of each condylar fossa. Coronal View

Medial Pole Bracing Medial pole bracing consistently reinforces the midmost, uppermost position. – For future reference: It also explains why an immediate side shift is not possible from the fully seated position of the condyles (CR).

Midmost, Uppermost Position Note the anatomy of the condylar fossa: – It is “V-shaped” – A horizontal axis through any part of the condyle other than the medial pole would result in translatory movements of the medial pole during a fixed rotational axis, which would be incompatible with the V- shape of the fossa. Looking up at the fossa from below

Mandibular movements Gothic arch tracing

MUSCLE CONTROL OF DISK ALIGNMENT

Opening As the inferior lateral pterygoid muscle (+) starts to pull the condyle forward, the superior lateral pterygoid muscle (-) releases contraction to allow the elastic fibers to start pulling the disk more to the top of the condyle.

Maximum Opening When the condyle reaches the crest of the eminence, the disk should be directly on top of the condyle. – At this point, the elastic fibers have rotated the disk back since the superior lateral pterygoid muscles are in a controlled release.

Closing As the jaw closes, the condyle starts to move back & up the steeper slope of the articular eminence. – To accomplish this, the superior lateral pterygoid muscle (+) starts its contraction as the inferior lateral pterygoid muscle (-) releases the condyle to the elevator muscles that pull it back.

KEYS TO MANDIBULAR MOVEMENT

Orthopedically stable joint position Musculoskeletal stable position Directional pull of the muscles applies interarticular pressure to the disc

Thick strong bone Thin squamotympanic plate Retrodiscal tissues

Articular Eminence Angle compared to horizontal (in Degrees) aka Articular Incline, Condylar Inclination Steeper incline = Quicker disarticulation of the posterior teeth in eccentric movements

Angle in degrees 45º 60º

A steeper ( ↑º ) articular eminence will cause the posterior teeth to separate More quickly when the mandible is moved forward.

Articular Disc Bi-concave - Aids in positioning during movements Dense fibrous tissue Few blood vessels or nerves - Weeping lubrication / nutrient exchange Divides the joint into upper (superior) & lower (inferior) synovial cavities

Inferior Synovial Cavity Disc and condyle are tightly joined by lateral and medial discal ligaments Rotational movement

Superior Synovial Cavity Articular Disc / Articular Eminence of the temporal bone Translational movement

Superior Retrodiscal Lamina Contains elastic fibers Retracts the disc posteriorly - the only structure in the joint capable of doing so Ligaments once stretched do not return to their original length

Superior retrodiscal lamina is stretched when the condyle is fully translated

Lateral Pterygoid Inferior LP - pulls condyle forward, is active during the opening cycle Superior LP - attached to condyle and the disc – could pull the disc forward or at least apply tension to it, but it does not. This muscle is active during the closing cycle.

Closing Opening

What role do the following two structures play in translation of the mandible? 1.Temporomandibular ligament – outer oblique portion 2.Inferior lateral pterygoid

The OOP restricts posterior movement of the neck of the condyle and acts as a mechanical initiator of translation of the condyle. Outer Oblique Portion of the Temporomandibular Ligament

Contraction of the inferior lateral pterygoid pulls the condyle forward; this is a muscular component of translation.

Inferior lateral pterygoid Bi-lateral contraction = Protrusion or opening of the mandible. Unilateral contraction = lateral movement to the opposite side.

CENTRIC RELATION Optimum Orthopedically Stable Joint Position

Definitions of Centric Relation Old Definition – Most posterior (retruded) position of the condyles – Denture patients – it was reproducible – Anatomy of the joint had not been examined New Definition – Most anterior superior position of the condyles in the mandibular fossa with the disc properly interposed.

Centric Relation The Glossary of Prosthodontic Terms “The maxillomandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective disks with the complex in the anterior-superior position against the slopes of the articular eminences.” – This position is independent of tooth contacts. – This position is clinically discernible when the mandible is directed superiorly and anteriorly. – It is restricted to a purely rotary movement about the transverse horizontal axis.”

Centric Relation Additional Clarifications At the most anterior-superior position the condyle-disk assemblies are braced medially, thus centric relation is also the midmost position. A properly aligned condyle-disk assembly in centric relation can resist maximum loading by the elevator muscles with no sign of discomfort.

