Armamentarium

Tray selection

All impression materials require retention in the impression tray. This can be provided for irreversible hydrocolloid by using an adhesive or by making perforations or undercuts around the rim of the tray. All types of trays are capable of producing impressions with clinically acceptable accuracy.¹ For irreversible hydrocolloids, the largest tray that will fit comfortably in the patient’s mouth should be selected. A greater bulk of material produces a more accurate impression (i.e., a bulky impression has a more favorable surface area/volume ratio and is less susceptible to water loss or gain and therefore unwanted dimensional change). In contrast, elastomeric impression materials work well with a relatively tightly fitting custom impression tray in which a uniform thin layer of material is used. This produces the most accurate impression (see Chapter 14).

Distortion of irreversible hydrocolloid can occur if any part of the impression is unsupported by the tray or if there is movement of the tray during setting. For these reasons, the tray may need to be extended and its perimeter modified with modeling compound (Fig. 2-2).

Fig. 2-2 Stock impression trays can be readily modified with modeling compound to provide better support for the alginate. The posterior border typically needs extension. If the patient has a high palate, the alginate should be supported here, too, although the compound should not block out the retentive area of the tray.

Impression making

For optimum results, the teeth should be cleaned and the mouth thoroughly rinsed. Some drying is necessary, but excessively dried tooth surfaces cause the irreversible hydrocolloid impression material to adhere. The material is mixed to a homogenous consistency and loaded into the tray, and its surface is smoothed with a moistened gloved finger.² Concurrently, a small amount of material is wiped into the crevices of the occlusal surfaces (Fig. 2-3A,B) before the tray is seated (Fig. 2-3C). Also, a small amount can be applied by wiping it into the mucobuccal fold. As the tray is inserted into the patient’s mouth and seated, the patient is reminded to relax the cheek muscles. If a patient continues to stretch wide open while the tray is being fully seated, impression material is often squeezed out of the mucobuccal fold or from underneath the upper lip.

Fig. 2-3 A to D, Making an alginate impression for diagnostic casts.

A loss of tackiness of the material (gelation) implies initial set. The tray should be removed quickly 2 to 3 minutes after gelation. Teasing or wiggling the set impression from the mouth causes excessive distortion as a result of viscous flow. Also, certain irreversible hydrocolloid materials become distorted if held in the mouth more than 2 or 3 minutes after gelation.³ After removal (Fig. 2-3D), the impression should be rinsed and disinfected, dried slightly with a gentle air stream, and poured immediately. For disinfection, spraying with a suitable glutaraldehyde and placement in a self-sealing plastic bag for approximately 10 minutes is recommended, after which it can be poured. Alternatively, the impression can be immersed in iodophor or glutaraldehyde disinfectant. The disinfection protocol is an essential precaution for preventing cross-infection and protecting laboratory personnel (see Chapter 14). It should be noted that irreversible hydrocolloid impressions carry significantly higher numbers of bacteria than do elastomeric materials.⁴ No significant loss of accuracy or surface detail is caused by the disinfection procedure.^5,6 To ensure accuracy, pouring should be completed within 15 minutes after the impression is removed from the mouth. Keeping an impression in a moist towel is no substitute for pouring within the specified time. Trimming off gross excess impression material before setting the tray down on the bench top is helpful. A vacuum-mixed ADA type IV or type V stone is recommended. The choice of the brand of stone is important because of the harmful surface interactions between specific irreversible hydrocolloid materials and gypsum products.⁷

After mixing, a small amount of stone is added in one location (e.g., the posterior aspect of one of the molars). Adding small amounts consistently in the same location helps minimize bubble formation (see the section on pouring stone dies in Chapter 17). If air is trapped, bubbles can be eliminated by poking at them with a small instrument (e.g., a periodontal probe or a wax spatula). While setting, the poured impressions must be stored tray side down, not inverted. Inverting freshly poured impressions results in a cast with a rough and grainy surface.⁸ Stone is added to create a sufficient base that provides adequate retention for mounting on the articulator. To achieve maximum strength and surface detail, the poured impression should be covered with wet paper and stored in a humidor for 1 hour. This minimizes distortion of the irreversible hydrocolloid during the setting period. The setting gypsum cast should never be immersed in water. If this is done, setting expansion of plaster, stone, or die stone doubles or even triples through the phenomenon of hygroscopic expansion (see Chapter 22). For best results, the cast should be separated 1 hour after pouring.

