Wrong Restorative Components in a Straumann RC Implant

Wrong Restorative Components in a Straumann RC Implant that Produced Two Abutment Screw Failures Inside of Two Years and Resulting in an IMR Abutment Screw Rescue

The patient presented for retrieval of a fractured abutment screw from what was described on our referral intake form as a Straumann 3.3 BL NC implant in the #6 site. This implant had been placed on 04.21.22 and originally restored on 10.13.22. Unfortunately, this was the second abutment screw fracture in this implant which happened between 10.31.23 and when she presented back to the restorative practice on 3.05.24.This range in dates encompasses the time the abutment screw loosened and when it actually fractured. In the first fracture episode, the screw fragment was backed out the implant with an explorer and the restoration was reseated with a new abutment screw. With the second fracture, the abutment screw was not mobile, so an attempt was made to recover the screw fragment by using an unguided Astra Tech 1.4mm fragment fork. This attempt was unsuccessful, and she was then referred to our practice in the hope of clearing the fractured screw and replacing the restoration. The thought at the time was the Straumann NC implant might be undersized for the cuspid location and the connection unable to handle the possible lateral forces in this site. However, this type of failure is secondary to cyclic loading and this screw failure had occurred twice in under two years in function. Fast mechanical failures seem to implicate iatrogenic induced failure as there just isn’t enough time to accumulate enough load cycles on the implant pillar. The question then becomes, what was the origin or mechanism of failure, and how did it occur? As the intake information was reviewed, it was noticed the implant had been restored with an Argen custom abutment. This brought in the possibility of a non OEM component, with possible unknown design and tolerance issues.

After an initial preoperative discussion of the recovery possibilities and protocols used in our practice, the site was anesthetized, and a conservative exposure of the implant top was completed. Surprisingly, preop photos were taken which showed the prior recovery attempt with the 1.4mm Astra fragment fork had been quite concentric on the retained screw fragment, with only minor scarring of the first implant thread in the 3 o’clock side of the implant. As the Straumann NC and RC implants both use M1.6 screws, it is important to note the predrill size for a M1.6 screw is 1.25mm. Therefore, any instruments introduced with a diameter greater than 1.25mm will invade the implant threads, even is the instrument is used completely concentric on the screw. This is extremely important, especially, if the top of the retained screw fragment is not residing above the first implant thread. However, all things considered and with a history of two fractured abutment screws in under two years, this implant appeared to be significantly more intact than anticipated.

In the preop clinical image on the right, what appears to be a dimple in the center of the screw is actually a protruding point created when the 1.4mm Astra fragment fork was used in the prior recovery attempt. This fragment fork is somewhat similar to a left hand, end and side cutting end mill, but it does not cut in the center, which is notched, and this feature created this protruding center pyramid. (See the photograph below) The shiny circle around the center point was actually slightly depressed or troughed. The 3 o‘clock darkness and minor thread issue, is not in good focus in this image but will be in subsequent images. As expected, the fragment was not mobile, so a concentric mobilization technique was initiated. Staying concentric on the center of the screw is the safest place to work, which is away from the implant threads. However, before drilling, a concentric spot is placed to direct the drill and prevent wandering of the drill location as the drill starts the hole. As this implant had this center protruding area, there was concern that the spotting drill would also have a problem staying on center. So, before using the spotting drill, a small concentric dimple was created with a surgical length ¼ round bur on a high speed handpiece. This was accomplished freehand as the access was tight, but with 25x microscope magnification to provide visual guidance.

Next a precision custom drill guide was placed and secured with light cured orthodontic resin, so it would be stable, but removable. The spot drill was used next and is always microscopically confirmed to be drilling concentrically before proceeding further. Unfortunately, the spot was not concentric, which is highly unusual. The guide was reset and in doing so it was discovered there was some instability in the guide as it was seated into the implant. The assumption was made that this was happening because the guide was “bottoming out” and not engaging the limited conical interface completely. In the Straumann BL implants, they have a very narrow zone of contact on the top “bevel” and in the NC it is only .5mm at 15 degrees, as measured in my machine shop on an optical comparator. Once the guide was shortened, I was able to seat it more positively, I thought, and the guide restabilized with new resin. The second test was better, but not best and the process was repeated until the spots produced were concentric.

