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Authors: Dr. Rathika Rai, Dr. Surojit Dutta, Dr. Vijay Anand
Abstract:

In few decades, implant dentistry has enormous success. Passive fitting prosthesis is one of the important reasons behind it. To achieve a passive fit, an accurate impression is mandatory. Implant impression needs specific components. These are called as impression copings. Various types and designs of impression copings are available in today’s field. This article describes the available impression copings in implant dentistry. Key words: Impression coping, open tray copings, close tray copings, tissue level impression copings, bone level impression copings.

Introduction

Dental implant, a sign of engineering excellence1, 2 designed to meet the desired dental excellence. Technically dental implants have locking means (externally or internally) which get interlock with corresponding locking means in the components fitted to them. Such mechanism prevents rotation of a component relative to the implant around the longitudinal axis of the implant. An implant also has a machined interface to match a corresponding surface to the superstructures like abutment or other components. This gives a tight seal preventing ingress of microorganisms like bacteria and ensures correct spatial orientation and alignment for prosthesis. Once the implant has become osseointegrated or achieves stable anchorage with the host bone, it becomes necessary to preserve in the impression the information describing the orientation of its non-rotational connection and machined interface. Recording the correct orientation is critical if an accurate master model of the patient's case is to be created in the dental laboratory. The component used to affect this information transfer is commonly called an “impression coping”.

Impression coping is well defined in Glossary of prosthodontic terms as that component of a dental implant system used to provide a spatial relationship of an endosteal dental implant to the alveolar ridge and adjacent dentition or other structures (Fig . 1). Impression copings can be retained in the impression or may require a transfer (termed an impression transfer procedure) from intraoral usage to the impression after attaching the analogue or replica. An accurate impression is an essential prerequisite for a successful dental implant treatment. Inaccuracy to record will result in a prosthetic misfit leading to unavoidable failure like screw loosening, fracture of implant restorations, or adverse loading to implants. In few decades, implant dentistry has evolved to extensive heights, promising a healthy and successful mode of treatment for partially or completely edentulous patients. Various components have been introduced to this field to overcome different clinical complications. Impression copings also exist in various forms and need to be addressed for better understanding and use.

Structural description and its importance..

Impression copings were invented to with a sole purpose of transferring the inclination and position of a dental implant fixture or abutment to a working model. It has a base portion which provides rotational locking to a fixture head or an abutment and an elongated upper part projecting from said base portion to provide for retention in a surrounding impression mould. The base portion comprises a downwardly projecting, unthreaded guide pin adapted for insertion into the internal bore in the fixture head or abutment during installation of the impression coping, thereby facilitating the centring and positioning of the coping during placement. (Fig . 2)

Basically two types exist. They are pick-up and transfer impression copings. If the coping used is a transfer type impression coping, it will remain in the patient's mouth when the impression material is removed along with the tray. This technique is known in the art as the “closed tray method” and requires the implant analogue or fixture replica to be mounted on the impression coping which is subsequently reinserted into the same socket before making the master cast.(Fig. 3) If the coping used is a pick-up type impression coping, it will remain within the impression when the material is removed from the patient's mouth. This technique is known as the “open tray method” and requires that the implant analogue is mounted on the impression coping, while the coping is still being positioned in the impression material, before making the master cast. (Fig. 4) The structural geometry changes accordingly. For pick up type, it needs features to engage the impression material with anti-rotating features. Such types are square impression copings with square shaped structures on the guiding pins. On the other hand, transfer type does not require such features as impression has to without distorting the impression while removing. This makes it tapered or conical for easy removal and placing back to the impression socket.

The geometrical fit has to be verified radio graphically before the impression procedure. A metal to metal contact is expected an ideal geometrical fit among the two components. Any misfit will be shown as a gap between the two margins of the metals i.e. between the collar and implant coping. An improper fit will result in faulty registration resulting in prosthetic complications like stress on the abutment implant interface, an misfit between abutment and fixture, screw loosening or even a failure in implant osseointegration in long term basis. The conventional method to record such fit is to use a chair side peri-apical radiograph. Studies have been done on this geometrical interrelationship using high resolution X-ray beams like Micro CTs and have found the existence of acceptable machine tolerance.3 The role of this machine tolerance is not significant in creating error during impression making.

Types of impression coping.

Impression coping can be broadly categorised as:

  • Abutment level impression
  • Implant level impression.