Evaluation

Although it is apparently a simple procedure, diagnostic cast fabrication is often mishandled. Seemingly minor inaccuracies can lead to serious diagnostic errors. Questionable impressions and casts should be discarded and the process repeated (Fig. 2-4). Voids in the impression create nodules on the poured cast. These can prevent proper articulation and effectively render useless a subsequent occlusal analysis or other diagnostic procedure.

Fig. 2-4 Diagnostic casts must be accurate if they are to articulate properly. A, Occlusal nodules may make proper occlusal analysis impossible. B, Proper technique ensures a satisfactory cast.

Articulator selection

Handheld casts can provide information concerning alignment of the individual arches but do not enable analysis of functional relationships. For an analysis, the diagnostic casts need to be attached to an articulator, a mechanical device that simulates mandibular movement. Articulators can simulate the movement of the condyles in their corresponding fossae. They are classified according to how closely they can reproduce mandibular border movements. Because the movements are governed by the bones and ligaments of the temporomandibular joints, they are relatively constant and reproducible. With most articulators, mechanically adjustable posterior controls are used to simulate these movements; in some, plastic premilled or customized fossa analogs are used. If an articulator closely reproduces the actual border movements of a given patient, chair time is significantly reduced because the dental laboratory can then design the prosthesis to be in functional harmony with the patient’s movements. In addition, less time is needed for adjustments at delivery.

On some instruments, the upper and lower members are permanently attached to each other, whereas on others they can be readily separated. The latter instruments may have a latch or clamplike feature that locks the two components together in the hinge position. Instrument selection depends on the type and complexity of treatment needs, the demands for procedural accuracy, and general expediency. For instance, when a fixed dental prosthesis is waxed, it is advantageous to be able to separate the instrument into two more easily handled parts. Use of the proper instrument for a given procedure can translate into significant time saving during subsequent stages of treatment.

Hinge axis recording

The hinge axis of the mandible can be determined to within 1 mm by observing the movement of kinematic facebow styli positioned immediately lateral to the temporomandibular joint close to the skin. A clutch, which is essentially a segmented impression-traylike device, is attached to the mandibular teeth with a suitable rigid material such as impression plaster. The kinematic facebow consists of three components: a transverse component and two adjustable side arms. The transverse rod is attached to the portion of the clutch that protrudes from the patient’s mouth. The side arms are then attached to the transverse member and adjusted so that the styli are as close to the joint area as possible. The mandible is then manipulated to produce a terminal hinge movement, and the stylus locations are adjusted with thumbscrews (superiorly and inferiorly, anteriorly and posteriorly) until they make a purely rotational movement (Fig. 2-10). Because the entire assembly is rigidly attached to the mandible, a strictly rotational movement signifies that stylus position coincides with the hinge axis. When this purely rotational movement is verified, the position of the hinge axis is marked with a dot on the patient’s skin, or it may be permanently tattooed if future use is anticipated or required.

Fig. 2-10 Hinge axis recording. A, Left and right styli are attached via a facebow to a clutch affixed to the mandibular teeth. When the mandible makes a strictly rotational movement, the stylus remains stationary if aligned with the actual axis of rotation. If the stylus is positioned forward or backward, above or below the actual axis, it travels one of the arcs indicated by the arrows when the mandible makes a rotational movement. Thus, the arc indicates in what direction an adjustment should be made to the stylus position. B, Hinge axis locator positioned. C, Set screws allow side arm adjustment. D, Adjustment continues until no arcing of the pointer is seen.

Kinematic facebow transfer

An impression of the maxillary cusp tips is obtained in a suitable recording medium on a facebow fork (Fig. 2-11). The facebow is attached to the protruding arm of the fork. The side arms are adjusted until the styli are aligned with the hinge axis marks on the patient’s skin. The patient must be in the same position that was used when the axis was marked to prevent skin movement from introducing any inaccuracy. A pointer device is usually attached to the bow and adjusted to a repeatable reference point selected by the clinician. The reference point is used later for reproducibility. The kinematic facebow recording is then transferred to the articulator, and the maxillary cast is attached.

Fig. 2-11 Kinematic hinge axis facebow. A, Clutch seated on the mandibular teeth. The clutch separates for removal into two components by loosening the screws on left and right sides. B, Kinematic facebow assembly positioned. C, Pointers aligned with the previously marked hinge axis location. D, Assembled kinematic facebow. E, Kinematic facebow aligned on the articulator.

The kinematic facebow technique is time consuming, so it is generally limited to extensive prosthodontics, particularly when a change in the occlusal vertical dimension is to be made. A less precisely derived transfer would then lead to unacceptable errors and a compromised result.

