Multiple Unparallel Implants: Making Impression a Challenge to Meet Out

Abstract

There are several implant impression techniques described in literature to transfer the working conditions of mouth to laboratory, as accurate impressions of implant analogue or abutment analogue, and the surrounding hard and soft tissues are paramount for fabricating aesthetic and biologically functional restorations. This article critically outlines and described the clinical efficacy of open impression tray technique in multiple or unparallel implants placed in partially and completely edentulous patients.

Keywords:Abutment, Custom tray, Implant analogue, Impression Technique, open-tray, Pick Up , Transfer Coping.

INTRODUCTION

Impression making in implant dentistry is to accurately record the spatial implant position, required to obtain a proper support to definitive restoration with passive fitting. This is influenced by use of an impression transfer coping which is attached to the implant or implant abutment. With appropriate technique selection and manipulation, accurate impressions can be obtained by the contemporary restorative dentist for fabrication of implant-supported restorations. When compared, Natural teeth have a PDL to compensate for minor inaccuracies while implants have not. So it is more critical to record 3- dimensional position of the implants. The benefits of implant impression technique in dentistry is that it accurately record the implant Position, Depth, Axis/Angulation, Rotation-Hex position ,and the Soft Tissue Contour that is the Emergence Profile. An inaccurate impression might result in an ill-fitting prosthesis, which may lead to mechanical and/or biological complications. Screw loosening and fracture, implant fracture, and occlusal instability have been reported as mechanical complications occurring due to prosthesis misfit.1-4 Biologically, marginal discrepancy from misfit might cause unfavourable soft- and/or hard-tissue responses arising from increased plaque accumulation.5-6 Even though it is not possible to obtain absolute passive fit,7 minimizing the misfit to prevent possible complications is a generally accepted goal of prosthodontic implant procedures. The most common techniques used to record the spatial implant position are open and closed tray impression technique. Closed tray impression technique is designed to transfer the soft tissue profile as well as the implant,s position and hex orientation; transfers copings remain attached to the implants when the closed-tray impression is removed from the mouth. The transfer copings used with this technique are parallel sided or slightly tapered for ease of removal of impression. The transfer copings are then retrieved from the implant, mated to the corresponding Implant Analog, and placed into its corresponding impression hole. To fabricate a working cast containing a replica of the implant in the patient,s mouth, the impression is poured in dental stone. Advantage of closed tray technique is that it does not require custom trays, and can be used in limited mouth opening cases, were there may not be sufficient space for access to the screws retaining pick up/open tray type impression. But the disadvantage is the lack of predictability with the possibility of coping dislodgement during impression making. Additionally abutments have to be fitted onto the copings and errors maybe introduced at this stage. Further soft tissue anatomy transfer is not very accurate and the size and shape of the abutment cannot be modified. Sometimes it is very difficult to remove the impressions from the mouth of the patient. Even the multiple implants bring along with them the problem of non parallelism and the use of the indirect/closed tray technique has shown to cause errors in the fitting of the framework. Therefore the aim of this article is to describe the clinical efficacy of Implant-level open-tray, direct, pick-up impression technique in multiple or unparallel implants placed in completely and partially edentulous patient.

Open tray impression Procedures for edentulous patients

This technique is designed to transfer the soft tissue profile as well as the implant,s position and hex orientation. In edentulous subjects, an open tray technique with functional impression should be considered due to the difference in resiliency between implant and mucosa.8-9 and the major advantage of this technique is that it provides an accurate relation of the implant components and the supporting tissues.9

