Introduction and Principles

Because implants are connected directly to bone, dentists placing implants do not enjoy the tolerances allowed by movement within the periodontal membrane. Discrepancies in fit between large superstructures and the supporting implants are thought to create undesirable stresses in the implant-bone interface, and soft-tissue irritation and inflammation at the abutment-restoration interface. Furthermore superstructures which are not seated correctly are more likely to become de-cemented, or suffer screw failure. Therefore impressions used to transfer information of implant and/or abutment positions to the laboratory must be intensely accurate.

Different methods available for transfer of implant/abutment position into the laboratory

Table 1 shows the main options available to the clinician for recording and transferring implant/abutment position to the laboratory.

If impressions are taken at first stage surgery, then the technician will choose the appropriate abutments, and construct a provisional restoration to fit. If the clinician chooses the abutments from stock, and fits and prepares them, he will have to adapt a provisional restoration to the abutments.

Splinting of copings is done by attaching acrylic pattern resin to the copings, and linking them thus.

Advantages and disadvantages presented by different techniques

The advantages of fitting abutments, and then taking conventional crown and bridge impressions in a customised tray are;-

1. Evidence is available to suggest that reducing the number of connections and disconnections from the implant head reduces the likelihood of bone loss and soft tissue recession around the head of the implant.
2. Binon1 observed in a study of 13 implant systems that manufacturing tolerances could lead to discrepancies of 100 microns in the fit of implant components, including impression copings and abutments. Taking impressions of fitted abutments removes this potential discrepancy.

The advantages of open trays over closed trays are:-

1. The clinician can be sure that the impression coping is correctly seated onto the implant head whilst the impression is taken, and
2. The impression copings do not have to be re-seated in the impression. Assif et al.2 in a carefully thought out in vitro study took 60 impressions of a model using different techniques but the same materials, and demonstrated that replacing impression copings into impressions led to unacceptable inaccuracies. Daoudi et al.3 in another well developed in vitro study, albeit for single tooth implants, demonstrated that the use of re-positioning and unsplinted pick-up impression techniques at implant level can be unpredictable, whereas connecting the impression coping to an open tray with duralay significantly increased the accuracy of the resultant casts. A further study by Vigolo et al. in 2004, of multiple implant impressions, backs up Daoudi’s findings.

Both these studies could be criticised for not being in vivo studies, but it is difficult to devise a realistic reproducible in vivo study capable of delivering meaningful results. Neither study draws attention to machining tolerances. One study that is out of step with most others is a very recent in vitro study by Conrad et al.4 in which thirty five closed tray impressions and the same number of closed tray impressions were taken of seven different models. This study suggests that there is no difference in average distortion between impressions of angled implants whether a closed tray or open tray system is used. However the study does not reveal which implant/impression system was used, and the large number of variations may have hidden any meaningful evaluation of results achieved.

Table 2 is taken from a 1991 study by Carr and compares the advantages and disadvantages of the closed (indirect) and open (direct) tray techniques

The advantage of splinting impression copings (providing the system allows removal of splinted non-parallel impression copings) is increased accuracy in the resulting casts as demonstrated by Cabral and Guedes5 in another very recent in vitro study involving 60 impressions taken using four different techniques. Sectioning and re-welding the splints to reduce inaccuracies induced by the setting of the splinting material did not discernibly increase accuracy of the resultant cast. Other studies including some of those already quoted also demonstrate the increased accuracy associated with splinting impression copings. One very recent study suggests that splinting impression copings produces a less accurate impression because of the greater displacement of impression material around the splinting material, but delivers a more accurate cast!

Accuracy of transfer using different techniques

As has been already demonstrated:-

1. Impressions of fitted abutments remove the inaccuracies associated with machining tolerances of implant components.
2. Open custom-made trays produce more accurate impressions than closed trays, or non-customised trays.
3. Splinted impression copings produce more accurate castings than non-splinted impression copings. (Furthermore they are more resistant to torque-induced inaccuracies when attaching the implant analogues in indexed systems, especially if they have square rather than round heads).
4. Copings that remain in the impression are more likely to lead to accurate casts than copings that need to be repositioned within the cast.

Clinical Application of these findings

To construct accurate superstructures supported by multiple implants, a clinician is more likely to be successful if they fit and prepare abutments, and then transfer this information to the laboratory by means of conventional bridge impressions within a customised tray. (This assumes that the implant system has a range of abutments that allows the margin of the superstructure to be placed just sub-gingivally). If clinical reasons make this technique impracticable, than an open customised tray with securely attached and splinted impression copings, is likely to produce the next most accurate castings. A non-indexed system which allows removal of the impression copings from differently angled implants is likely to give the most accurate castings from an implant-head impression.

Conclusion

There are a large number of techniques available for transferring information regarding implant/abutment position to the laboratory, but clinicians need to be aware of the relative inherent inaccuracies in each technique.

References:

1. Binon P. Evaluation of Machining Accuracy and Consistency of Selected Implants, Standard Abutments, and Laboratory Analogs. Int J Prosthodont 1995;8:162-178.
2. Assif D, Fenton A, Zarb G, Schmitt A. Comparative Accuracy of Implant Impression Procedures. Int J Periodont Rest Dent 1992;12:113-121.
3. Daoudi M, Setchell D, Searson L. An Evaluation of Three Implant Level Impression Techniques for Single Tooth Implant. Eur J Prosthodont, Rest, Dent,. Vol 12, No.1, 9-14.
4. Conrad H, Pesun I, DeLong R, Hodges J. Accuracy of Two Impression Techniques with Angulated Implants. J Prosthet Dent. 2007 Jun;97(6):349-56.
5. Cabral L, Guedes C. Comparative analysis of 4 impression techniques for implants. Implant Dent.2007 Jun;16(2):187-94.