Following placement and successfully achieving osseointegration, the next most important step in the process is the accurate transfer of the implant position to the laboratory. There are a number of methods by which the precise location of the implant can be established in a laboratory model. Impressions can be taken at the abutment-level or at the implant-level using two basic techniques. Firstly, a repositioning technique in which a tapered or parallel sided coping is replaced in the impression material ready for model making. Secondly using a pick-up technique the transfer coping becomes an integral part of the impression or impres sion tray upon removal from the mouth, after which the implant or abutment analogs are connected without repositioning within the model.
Generally the greatest attention in the literature has been focussed on impression tech nique and accuracy for multiple implants, whilst single-tooth restorations have been largely ignored. It was therefore the purpose of this study to investigate the accuracy of pickup impressions taken at the abutment level and the repositioning method at the implant level using two different elastomeric materials
Materials and Methods From a master model of a single central incisor implant, 40 impressions were made and stone casts (Silky Rock, Whip Mix) were poured incorporating laboratory implant or abutment analogs. These were produced from either a repositioned impression coping technique taken at the implant-level using the Nobel Biocare tapered implant impression coping (DCA 448) with Impregum (a polyether) or President (a polyvinyl siloxane) impression materials; or a pickup impression technique taken at the abutment level using a CeraOne abutment (SDCA 334) and a plastic impression coping (DCB 119). Again both impression materials were used. New components were used for each test and each model was completed by securing a CeraOne abutment to the implant analog.
Working cast variability was recorded using a Reflex microscope to record the position of optical targets incorporated into a coping that could be attached to each abutment and cross-referenced with similar targets incorporated within a master reference plane taken from the occlusal surface of the master model.
Results The results showed that the repositioning technique produced greater variation in analog position than did the pickup technique. No significant differences were found for either type of impression material although the polyether Impregum did show slightly greater accuracy. Rotational changes in implant position were the most common variations observed and to a lesser extent axial deviations, however the patterns of inaccuracy were subject to considerable scatter.
In the repositioning technique rotational changes accounted for inaccuracies ranging from 0 – 25º compared to 0 – 4º for the pickup technique. Changes of inclination accounted for inaccuracies ranging from 0 – 10º compared to 0 – 4º for the pickup technique.
Discussion and Conclusions Positional errors for a single tooth restorative system such as the CeraOne will not affect the fit of the crown to the abutment, however rotational or inclination errors will affect occlusion, contact points and aesthetics. Whilst the problem was more obvious for the repositioning technique significant errors were also possible with the pickup technique. With the growing popularity of implant-level impression and repositioning techniques due to the ability of the technician to select and adapt the abutments, the authors advise particular caution as at this level significant errors are a clinical possibility.
One interesting finding for all techniques was that the analogs were at a slightly increased distance from the occlusal plane which would result in restorations that are in supraocclu sion when delivered to the oral cavity.
UK Vol. 14 No. 2 pp 152-158 A laboratory investigation of the accuracy of two impression techniques for single-tooth implants Int. Journal of Prosthodontics