TMJ anatomy and biomech

What is abnormal dental occlusion ?

*deep dental horizontal overbite* (overset) > 3-5mm --> excess protrusion of the u/jaw. Common in TMJ dysfunction * horizontal underbite* --> excess protrusion of the L/jaw * Cross bite* --> mandibular teeth are lateral to the maxillary teeth on 1 side and medial on the opposite side

What are the biomechanics of the TMJ with protraction and retraction ?

*protraction* of the mandible: condyles and discs glide ant-inf *Retraction* of the mandible: condyles and discs glide sup-posterior both protraction and retraction occur and the superior joint space ex. with decreased anterior translation and rotation on the right side means that protraction will also be limited on the right side

What is the functional relevance of the upper and lower compartments of the TMJ?

*supperior compartment* - plane - allows for gliding (translation) *inferior compartment* - hinge joint (permits movement only in one plane - permits *rotation* of the condyles

What does normal function of the TMJ depend on ?

1. *Disc morphology*--> the middle part of the disc needs to be thinner, the ant and posterior aspects need to be thicker 2. *Posterior disc attachments* --> holding disc posteriorly 3. *Anterior disc muscle* coordination --> (lateral pterygoid ms coordination)

What muscles open the mouth /

1. *Lateral pterygoid* - initiates movement - inferior head attaches to condyle - sup head attaches to anterior medial portion of the disc - *with rotation* inferior head contracts and superior head relaxes - *with translation* superior and inferior heads contract together 2. *Suprahyoid ms* (digastric, hyoid)

What are the *anterior attachments* of the TMJ disc?

1. *Superior head* of the *lateral pterygoid* ms --> pulls disc anterior 2. TMJ joint capsule --> is highly vascularized and innervated and thus can cause pain

What is important to consider about anterior translation that occurs at the onset of mouth opening ?

1. Movement occurs in the lower compartment of the joint 2. Condyles rotate on stationary disc until collateral ligaments are taut --> only the condyle moves initially with rotation 3. Lateral pterygoid muscles *inferior head* contracts, *superior head* relaxes

What general info about TMJ biomechanics are important ?

1. TMJ is a modified ovoid joint 2. Bicondylar (has two compartments 3. *Capsular pattern*: 1. decreased ROM mouth opening with deviation toward involved side 2. Decreased protrusion with deviation towards involved side 3. decreased lateral translation to Uninvolved side 3. Closed packed position: teeth slightly clenched 4. Resting position: mouth slightly open, lips together, teeth not in contact

What are the muscles involved with TMJ?

1. Temporalis 2. Masseter 3. Lateral Pterygoid (sup and inf heads) 4. Medial Pterygoid 5. Suprahyoid muscles

What muscles close the TMJ ?

1. Temporalis (posterior fibres initiates closing) 2. Masseter 3. Medial Pterygoid --> these muscles may become hypertonic 4. Lateral Pterygoid (superior belly) --> work eccentrically to ensure posterior glide of disc (follow condyle)

What structures form the temporal mandibular joint ?

1. The articular eminence of the temporal bone 2. Condyle of the mandible 3. Intra-articular disc

AT what ROM of the mouth does the joint begin to be dysfunctional ?

> 40 mm opening gets 1. increased load on anterior structures 2. increased lension on posterior structures ??? 70% of max opening (2 PIP joints) ... 35mm --> should use complete rotation but only slight translation to prevent too much stress on soft tissues, muscles and articular cartilage???

What is important to consider about the anterior translation that occurs after the rotation component?

After the initial 25mm of anterior rotation, approximately 10-15 mm of anterior rotation occurs - movement happens at the *superior joint* - movement occurs in the *condyles* and the *discs* until the posterior and collateral ligaments are taught - opening greater than 35mm of translation and over rotation causes increased stretching of the ligaments -With translation --> *superior and inferior heads of pterygoid* contract together; *posterior ligaments* pulls on the disc to keep it over the condyle

What is the biomechanics of the TMJ during Lateral deviation ?

Contralateral condyle --> moves anterior, inferior and medial Ipsilateral condyle --> remains relatively stationary Depression, lateral deviation and protrusion should function with a ratio of 4:1:1 i.e. limitation of anterior translation on the right means that depression on the right, protrusion on the right and lateral deviation to the left will be limited

What muscles protract the mandible ?

Lateral pterygoid (inferior belly) and medial pterygoid Supra hyoid muscles

What is the orientation of the TMJ muscles from outside to inside ?

Master is most superficial, temporalis, lateral pterygoid, medial pterygoid is most deep remember that the angle at which the medial pterygoid and the master muscles orient themselves are *equal* and *opposite* of each other

What is the origin and insertion of the lateral pterygoid?

