Biomechanics TMJ

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TMJ ● Temporomandibular Joint ● TMJ is not the name of the disorder it is the name of the joint; the name of the disorder is Temporomandibular Dysfunction or Disorder (TMD) ● Anatomy Review ○ Osseous ○ Muscular ○ Articular ● Biconcave disc allows for stability ● Anterior half - convex condyle ● Below space/disc - inferior compartment ● Above space - superior compartment ● 2 movements that occur in TMJ ○ Translation ■ Superior compartment ○ Rotation ■ Inferior compartment Downloaded by Vegeta Ouji ([email protected]) lOMoARcPSD|5612835
● Condyle has to go down slope for jaw to open ● Condyle of mandible is convex ● Going from convex convex articulation, so need biconcave disc as open jaw further → keep TMJ stability Dentists and PTs ● Dentists starting to realize the craniocervical part of TMD ● Physical therapists can assist dentists with their patients because of our knowledge of the TMJ and concurrent effects with the cervical spine Basic features ● Compound synovial joint between the mandible and cranium ● Articulating surfaces are covered by avascular fibrocartilage which can withstand large amounts of compressive and shearing forces ● The active joint surface is from the superior part of the fossa to the anterior / inferior slope of the eminence all the way to the apex of the eminence ● The articular disc divides the TMJ into an upper and lower compartment ● Innervated by the mandibular division of the trigeminal nerve ● In the sagittal plane, The convex condyle does not articulate with the concave temporal or mandibular fossa but against the convex slope and apex (convex with convex) ● In the Horizontal Plane, the direction of the temporal joint is in an oblique and sagittal direction ● This orientation causes the condyles to translate laterally as we open our mouths as the condyles slide forward ● Palpation of this movement Articular Disc ● Plays a central role in the stability of the TMJ by making up for the congruency problem between the two joint surfaces ● The disc is biconcave and oval in shape ● This orientation presents the disc to be thinner in the central portion and thicker in the peripheral portions; self seating capacity of the disc ● Excellent shock absorber ● Laterally and medially the disc is stabilized by collateral ligaments ● The disc divides the joint into two compartments ● Rotation takes place in the inferior compartment ● Translation takes place in the superior compartment Muscles of Mastication ● Temporalis: ○ Muscle fibers in antero and postero inferior directions ○ Origin: entire temporal fossa ○ Insertion: coronoid process and anterior border of the ramus and some fibers to the skeletal orbit of the eye Downloaded by Vegeta Ouji ([email protected]) lOMoARcPSD|5612835
○ Function: ■ Bilateral contraction closes the mandible ■ Unilaterally deviates the mandible to same side ■ Posterior fibers minimally retract the mandible from rest position ■ All fibers retract mandible if it is fully protruded; helped by suprahyoid ■ Guides biting motion ● Lateral pterygoid ○ Prime mover of TMJ and guides disc both during eccentric and concentric contractions ● Medial pterygoid ○ Similar to the masseter but smaller ○ Function: ■ elevates the mandible; ■ unilaterally deviates mandible toward opposite side; ■ participates in chewing motion Biomechanics of the TMJ ● Functional unit ○ Cranio-cervical joint posteriorly ○ Cranio-mandibular joint anteriorly ● TMJ problem, check the cervical spine and vice versa ● Complex synovial joint: moves in three planes ● 4:1:1 ratio between opening, lateral excursion, and protrusion ● For 40 mm if opening, there is 10 mm of lateral excursion and 10 mm of protrusion ● When we measure left lateral excursion, we are measuring the function of the right TMJ ● Joint surfaces and the articular disc ○ Stability based upon on the shape and relationship of the joint surfaces ○ TMJ is unstable ○ The anterior slope of the temporal surface is convex and the condyle is convex: so convex on convex relationship: not stable ○ Not a problem with rolling but a problem with sliding: grinding and instability ○ Disc solves this problem by being biconcave and creates two compartments ● The 2 departments of the TMJ ○ Disc-condylar (inferior) and disc-temporal/eminence (superior) joints ○ Inferior department: rotation ○ Superior department: translation ○ Disc will follow the condyle as long as there is apposition of the two joint surfaces and there is no subluxation ○ Centric relation: term that refers to the most congruent contact between disc/eminence and disc/condyle with the disc interposed TMJ movements Downloaded by Vegeta Ouji ([email protected]) lOMoARcPSD|5612835
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