Sir, the BDJ has played a significant role introducing the concept of evidence-based dentistry. It is disappointing therefore to read so little scientific evidence supporting the processes and materials recommended in Impressions in implant dentistry (BDJ 2011; 211: 361–367). I have to say at the outset that 'Dispatching the impression(s) in a sealed box, rather than a polythene bag will ensure that they are not damaged and delicate components are not dislodged' does not engender much confidence where any science is concerned!

Nowhere in the article do we find any scientific evidence in support of the chosen materials and methods used for taking accurate impressions and manufacturing a precise replica or 'working model'.

Historically, the choice of dental impression and model materials in practice is often made on an empirical basis and as the article acknowledges, the replication of the geometry of the clinical landmarks is pretty critical. Any errors in location of the implant supported frameworks may result in loosening of the implants themselves. It is incredibly difficult choosing the right combination of materials to use in conjunction with a customised impression matrix to capture reliable data for use in the manufacturing of precisely fitting dental prostheses. The dental impression is only the beginning of a complex set of operations that must be the subject of an overall manufacturing process control (CADCAM) designed to create and harvest validated clinical data.

A significant amount of research has been carried out at Renishaw Plc into the veracity of impression and replicating materials where we have been able to compare the replicated models with the clinical geometry using Mitutoyo Coordinate Measuring Machinery (CMM) and Incise Contact Scanners calibrated to ISO 10360 Pt IV. A property that is not mentioned in the article, but which we found to be of very great significance, is the viscosity of the impression materials themselves and the design of the impression trays, which should be perforated. The best results were consistently within an error budget of 25 microns and this research has been reported in a Monograph.1

Now that we have the computerised tools to engineer dental prostheses and mill implant frameworks to fit within precise metrology limits, it would be beneficial for BDJ readers to see the scientific evidence in support of methods prescribed for taking accurate impressions and creating precise replica models for use in dentistry.