O: 1. *sup head* sphenoid bone , 2. *inf head* maxilla (lateral pterygoid plate) I: 1. *sup head* disc and anterior caps on TMJ, 2. *inf head" anterior neck of mandible **if hypertonic/stiff can cause *MALOCCLUSION OF TEETH* only the INFERIOR head of the lateral pterygoid is palpable

What is the origin and insertion of the medial pterygoid?

O: Lateral pterygoid plate (under lateral pterygoid muscle inf head AND Maxillary tuberosity I: Inside angle of RAMUS of the mandible

What is the origin, insertion of the *temporalis* ?

O: Temporal Fossa (sup to zygomatic arch) I: Passes underneath zygomatic arch to insert at the *coronoid process* of the mandible

What is the origin and insertion of the Master ?

O: zygomatic arch I: ramus of the mandible

What are the *posterior attachments* of the TMJ?

Posterior aspect of the TMJ = *bilaminar region* Posterior structures include the posterior ligaments 1. Superior Head of PL --> attaches to posterior aspect of the *temporal bone* --> prevents anterior translation of disc holding it posterior 2. Inferior head of PL --> Attaches disc to *neck of mandibular condyle* 3. Between the two heads of the posterior ligaments is a *retrodiscal pad*

What are the biomechanics of the TMJ during closing?

Posterior/superior translation --> discs and condyles glide on temporal bone Posterior rotation --> condyles move under disc Lateral Pterygoid (superior head) --> contract eccentrically to ensure that the disc follows condyle (relaxation) Elastic part of posterior ligament : PULLS DISC POSTERIORLY during closing *EXCESS closing should be avoided to prevent injury to the articular cartilage*

What is significant about the disc in terms of the functional anatomy of the TMJ?

Separates the TMJ in to 2 distinct compartments 1. Upper and 2. Lower compartments

What is the function of the Temporal Mandibular Ligament ?

Strongest ligament of the TMJ Connects the disc to the condyle limits movement of lower jaw - limits *posterior-inferior* movement of *condyle* - limits posterior movement of *disc*

What is the function of the Suprahyoid muscles ?

Stylohyoid, geniohyoid, mylohyoid work to 1. *When hyoid if fixed* = open mouth 2. *When mandible is fixed* = elevate the hyoid bone

What muscles Retract the mandible ?

Temporals (posterior/medial fibres) Masseter Suprahyoid

Why is the position of the TMJ important for healthy TMJ function ?

The disc is *sellar shape* medial lateral convex, A/P concave The "middle band" should be on the "anterior 2/3" of the condyle The posterior band should be on the "APEX" of the condyle the proper placement of the middle and posterior bands ensure optimal biomechanics of the TMJ. When the disc moves too far anterior or posterior, dysfunction occurs

With opening of the mouth, what are the two basic movements of the jaw that occur ?

The normal for jaw opening is *35-40 mm* first there is ~ *25mm of Anterior rotation* Second there is *10-15mm of Anterior translation*

What part of the TMJ joint does protraction and retraction occur ?

The superior joint only

What is normal dental occlusion ?

The upper teeth should fit slightly over the lower teeth The points of the molars should fit the grooves of the opposite molars *vertical overlap/overbite* --> maxillary (superior) anterior teeth overlap almost half (1/2) (2-3mm) the length of the mandibular inferior anterior teeth *Horizontal overbite/overjet* --> Maxillary (sup) anterior teeth lie anterior to the mandibular (inferior) anterior teeth --> *normal = 2-3 mm*

What muscles laterally deviate the mandible?

With L lateral deviation (R) lateral pterygoid inferior belly and (R) medial pterygoid muscle (L) Temporalis holds the resting position of the condyle preventing the mandible from deviating anteriorly

What is important to consider about the anatomy of the intra-articular disc?

has three bands 1. Middle band --> thinner, avascular, aneural (not pain sensitive), is the weight bearing part 2. *Posterior band* 3. *Anterior band* 2 and 3 are both *two times thicker* than the middle band, *well vascularized* and *innervated* --> i.e. are able to heal relatively well and can cause pain

What is the effect of a malocclusion on the TMJ and spine ?

if any malocclusion is present, muscles, joints must accommodate the malocclusion/abnormal pattern this will lead to structural disorganization of the biomechanics at the level of the 1. Cranial vertebral Spine, 2. Cervical Spine 3. mandibular spine (TMJ stress and myofascial tension/pain) IMPORTANT - there is a functional and an anatomical interrelationship between TMJ and CX spine - Poor posture, Cx spine dysfunction may lead to TMJ dysfunction - it dictates that a thorough Ax and Rx of both regions is necessary to obtain positive clinical outcomes