TMJ & Mandibular Movements MCQ

The temporomandibular joint is the foundation of occlusion. It is a synovial joint with a fibrocartilage disc that divides it into an upper compartment (translation) and a lower compartment (rotation). The mandible moves by combining rotation about a hinge axis with translation forward and downward along the articular eminence. Posselt's envelope describes the anatomic limits of mandibular motion. Working vs non-working side, Bennett movement, and the determinants of mandibular movement together govern how teeth pass each other in eccentric movements.

TMJ Anatomy

• The TMJ is a synovial joint with an articular disc made of dense fibrous connective tissue (fibrocartilage); the articular surfaces of the condyle and the temporal eminence are also fibrocartilage, not hyaline cartilage like most synovial joints.
• The disc has a thicker posterior band and thinner intermediate zone; posteriorly the retrodiscal tissue (bilaminar zone with elastic fibers superiorly and dense fibrous tissue inferiorly) is highly vascular and innervated.
• Ligaments include the temporomandibular ligament (lateral; the main reinforcement of the capsule, limits inferior/posterior condylar movement) and the accessory sphenomandibular and stylomandibular ligaments (less mechanically restrictive).
• Innervation: branches of the auriculotemporal nerve (from V3) and the masseteric nerve provide TMJ sensory innervation; blood supply comes from the superficial temporal and maxillary arteries.

Mandibular Movement: Rotation + Translation

• Mandibular movement combines ROTATION (lower compartment between condyle and disc) and TRANSLATION (upper compartment between disc + condyle complex and articular eminence).
• The first ~20-25 mm of opening is predominantly rotation about a hinge axis; further opening adds translation forward and downward along the articular eminence.
• The terminal hinge axis is the rotational axis through both condyles in centric relation; recording the terminal hinge axis allows reproducible registration of CR.
• The mandible is a CLASS III LEVER: the fulcrum is at the TMJ, the effort is from the masticatory muscles (closer to the fulcrum than the load), and the load is at the bite point (farther from the fulcrum); this geometry means muscle force is amplified at the bite point relative to small condylar excursions.

Posselt's Envelope of Motion

• Posselt described the envelope of mandibular motion in the sagittal plane traced by a mandibular incisor: anteriorly the protrusive border, superiorly the contact (intercuspation) limits, posteriorly the retruded border, and inferiorly the maximum opening.
• Border movements (the outer perimeter) are the limits of motion; functional movements (chewing, speaking) take place within the envelope, not at the border.
• Key landmarks within the envelope include the CR-CO slide (the short anterior-superior movement from CR to MIP/centric occlusion) and edge-to-edge (incisor edge-to-edge contact during protrusion).
• Maximum opening varies but is typically about 40-55 mm of interincisal distance in adults; less than ~35 mm warrants further evaluation for restriction.

Working vs Non-Working Side and Bennett Movement

• When the mandible moves laterally, the side TOWARD which it moves is the WORKING (laterotrusive) side; the opposite side is the NON-WORKING (balancing, mediotrusive) side.
• On the WORKING side, the working condyle stays largely in place and rotates with a slight lateral shift (the Bennett movement). On the NON-WORKING side, the non-working condyle translates forward, downward, and medially down the articular eminence.
• The BENNETT ANGLE is the angle between the path of the non-working condyle and a sagittal reference plane (typically 7-15°); it affects the shape and inclination of posterior cusps that must clear the opposing teeth in lateral movement.
• In mutually protected occlusion, the working canine guides the mandible laterally and DISOCCLUDES the posterior teeth on both sides; non-working contacts in a dentate adult are typically interferences.

Determinants of Mandibular Movement

• Posterior (condylar) guidance: the slope of the articular eminence and the Bennett angle determine the condylar path during translation; these are fixed by the joint anatomy.
• Anterior guidance: the contact pattern of the maxillary and mandibular anterior teeth determines the path of the mandible during protrusion and eccentric movement; this is set by the dentist in restorative work.
• Posterior tooth occlusion: cusp height, fossa depth, marginal ridge anatomy, and contact pattern (centric stops) translate the anterior + condylar guidance into the path that posterior teeth take past each other.
• Together these are 'the determinants of occlusion' (sometimes summarized in Hanau's quint of five factors: condylar guidance, anterior guidance, plane of occlusion, curve of Spee, cusp height); they are the inputs to articulator setup and to occlusal scheme design.