It can be also categorized based on the level of application as follows:

Abutment level impression:

Two systems exists as multi-unit abutment system where on the fixture a additional transmucosal extension is screwed and over that prosthetic abutment is attached whereas in conventional system, there is direct connection of the prosthetic abutment to the implant fixture.

a) Multi unit abutment level impression copings:

A multi-unit abutment system demands different type of impression coping as the impression has to make at abutment level. (Fig. 6) It is to know the three dimensional orientation of the abutment in the jaw bone.

i) Direct Pick up or open tray impression coping : It is named direct as it can be removed directly from the patient’s mouth along with the impression and can be tightened with a multi-unit analogue without disturbing its position in impression. In this, the base of the impression coping fits to the abutment and there exist a long, smooth guiding pin with an access hole for the screw. It is placed over the abutment and a desired tray is selected for making impression. A window is created on the specific site of interest and impression is made. The long guiding pin extends outside of the tray and can be removed externally by unscrewing it from top.(fig 7)

ii) Indirect transfer or close tray impression coping: It is named indirect as it remains in the abutment and the impression alone is removed without the coping. Then it is removed from the abutment and then attached to the multi-unit analogue. It lacks the long guiding pin and usually tapered in shape. It lacks the retentive groove that can hinder while removing from the patient’s mouth. ( Fig. 8)

 
b) Conventional abutments level impression: Accurate placement of abutment back to the impression coping is a challenging procedure and may lead to undesirable accuracies. To enhance its accuracy and ease of transfer, researchers have developed another two types of copings known as snap top copings and abutment with a extender.

 

i) Snap top copings: These are nylon based cap which comes along with the abutments. These nylon caps have a snug fit and manufactured according to the shape of the machined abutment. The complete fit is ensured with a “click” sound when it is placed on the abutment. While making impression with this snap on technique, the abutment remains in the mouth but the snap top comes out along with the impression.( Fig. 9 )

ii) Abutment with an extender: An extender is an addition feature which is attached on the top of the abutment such that it can be used as a direct impression coping. The extender has an access hole on the top like that of a normal open tray impression coping Impression is made after tightening the abutment to the fixture and then can be removed as like an open tray coping. After placing in the master model, the extender is removed. Hence it acts like a normal prosthetic abutment. (Fig 10)

II) Implant level impression: It is made to record the orientation of the hex and the implant fixture angulation. It is made at the fixture level where the impression coping is directly attached to the fixture. It also consists of both type of impression copings i.e pick –up or open tray (fig 11) and transfer or closed tray(Fig 12) . The mechanism is the same as for multi -unit abutment level impression, but here the copings are screwed to the fixture directly.

Daoudi et al.4 Compared the closed tray technique at the implant level with the open tray technique at the abutment level for single tooth implants and found the open tray technique to be superior and more predictable. The closed tray technique had discrepancies in axial rotation and inclination of the analogs. Several authors have reported the superiority of the open tray technique.5-10 Carr 11 compared the open and closed tray techniques with a 5 implant mandibular cast where the interabutment divergence angles were all less than 15 degrees. The open tray technique was found to be superior as it provided the most accurate working cast.

Material aspect of impression copings: It is made by either of the following materials:
  • Titanium
  • Plastic
  • Anodized aluminium.
Surface modification of the impression copings :

Various surface modifications can be done to enhance the bonding of elastomeric impression materials to the coping surface. It can categorized into two :

  • Chemical method
  • Mechanical method

a) Chemical Method: tray adhesives are used in regular implant and fixed dental prosthesis as an aid to increase the retention between the impression material and tray. The elastomeric adhesive enhances the bonding of the impression to the metal coping.

b) Mechanical method: Mechanical method includes those procedures which can increase the surface contact area by additive method or by subtractive methods. Various methods like sand blasting, acid etched, plasma spraying and laser lok can be done to increase the bond strength of the coping to impression material. Such surface modifications reduce the chance of mechanical error, leading towards a more accurate impression.

Few systems like Straumann and noble biocare offers two another forms of impression copings namely:

  • Tissue level impression copings
  • Bone level impression copings
I)Tissue level impression copings: It also has two forms i.e. implant level and abutment level impression.

a) Implant level impression: Implants come with different neck diameter. It can be of three types i) narrow neck ii) medium neck and iii) wide neck. Different colour code of impression copings are used according to the colour code and neck dimension of the implant. For example noble biocare has pink for narrow neck, yellow for regular neck and blue for wide neck. In Straumann implant system narrow neck implant fixture level impressions can be carried out with a metal, silver coloured direct impression coping and are screwed to the implant or with a white nylon snap on impression coping to be captured in the impression material. Regular neck fixture level impressions can be conducted with traditional engaging metal, red coloured impression coping or a two component plastic impression coping to be captured in the impression material. The two part snap on impression coping consists of a white nylon basket that snaps over the shoulder of the implant and a red plastic positioning that slides inside the nylon basket and engages the internal octagon of the implant . Wide neck implant fixture level impressions can be executed with traditional engaging, metal direct impression copings or a two component plastic impression coping to be captured in the impression material. The two part impression coping consists of a white nylon basket that snaps over the shoulder of the implant and a white plastic positioning cylinder that slides inside the nylon basket and engages the internal octagon of the implant.( Fig 13 and 14 )

b)Abutment level impression: copings for regular neck implants are a two part set, consisting of a white nylon basket and a solid abutment height dependent colour coded positioning cylinder. Positioning cylinder colours correspond to the installed solid abutment colour. 4mm height abutments employ a yellow cylinder 5.5mm abutments require a grey cylinder and 7mm abutments utilize a blue cylinder.( Fig 15 )

