How does it work?

Sir, I was pleased to see the excellent article by C. D. Lynch and P. F. Allen on a contemporary impression technique (BDJ 2006; 200: 258–261). This technique is very useful in practice and certainly works;¹ the question is HOW does it work? The laws of hydraulics state that the liquid that is the impression material (yes, it is a liquid, albeit one in which the viscosity is changing) can only exert one pressure. So if you take a wash impression in the adapted tray, it is the same pressure over the close fitting area as over the spaced area. So why does the technique work? It is my belief that the difference in displacement in these two areas comes about because the set heavy bodied impression material touches the residual ridge and displaces it; the light bodied wash then records the distorted position. Thus this technique may produce a highly mucodisplacive impression over the heavy bodied silicone and relatively mucostatic impression over the flabby ridge. This contrasts with the Watson² technique which may create a true differential pressure impression at lower overall pressures. Clearly this is an area where more research is required. I believe that in specialist practice the Watson technique may still have a place. I agree with the authors that the technique they describe is very useful in general dental practice and I applaud their presentation of the technique in this paper.