Mechanical Assessment of a Straumann RC BL Case Where Two Abutments had Failed in Two Years

August 2024

Case History

The patient presented on referral for retrieval of a loose crown and abutment from a Straumann 4.8 BL RC implant in the #19 site. Our Referral Intake Problem Summary noted the implant crown on #19 had loosened during the extraction of #17 and #18. He was referred to our office when the attempt to locate the abutment screw to recover the crown was unsuccessful. However, the patient related to us the crown had felt loose prior to the extractions. The patient was probably correct and when the distal contact was lost with the extraction of #18, more mobility was noticed. This implant was placed along with another in the #20 site on 04.27.22. Both were restored with Tibase abutments as independent restorations following osseous integration.

On 06.10.2023 he presented back to his restoring dentist with an abutment fracture in #19. This fracture occurred after approximately 1 year in function. The mechanics of this fracture will be discussed in detail below.

Recovery

Recovery of the crown with microscope visualization revealed the access opening had to be modified to the lingual to accommodate lingual implant pillar angulation. This allowed the driver to engage into the SCS screw connection on the long access of the screw. Once the screw was retrieved, the abutment and crown were recovered. As suspected prior to the appointment, the crown looseness had been secondary to a fractured abutment. Following cleaning of the internal aspects of the implant, the implant was examined under 25x microscope magnification and found to be free of any structural defects visible at that level of inspection. He was referred back to his restoring dentist following routine placement of a healing abutment.

This case became significantly more “interesting” when I realized the vertical location where this abutment had fractured and the short duration in function to produce this failure. While the recovery phase of this case was straight forward, consideration as to how to restore for better long term stability is more complicated. The current issues then become, why did these abutments fail, the locations where the fractures occurred, and the short time span of just 1 year in function for each abutment. I believe we need to examine all these issues.

Explanations of etiology

I have seen numerous cases that have suffered abutment fractures, and the two major factors seem to be the magnitude of force applied to the mechanical system and the design and size of the mechanical system resisting it. Force is the accelerating factor and without it, nothing fails mechanically. Torsional loading, or bending moment, is the multiplying factor that magnifies the force and is very destructive to the mechanical system. Torsional loading is simply force x leverage and the multiplying effect of the leverage arm is really the dangerous gorilla in the equation. While the physics is basic, ignoring this produces forces that are large enough to be very significant and dangerous. The following photographs taken of the recovered #19 crown on an analog illustrate the point. The facial view shows a well centered implant with the spacing is about equal on the mesial and distal. This will distribute the inevitable torsional loading of a molar sized crown. However, when viewed from the mesial, the lingually inclined implant produces an additional facial cantilever under the buccal working cusps. This additional torsional loading force has to be resisted by the components in the implant stack. At this point, the questions of bruxism and heavy posterior function seem to always enter the conversation and are often conveniently or erroneously cited as the only explanation for the failure. While this is an accelerant and definitely a consideration on the side of loading, so is normal routine use depending on diet consistency and load cycles, especially when coupled with the magnified torsional loaded force. The point is mechanical stability also plays a significant roll and having excessive cantilevers on any system will deliver increased loads in all scenarios.

Evaluating many abutment failures has led me to understand there are specific failure trends which are generated by component size, and implant design. When mechanical stability of the abutment has failed, these failures seem to concentrate where a smaller or weaker abutment cross section joins to a larger or stronger cross section below it in the implant pillar. As I have routinely measured fracture surface areas, they also strongly correlate to the area of the fracture zone. Both of these observations should be rather intuitive mechanically. With everything else equal, a smaller area will fracture more easily than a larger one. Therefore, if the abutment to implant joint is stable and the abutment diameter is weaker at the implant top, the fracture will routinely be seen through the abutment at the implant top. An example of this would be in the Ankylos implant system and also in the mechanical clone of that system, the Neodent CM. In both cases, abutment fractures occurring at the implant top are prevalent while the very stable conical connection below stays intact. These systems use a small common abutment diameter of 2.5mm in all of their implant diameters. As in this case, TiBase abutment fractures are often seen above the implant top at the base of the vertical cylinder and shoulder. This is where the wall thickness is thin and joins a larger cross section below at the abutment shoulder. Two prior cases below illustrate this point. The top images are a fractured Straumann Variobase abutment in which the measured fracture zone was 2.64sq.mm. The second case below is another Straumann 4.8 RC implant but restored with a non OEM abutment from an unknown manufacturer. Both of these abutments suffered the same fate, and this totally explains the failure of the first TiBase abutment in this current case.

Applications to the current case

The second abutment that failed in this case was a custom abutment that had failed due to a horizontal fracture which traversed below the small .7mm high,15 degree (30 degrees inclusive) top bevel in this implant. I have seen several abutment fractures in RC bone level abutments, that have occurred at the implant top, obliquely through the top bevel or just below the top bevel, mostly in molar applications where forces are greater. Using the observations described in the above paragraph, the below bevel fracture leads me to believe the lead in bevel adds little to the stability and resistance to abutment fracture of this interface. In this case, it happened to be at the base of the top bevel. This zone is limited in strength, as the 4.8 RC connection is “ganged” with the 4.1 RC connection. The cross section of the abutment at the implant top is 6.11sq.mm. The cross section of the fracture zone in this abutment failure was about 3.97sq.mm, decidedly less than the cross section at the implant top and much closer in size to the fractured TiBase abutments described above.

There was no Straumann laser marking on this abutment and the last custom abutment I received from Straumann Scan and Shape was laser engraved. I subsequently learned this case was manufactured by Glidewell. While this potentially introduces additional mechanical questions, the following case photographs, from another previous case with essentially the same abutment fracture, clearly illustrates this issue in a laser engraved Straumann abutment. This case did not present with the restoration, so other force factors could not be considered. The following photographs clearly show the location of the abutment fracture below the conical connection.

While the abutment dimension at the implant top seems reasonable, when compared to several other systems, it is definitely less than some other 4.8mm implant designs. A prime example is the 4.8mm Dentsply Astra EV or Prime taper which has a cross sectional area at the implant top of about 9.98sq.mm. This implant connection is not ganged with their smaller 4.2 EV implant, which is slightly larger size than the Straumann RC connection at 7.1sq.mm. This 1sq.mm only adds about 16% more surface area, but if the 4.8mm implants are compared the difference jumps to a 63% gain. This is substantial especially when restoring molar sites with the inevitable torsional loading forces.

Conclusions

When considering the solution for adding increased mechanical strength and resistance to torsional loading, the most practical solution is probably to splint #19 and 20. This will substantially decrease the mesial to distal torsional loads and better distribute the buccal to lingual loads. The patient was not planning to add additional implant support distal to #19, but that might be revisited with consideration to splinting to #19 distally. If another implant is decided on, additional consideration as to what is placed may give additional help to #19.

One final note, when considering how to design a splinted case, as either cement or screw retained. I have seen several splinted cases on Straumann RC implants that were delivered screw retained with nonengaging abutments. They have not been mechanically stable as this connection is dependent on the length and tolerance fit of the cross fit connect for abutment to implant stability. This should be obvious now as I have already illustrated the lack of stability in the top conical bevel. This does not eliminate the use of a screw retained concept with nonparallel implants, but it does complicate the construction with the use of additional copings and screws. If this complication is avoided with cement retention, the issue of retained cement is real as discovered in this recovered abutment in the distal proximal concavity shown below. CAM

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