Clinical Techniques and Protocols for Primary Alginate Impressions

Criteria for Tray Selection

• When selecting a tray for a patient, there are two primary methods based on the patient's dental status:
  • Edentulous Patients with Existing Dentures: Align the edentulous stock tray directly over the patient's existing lower and upper dentures to determine the appropriate size.
  • Dentate Patients: If the patient has teeth and the correct size is uncertain, use fingers to feel for the width of the ridge. Compare this physical measurement to the width of the trays available. This method is more efficient than the trial-and-error approach of testing small, medium, and large trays.
• Tray selection should always be performed immediately following the oral examination.

Stock Tray Modification and Extension Goals

• Stock trays typically come in $3$ standard sizes. Because of this limited variety, a perfect fit is rare, and modifications are often necessary.
• Periphery Wax Application: To provide further extension from the stock tray when it is short of the required anatomy, apply periphery wax to the deficient areas.
  • Modification does not require heat.
  • Squeeze the wax on with fingers and manual molding to adapt it to the tray's edge.
• Clinical Goal of Primary Impressions: The objective is to obtain an overextended impression of the mouth. This ensures that all relevant anatomy is clearly visible for the construction of a special tray.
• The Refinement Process: The primary impression is the first step in a sequence where every subsequent action is a refinement toward the final denture shape. The special tray made from the primary cast will be used for a more refined secondary impression.

Insertion Techniques and Mandibular Support

• Tray Entry: Do not attempt to insert the tray directly into the mouth. Instead, use a rotational movement to clear the lips and position the tray correctly.
• Retracting the Lips: Hold the corner of the mouth open using either fingers or a mouth mirror, depending on personal clinical preference.
• Mandibular Support: When seating a lower tray, support the mandible by placing thumbs underneath it and using fingers to keep the tray steady. This prevents unnecessary pressure on the patient's Temporomandibular Joint (TMJ), which is particularly important for elderly patients.
• Handle Position: The handle on the upper tray comes down from the alveolar ridge and then projects outward. It is critical to note the tray's position relative to the lip before loading it with material. The tray must be maintained in that same alignment during the actual impression taking.

Primary Impression Procedure: Lower Arch

• Retention Mechanisms:
  • Mechanical Retention: Stock trays have slots cut into them to mechanically lock the alginate in place.
  • Chemical Retention: Apply a thin coating of an adhesive called Fix to the fitting surface of the tray. Only the slightest contact is necessary to ensure the alginate does not pull away from the tray during removal.
• Loading and Sequencing: The lower impression is typically taken first to help the patient become comfortable with the procedure and to minimize the risk of a gag reflex, which is more common with the upper arch.
• Loading Technique: Load the tray from the sides and bring it toward the center. Push down firmly during loading to ensure the alginate flows through the mechanical retention slots.
• Patient Exercises for Border Molding: To capture the lingual extension and create a rolled margin:
  • Ask the patient to stick their tongue out as far as possible.
  • Ask the patient to move their tongue into one cheek and then the other.
  • These movements lift the floor of the mouth, defining the lingual aspect of the denture extension.
• Removal: Test the set of the alginate before removal. Break the seal by placing a finger in the $\text{lower 6}$ area and lifting. Rotate the tray out of the mouth in the same manner it was rotated in.

Primary Impression Procedure: Upper Arch

• Volume Control: Although standard mixing might involve $3\,\text{scoops}$ for an upper and $2\,\text{scoops}$ for a lower, do not feel obligated to use all the material. Overloading the tray increases the risk of triggering the gag reflex.
• Seating Technique: Rotate the tray into the mouth. Seat the posterior portion first, followed by the anterior. This directed pressure forces excess material toward the front of the mouth rather than the throat.
• Pressure and Manipulation: Maintain constant finger pressure in the middle of the palate. Simultaneously manipulate the patient's cheeks and lips to allow the alginate to fill the sulcus (vestibule).
• Clinical Significance of the Seal: In patients like Peter, who may have minimal saliva, the peripheral seal created by the contact between the denture and the lips/cheeks is vital. This maximizes the cohesion-adhesion effect required for denture retention.
• Removal: Turn the handle and place a finger up in the $\text{6 area}$ to break the suction seal before rotating the tray out of the mouth.

Post-Impression Evaluation and Decontamination

• Criteria for Success: A successful primary impression must show:
  • Full extension around the entire periphery.
  • Clear recording of the posterior seal.
  • Visible hamular notches, rugae, and the midline.
• Decontamination Protocol:
  • Ask the patient to rinse their mouth after the procedure.
  • Immerse the impressions in a disinfectant; for example, the hospital uses Release Plus.
  • The impressions must remain in the disinfectant for exactly $10\,\text{minutes}$.
• Laboratory Casting: After disinfection, the impressions are sent to the dental laboratory to be cast in plain plaster. These plaster casts are then used to fabricate the custom special trays required for the secondary impression phase.