Impressions are used to transfer the three dimensional spatial relationship of implant(s) to the laboratory so that a prosthesis may be constructed.
The impression technique should confer to the technician the position of the implant, its angle and orientation and relate the relationship of the implant to other relevant structures such as bone, teeth, other implants and soft tissues.
It is important that the impression should accurately record the head of the implant or abutment in a way that allows for precise duplication. The choice of the material used impacts on the accuracy with recommendation being given to medium consistency polyether and high consistency addition silicones. (1)
The impression record should be accurate, dimensionally stable, resist rotational force and reproducible (1).
It has been demonstrated in a multiple implant impression situation, that the transfer copings when splinted with an acrylic resin were more accurate than when acrylic was not used and this reinforced when non smooth, angular impression transfer copings were used. (2,5).
Transfer of implant position may be achieved by taking an impression of the head of the implant using;
1. Screw retained impression coping technique, either open tray or closed tray. The open tray impression technique requires the insertion of an impression post into the head of the implant. This post is then secured with a pin. The impression is taken with the securing pin passing through a prepared hole in the impression tray and when set; the securing pin is removed allowing the impression to be withdrawn, the impression post being securely retained in the set impression. The technician inserts a matched implant analogue onto the impression coping and the model is cast.
Advantage: Simple technique. Dentist does not need to hold a stock of abutments. Ideal in poor access situations or patients with strong gag reflex. The prosthetic (crown) should accurately fit the abutment as it has been made on the abutment present.
Disadvantage: Increased laboratory cost, laboratory decide on position of margins. Implant replicas are not identical to the implant and so micro positional error can occur which may be significant when planning splinted cases as the abutment position and orientation may differ slightly when comparing the model to the intra oral position. Splinted cases may require sectioning and re-soldering in the metalwork stage to ensure the “Sheffield test” criteria are met.
A closed tray impression post involves the placement of an impression post as a single unit being inserted into the head of the implant and an impression taken with a special tray. The impression is removed and the closed tray impression post removed from the implant, fitted with a matched implant replica and re-positioned into the impression before casting.
Advantages are as with the open tray technique. Easier to use further back in the mouth where it may not be possible to remove the securing screw from an open tray impression post.
Disadvantage. Implant replicas are not as accurate as the placed implant. Also difficulty may noted in re-fixing the impression post into the impression and this can result in the restoration produced being high in the occlusion and create significant discrepancies with regard to rotational inaccuracies(3) The possibility of non identical replication of position is a concern, particularly in multiple unit placement.
Some operators have developed a technique of using the implant carrier as an effective closed tray impression technique.
In a bounded saddle situation it is possible to transfer the information required by using pattern resin to locate the position of the implant to adjacent teeth.
The technique involves, prior to placement, taking an accurate planning cast. At placement, pattern resin is used to connect the carrier to adjacent teeth. No impression at operation is taken. The carrier is then removed and fitted with an implant analogue and the pre-operative model modified to allow placement of the analogue in the correct position once the pattern resin matrix is fitted over the adjacent teeth.
Advantage: Cross infection control issues are minimized, quick, simple, and inexpensive. Disadvantage; manipulation of soft tissues is compromised as it is difficult to assess the soft tissue contour; implant replica may not be accurate.
“Clip in” or “snap over” closed tray impression caps are available from some implant manufacturers. These manufacturers have produced plastic caps that “push fit” onto the head of the implant and or implant abutment. A standard silicone impression is taken and an implant abutment replica fitted into the impression cap along with implant replica prior to casting.
Advantage; quick and easy.
Disadvantage; inaccuracy of fit due to multiple transfer errors. (impression, plastic cap, inaccuracy of replica). In addition machined tolerances between implant components range from 20-1001.tm, and that second generation components have a lower tolerance than first generation components. (4)
The above techniques may be applied at first stage surgery. i.e. at time of implant placement, or after an appropriate healing period. If these techniques are applied at first stage surgery consideration must be given to the issues surrounding infection control.
Information may also be transferred to the laboratory once an abutment has been placed and this may be achieved by;
Securing the abutment in place so that it may be regarded as in the final position and conventional preparation of the abutment may be made followed by conventional crown and bridge impression technique. This technique reduces the errors associated with component and replica tolerances but can limit operator in management of the soft tissues.
Securing an abutment into position and taking an impression. Once the impression has set the abutments may be removed and implant analogues fixed to the abutments before the abutments are re-inserted into the impression and cast. Errors can occur when using a replica analogue (4) and repositioning the abutment into the impression i.e. acting as a closed tray impression component. In addition, precise placement of the implant abutment into the exact position it was in at the time the impression was taken is unpredictable even when using an indexed system.
Comparison of impression materials for direct multi-implant impressions. Alvin G Wee BDS MS. Journal Of Prosthetic Dentistry 2000; 83:323-31
Comparative Accuracy of Implant Impression Procedures David Assif DMD, Aaron Fenton DDS, MS, FRCD
George Zarb, C Chd, DDS,MS,MS, Adrianne Schmitt, MSc DDDS. International Journal Periodontal & Restorative Dentistry 1992; 12:113-121
A laboratory investigation of the accuracy of the repositioning impression coping technique at the implant level for single-tooth implants. Daoudi MF, Setchell DJ, Searson LJ European Journal of Prosthodontic & Restorative Dentistry. 2003 Mar;11(1):23-8.
Tolerance Measurements of Various Implant Components Tsun Ma, DMD, MS, MDS Jack I Nicholls PhD Jeffery E Rubenstein DMD MS International Journal Oral & Maxillofacial Implants 1997; 12:371-275
An evaluation of impression techniques for multiple internal connection implant prostheses. Vigolo P, Fonzi F, Majzoub Z, Cordioli G. Journal of Prosthetic Dentistry. 2004 Nov;92(5):470-6