The Settling Effect

Jeffrey Caso

12 years ago

Remember back to the last time you had a tire replaced on your car. The mechanic used a torque wrench to tighten down the lug nuts to the proper specifications. Then he gave you specific instructions to return after driving 25 miles to have the lug nuts re-torqued. Did you ever wonder why? Well, there is a real reason behind this and it is called “The Settling Effect.”

As CEREC dentists we have honed our skills to such a high level that the placement visit has been shortened considerably. I find that when placing my favorite restoration (the screw-retained e.max crown), I have minimal adjustments and minimal chair time. Well, that might be something that needs to be reconsidered. In an article published in The Journal of Oral Implantology by Sheldon Winkler, D.D.S. et al. The Settling Effect is discussed and offers a compelling reason for implant screws loosening. It is something we all need to know about and take steps to counter.

So, we all know that we need to torque our implant screws to the manufacturer’s recommendation. That is called pre-loading the screw. Torquing actually elongates the screw and pulls the abutment and implant together, and essentially acts like a clamp. Fighting this pre-load is occlusal forces or joint separating forces. So we have this fight going on between the implant and abutment at all times. The pre-load generally wins this battle and the restoration is a success.

There is a phenomenon which can tip the tides against us. As finely machined as implants and their components are, on a microscopic level there is a roughness. As the components mate and the screw elongates to pull them together. This roughness prevents full seating of the abutment to the implant on a microscopic level. Soon after torquing of the screw, the rough areas can collapse, making the fit between the implant and the abutment more intimate. This is a huge concept to understand. If the implant and abutment slide together more over time, the torqued screw is no longer torqued to the proper specifications. This article found that the torquing or pre-load can be decreased by 2 percent to 10 percent. This is The Settling Effect.

Add to that the other main reasons for implant screw loosening like excessive or misplaced occlusal forces or inadequate design of the restoration, and you are providing a force which will further increase the micro-seating of the abutment and thus a further loss of the pre-load. Now you have a screw which is functioning without the proper torque, and it loosens. Leave that screw in function for a while before the patient is aware of it and you can have a failure of the screw itself. I will also add my own thought, and that is while e.max is a wonderful material, it is unforgiving to the bite and marks poorly with articulating paper. This can further affect the preload and lead to a loose or failed screw.

Well, what is the answer? It's really simple. The article found that much of the pre-load is lost in the first 10 minutes after the initial torquing. So, just wait 10 minutes and re-torque the screw. It really is that easy. Also, check and recheck your occlusion and relieve any interferences. Design your restoration with the proper anatomy and do not over-build the occlusal table in a buccal lingual direction. That should solve the majority of loose screw problems.

This phenomenon proved itself to me on a larger scale a few years ago. I had a tire replaced before a vacation and the mechanic told me to come back after driving 25 miles to re-torque the lug nuts. I told him that I was traveling and that wasn't possible. He said that I must retighten the lugs, and was insistent. I stopped after about 75 miles and grabbed my lug wrench. I was amazed. Every lug nut was loose and not just a little. The Settling Effect is real; it's dangerous on a large scale and inconvenient on a small scale. Slow down, do a hygiene check, talk more or grab a coffee, but re-torque that screw in 10 minutes.

The article can be read here.