A flat side inside the cylinders correlates to the flat side machined on to the solid abutment. The white nylon basket is slipped over the solid abutment, engaging the shoulder of the implant through the snap on feature and the positioning cylinder is pushed inside the basket over the abutment lining up the flat side of the abutment with the flat side of the coloured cylinder. Orientation of the flat side inside the cylinder is accomplished through a raised knob on top of the positioning cylinder. Wide neck implant (WN) solid abutment impressions follow the same technique as RN implants but a different colour scheme.

II )Bone level impression coping: In this Traditional fixture level impressions can be accomplished with platform specific direct and indirect, engaging impression copings. The impression coping bodies are not colour coded, only the fixation screw and in case of indirect impression copings a plastic cap are colour coded for identification.  Narrow connectcion (NC,yellow) direct impression copings consist of a metal coping body and a separate screw whereas the indirect impression coping comprises a coping body, the fixation screw and a disposable yellow plastic cap. This plastic cap is supposed to be captured in the impression material and is meant to stay in the impression at all times after impression taking. Two flat sides inside the plastic cap engage corresponding flat sides on the indirect impression coping body and the cap is pressed on to the coping body after the impression coping body is securely fastened on to the implant with the fixation screw. A double arrow indented on top of the plastic cap indicates parallelism with the flat sides inside the plastic cap to ease proper orientation of the cap on to the impression coping bodies flat sides. The regular neck has the same procedure except for the colour coding. (Fig 16)

Abutment level impressions are performed in a similar fashion as tissue level solid abutment impressions. Colour coded plastic impression copings are snapped over already installed abutments intended for this simplified Plastic impression copings are colour coded according the implant/abutment platform diameter, abutment height and emergence profile of the abutment.

An attached, secondary colour coping is also attached on the top of the plastic impression coping. For example ,black plastic piece on top of the plastic impression coping denotes a 4mm abutment height in compliance with a black band etched on to the actual abutment. A attached secondary white plastic piece on top of the plastic impression coping denotes a 5.5mm abutment height in compliance with a white band etched on to the actual abutment.( Fig. 17).

A problem that exists with current impression components is that the same components used for pick-up type impression coping cannot be used for transfer type impression coping and vice versa This forces manufacturers to produce two separate lines of impression coping components—one for pick-up type and one for transfer type. Moreover, clinicians are forced to maintain separate inventories of components to be used with either method. In addition several manufacturers have a set of 3 or more different screws heights where there is restricted access to the impression coping screw possibly because of a limitation in mouth opening for low-seated implants in order to get access to the impression coping screw.

A new impression coping is invented which comes with a detachable extender. The impression components of the present invention include the improved feature of only requiring one single sized coping screw which is used to attach the coping component to the implant. By achieving access through the impression material to the coping screw by a mounted extender or through a superstructure on the coping screw or on the impression coping the coping component may be used as a pick-up type impression coping. However, the same or an identical coping component and coping screw may be used as a transfer type impression coping if no extender or superstructure is used. Therefore, the present invention allows one single coping and one single coping screw to be used for either pick-up type or transfer type impression coping by achieving access to the coping screw through the impression material.

Conclusion:

Implant restoration is a multi- step procedure where making accurate impression plays a major role in the success of implant restoration. Impression copings were developed to fulfil the desired demands. This article describes the various types of available impression copings in the field of implant dentistry.

References:
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  • Daoudi MF, Setchell DJ, Searson LJ. An evaluation of three implant level impression techniques for single tooth implant. Eur J Prosthodont Restor Dent. 2004;12:9-14.
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  • Lorenzoni M, Pertl C, Penkner K, Polansky R, Sedaj B, Wegscheider WA. Comparison of the transfer precision of three different impression materials in combination with transfer caps for the Frialit-2 system. J Oral Rehabil. 2000; 27:629-38.
  • Hsu CC, Millstein PL, Stein RS. A comparative analysis of the accuracy of implant transfer techniques. J Prosthet Dent. 1993;69:588-93.
  • Liou AD, Nicholls JI, Yuodelis RA, Brudvik JS. Accuracy of replacing three tapered transfer impression copings in two elastomeric impression materials. Int J Prosthodont. 1993;6:377-83.
  • Carr AB. Comparison of impression techniques for a five-implant mandibular model. Int J Oral Maxillofac Implants. 1991;6:448-55.

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