An Iatrogenic Mechanical Failure When A Conical Abutment Was Used On A TSV Implant

The patient was referred for retrieval of a fractured abutment screw from an unknown implant in the #30 site. This implant was placed and restored in Tijuana, Mexico in 2019. The patient and referring doctor were not able to get any implant information from the dental clinic in Mexico prior to the appointment in this office. The implant was thought to be a Hiossen implant, but the radiograph sent appeared to be a TSV design in the 4.5mm diameter range. This implant had a very unstable mechanical history from the start, with the crown replaced once and the abutment screw tightened at least 3 times. An accurate accounting of these dates and corrections was not possible to obtain.

Fortunately, when she presented, she had the recovered abutment and crown that had displaced when the abutment screw fractured. This abutment was a conical design of approximately 11 degrees and an internal hex. Clinically, the implant was clearly a TSV design with the abutment screw fractured high in the implant, well above the implant threads. This abutment screw fragment was not mobile. The position of this fragment was a Type II abutment screw fracture in my treatment algorithm and therefore required concentric mobilization to recover. As a drill guide could not be placed into the implant due to the obstruction of the long abutment screw fragment, the fragment was concentrically drilled, by hand, with a .8mm drill and microscope guidance. A .8mm screw extractor was engaged and torqued until the extractor tip fractured. This abutment screw fragment was very tightly wedged into the implant. The screw fragment was then lowered to the level of the threads and a 4.5 TSV custom drill guide was placed and stabilized with light cured resin. With this precision drill guide, a .8mm hole was concentrically placed through the fragment. The hole was enlarged to 1.1mm and another screw extractor was engaged. Again, significant torque was applied, but the procedure was terminated before the screw could be recovered for fear of fracturing the second extractor. At this point it was clear this fragment could only be recovered in pieces. Knowing the implant was a 4.5 TSV it was assumed the threading in the implant was a 1-72 english thread. The dimensions of this thread are almost identical to a M1.8 metric thread, so the fragment bore was enlarged to 1.45mm, the predrill size for a M1.8 screw. Once completed, the residual male threads were picked out of the female implant threads, until the implant threads were clear. A 1-72 tap was then passed through the implant threads to ensure they were clear. A 4.5 TSV open tray pickup was then successfully trial seated, removed and a silicone plug was placed to hold the tissue stable until a titanium healing abutment could be placed.

Etiology of the Failure
When a mechanical failure happens early after delivery it triggers the thought that an iatrogenic mechanical failure is involved. This case is a prime example of this, when restorative components were used from two completely different implant systems and designs. The following photographs of the recovered abutment and another with this abutment on a sectioned 4.5 TSV analog are quite self-explanatory. The difficulty in recovering the abutment screw fragment was most likely secondary to placing a metric threaded abutment screw into the 1-72 english threaded implant. This creates a mismatch of thread pitch and creates a cross thread or minimally a series of very wedged threads. The screw fracture is due to the total lack of stability in the abutment connection, so all of the occlusal force has to be resisted by only the abutment screw. It’s hard to image how this error occurred, as the fit is incredibly bad. It’s so bad I would have thought it could not have been possible, except the presenting abutment had beveled wear on the end that totally matched up with the 4.5 TSV implant shoulder at the top of the internal hex. CAM