Dental Impressions and Study Cast Trimming
Study casts accurately represent the teeth, their supporting tissues, and the relationship between upper and lower teeth in centric occlusion. They contribute greatly to diagnosis and treatment planning and are valuable instructional and illustrative aids during a consultation with patients. Even if you are observing a young patient prior to the onset of treatment, study casts are useful three-dimensional records for a growing and changing patient. Study casts are among the most important records taken prior to, during, and after orthodontic treatment. For treatment planning, casts are indispensable. You must study the positions of the maloccluded teeth, to plan how and where the teeth need to be moved during treatment. After treatment, study casts will show the changes that occurred during treatment. You need high-quality working casts for appliance fabrication.
With advances in digital model technology, dentists will eventually no longer take impressions and trim plaster diagnostic casts as described in this chapter. Even the laboratory fabrication of orthodontic appliances will be accomplished through digital technology. Several companies are selling equipment designed to capture digital images of individual teeth and arches for restorative dentistry (Helvey 2009). This technology is reducing errors commonly made in recording margins for crowns made in dental laboratories (Shannon, Qian, Tan, and Gratton 2007). Services and equipment that digitize orthodontic casts and alginate impressions are being marketed to orthodontists. A clinician can send plaster casts or impressions to a company for digitizing. Cone beam computed tomography machines can create digital casts. Digitized casts can be forwarded electronically to another clinician when patients transfer from one office to another. Through CAD/CAM (computer-aided design/computer-aided manufacturing) procedures, a three-dimensional cast can be created from a digital model.
The accuracy of measurements taken from digital models has been reported in several publications. The reports agree that the accuracy of currently available digital models is very good and quite acceptable for use in orthodontic diagnosis and treatment. With further hardware and software developments, improved accuracy will be available. One study compared tooth width measurements on digital and plaster models and found some statistically significant differences, but the differences were clinically acceptable (Stevens et al. 2006); a second study found no significant differences in tooth widths (Mullen, Martin, Ngan, and Gladwin 2007); and a third study found only significant differences for canine tooth widths, recommending a smaller rotational angle during scanning in the canine region to improve accuracy (Nouri et al. 2009). One study compared digital and plaster cast measurements of arch length and reported significant differences that were clinically acceptable (Mullen et al. 2007). One study compared space analysis in digital and plaster casts and found no difference in the mandibular arch but a significant difference in the maxillary arch that was considered clinically acceptable (Leifert, Leifert, Efstratiadis, and Cangialosi 2009); a second study of space analysis reported no difference in the maxillary arch for four segment and six segment arch lengths and found no difference for six segment arch lengths in the mandibular arch, but found a difference in the lower arch when using four segment arch lengths (Goonewardene et al. 2008). Arch widths were compared in digital and plaster casts, with one study finding no differences (Gracco, Buranello, Cozzani, and Siciliani, 2007) and another study reported no differences in lower intercanine widths but significant differences in intermolar widths (Asquith, Gillgrass, and Mossey 2007). Two studies found that digital measurements were more quickly taken than manual measurements with calipers (Gracco et al. 2007; Mullen et al. 2007).
To obtain high-quality casts, you must obtain high-quality impressions. The objectives in making impressions for orthodontic study casts are somewhat different from the objectives in making impressions for restorative and prosthetic patients. We want accurate impressions of the teeth and much more coverage of the surrounding anatomic structures of both upper and lower arches. The impressions should record as much of the upper and lower arch as possible. This is accomplished by displacing the soft tissue upward and outward beyond the mucobuccal folds in the upper impression and downward and outward in the lower impression. Use perforated trays of the proper size for each arch. Trays need to be large enough to extend at least ¼ inch beyond the most distal tooth in each arch and wide enough so that teeth do not come into contact with any part of the impression tray. Add soft wax strips to extend the tray flanges into the mucobuccal fold and to act as stops to keep the tray from contacting teeth. Wax is sometimes added to the palatal surface of an upper tray to obtain a satisfactory impression of a high palatal vault. The goal is a good impression of both the teeth and the supporting structures with no voids. If the tray is seated far enough to contact teeth, a clicking sound is heard as the incisal edges or cusps of teeth hit the bottom of the tray. This will result in a poor impression and poor casts because the impression will be perforated at the places the teeth contact the tray.
Any good alginate impression material will produce a good impression if you are familiar with the working properties of the impression material. Always mix the material according to the manufacturer’s directions. After the impression material is mixed, it is placed in the tray and should be smoothed with wet fingers. The patient’s teeth should be clean, and the patient should rinse his mouth thoroughly before an impression is made. Before seating the filled impression tray, you can smear alginate on the occlusal and lingual surfaces of the teeth and the palate with your finger to reduce the occurrence of saliva bubbles on these surfaces.
