Recently I received a call from my laboratory questioning a prescription they had received. The patient had two single unit implants on her lower left replacing the first and second molars. ” The prescription says screw retained and we wanted to call and make sure you really wanted screw retained restorations?” I answered that I did indeed want two screw retained restorations and then was curious about why they questioned this prescription so quizzically. The response I received was that they just don’t get asked to do that very much anymore. I’m not sure what the percentage of cement retained to screw retained crowns are placed these days on implants. In the early days of single tooth implants screw retained was the solution, over time we moved to cement retained. I believe there are still clinical situations where one is preferable over the other as they have unique risks and benefits.
Screw Retained Implant Crowns
Advantages:
- Ability to be used with limited inter-occlusal distance
- Absence of cement under the gingival tissues as a potential irritant
- Predictable retention
- Retrievability
Disadvantages:
- Screw access compromise of occlusal function
- Esthetics
Cement Retained Implant Crowns
Advantages:
- Passive Fit of the Implant Crown
- Esthetics
- Ideal occlusal form
- Flexibility of fixture placement
Disadvantages:
- Subgingival Cement Line
- Cleaning of Excess Cement
- Crown Retention
Choosing between the two options requires a knowledge of the clinical situation and both the patient’s and practitioner’s preference.
What is the minimum occlusal height distance in order to use cement-retained implant crowns?
This is a challenging question to answer as the required space varies between implant types and the restorative components you select. If we simply break down the numbers you need a minimum of 2mm for the occlusal table of the crown to have adequate thickness of restorative material. If you are depending on retention form you will need a minimum of 3mm of opposing vertical walls buccal and lingual. Most often the buccal and lingual walls are compromised for the screw access and the margin due to the diameter of the fixture, so to get 3mm you may need 4-5 of actual space for the abutment. If we add this up we are looking at 5-6mm as an absolute minimum and that works only if the abutment parts are available. The difference is that when you have limited space, retention becomes an issue with cement retained because you have to compromise the height of the abutment, this is not an issue with screw retained.
Thanks for the “pros and cons.” To add my opinion: The biggest problem I have with screw retained is when the contacts are too tight. Adjusting them takes a long time as I have to screw to place, check, unscrew, adjust, repeat. If your lab consistently produces spot on contacts, great! The biggest problem I have with cement retained is in the rare event of screw loosening. Even when mixing a little vaseline with the tempbond, there is usually such good retention the crown is hard to or impossible to retrieve. Then it is time to create an access opening and turn the restoration into screw retained! All that said, if I have two adjacent implants in the posterior with adequate occlusal clearance and any question about the quality of bone and/or parafunction, I will cement splinted crowns to custom abutments.
Why are the contacts too tight? A good lab will not have this issue and, I hate to say this, a digital impression would eliminate this problem.