for restoration of only a small part of the occlusion generally is done with the teeth in a position of maximal intercuspation.
Centric Relation Record
To mount the mandibular cast on the articulator, it is necessary to record the relationship of the dental arches to each other. There are three techniques that are frequently used in locating the centric relation position: (1) chin point guidance, (2) bilateral manipulation, and (3) the unguided method. With a computer-assisted three-dimensional mandibular recording device, Hobo and Iwata3 analyzed condylar position achieved by the three methods. Chin point guidance puts the condyles in the most posterior and superior position, while the bilateral and unguided methods allow the muscles to guide the condyles into a physiologic anterosuperiorly braced position on the articular disc along the articular eminence.
The unguided method produces a physiologic “muscle position,” but it can be difficult to achieve consistent results because of the patient’s muscle activity. Muscle proprioception is minimized by separating the teeth with a leaf gauge composed of several 0.1-mm-thick plastic strips, which help to eliminate direct proprioceptor responses. While the patient occludes with light pressure, strips are added one at a time in the anterior region until the patient no longer feels any posterior tooth contact. This permits the muscles to act freely and allows the condyles to move into a physiologic position.4,5 Then the muscles will rotate the mandible anteriorly and superiorly.
Armamentarium
Cotton rolls
Green stick compound
Polyvinyl siloxane registration material
Impression material dispenser
Laboratory knife with no. 25 blade
28-gauge green wax
No. 10 red-inked silk ribbon
Technique
The most consistent, repeatable results can be accomplished using the technique of “bimanual manipulation” described by Dawson.6,7 The neuromuscular system monitors all sensory impulses from the teeth and jaws and programs occlusal contact to occur where the protective stimuli are minimal. This position, through repeated closures, becomes habitual and is maintained at the expense of normal muscle function. 8 To enable the condyles to be placed in an unstrained position, the musculature must first be deprogrammed from its habitual closing pattern.
A simple means of doing this is to place a cotton roll between the anterior teeth and instruct the patient to “bite on your back teeth.” It should be confirmed that there is no contact of the posterior teeth. If the cotton roll is placed as soon as the patient is seated, the operator and assistant can prepare the materials for the subsequent interocclusal record during the 5 minutes that the patient’s jaws remain closed. After this time, the “memory” of the position in which the teeth intercuspate fully will likely have been lost, and the mandible can be manipulated more easily into its optimum position. As soon as the cotton roll has been removed, mandibular manipulation should be initiated. The patient should not be allowed to close the teeth together again, as this will permit the musculature to readapt to a tooth-guided closure.
Fig 4-1 The fingers are placed along the inferior border of the mandible.
Fig 4-2 With the thumbs in position, the mandible is manipulated into a centric relation position.
Fig 4-3 The temporomandibular ligament (TML) acts as the posterior limit and fulcrum (F). The downward force of the thumbs and upward force of the fingers help to seat the condyles in the posterosuperior portion of the glenoid fossa. (Modified from Dawson7 with permission.)
The patient should be seated with the chair back approximately 45 degrees from the floor. The patient’s head should be tilted back with the chin up so that the face is parallel with the floor. This position tends to keep the patient from protruding the mandible. The dentist should take a position behind the patient that will facilitate stabilization of the patient’s head between the dentist’s rib cage and forearm. The patient’s head must not move while the mandible is being manipulated. All four fingers of each hand should be placed on the lower border of the mandible, making sure that the fingertips are in direct contact with bone (Fig 4-1).
The thumbs should be placed lightly over the mandibular symphysis so that they touch each other at the midline. The patient is instructed to open approximately 35 mm and then asked to relax the jaw as the dentist closes it, guiding the mandible posteriorly into a terminal hinge relationship with a gentle motion (Fig 4-2). Observation of the patient’s mandible will demonstrate that it shifts posteriorly with this gentle motion.
When the mandible has “dropped back,” firm pressure is applied to seat the condyles anterosuperiorly in the glenoid fossae (centric relation). An upward-lifting force is applied on the inferior border of the mandible by the fingers of each hand while a downward force is applied to the symphysis by the thumbs (Fig 4-3). With firm seating pressure, the dentist should once again open and close the mandible in small increments of 2.0 to 5.0 mm while gradually closing the mandible to the point of first tooth contact. The mandible should not be allowed to deviate from this arc while closing. This position of initial tooth contact with the mandible in the optimum position is the centric relation contact position (CRCP).
Fig 4-4 Green stick compound is molded to form the anterior programming device.
Fig 4-5 The patient is guided into centric relation to complete formation of the anterior programming device.
A piece of 28-gauge green wax is lightly adapted over both quadrants of the maxillary teeth, and the mandible is again manipulated into centric relation. At this position, the teeth are tapped together lightly until perforations are made in the wax at CRCP. The wax is removed and stored in a cup of cool water. This is used in the articulation of casts in chapter 5.
An anterior programming device, or jig, is made to establish a predetermined stop to vertical closure with the condyles in optimum position. The absence of deflecting incline tooth contact allows muscle function to be reprogrammed to eliminate the adaptive arc of closure. A 2.5-cm (1.0-inch) length of green stick compound is softened in hot tap water and bent into a J. The compound is placed over the midline between the two maxillary central incisors, with the short leg of the J on the facial surface, extending approximately halfway between the incisal edge and the gingiva. While the compound is still quite soft, it should be quickly adapted to the maxillary teeth using the following three-step procedure:
1 The facial portion of the compound should be firmly adapted into the labial embrasure with the thumb while the compound is thinned out to an approximate thickness of 2.0 mm.
2 Both thumbs should be placed on the facial and both index fingers on the lingual, with approximately 6.0 mm (1/4 inch) of space between the tips. The compound should be molded to the lingual surface by squeezing tightly (Fig 4-4).
3 While