I splint multiple adjacent implant restorations together, particularly in the posterior. Why? Because an implant is not so great at receiving forces that are off its long axis. Lateral pressure is concentrated at certain areas instead of being evenly distributed. For more on this topic, check out my three-part series on implant occlusion: How an Implant is Different from a Tooth, Cantilevers are Your Worst Enemy, and Cantilevers are Your Best Friend. That series presents a quick overview of implant-protective occlusion and why splinting is important.
Okay, so we’re agreed that we should splint and we’re ready to make our restoration. But beware! Connecting multiple implants together makes it more difficult to achieve passive fit. Picture a single, screw-retained implant restoration. We insert the restoration into the implant and tighten the abutment screw. Imagine that one of the interproximal contacts was way too bulky; it’s putting pressure on the adjacent tooth and the abutment screw as we tighten it down. This would be non-passive fit, also known as active fit. Even if we fully tighten that sucker down, we have introduced stress into the system that has to release somehow. This leads to several potential complications such as screw loosening, component fracture, pain, etc.
Now imagine a restoration that engages multiple implants. This prosthesis has to insert correctly into each implant at exactly the same time. If one of the angulations is even slightly off, then tightening down the prosthesis will introduce stress into the system. The more implants we have in our splinted restoration, the more opportunities there are for an incorrect angulation. So passive fit is harder to achieve and the the greater the chance for prosthetic complications.
How do we achieve passive fit? We can splint the impression copings together to help ensure that the implant angulations are correctly recorded, but we still need to to verify that the prosthesis fits passively at each visit. Every time the prosthesis take a trip to the lab there is a chance that inaccuracies will be introduced.
There are a few methods of verifying passive fit, including radiographs and tactile sensation with an explorer tip. However I’ve found the most accurate to be the quarter turn test. Here’s a example using a metal framework:
Three-unit restoration at the metal framework stage.
Tighten down the anterior abutment screw. Now place your driver in the posterior abutment and screw down until the first point of resistance is met. Note this position.
Continue to hand tighten until full resistance is met. If this is less than a quarter turn (90 degrees) from the first point of resistance then we have passive fit.
An example of non-passive fit. Full resistance was not met until about 200 degrees from the point of first resistance.
Each abutment screw is verified using this technique, one at a time. This test is done at each try in visit to make sure that the most recent laboratory work did not distort the prosthesis. If the prosthesis fails this test, we can verify with radiographs that there is a misfit. The prosthesis is sectioned and luted together with GC resin for welding in the lab.
What if the full resistance is met just a little past the quarter turn point? Well, this was first described by Jemt in 1995 as the “screw resistance test” using a half turn (180 degrees) as the cut off point. In my opinion, I am perfectly fine with a prosthesis that meets full resistance within a half turn. I’d prefer it to be less than a quarter turn, but research has shown that we’re still safe up to 180 degrees.