Think about the rest seats of a tooth as one of these chinese wok pans. Now imagine a small metal ball which you are trying to stabilize on that pan. If you flip the pan (so it will be present a convexity) and put the ball on its top it will have the tendency to fall down ('coz you know physics). I am not saying it is impossible to stabilize it on , it is just very difficult. Now put the pan in normal fashion (so it goes back to concavity again) and let the ball just drop. It will immediatelly find the lowest spot ('coz you know physics) and stay comfortably there. Whit that said, I have done both, I regret doing a partial without the rest seat and ended up redoing it. Your choice.
Alginates: Lets imagine the scenario of uncontrolled bleeding, tons of saliva, all the things that make the a nightmare case for a PVS impression and lets imagine that you do not have the polyether available with its alleged hydrophilic properties. What would you do? What did our old professors use to take an impression for a golden crown back in the days? They used plaster or wax based materials or the combination of both. This means paris stone, variety of waxes and eventually alginates . Don't believe me? Check it. We are being spoiled by all the sales reps and big companies. Do they give us a supreme product? Hell, yeah. Can we work with an outdated product? if you have survived dental school, you know that creativity matters. If you are not happy with your PVS impression and you think the moisture control is the issue, take an alginate back up. Here comes the skinny though, there are 2 reasons alginates are not the preffered impression materials anymore. 1. Not accurate enough (I tend to disagree with this). 2. Greater dimensional changes.
The way to fight #2 is to know how great exactly and if you know this you need to find a plaster/stone in your office that will offset this change.
Cheers!
