Troy you and I talked on the phone on this so you know my thoughts but I will share with the group.
I have never had a ti base come loose on a single unit implant case, hell on some occasions when we needed to add a contact after it was cemented it was damn near impossible to separate the base.
Now for bridges I admit there has ben some issues, I have been forced to change my protocols to make sure my clients are doing their jobs so no finger pointing happens. Like our clients I feel "married" to these large cases. I saw the writing on the wall early and realized I needed to be able to manage my risk on them.
We all know on any bridge its the accuracy of our master model that affects the success or failure of a case. Now there are levels of failure and some propagate immediately, some take time. On Metal ceramic cases the fudge factor of clinical competency can be off to allow for bridges that are slightly not passive. Over time the screw will be the weak link and screw loosening happens. On screw retained cases the doc would often times just tighten the screw dismiss the patient until the next time. On any other material that is utilizing a cemented base we do not have this luxury.
What I have found is that even though we go through the steps to verify the master model it makes sense that mistakes can happen. Jigs can appear seated in radiographs if x-rays are taken at the wrong angle, Also the new bone level fixtures with offsets in the base can be a bit of a challenge to know if they are all the way down. After some issues with bases I started requesting copy's of the radiographs to verify and add to my file. This allows me a better way to track our failures.
As I see it the issues is not with the cement, hell we need a bit of a week link some where right? Since the caps are the screw seat if the fixture is slightly off high or low then the cap will always be under tension once the screws are tightened on the appliance, of course eventually it fails. Creating mechanical retention to try and lock things in will lead to more frames being under tension and thats not a good thing for ceramics. While I know the manufactures of these parts realized this they still offer solutions to make us feel better. (Why do you think NT trading added ribs to their NEW caps?)
A bit of a tip for guys doing these large cases:
Often times we come down to the wire and cement these large cases on the day they need to ship. Many of us choose to ship these cases assembled on the master soft tissue model. On maxillary cases often times the soft tissue will be under a bit of tension as we screw the appliance down. Since all of these cements were designed to be cured in the mouth when your working in a 75* lab it will take MUCH longer to fully set. Your light source will not fully penetrate to get a full cure so we are forced to wait for the self cure properties to fully harden. I have found that even the slight pressure of the soft tissue on the bases prior to full set of the cement can make you look foolish when the case arrives with a base or two loose. Now we cement one day prior to the ship date, on a model with no tissue in place. We let the case fully set overnight before we remove it from the model. Then and only then will we separate it and clean up all the cement and try the case with the soft tissue back in place. Does this protocol make a big difference? Big is a relative word but it allows me to sleep a bit better knowing I am thinking past just slapping some cement in and throwing it in a box. It also takes one more variable out of the equation for me as well.
The new materials are awesome, they can offer our clients really amazing results when used correctly but we have to be able to track out failures better and to understand WHY things fail. Its rarely the material that causes our issue but the way its being used that is the problem.
