Implant-supported restorations are either screw-retained or cement-retained. While screw-retained restorations offer easier retrievability, the screw access hole can affect esthetics. Additional problems with screw-retained restorations include screw loosening and issues with achieving a passive fit. In comparison, cement-retained restorations are generally less expensive, as they require fewer components and less chair-side time.
One major drawback when fitting a cement-retained restoration is the extrusion of excess cement. This residual cement can be extruded into the sulcus. Excess cement located at a depth of more than 3 mm can be tricky to see and thoroughly remove. If it’s not properly removed, it can initiate peri-implant disease, an inflammatory process that causes implant failure. Another potential problem when removing excess cement is inadvertently scratching the implant surface with metal or plastic scalers. Controlling cement flow is vital when using custom or stock implant abutments.
One easy method is to use die spacers, while another technique is to use PTFE, or plumber’s tape. Both methods will ensure a space of 50 µm thickness, which is approximately the width of a human hair. With both techniques, the crown is first tried in and adjusted as necessary. Ideally, a retraction cord is packed to just below the margin of the abutment. A retraction cord minimizes the risk of excess cement entering the peri-implant sulcus during the cementation process. Once the crown is in place and the cement is set, the retraction cord is removed, and the crown is checked for any remaining excess cement. At this point, the crown is modified if necessary and polished. Afterward, it’s worthwhile to take a digital X-ray to check that all excess cement is removed. However, excess cement may not show unless it is on the mesial or distal of the implant, and radiopacity of cement can vary tremendously.
Using Die Spacers to Minimize Excess Cement - Step By Step
Place the implant analog on the dental model, so you can carefully check the fit of the crown restoration.
Next, apply die spacer to the fitting surface of the crown. It normally takes three or four applications of die spacer to form a thickness of approximately 50 µm. It is important that the die spacer evenly covers all internal surfaces of the restoration.
Fill the restoration with a temporary restorative material and insert a pin with a small diameter tip into the uncured material. This forms a handle and should be secured in place until the acrylic material is fully cured.
Once the material is cured, remove the restoration and should carefully check it for any anomalies between the implant abutment and the abutment. It is important to check that there aren’t any air bubbles on the duplicate abutment and, most importantly, that the finish line of the abutment has been accurately duplicated.
Thoroughly clean and carefully check the fitting surface of the crown to ensure no residual die spacer remains. After applying the appropriate luting agent, fill the crown with a small amount of cement. Gently fit the crown on the acrylic duplicate abutment. At this stage, a residual layer of cement on the duplicate abutment can be easily wiped away.
If there isn’t a cement layer, then you may need to add a little more cement to the fitting surface of the crown, filling any areas that look a little thin. The crown is now ready to be placed on the abutment in the mouth, and by taking these precautions there should be minimal or no excess cement.
By using a quick and economical duplicate abutment fabricated during the appointment for the crown fitting and cementing, you more easily control the cement flow. One of the main benefits of extra-oral cementation is that it allows excess cement around margins to be indirectly removed more easily. Using die spacers creates the ideal 50 µm cement space. One disadvantage of using this technique is the extra time required, so be sure to choose a luting agent with a longer working time. If you’ve chosen a custom abutment, the dental lab can make an analog using acrylic resin, which would obviously increase the lab costs.
Using PTFE and Impression Material - Step By Step
Instead of die spacer, you can adapt PTFE, or plumber’s tape, to fit the surface of the crown, as it is 50 µm thick. First, paint the crown with a lubricant like Vaseline or KY Jelly (if you choose Vaseline, then it must be properly removed afterward), which helps the tape stick to the inside of the crown. Adapt the tape to the inside using a small brush. Complete the adaptation of the tape by carefully inserting the abutment into the crown.
Gently remove the crown and check the tape. Fill the crown with bite registration material or a fast-setting impression material. It’s best to use a small tip nozzle and to overfill the crown so it creates a handle consisting of excess material. Alternatively, you can place a pin into the impression material to create a handle.
Once the impression material is set, you will have a chair-side copy abutment that is approximately 50 µm smaller than the inside of the crown. Carefully check the copy abutment with the proper abutment for any discrepancies. Place the original abutment into the patient’s mouth and torque the screw to the appropriate value.
Load the crown with appropriate cement and push the copy abutment into the crown. Any extruded excess cement can then be easily removed. Before seating the crown, check that there is an even cement layer and add extra to compensate for any bare areas. Then seat the crown in the mouth. This technique is quick, inexpensive, straightforward, and can be used for custom or stock abutments.
Sometimes stock abutments are undersized, and crown preparation margins are located sub-gingivally. Although this prevents any metal from being displayed, it makes it much more difficult to thoroughly check that cement is removed. Generally, when choosing a cement-retained restoration, a custom abutment is preferable.
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