Because patients usually tolerate lower arch impressions better than they do upper arch impressions, you should take the lower impressions first. Seat the patient upright in the chair. Stand in front of the patient. Ask the patient to roll back his tongue as you put the lower arch impression tray into the mouth and ask him to move his tongue forward above the impression tray after you seat the tray fully. This prevents the tongue from getting trapped beneath the impression tray and allows the tongue to mold the lingual alginate. As you seat the impression tray, center the tray handle in line with the nose and keep the tray level with the occlusal plane. The patient may be instructed to hold his head forward and down slightly; this will help the patient breathe and, if necessary, to drool his saliva onto the napkin while the tray is in the mouth. When the leftover alginate in the mixing bowl is set, the impression can be removed from the mouth. Grasp the tray by its handle and roll it back and forth gently to break the seal. In order to overcome the suction that holds the alginate impression in the arch, you may need to place your finger under the buccal rim on one side of the tray to forcibly pull it upward. If taken properly, the impression should have no large voids and the alginate should not have pulled away from the tray (Graber and Swain 1985, Monetti 1993).
After removing the impression from the mouth, rinse it thoroughly with cool tap water to wash out saliva and debris. Shake or blow out excess water from the impression and inspect the impression for voids. Determine if all desirable anatomic parts of the impressed arch have been duplicated accurately. Follow proper disinfecting procedures and place the impression into a plastic bag for transport to the laboratory for pouring of the cast. If the impression must sit for more than 15 minutes after removing it from the mouth, it must be placed in an airtight container to keep it from drying out, which causes distortion of the impression.
Put only enough alginate in the upper tray to make a good impression. If you overload the tray and place the tray over the anterior teeth first, the excess alginate will flow down the soft palate as you seat the tray over the posterior teeth. Most patients gag when alginate flows freely down the surface of the soft palate. Stand behind the patient and bring the tray to the upper arch so that the alginate contacts the occlusal surfaces of all the teeth. Center the tray handle on the nose. Hold the tray level with the occlusal plane. Position the tray so that the alginate can flow evenly upward into the mucobuccal fold area. When a patient has flared upper incisors, position the impression in the molar region first to achieve an adequate flow of alginate into the anterior mucobuccal fold. Pull the upper lip of the patient over the tray flanges to keep the lip from becoming trapped beneath the tray. Ask the patient to breathe through his nose when you take the impression. This makes the procedure more comfortable and takes the patient’s mind off gagging. Always ask the patient if he can breathe through his nose before you take an upper arch impression. Patients who have nasal airway blockage are poor candidates for upper arch impressions. Have the patient close his mouth lightly by saying, “You may close your mouth until your lower teeth lightly touch my fingers.” Closing the mouth slightly allows the muscles of mastication to relax, making the patient more comfortable (Graber and Swain 1985; Monetti 1993).
Remove the tray after the alginate has set by following the procedures described earlier for the mandibular arch.
Record of Centric Occlusion
After the impressions are taken, ask the patient to bite into a piece of wax to record the relationship of the teeth in centric occlusion (maximum intercuspation). The patient must bite through the wax into full tooth contact. The wax bite registration serves as a guide in the cast trimming process. Rinse the wax bite with cool water, disinfect it, and place it into the plastic bag with the upper and lower impressions.
Pouring of Plaster Study Casts
Casts should be poured shortly after the impressions are taken. In pouring a cast, two pitfalls must be avoided: (1) lack of proper density of gypsum material and (2) voids or bubbles within the gypsum. Proper density is obtained by mixing the correct amount of plaster with the correct amount of water as prescribed by the manufacturer. Normal-size upper and lower impressions for study casts will require about 600 grams of powdered gypsum. Plaster can be weighed and stored in bags, so that it can be quickly mixed with the appropriate volume of water. Mix enough plaster for both impressions in a metal mixing bowl. Bubbles can be minimized by incorporating the gypsum powder into the water with a hand spatula, followed by 25 or 30 seconds of mixing with a vacuum power mixing machine. After mixing, remove the vacuum hose. Vibrate the mixing bowl and remove the mixing blade from the metal bowl, and vibrate the mixed plaster from the blades into the bowl.
Remove the alginate impressions from the plastic bag and rinse them under cool running water to remove disinfectant and debris. Shake out excess water. The surface of the impre/>