1. Preliminary impression of the edentulous maxillary arch was made with healing abutments in place after healing from second-stage surgery.(Figure:1)
2. Individualized maxillary custom acrylic resin tray was fabricated on the resultant cast, leaving an opening in the areas of the implants. The edentulous portion of the tray needs to be prepared without holes for pressure application during the functional impression. (Figure:2)
3. Healing abutment was removed. (Figure:3)
4. Border molding of maxillary arch was done so as to obtain the functional impression. (Figure:4)
5. The open tray transfers coping were screwed onto the implant bodies, and were secured in place by help of floss and pattern resin to ensure that the transfers copings are held rigidly within the tray (Figure:5)
6. A tray was placed to check that all the open tray copings project through the opening and is not in contact with the tray. Vinyl polysiloxane polymer adhesive was painted inside and on the border of the tray and Functional impression of the alveolar mucosa with Combination of putty and Medium body addition silicon material was made. (Figure:6)
7. The central transfer screw was removed before the impression was released from the mouth.
8. The Implant analog is connected to the transfer coping embedded within the impression, the impression is poured in dental stone to fabricate a working cast containing a replica of the implant in the patient,s mouth. (Figure:7)

Open tray impression Procedures for partially edentulous patients

1. Primary impression of partially edentulous maxillary arch was taken with healing abutments in place after healing from second-stage surgery and cast was made.
2. Healing abutment was removed. (Figure:8)
3. Open tray transfer coping were attached and secured with floss and pattern resin. (Figure:9)
4. Prepared maxillary custom tray with opening over the implant area was placed to check the adaptation. (Figure: 10)
5. Light body impression material was injected around the transfer copings and tray with putty materials was used to take full arch impression. Holes of open tray transfer coping that protrude out of tray should be visible for retrieval of impression tray from mouth. (Figure: 11)
6. Implant analog was attached to transfer copings and send to laboratory for fabrication cast. (Figure: 12)

Fig. 1: Primary impressions of maxillary arch.	Fig. 2: open Impression tray fabricated for completely edentulous maxillary arch
Fig. 3: Healing abutment removed	Fig. 4: Border moulding done for maxillary arch
Fig. 5: Open tray transfers copings were attached over implants and secured with pattern resin.

Fig. 6: Combination of putty and Medium body addition silicon are used for secondary impressions made with open tray transfer copings.	Fig. 7: Master cast poured for further laboratory steps

Fig. 8: Healing abutment removed for final impression	Fig. 9: Open tray transfer coping attached and secured with pattern resin.	Fig. 10: Custom tray with opening over the implant area placed to check the adaptation of tray.
Fig. 11: Full arch impression was made and retrieved of through the holes of open tray transfer coping that protrude out of tray		Fig. 12: Implant analog was attached to the transfer copings and was send to laboratory.

DISCUSSION

Inaccurate impression of multiple implants can severely compromise the final prosthesis. The selection of the impression technique, material and henceforth the tray also influence clinical accuracy of the impression. Wee at el10 also stated that when using pickup/open tray technique, the impression material used should be rigid to hold the transfer copings and the torque required to rotate the copings in impression should be higher. Therefore the recommended impression material for open or close tray technique is polyether.11 Nevertheless, the use of the polyethers in a partially edentulous arch could lead to an increased difficulty for an intraoral removal of the impression.12 The addition silicons, because of the more favorable, lower modulus of elasticity, could be considered as a viable alternative allowing for the easy removal of the impression and reducing the permanent deformations caused by the stress between the impression material and the copings,13 particularly when nonparallel implants are present and implants with deep internal connection are used. Multiple implants bring along with them the problem of unparallelism and the use of the indirect or close tray technique has shown to cause errors in the fitting of the framework. Bone loss and even loss of integration has been attributed to this misfit.14 To increase the accuracy of the impression technique for highly unparallel implants, an open tray technique with splinted impression copings is highly recommended as it was found to be accurate then the other techniques.

CONCLUSION

This technique allows direct access to the screws that hold the transfer copings correctly positioned against each implant. The principal advantage of this technique is that the transfer coping remains in the impression and the inaccuracies of transferring are avoided. When multiple implants are present and there is a non-parallel configuration this technique is ideal as one the impression is easily retrievable and secondly there is minimum distortion of impression material. On other hand the disadvantages are that there is blind fastening of analog,15 even this technique required the custom tray with access to the impression coping screws or a metal tray with windows which increases the time

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