1b- functional anatomy and biomechanics

What is the TMJ?

Area connecting jawbone to skull

Mandibular condyle articulates in base of skull through temporal bone

Fibrocartilage surface- greater repair capacity than hyaline cartilage

Function of ginglymoid?

Hinge movement- rotation in inferior compartment

Function of arthrodial?

Gliding movement (translation) in superior compartment

Label this

Temporal bone

Zygomatic bone

Glenoid/Mandibular fossa

Articular eminence

Condyle

Disc

What is the glenoid fossa?

Area formed by concave mandibular fossa where condyle is situated

Thin roof

What is the articular eminence?(4)

Protruding dense convex bone in front of the fossa

Varying level of convexity

Guides movement of condyle, primary load bearing area

Ginglymoarthrodial joint- Ginglymoid (hinging) AND arthrodial (sliding joint)

What is the articular disc made of? its structure and mechanism?

Dense fibrous ct

Intermediate zone is its thinnest points, functional contact between condyle and eminence

Has self centring mechanism- inter articular pressure aligns the thin zone with condyle

Thick posterior band prevents displacement

What are the dynamics of synovial lubrication?

Weeping lubrication- Fluid expelled from tissues under load- metabolic exchange

Fluid moves to surface during movement- reduces friction

1.2ml vol- only nutrient source for avascular disc

Clenching (static loading) exhausts weeping lubrication- leads to hypoxia

Describe the parts of the bilaminar zone/retrodiscal tissue

Superior retrodiscal lamina- elastic fibres, opposes forward translation

Inferior- collagenous (nonelastic), limits rotation

Venous plexus- rapid volumetric filling during condylar translation

Rich in nociceptors

If compressed here- causes intracapsular pain- retrodiscitis

Function of collateral (discal) ligaments?

Passive restriction- they elongate or tear but don’t stretch

Anchor disc to condyle poles

Allows disc to rotate with condyle

Function of capsular ligament

Retains synovial fluid and resist separation forces

What is the temporomandibular ligament made of and its effect?

Outer oblique portion- limits pure rotational opening to 20-25mm

This forces condyle to translate after initial rotation (move down eminence)

Inner horizontal portion- limits posterior movement- protects retrodiscal tissue

This prevents compression of vital neck structures during opening

What are the jaw elevator muscles?

Temporal

Masseter

Pterygoid

What are the jaw depressor (opening) muscles?

External or lateral inferior pterygoid muscle

Suprahyoid- digastric, stylohyoid, mylohyoid, geniohyoid

Infrahyoid

Structure and function of temporal muscle

Fan shaped

Anterior, medial, posterior portion

Contraction of anterior portion elevates jaw , medial and posterior elevate and retract

Structure of masseter muscle- where, insertion, portions

Rectangular muscle that goes from zygomatic arch to inferior edge of mandible

Insertion in jaw from 2nd molar at lower edge to angle

Superficial (for protrusion) and deep (retrusion) portions

Explain the lateral pterygoid paradox

Inferior head is active during opening- pulls condyles forward for translation

Superior head active during closing- stabilises disc against eminence during biting

Incoordination causes disc displacement

What muscles are responsible for- mouth opening, mouth closure, protrusion, retrusion?

What is the rotation phase constrained by? Its dynamic?

Limited by tightening of TML outer oblique portion

Condyle rotate, disc remains superiorly stable

What is translation phase triggered by and stopped by?

TML tightens forcing condyle to translate

Elastic superior retrodiscal lamina stretches to control movement and recoil

What is the criteria for CR (orthopedic stability)?

Most superior posterior position

Loading on intermediate zone

Elevator muscles released, minimal tonicity in resting position

MIC coincides with this joint position