Anterior reference point

The use of an anterior reference point (Fig. 2-13) enables the clinician to duplicate the recorded position on the articulator at future appointments. This saves time, because previously recorded articulator settings can be used again. An anterior reference point, such as the inner canthus of the eye or a freckle or mole on the skin, is selected. After this has been marked, it is used, along with the two points of the hinge axis, to define the position of the maxillary cast in space. This procedure has the following advantages:

Fig. 2-13 Anterior reference point. The Denar Slidematic facebow uses a mark 43 mm superior to the incisal edge of the maxillary central incisor as an anterior reference point. Other systems use the infraorbital foramen or nasion. The mark serves as a reference to average anatomic values. It also allows subsequent casts to be mounted without a repeat recording.

(Courtesy of Waterpik Technologies, Inc., Fort Collins, Colorado.)

Facebow transfer

Step-by-step procedure

1. Add modeling compound to the facebow fork (Fig. 2-14A).

2. Temper in water and seat the fork, making indentations of the maxillary cusp tips. The facebow fork is positioned in the patient’s mouth, and an impression is made of the maxillary cusp tips. The impression must be deep enough to permit accurate repositioning of the maxillary cast after the facebow fork has been removed from the mouth. Only the cusp tips should be recorded. It is not necessary to get an impression of every cusp, or even an entire cusp—just one that is sufficient to position the diagnostic cast accurately. If the impression is too deep, accurate repositioning of the cast can become problematic because the diagnostic casts are not absolutely accurate reproductions of the teeth. In general, the tips are reproduced more accurately than the fossae.

3. Remove the fork from the mouth. Chill and reseat the fork, and check that no distortion has occurred (Fig. 2-14B). The inclusion of details of pits and fissures in the recording medium leads to inaccuracies in trying to seat the stone cast. Trim the recording medium as necessary before reseating. After reseating, check for stability.

4. Have the patient stabilize the facebow fork by biting on cotton rolls. As an alternative, wax can be added to the mandibular incisor region of the fork. The mandibular anterior teeth stabilize the fork as they engage the wax.

5. Slide the universal joint onto the fork and position the caliper to align with the anterior reference mark (Fig. 2-14C).

6. Tighten the screws securely in the correct sequence (Fig. 2-14D).

7. If the articulator has an adjustable intercondylar width, record this measurement (Fig. 2-14E). Remove the facebow from the mouth.

Fig. 2-14 Facebow technique. A to E, Denar Slidematic Facecow Technique. A, Indentations obtained in compound. B, Facebow fork positioned. C, Facebow attached to facebow fork, toggles tightened. D, Transfer complete. E, Width measurement is read from the top of the facebow. F to K, Whip Mix Quick Mount Facebow Technique. F, Armamentarium. G, Automixed elastomer is applied to the transfer fork. H, Adapting the facebow fork to the maxillary teeth. I, The obtained record is trimmed with a sharp blade to facilitate seating. J, Nasion relator being positioned. K, Knobs and toggles are tightened.

(B to D, Courtesy of Waterpik Technologies, Inc., Fort Collins, Colorado. F, H to K, Courtesy of Whip Mix Corporation, Louisville, Kentucky.)

The technique is slightly different with other arbitrary facebows (Fig. 2-14F to K).

Centric relation record

A centric relation record (Fig. 2-15) provides the orientation of mandibular to maxillary teeth in CR in the terminal hinge position, in which opening and closing are purely rotational movements. Centric relation is defined as the maxillomandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective disks with the condyle-disk complex in the anterosuperior position against the articular eminences. This position is independent of tooth contact.

Fig. 2-15 A centric relation (CR) record transfers the tooth relationships at CR from the patient to the articulator.

MI may or may not be coincident with the CR position. The CR record is transferred to the maxillary cast on the articulator and is used to relate the mandibular cast to the maxillary cast. Once the mandibular cast is attached to the articulator with mounting stone, the record is removed. The casts then occlude in precisely the CR position as long as the maxillary cast is correctly related to the hinge axis with a facebow (see Fig. 2-14). When the articulator controls are set properly, through the use of appropriate excursive records, translated mandibular positions can be reproduced from CR. A CR/MI slide is readily reproducible on casts that have been articulated in CR. Thus, premature tooth contacts (deflective contacts) can be observed, and it can be determined whether an occlusal correction is necessary or appropriate before fixed prosthodontic treatment. Casts articulated in the MI position do not enable the evaluation of CR and retruded contact relationships. Therefore, the articulation of diagnostic casts in CR is of greater diagnostic value.

