With regards to Screwmentation technique, I have 2 questions: 1. Once the crown is bonded to the abutment either extra/intraorally, does it sometimes that the whole piece is hard to be removed from the model or mouth due to different paths of withdraw of the abutment and crown? How to predict this is gonna happen? 2. Is the screwmentation technique exactly the same protocol for 3-unit bridge and single crown cases? Again, is there a RISK that the whole unit of the bridge and abutments once bonded can not be taken off from the implants/analogs due to slight different paths of withdraw of the implants?
Editors Note: Screwmentation is a technique in which a large implant supported prosthesis is attached via a combination of screw retention and temporary cementation. You cement anterior crowns typically due to angle an screw retain posterior ones. There is a full explanation of it by Uwe Mohr on DentalTown. For a further discussion of the Screwmentation technique, please see Dental Implant Prosthetics: Achieving Retrievability and Reducing Treatment Complications By Using a Modified Installation Technique, Emil L. A. Svoboda, PhD, DDS.
Alejandro Berg comments:
hi, we do this technique almost exclusively, and when we try in, we do temporary bonding with provicol from vocco and after all is said and done( colour, shape and emergence profile even oclusion)... remove temp cement, very clean and full cement, then place in site.
Emil Svoboda comments:
There are many variations of the screwmentation technique. Most will frustrate the efforts of the dentist because the root causes of misfit margins, poor contacts, and subgingival cement are not specifically addressed. Those root causes are Prosthesis Dimensional Error (PDE) and the Tissue Effects: Resistance to Displacement Effects (RTDE) and the Gingival Effects (GE). See New Dental Terminology ... www.ReverseMargin.com.
The basic objective of Screwmentation is to allow the dentist to optimize the fit of the abutment before having to manage contacts, open and overhanging margins and subgingival cement. The same abovementioned root causes of complications are also responsible for implant-abutment and abutment-prosthesis connector misfits, loose screws and poor contacts inherent to the Screw-in Prosthesis installation technique. They are all known risk factors for peri-implant disease.
- installing the abutment in the mouth: this allows the dentist to optimize the fit of the abutment without the prosthesis attached. This simplifies installation because there is no prosthesis involved to frustrate the optimal seating of the abutment - no contacts to deal with.
- Installing the crown involves managing PDE -adjusting contacts while pushing adjacent tissues tissues out of the way of the prosthesis (RTDE) and then cementing the crown in the presence of tissue fluids and working blindly.
When margins are subgingival and the abutment profile is narrower than the crown profile it can be difficult to displace the adjacent gingiva and this can prevent seating of the crown. This problem is exacerbated by tissue fluids and working blind. Of course the in and out of the prosthesis can can traumatize the gingival and cause swelling and bleeding.
Hence we have two variations - a) after adjusting contacts the abutment-prosthesis complex is removed from the mouth and cemented into place on the model … before being reinstalled. Unfortunately this reintroduces PDE cause by model inaccuracy and that can cause implant-abutment misfits and poor contacts.
b) the prosthesis is now cemented into the mouth while trying to manage the RTDE. Was the prosthesis ever seated optimally? How do we know? We now are cementing in the presence of blood or tissue fluids and must use high pressure cementation to displace tissues, while working blind. When the abutment-crown complex is removed we can remove excess cement but how do we fix open margins?
The Svoboda Variation of the Screwmentation is presented in Comparing the Chamfer Margin with the Reverse Margin System (RM) at the above website. This system was designed by the author to mitigate the root causes of problems. Thus, once the abutment is installed optimally, the abutment and prosthesis design prevents the gingiva adjacent to the prosthesis margins from interacting with the prosthesis. As well the abutment margin trough allows for some tolerance of PDE without causing open and overhanging margins. So, once the abutment is in, the dentists can adjust contacts and seat the prosthesis without displacing or otherwise traumatizing adjacent gingiva. This is easy. Then when cementing, the increased cement space and no need for displacing tissues, allows the dentist to use controlled low pressure cementation to seat the prosthesis. The ledge on the RM abutment makes it easy to remove residual cement. There are no open and overhanging margins to deal with. This is great!
Now, the dentists can still have an occlusal plastic covered screw-access hole to remove the abutment-prosthesis complex and reshape the margin area before reinstalling it. The author prefers to leave the occlusal intact. When the implant-abutment connection is optimized, there is little need to tighten screws. If and when that is necessary, it is easy to drill a hole through the crown to do so.
Emil Svoboda comments:
Now to answer the 2 Questions:
1) Retrievability is the consequence of placing implants parallel to each other and the path of insertion. If the system was designed to be easily retrievable when installed by the screw-in system, it will also be retrievable when installed by the cement-in system. Perhaps the use of multi-unit abutments would help the clinician remove a prosthesis where the implants and prosthesis were not quite parallel. These were developed to support retrievability for the screw-in system.
When using temporary cement, it is possible to remove the prosthesis without removing the abutments. For this system, it is more important that the abutment retainers are parallel to each other and the path of insertion as determined by remaining teeth. By the way, it is not necessary to remove a prosthesis to tighten or replace abutment screws.
- the concepts underlying removal of a multi-unit prosthesis are the same as above. It just gets a bit more complex when aligning many implants with the path of inserting. Perhaps the use of many multi-unit abutments and better planning of implant placement would help if easy retrievability is desired. The mode of installation does not determine the retrievability status of a prosthesis … rather it is the inclusion of retrievability features like multi-unit abutments, angled screw channels and alignment of implants and the path of insertion.