is a distortion in the registration or the cast.
The reference pin is fastened to the underside of the face-bow by tightening the thumbscrew (Fig 5-41). The clamp marked with a 2 should be on the patient’s right (with the left side of the instrument viewed from the front).
Fig 5-42 The dentist places the clamp over the bite fork shaft while the patient inserts the earpieces.
Fig 5-43 The thumbscrew on the front of the facebow is tightened.
Fig 5-44 The reference pointer is aligned with the reference mark.
Fig 5-45 The clamps in the facebow assembly are tightened.
The bite fork is placed in the mouth, and the patient is instructed to hold it securely between the maxillary and mandibular teeth. The patient should grip both arms of the facebow to guide the plastic earpieces into the external auditory meati, in the same manner as one would place a stethoscope into the ears (Fig 5-42). While the patient is inserting the earpieces, the operator should slide the clamp marked with a 2 onto the shaft of the bite fork. The clamp should be positioned above the shaft. The single thumbscrew on the front of the facebow is tightened (Fig 5-43).
The anterior reference pointer is extended while the facebow is moved up or down. When the pointer is properly aligned with the anterior reference point, the thumbscrew is tightened (Fig 5-44). With the facebow still supported, the set screw on clamp 1 is tightened on the vertical reference pin (Fig 5-45). Then clamp 2 is tightened on the horizontal reference pin. For added stability and peace of mind, the patient can continue to support the facebow by holding the side arms. The facebow should not be allowed to torque or tilt during the tightening procedure.
The thumbscrew on the front of the facebow is loosened by a quarter turn. As the patient opens the mouth, the assembly is removed from the head. The clamps are rechecked and tightened. The bite fork assembly is removed from the underside of the facebow by loosening the set screw on the clamp by a quarter turn. Only the bite fork assembly needs to be used for mounting the maxillary cast and should be disinfected at this time. The facebow, after being disinfected with a hospital-grade disinfectant, is ready for use on another patient.
Fig 5-46 The articulator index is placed on the lower member of the articulator.
Fig 5-47 The maxillary cast in the bite fork is positioned by the articulator index.
Mounting the maxillary cast
The incisal guide block is removed from the articulator and replaced with the articulator index (Fig 5-46). The vertical reference pin of the bite fork assembly is inserted into the hole on the top of the articulator index. The reference pin has a flat side, which will match a flat side on the hole. The numbers 1 and 2 on the clamps of the bite fork assembly should be upright. The set screw at the front of the index is tightened.
Clean mounting plates are secured to the upper and lower members of the articulator. The articulator is assembled by placing the fossae over the condyles. The incisal pin is placed at the zero position. The long incisal pin for the dimpled guide block will rest in the recessed center of the index. The short pin used for flat guide blocks will contact the sliding metal piece in the middle of the index. The incisal pin with the adjustable foot sits on the posterior section of the index. The upper member of the articulator is removed and set on the top of the bench with the mounting plate up.
The maxillary cast is soaked, tooth side up, in a plaster bowl containing only enough water to wet the sides and bottom of the cast. The cast is seated into the wax registration on the bite fork (Fig 5-47). Mounting stone is mixed to the consistency of thick cream. A golf ball–sized mound of stone is applied to the base of the cast and the mounting plate. The articulator is assembled by placing the fossae over the condyles. The upper member of the articulator is closed into the soft mounting stone until the incisal guide pin contacts the appropriate spot on the articulator index. The centric latch is locked by pushing it into the down position.
The mounting stone will engage undercuts in the mounting plate and cast. Additional stone can be added if needed to secure the mounting. When the stone has set completely, the transfer jig is removed from the articulator. The incisal guide block is replaced in the articulator. The transfer jig is then rinsed with a hospital-grade disinfectant and stored until ready to be used again. All registration material is removed from the bite fork, which is then placed in a sealed sterilization bag and submitted for steam sterilization.
Mounting the mandibular cast
The incisal pin is adjusted for a 2-mm opening to accommodate the thickness of the interocclusal record. The articulator with the attached maxillary cast is inverted, with care taken to ensure that the centric latch is engaged. The centric relation interocclusal wax record is placed on the maxillary cast. The teeth should seat completely into the record.
The mandibular cast is placed into the interocclusal record, with care taken to ensure that the teeth are fully seated. There should be no contact between the maxillary and mandibular casts. The mandibular cast is removed, and the bottom and sides of it are soaked in a partially filled bowl of water for about 2 minutes.
The soaked mandibular cast is reseated into the interocclusal wax record. Some mounting stone is mixed to a thick, creamy consistency, and a mound of it is placed on the inverted bottom of the cast. Some mounting stone is applied to the mounting plate on the lower member of the articulator, which is hinged down into the soft stone on the cast until the incisal guide pin makes firm contact with the incisal guide block. The mandibular cast is stabilized by hand to keep it securely in the interocclusal record until the mounting stone sets (Fig 5-48). Rubber bands or sticky wax can also be used, but they are more likely to slip and produce a mounting error.
Fig 5-48 The cast is held in the interocclusal wax record until the mounting stone sets.
The casts and articulator are examined for the following:
The condyle is located against the posterior and superior walls of the condylar guide.
Both casts are completely seated in the interocclusal record.
Mounting stone engages undercuts on both the base of the cast and the mounting plate.
When the mounting stone has set completely, the accuracy of the mounting is confirmed. The articulator is opened, the interocclusal record is removed, and the incisal guide pins are raised 1 inch. A 2-inch piece of no. 10 red-inked silk