Dental Implant Impression Techniques and Overdenture Bar Fabrication

Item 8: Dental Implant Impression Techniques

Impression Definitions

The term transfer describes the process of accurately replicating the position of an implant or abutment onto a dental model. Transfers can be categorized as either pick-up or transfer types.

Transfer Types

• Pick-up Transfer: A two-piece system where the transfer is fixed to the implant with a screw. The screw is unscrewed to extract the impression using a fenestrated tray.
• Transfer Coping: A single-piece component placed on the implant to facilitate the impression. This method utilizes a full-arch tray without fenestration.

Impression Methods

Direct Method

The direct method utilizes a fenestrated tray and pick-up transfers, verified by X-ray. The transfers are connected with wire and low-shrinkage resin (approximately 10 minutes) to create a rigid bar. Once set, the transfer screws are loosened to remove the tray. This method requires a custom tray for each patient and is considered more accurate.

Indirect Method

The indirect method uses a standard tray and transfer copings. The tray is removed after the material polymerizes, leaving the transfers in the mouth. The transfers are then removed and placed into the corresponding holes in the impression. A primary disadvantage is the potential for distortion when transferring the components from the mouth to the model.

Overdenture Bar Fabrication

After receiving the impression with the analog attachment, the cast is poured. Once cured, the model is separated, and the impression pins are removed. Analogues are secured into the working model with attachment screws. The bar is waxed to connect the attachments, ensuring the contact area remains open to prevent tissue impingement. The assembly is then invested, cast in metal, and finished.

Verification and Passive Fit

Verify the casting on the model to ensure a passive and precise fit. A tight fit can cause stress concentrations, potentially leading to implant failure. Furthermore, significant screw loosening may indicate a lack of passive fit rather than structural adjustment. The final work should be sent to the prosthodontist for clinical verification and adjustment in the patient's mouth.