In theory, when a kinematic facebow is used, the thickness of a terminal hinge record is unimportant; a thicker record merely increases the amount of rotation. When an arbitrary facebow is used, any arcing movement results in some degree of inaccuracy. Both techniques are subject to small errors, which can be minimized by keeping the record thin.^11,12 However, it is essential that the teeth not perforate the record. Any tooth contact during record fabrication can cause mandibular translation because of neuromuscular protective reflexes governed by mechanoreceptors in the periodontium, rendering the resulting articulation useless.

Jaw manipulation

Accurate mounting of casts depends on precise manipulation of the patient’s mandible by the dentist. The condyles should remain in the same place throughout the opening-closing arc. Trying to force the mandible backwards leads to downward translation of the condyles, and restorations made to such a mandibular position are in supraclusion at the evaluation stage (Fig. 2-16).

Fig. 2-16 Incorrect centric relation recording. A, If the mandible is forced backward (F), the condyles are not in their most superior position but are moved backward and downward (arrow). B, Any restorations made on casts related with this CR record are in supraclusion when tried in the mouth. C, Note the relationship of the anterior teeth.

The load-bearing surfaces of the condylar processes, which face anteriorly, should be manipulated into apposition with the mandibular fossae of the temporal bones, with the disk properly interposed. The ease with which this can be accomplished depends on the degree of the patient’s neuromuscular relaxation and on sound technique. The latter, in turn, depends on the patient’s permitting the dentist to control the mandible. Attempts to force or shake the mandible lead to a protective muscle response by the patient.

The bimanual manipulation technique described by Dawson¹³ is recommended as a reproducible technique¹⁴ that can be reliably learned.¹⁵ In this technique, the dental chair is reclined and the patient’s head is cradled by the dentist. With both thumbs on the chin and the fingers resting firmly on the inferior border of the mandible (Fig. 2-17A), the dentist exerts gentle downward pressure on the thumbs and upward pressure on the fingers, manipulating the condyle-disk assemblies into their fully seated positions in the mandibular fossae. Next, the mandible is carefully hinged along the arc of terminal hinge closure. Note: It is more difficult to ensure that the condyles are properly located when the single-handed approach (Fig 2-17B) is used with the fingers exerting upward pressure, although this technique does allow the other hand to hold the record.

Fig. 2-17 Manipulating a patient’s mandible into centric relation. Note the position of the dentist’s thumbs and fingers on the mandibular border. The bimanual (A) and the single-handed technique (B).

Anterior programming device (Fig. 2-18)

In some patients in whom CR does not coincide with MI, resistance may be encountered when hingeing the mandible. Because of well-established protective reflexes that are reinforced every time the teeth come together, such patients do not allow their mandibles to be manipulated and hinged easily. If tooth contact can be prevented, they “forget” these reflexes, and manipulation becomes easier. The teeth can be kept apart with cotton rolls, a plastic leaf gauge, or a small anterior programming device made of autopolymerizing acrylic resin (also known as a Lucia jig).¹⁶

Fig. 2-18 An anterior programming device is used to facilitate centric relation recording. A, Autopolymerizing resin is mixed and adapted to the maxillary central incisors. The patient’s mouth is guided into closure and stopped when the posterior teeth are about 1 mm apart. B, The indentations are used as a guide during trimming of the device. The completed device (C) should allow the patient to make smooth lateral and protrusive movements. An inclined contact area must be avoided, because it will tend to cause the mandible to retrude excessively. D, Alternatively, a thermoplastic material can be used. E, After softening and positioning, the mandible is guided into centric relation closure. F, The device is trimmed with a scalpel. G, Again, posterior disclusion is verified. H, Cross-section through device. I and J, Use of a plastic leaf gauge to prevent habitual closure into maximal intercuspation.

If the mandible cannot be manipulated satisfactorily after an anterior programming device has been in place for 30 minutes, marked neuromuscular dysfunction is likely. Normally, this is relieved by providing an occlusal device (whose fabrication and adjustment are described in Chapter 4).

Centric relation recording technique

Different techniques can be used to make a CR record. The choice of recording medium is, to some degree, a function of the casts to be articulated. For instance, very accurate casts made from elastomeric impression materials can be articulated with a high-accuracy interocclusal record material such as polyvinyl siloxane. However, less accurate diagnostic casts poured from irreversible hydrocolloid are bett/>