Unlike a tooth where a crown can only be cemented to the prepared tooth or post and core, in implant dentistry a crown may be either cemented to an abutment or screwed to an abutment.
An implant-retained crown may be cemented onto an abutment, the abutment being one of many types. What is the rational for cementing crowns rather than fixing them with screws?
The cement-retained crown has the following advantages:
i) The crown will have an unbroken surface, which allows the porcelain to retain its strength
ii) The crown can be made less bulky as there is no screw hole to be accommodated
iii) There is no compromise of aesthetics as the porcelain has complete continuity
iv) Screws that retain crowns or bridges are small and can fracture.
Figure 1 demonstrates the Astra prepable abutment. Having been prepared at the laboratory and having the implant axis not co-incident with the crown, cement retention is the only option here.
A screw retained crown is held onto the abutment or implant by a screw. This can be a small screw which fits into the abutment, or the crown may be made as a part of the abutment at the laboratory, and the crown is then fixed to the implant using the abutment screw. What is the rational for this method?
i) A screw retained crown can be removed and the implant stack disassembled should the need arise.
ii) A screw retained crown with a deep sub gingival margin does not require cement retrieval at the time of cementation, which can be very difficult
iii) If porcelain fails on a screw retained crown, it may be easily removed before being repaired at the laboratory.
Figure 2 demonstrates the opening in the crown for the screw. This crown has been made as one unit with the abutment, and the unit is held onto the implant using an abutment screw.