The Sheffield Test for Implant Models



Confirming the accuracy of the analog model for implant bridges or implant bar cases is a critical cost saving piece of the protocol. This is especially necessary for screw retained multiple fixture situations whether they are milled or cast and it is imperative that they fit passively. An ill-fitting restoration will result in a constant lateral load of all connected implants. This is destructive to crestal bone and will expedite the demise of implants.

Most verification bridges are made using a product like the Zirlux non-engaging bite verification  cylinders and an acrylic connecting bar. These can be done using CAD/CAM or conventional fabrication. If you’re constructing the jig using conventional techniques, you should consider ways to ensure dimensional stability yet still be able to section the jig if needed to reorient the fixture positions. One alternate technique utilizes prefabricated plastic bars and connects them to the cylinders with pattern resin. Either way, these jigs should be sectioned with an ultra-thin disc and be allowed to fully mature and release any dimensional instability for at least 12 hours. Once they have fully set you can reconnect the cuts with a minimal amount of well-condensed pattern resin. Always clean the jigs mating surfaces and the individual screws meticulously to ensure complete seating and remove any variables in the fit.

When checking the fit of an implant structure the most common technique is called a Sheffield Test. The way the test is performed, the clinician seats the structure onto the implants and lightly tries to tighten the fixation screws. The screws should not bind or resist the tightening process. Allow the tissue to relax and ensure that there is no tissue trapped between the interface of the structure and the implant platform. If you experience resistance, do not proceed until you determine the source. Once you feel the structure is seated, you need to determine if it is seated 100% and if it is truly passive. The most definitive test is leaving one of the most distal screws tightened and loosening all of the other screws at least one and a half turns. The structure should not change position when the other screws are loosened. Shoot a couple of radiographs to ensure that the structure does indeed fit and is seated with excellent adaptation to the interfaces of all the implants.

Depending on the structure and the results of the Sheffield Test, you are then either ready to proceed with your definitive structure or if the jig fails the test, you need to have an intervention to correct the working model to match the intra oral situation. You can alter the cast by repositioning an analog using the jig that has been reoriented in the mouth. Your best bet is to alter the cast only when it is a single fixture that is out of position and preferably if it is the terminal fixture. When the clinician needs to cut the jig in more than one spot, it is hard to know which positions on the cast are constant and which need to be repositioned. If there is more than one cut, the best bet is to take a new impression with the reconnected jig tightened down using an open tray technique. Of course the verification process should still be taken to ensure the new cast is now accurate before proceeding to the final structure. This can be done by the clinician or if needed, the jig can be retrieved from the impression and the Sheffield Test can be done on the model. If you are using a centralized milling center to mill a bar or bridge, the fit is normally guaranteed on the model not in the mouth. A mistake in the process has the potential to be very costlyThis is a step that is critical to success – do not be strong armed into proceeding without the cast being fully vetted. The good news for the clinician is once you have a verified cast, there is no need to try in the processed structure for fit which typically would require an additional step at that point. Explain that the verification process doesn’t add a step. It merely places that step at the proper point. Like any test the key to success is careful preparation. 

Thanks for reading,

John Ackley

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