This post is a continuation of my previous two posts. My initial post talked about how I deal with broken screws. Then I expanded on some of the possible causes for broken screws in my following post, dealing specifically with issues with inserting single tooth implant restorations. For this post, I like to talk about common issues on multiple unit implant restorations
One case involved a four unit implant bridge supported by two non parallel implants. One implant was angled buccally and the other relatively parallel to the long axis of the prosthetic tooth. The case was restored with a screw retained implant restoration. The pontic space was not excessive and I thought the treatment design was adequate. I would have corrected the off angled implant with a custom abutment before connecting it together. See my sample case below. I think it would have made it easier. In any case, according to the patient, the bridge would loosen every few months requiring frequently tightening. Finally, the screw broke and the dentist was unable to retrieve it. The patient got referred to my office for evaluation. When I evaluated the case, the remaining fragment is below and inside the implant fixture. I had to use the screw removal kit from the corresponding implant company to essentially drill out the entire screw and to re-tap the internal threads. It was not fun but it had to be done. When I finally did put the prosthesis back in with new screws, it was clear to me that the implant bridge never seated passively. It was rocking between the two implants. It became clear to me that the screws were merely holding down the prosthesis down and so would undoubtedly loosen up over time. So I told the patient even if I put brand new screws in this case, it will continue to have the problem. The only solution is to remake the implant bridge and to make sure it has a passive fit.
So what do we mean by a passive fit and can we achieve it with the current clinical and laboratory steps involved? Some literature will say that it’s next to impossible to have a passive fit for screw retained restoration, especially if the implants are not parallel to one another. As we move from single tooth restoration to multiple tooth restorations, it’s natural to splint several units together. However, as the number of teeth involved increases, the longer span increases, so does the amount of errors increase. You may ask what kind of errors? Before the days of digital dentistry, there are impression errors, stone expansions errors, casting errors, and the list goes on and on. The difference is when you are dealing with a small single unit restoration, be it a tooth crown or implant crown, you will hardly notice the errors. The errors are there but minimal. Even if there are, most of the errors may have already been corrected by your laboratory technician before it lands in your hand in your office. But as you move to longer span prosthesis, the errors will start to accumulate to the point it will become more noticeable. For example, the framework will not seat on the model after the initial casting; unless it is cut and soldered/laser welded.
So going back to that implant bridge that was rocking between two implants. What was the problem? It can be many things but most would agree that there must be a discrepancy on the model this bridge was made from and the patient. And we know that can happen. But what do we do about that? We implement steps in the prosthodontic procedures to minimize these errors.
When implants were first introduced in North America, they were primarily used for fully edentulous cases with the use of external hex implants. Great efforts were made to ensure that the impression was accurate so the subsequent prosthesis will fit nicely on the model as well as in the mouth. The additional procedures that developed subsequently included splinting impression copings with self-curing acrylic resin and making verification jigs prior to fabricating the framework. These steps are designed to counteract the possible errors in impression taking and stone expansion. To counteract the casting error that may occur, we double check the fit of the framework to see if it needs to be cut and soldered/laser welded prior to moving to the next step of porcelain application. These are steps designed to check for errors that we know exist and we want to correct them before they become clinically relevant. One of my professors, Dr. Hussein Zaki, once told me, all the prosthodontic procedures are designed to counteract errors that occur at every clinical and laboratory steps. I think it still holds true today even in the digital era.
An example of three unit implant bridge 36-x-34; Implant 34 slightly too buccal which would have resulted in screw access on the buccal aspect of the restoration. Instead, a custom abutment was made to correct the angulation on implant 34. The implant bridge was connected with screw connection on 36 and cementable crown on 34.
My next post will be focusing on occlusal designs and manufacturing issues relating to implant restorations. Thank you for your time in reading my post and I look forward to your comments.
