May 2024
Another Example Of An Initial IMR Attempt Without Microscope Guidance Or Precision Tooling
The patient presented for retrieval of a fractured abutment screw in an Astra 4.0 Tx implant in the #8 site. This implant was placed in Milwaukee, WI, along with #7, on February 17, 2015. #7 implant is an Astra 3.5 Tx. The restorative doctor’s information was not available, so exact abutment types and manufacturer is also unknown. It is assumed the case was restored after allowing for the appropriate healing time. The case was restored with single tooth restorations. These implants were placed as part of a post-accident reconstruction where there was significant vertical alveolar bone loss, which necessitated additional combined height in the restorative abutments and crowns. Recently, #8 crown displaced with no previously noticed mobility. He presented to his current general dentist who diagnosed the fractured abutment screw in #8 and attempted to retrieve the screw fragment. Unfortunately, the effort was unsuccessful, and his new dentist referred him to a local periodontist , who then referred him to our practice.
The initial radiograph post fracture, and pre recovery effort, shows the fracture zone in the screw fragment to be slightly above the first implant thread. In my screw retrieval algorithm, this is either a Type I case or Type II case, depending on the mobility of the fragment. I have seen many of these situations and the predominate case is a Type I mobile fragment, which can be recovered without rotary instruments. However, if it happens to be immobile using an endodontic explorer, then our protocol is to mobilize the fragment with a concentric drilling technique. This is always done with microscope assistance, precision custom drill guides, and constant verification to ensure the effort is proceeding concentrically on the fragment.
In the above left, preop image, the position of the fragment is above the first implant thread. The center radiograph, post initial recovery effort, shows the fragment is now even with the first implant thread and there is a space on the right side, mesial, between the screw fragment and the implant. The image on the right is our preop clinical image at 25x, which is a Helicon Focus produced focus stacked image from about 8 photos, where the focus point was progressively changed in each picture. At 25x the depth of field is limited, and using this method can almost produce a 3d effect. It is clear there were multiple drilling efforts, and none were concentric. There were two notable ones. One at 2 o’clock and the other more significant one at 8 o’clock. The other two efforts at 5 and 6 o’clock were minor and of no real significance. Unfortunately, when this eccentric drilling occurs it almost always locks the fragment, but to varying degrees. This occurs when the implant and screw threads are cut and distorted. Just a nick into one thread can immobilize a fragment and force a change in the recovery protocol.
The first step in recovery is to establish the center of the fragment and place a concentric dimple. Then a precision custom drill guide is set up to drill the fragment concentrically. This was done and an .8mm hole was bored completely through the fragment. Note the difference in the position of the previous efforts and the new .8mm bore.
The bore was enlarged to 1mm in the top and 1mm deep and a .8mm screw extractor was engaged in an attempt to dislodge the fragment and unthread it. The extractor engaged well, but the in the process of ramping up the applied torque, the last 1.8mm of the screw extractor tip fractured in the screw fragment. This was a setback, as now the extractor fragment had to be drilled out to clear the original bore. At this point,
it was obvious this fragment was so locked it would require a complete drill out to clear the implant. Therefore, the extractor fragment was slowly drilled out and the bore was enlarged from .8mm to 1.25mm, which is the predrill diameter for an M1.6 tapped thread. This was completed successfully and then the recovery shifted to retrieve the remaining male screw thread fragments from the implant threads. Using a tap guide along with a variety of specially ground taps, the threads were slowly cleared.
To further evaluate the previous thread damage, a polyvinyl impression was taken of the cleared implant. The images below show the impression every 90 degrees. An impression of an intact implant as control is on the left.
The previous recovery attempt was made with tooling from a USDent screw retrieval kit. Reviewing these instrument on the web, they appear to be identical to the ones in the Neobiotech SR kit. While I’m highly suspicious this tooling is not adaquate to prevent eccentric drilling on the M1.6 screw in this 4.0Tx implant, none of the above has yet to be confirmed in my machine shop with the actual tooling. I’m hopeful I will be able to examine this kit and report further information on it in an addendum to this report.
A couple of thoughts on the replacement restoration
The more I have studied the initial radiographic images, the more I’m convinced the replacement restoration should have both implants splinted to provide additional stability for these abutment connections. As there was bone loss which forced the implants to be placed deeper and slightly lingual, the amount of torsional loading these relatively small connections have to resist is a contributing factor toward this failure. While I don’t think the amount of thread loss on #8 is highly significant, it is just an educated guess from a lot of experience. Splinting will decrease the loading on both connections, which includes both abutment scews. I believe the restoration can be fabricated very esthetically, so splinting will not affect the esthetic result.
If possible, I would also suggest a screw retained approach. If the implants are parallel enough, the first choice would be to use Atlantis Custom Base abutments with the ASA screw option, and build the case screwmentable. Atlantis milling is beautiful and the ASA option perfect for a restorative dentist, as it gives up to 30 degrees of off axis placement for ideal screw access openings, while maintaining the ability to fully torque the screws. I would advise to definitely confirm the parallelism of the implants before proceeding and this can be done by simply luting together two open tray pickups with GC resin. If they draw, you’re in business, if not, its easy to cut them apart and look for another option.
Lastly, I was looking at his old crown from #8 and it appeared the lingual access opening may actually have prevented a straight line access to the abutment screw. This may have been a possible factor in the failure, in that original screw torque may not have been able to be delivered to full torque at 25Ncm. CAM