Diagnostic Casts – Remember, the purpose of the mounted casts in the treatment planning phase is to see how the mandibular teeth relate to the maxillary teeth when the condyles are in CR. – Mounted casts make it possible to determine the best treatment approach for bringing the teeth into harmony with the correct maxillo-mandibular relationship.

Diagnostic Casts Ignoring the position of the TMJs when examining the occlusion is not acceptable. It does not show what must be done to achieve harmony between the occlusion and the TMJs. Unmounted casts are responsible for many mistakes in restorative treatment.

Diagnostic Casts Analysis of the mandible-to- maxilla relationship when the condyles are in CR presents a completely different picture from MI (ICP).

Protrusive and Lateral Movements

Movement of the Condyles Activity in the right and left TMJ’s is relatively equal in opening, closing and protrusive movements Right and left joints are acting independently and differently in all other eccentric movements

Protrusive Movement Mandible moves forward Condyles slide downward on condylar incline (eminence) Anterior teeth should separate the posterior teeth, therefore only incisors and canines should touch in a protrusive movement

Protrusive Movement 1.Articular eminence 2.Anterior guidance

Lateral Movements of the Mandible

Left Laterotrusive Nonworking side (right) Immediate side shift Translates “Travels” downward, forward & inward Downward = condylar inclination Inward = Bennett Angle (or progressive side shift) Working side (left) ISS rotate around vertical axis (mostly) & sagittal axis (some) Relatively fixed in position

Immediate Side Shift “Mandibular lateral translation”- Okeson The bodily shift of the mandible in the direction of the working side occurring in the initial phase of the movement. Can be programmed into fully adjustable articulator.

Bennett angle Definition: The angle in the horizontal plane between the condylar path and the sagittal plane. Related terms: Bennett Movement

R L

Bennett Angle The distance the translating (NW) condyle moves inward or medially – measured in degrees compared to a straight forward path. 22º

Downward (inferiorly) Movement of the Translating Condyle Down the articular eminence

Forward Movement of the Translating Condyle Translation

Inward Movement of the Translating Condyle Movement of a fixed radius around the rotating condyle

Occlusal Interferences Centric Working (Laterotrusive) Nonworking (Mediotrusive) Protrusive

Occlusal Interferences: Definition Tooth contacts in eccentric movements other than the anterior guidance on canines or incisors.

Vertical force: along the long axis Lateral or horizontal force: at an angle to the long axis  X

Why would we like to avoid having eccentric contacts on posterior teeth? Posterior teeth do not tolerate lateral forces as well as anterior teeth 1. Root morphology & bone density 2. Proximity to fulcrum 3. Reflexes / proprioception

Posterior teeth don’t tolerate lateral forces as well as anterior teeth – Why Not? 1.Root morphology & bone density - Canines have the longest, largest root in the arch - Canines and incisors are anchored in dense cortical bone - Molars have smaller, shorter, divergent roots - Anchored in less dense medullary bone

Premolars From a morphology standpoint, premolars are transitional between the canines and the molars 1)Occlusal surface alignment similar to molars allow axial loading 2)Root form similarities to anterior teeth that allow them to share in lateral guidance (group function)

Classification of Levers Class 3 - The fulcrum is at one end, the load at the other end and the effort lies between the load and the fulcrum

#2) Why would we like to avoid having eccentric contacts on posterior teeth? 2. Closer proximity to the fulcrum = ↑ forces

#3) Why would we like to avoid having eccentric contacts on posterior teeth? 3. Proprioception or sensory feedback to the muscles - Canine to canine contact in eccentric movement: results in less activity or contractive force in the closing muscles - Posterior teeth contacting in eccentric movement: results in more activity or contractive force in the closing muscles

With the condyles in the centric relation position – close together – the teeth that contact are centric interferences. If all teeth contact perfectly, then MI = CR.

Working or Laterotrusive Interference A contact on the working side other than canines or group function “Similar” cusps will contact: Lingual / Lingual or Buccal / Buccal

Non-working or Mediotrusive Interference Any contact on the non-working side “Dissimilar” cusps will contact: Lingual / Buccal

Protrusive Interference Any posterior tooth contact during protrusion

Role of Stress in TMJ Disorders Increase in stress can lower an individual’s ability to accommodate occlusal interferences If stress cannot be relieved there may be a need to alter or refine the occlusion via occlusal adjustment, bite guard therapy or restoration of the teeth

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