Full and partial dentures are still a popular way to restore wholly or partially edentulous patients who do not wish to have dental implants or who are unsuitable for treatment. An accurate denture impression can ensure your restoration will achieve three critical factors:
Preservation of the Soft tissue and Alveolar Ridge
Good Stability and Retention
- Patient approved esthetics
Preservation of the Soft Tissue and Alveolar Ridge
Preserving the remaining residual ridges is crucial, keeping in mind the effect of the denture base on the health of the hard and soft tissues. Since alveolar bone eventually resorbs, unwanted pressure could accelerate bone resorption and cause soft tissue damage.
Good Stability and Retention
Good Stability stability decreases when vertical height is diminished and where movable soft tissue increases. Increased stability is achieved by ensuring the denture is supported by the underlying bone.
Good retention relies on the underlying soft tissues to hold the denture in place when it is seated. Retention is affected by several different factors, which include the patient’s muscle control, mechanical retention, and adhesion and cohesion.
It is vital to ensure that the denture impression records the full depth and width of the sulcus to allow your technician to fabricate for maximum retention and stability.
Taking the Primary Impression
Most primary impressions are taken with a stock tray, but when a patient's arch is too large, a custom tray should be used instead. The resulting model is utilized for diagnosis, treatment planning and for the fabrication of the custom tray.
The final impressions are taken in the custom trays using a light body PVS impression material and its objective is to record all areas to be covered by the denture, as well as nearby landmarks.
The maxillary impression must include frenum attachments, the full palette, fovea palatina, the hamular notches, and the buccal and labial vestibules.
The mandibular impression must include external oblique ridges, the retromolar pads, frenum attachments, sublingual space, the disto-lingual region, and the buccal and labial vestibules.
Preparing the Patient
Make sure the chair is in the correct position, with the patient upright to help to prevent impression material running to the back of the mouth and stimulating the gag reflex. Patients that have a particularly strong gag reflex can also be advised to lean forward minimally. Laying back in the chair is never recommended, as the orofacial musculature can be slightly affected in this position. Usually, a lower impression is taken first because it is less likely to cause gagging. When taking a lower impression, raise the height of the chair, so the patient’s mouth is between your elbow and shoulder height. The chair should be lowered to elbow height for the upper impression.
Selecting a Stock Tray
After assessing the patient’s dental arches, select a suitable impression tray and try it in without any impression material. Some stock trays may require modification, notably lower stock trays which may need wax added to the border to extend the tray and capture the full ridge. Thermoplastic trays may also be modified by heating over a flame and bending to the needed contour.
Choosing and Mixing the Impression Material
Usually, alginate is used for first the first impression, and it is essential to ensure the alginate is mixed correctly. Follow the manufacturer’s instructions- never eyeball and guess. Room temperature and water temperature will affect the setting time so will may need to adjust water temperature accordingly. The impression tray should be filled level and not overfilled. Alginate should always be poured up immediately for the most accuracy. After the model is poured, it can be useful to use a pen to mark the required extension of the custom tray.
Taking the Second or Master Impression
Before the patient arrives, check the custom tray to ensure it is as requested. It is preferable if the patient doesn’t wear their existing denture for approximately one and a half hours before their appointment to give their mucosa a chance to recover and return to its normal shape. Final impressions should be taken in light body PVS impression material in the custom tray. Properly mix the material to manufactures instructions and add it to the tray spreading it evenly and slightly overlapping the borders. Seat the impression and assure that all loose tissue is free from the tray by gently pulling the corners of the patients lips for upper impressions and by asking the patient to place their tongue on the roof of their mouth for lower impressions.
While the impression material is setting, it is useful to distract the patient with conversation and essential to check if they are comfortable. Keep a small piece of impression material to one side so you can see when it is set and ready for removal.
Once the impression is removed, rinse thoroughly to get rid of debris or blood and scrutinize the impression carefully, making sure all relevant structures are captured. It is crucial to ensure the lower denture impression captures the full posterior ridge and that both impressions accurately record the entire width and depth of the sulcus as this can greatly affect the stability of the finished denture.
Common Errors When Taking a Denture Impression
Common errors include:
Not ensuring the tray is level horizontally - this results in an incomplete capture of the posterior ridge
Incorrectly rotating the tray, resulting in an incomplete capture of the distal on one side
Not fully seating the tray in the anterior, resulting in an incomplete capture of the buccal vestibule
Voids in the material due to incorrect amount of impression material added to the tray
It is possible to fill small voids with wax, but generally it is better to aim for a perfect impression. Even small discrepancies can affect the quality of the custom tray.
Taking a good denture impression is a crucial step in fabricating a well-fitting and comfortable denture. It is essential for the master model to accurately replicate the patient's morphology, to avoid problems with the fit and function of the finished dentures.
DDS Lab assesses and grades every impression to ensure it meets the standards required for fabricating an accurate prosthesis. If you need any advice, our technical team is always available for case consultations.