I will always opt for a screw-retained restoration over a cement-retained restoration for implants.
However one of the challenges for screw-retained restorations is achieving a passive fit of the restoration/abutment to the implant. In an ideal world, the restoration introduces no stress into the implant and it’s surrounding structures as it is torqued. Academically speaking, this never happens.
But we can take steps to make the fit as passive as possible and thereby reduce or eliminate the complications associated with non-passive fit. That’s good because those complications are ugly: screw loosening, crestal bone loss, and prosthesis fracture, to name a few.
(1) Make an Impression Jig
Your impression material may not accurately record the three-dimensional relationships of the implants to one another. This can be reduced by splinting the impression copings together before you take the impression. Here’s how I do it:
First, tie dental floss around the impression copings:
Next, apply pattern resin to the floss and copings:
Now go get a cup of coffee. Seriously. This needs to stay in the mouth for a good ten minutes to fully set. If the resin is still setting after you take the impression, you will distort the relationship of the implants and defeat the whole purpose of doing this!
(2) Splint with GC Resin, Not Duralay
As far as pattern resins go, I prefer GC Resin. Studies have shown that GC Resin has better dimensional stability than Duralay. Hey, every little bit counts.
(3) Use a Verification Jig
So now you have a model of the implants. Before you fabricate a framework, make an appointment with the patient to verify that the model is accurate. Trust me, it’s less expensive than cutting a framework.
Before the patient comes in, reinforce your impression jig with more GC Resin on the model; it’s now a verification jig. Alternatively, you can have the lab make a verification jig. Either way, you’ll have plenty of time for the resin to set to eliminate any possible distortion.
At the patient’s appointment, try in the verification jig and test for passive fit. There’s a chance you’ll have to section the GC Resin in a spot or two to achieve the fit you want. No big deal; re-apply GC Resin to those spots and wait for it to set. I’d rather cut GC Resin than a metal framework.
If no adjustments are needed, your model has been verified. Go to framework!
If you needed to cut the verification jig and make some changes, the lab can then pour up a new model. Go to framework!
In the case below, I had a new model poured based upon the verification jig that was tried intra-orally. Notice that this model has no anatomical detail (palate, alveolar shapes, soft tissue scallops, etc); it’s just an accurate account of the implant positions. I’ll use this model to fabricate the framework.
At subsequent visits I will still verify passive fit each time. I’ll address my tests for passive fit in a future post.