The importance of passivity of prosthetic fit in implant restorations has been well documented.1-4 The lack of passivity can result in prosthetic complications with increased stresses in the internal components like abutment screw-joint connection and alongside the bone-implant interface, resulting in technical complications like screw- loosening, screw fracture or a disharmonious occlusion.5,8 The likelihood of a misfit can happen at various stages of prosthetic rehabilitation: from impression making procedures to fabrication of framework and final prosthesis.5 Apart from impression accuracies resulting from technique or materials, the clinician’s application of torque while

connecting the implant analogue to the impression coping can result in distortion: too much can rotate the copings and too little can loosen the copings while pouring the cast.6 It can be noted that implant components have an inherent freedom of fit for upto 100 microns.7

Splinted Vs Non- Splinted Impressions: The use of splinted and non-splinted impression techniques have often been debated, with a systemic review favouring splinted techniques for multi-implant impressions.8

Parallel vs Angulated Implants Impressions: An in vitro study by De La Cruz JE and colleagues showed that verification jigs were not superior to standard procedures when using parallel implants. However, in case of mis-aligned implants and presence of tissue undercuts verification jigs might help improve overall accuracy.9 Filho HG and colleagues, however, reaffirmed accuracy while splinting implants in both straight and angulated implants

with a prefabricated acrylic bar when compared to non-splinted copings.10

Materials for Splinting: Autopolymerizing acrylic resin 6,10,11, flowable composite 6,11, bite registration material11 (bite registration-polyether more accurate than bite registration-addition silicone)12. However, an in vitro study by Del Acqua MA and colleagues found metal splinted impressions to be more accurate than acrylic splinted.13 Worn burs can be modified with flattened edges to be used as metallic splints.6

Acrylic resin splints: Require a scaffold made of floss or any other material. Can be used either directly with intra-oral splinting using Nealon14 or bead brush technique. After polymerisation, sectioning and rejoining of segments must be done prior impression making in order to minimise polymerisation shrinkage.10,11 However, an implant verification jig in the form of a prefabricated acrylic bar (lab fabricated) has been found to be more accurate when compared to sectioned and re-splinted acrylic.10

The use of an additional implant verification cast can aid in checking passivity of framework fit prior to scheduling the patient, with the steps of fabrication described by Simon and Marchack. The passivity can be verified using the Sheffield Fit Test (Part 1- identifying a misfit and Part 2- determining whether it is clinically acceptable)

Clinical Tips

# It is better to splint impression copings while making multiple implant impressions.

# When using acrylic resin (pattern) in the direct technique, section and secondary splinting is necessary to minimise polymerisation shrinkage. Wait for complete polymerisation.

# Controlled torquing while connecting the analogue with the impression coping is essential to avoid distortion.

# Polyether and polyvinyl siloxane impression materials are suited for implant impressions.

# Always follow manufacturer’s recommendations for impression/splinting material selected.

# Use a lab fabricated verification jig for accuracy and convenience.

#  Always verify the passivity of the implant framework.