As there was potential for some minor preop thread damage, a M1.6 bottoming tap was passed through the threads, without difficulty. The implant was cleaned, and the supplied healing abutment was placed finger tight. She was referred back to her restorative dentist for consideration of crown replacement. At the end of the appointment, I was still uncertain as to the etiology of the two abutment screw fractures. As described earlier, early mechanical failures, as in this case, are most likely iatrogenic in origin. However, the Argen abutment seemed to fit very well into a Straumann NC analog, so the thought of a non-OEM component mismatch seemed unlikely.

The day following the retrieval, her restorative dentist contacted me and related that he had just learned the implant was a Straumann 4.1 RC implant and not a 3.3. This information made total sense as it explained the failure issue and the difficulty with achieving a concentrically guided drill in the recovery appointment. My custom drill guides do not interface into the cross fit connection, but rather are carefully sized to interface internal diameters without the need to interface with indexing geometries, as in this case the cross fit connection flats. That is precisely why the lack of indexing was not noticed, as there was none. We were very fortunate to have recovered this screw without introducing any additional thread damage. The primary reason for our success was that we relied on our microscopically verified concentric technique and because extreme care was taken to ensure the drilling procedure started and stayed concentric through the compete process.

So, what happened to create this failure?

Following the recovery appointment and subsequently learning of the component size mismatch, I was interested in how this error occurred. I have seen numerous times where a fractured solid Locator abutment has occurred in a Straumann RC implant, with the etiology actually an NC abutment which had accidentally been delivered into an RC implant. The non indexed, solid NC Locator can be threaded into an RC implant, but it drops into a deeper position and the rotation is stopped with the screw bottoming out, just before the conical bevel can engage. Until this case, I had never seen a situation were an RC implant was restored with an engaging NC abutment. So, to see how this could even occur, I took the cross section photos below.

The left photo has the existing restoration in an opened NC analog. The right photo has the same restoration in an open RC analog. Note how the top .5mm, 15 degree conus seats correctly in the NC analog but slides by the seat to somewhat interface with the abutment above when in the RC analog. The screw has also seated deeper and has bottomed out in the analog threads. This test would be slightly better if completed
on actual open implants as sometimes the threading in analogs is not exactly the same as the implant they duplicate. However, this is close enough to understand the situation at hand. It is also important to note the indexing cross fit connection did not engage the RC analog at all, with the restoration becoming somewhat secure by wedging on the implant top. This means the abutment screw had to have barely enough threads in the implant to accomplish this. I would suspect if this was done on an actual implant, the abutment, screw threads would be slightly distorted in the seating process. To carry this one step further, the same analysis was completed using an NC open tray pickup.

So, the analysis is the same as the above old restoration, except the perplexing issue is the NC open try pickup does not ever engage the implant top, nor the cross fit indexing feature, but the screw does get tight. This is because it has bottomed on the analog threads. With the screw tight, the body of the transfer can still rotate freely. I would think the misfit error would have been picked up at this point in the initial impression process. However, I need to point out, I didn’t think of the error of a mislabeled implant either. I had the total mindset that it was a 3.3 NC implant, and it was my guide that had the problem. The only difference is that my guides do not catch the indexing feature and the pickup transfers do, as this is a key feature in exact transfer of the implant position into the master cast.

I believe this case points out how compounding communication errors resulted in the delivery of a restoration with the wrong components. Initially, an inadvertent clerical error resulted in the wrong implant size being reported to the restorative dentist. This was discovered, but there was then another error to get the correct communication forwarded to avoid the problem. Lastly, I still do not know how the impression process was completed so the smaller NC pickup could accurately relate the implant position so the
abutment and crown could be accurately delivered.

The mechanical failure is easy to understand. The platform error eliminated most, if not all, of the abutment connection stability, resulting in significant micro and macro motion in the abutment to implant connection, which then allowed for fast abutment screw failure. Fortunately, this problem was controlled before the implant was damaged, and the implant can now be restored with the properly fitting components. Also, going forward, the implant prognosis should be unchanged as it is essentially free of defects, at least at the 25x level of inspection used to examine it with. CAM