Articulation & Articulators MCQ

Articulators reproduce mandibular movement outside the mouth so that restorative work can be designed without the patient. The facebow transfers the maxillary cast to the hinge axis; the CR record mounts the mandibular cast to the maxillary cast at centric relation; protrusive and lateral records set the condylar inclination and Bennett angle on a semi-adjustable articulator. Chair-side verification with articulating paper and shimstock checks the articulator-derived occlusion against the patient. T-Scan digital occlusal analysis records the sequence and intensity of contacts over time.

Articulator Classes

• CLASS I (non-adjustable, hinge): a simple hinge that allows only rotation; suitable for simple single-tooth restorations where condylar movement is not needed.
• CLASS II (average value): condylar inclination and Bennett angle are FIXED at population-average values; better than Class I for casts but less precise than semi-adjustable for complex work.
• CLASS III (semi-adjustable): condylar inclination and Bennett angle are ADJUSTABLE based on the patient's records; the workhorse of fixed and removable prosthodontics, balancing precision with practicality.
• CLASS IV (fully adjustable): reproduces the patient's border movements precisely from pantographic tracings; used for complex full-mouth rehabilitations where the highest precision is needed.

Facebow Transfer

• The FACEBOW is an instrument that captures the spatial relation of the maxillary arch to the patient's hinge axis (or to an arbitrary reference); it then transfers this relation to the articulator.
• Reference planes for the facebow: FRANKFORT HORIZONTAL (Porion to Orbitale) is the most common; the AXIS-ORBITALE PLANE and other planes are used in some systems.
• An ARBITRARY facebow uses an average hinge axis location (typically 13 mm anterior to the tragus of the ear on the canthus-tragus line); a KINEMATIC facebow records the patient's actual terminal hinge axis (more accurate for full-mouth rehabilitation).
• Facebow transfer is the FIRST step in articulator mounting; without it, the maxillary cast is placed arbitrarily on the articulator and the relationship to the hinge axis is lost.

Centric Relation Record

• The CR RECORD mounts the mandibular cast to the maxillary cast at the chosen CR position; combined with the facebow it positions both casts correctly on the articulator.
• CR-recording techniques (covered in Mod 2): bimanual manipulation (Dawson), leaf gauge, anterior deprogrammer (Lucia jig); the patient closes lightly under operator guidance to first tooth contact in CR.
• Materials: polyvinyl siloxane (PVS) bite registration is the modern standard for routine cases; wax (extra-hard or aluwax) is used in some systems; the record must be RIGID, DIMENSIONALLY STABLE, and EASY to seat back on the casts.
• The CR record is checked by trial seating on the casts; rocking or non-passive seating indicates a distorted record that must be remade.

Setting Condylar Inclination (Protrusive Record)

• CONDYLAR INCLINATION (also called horizontal condylar guidance angle) is the angle of the condylar path relative to the Frankfort horizontal during protrusion; it is set on the articulator from a PROTRUSIVE interocclusal record.
• TECHNIQUE: the patient protrudes 4-6 mm (until the incisors are edge-to-edge or past it), a PVS or wax record is captured at this protrusive position, and on the articulator the condylar elements are adjusted until the casts pass smoothly into the protrusive record.
• Average condylar inclination is about 30-40° to the Frankfort horizontal; steeper inclinations (older patients, deep overbite, brachyfacial) demand steeper posterior cusps to clear opposing teeth in protrusion without interferences.
• Setting condylar inclination accurately matters most for FULL-MOUTH and COMPLEX RESTORATIVE work; single restorations can often use average-value or arbitrary settings.

Setting Bennett Angle (Lateral Record)

• BENNETT ANGLE is the angle of the non-working condyle's path to a sagittal reference plane during lateral movement; it is set on the articulator from LATERAL interocclusal records.
• TECHNIQUE: the patient performs a lateral excursion (e.g., to the right) and a PVS or wax record is captured; the maneuver is repeated on the opposite side. On the articulator, the Bennett angle for each non-working condyle is adjusted to seat the casts in the lateral record.
• Average Bennett angle is about 7-15°; steeper Bennett angles demand wider mandibular lingual cusps and wider maxillary palatal embrasures to clear opposing teeth on the non-working side without interferences.
• HANAU'S FORMULA L = H/8 + 12 (Bennett angle L in degrees from condylar inclination H in degrees) provides an alternative estimate when lateral records are not available.

Chair-Side Verification: Articulating Paper, Shimstock, T-Scan

• ARTICULATING PAPER (typically thin, two-sided, e.g., 12-40 μm) marks contacts on tooth and restoration surfaces; light pressure and heavy pressure together distinguish centric stops from interferences.
• SHIMSTOCK is a thin metal foil (typically 8-12 μm) used to verify that a contact is firm enough to grip the foil; a foil that pulls through easily indicates absent or too-light contact.
• T-SCAN is a digital occlusal analysis tool that records the sequence and intensity of contacts over time; it captures dynamic information that paper cannot (which contact closes first, where force concentrates).
• Chair-side verification protocol: mark centric stops with one color of paper at light bite, then heavy bite; mark working and non-working excursions in different colors; check shimstock at the centric stops; use T-Scan for refractory cases.