Case Study - Reverse Abutment

Case Study: A new approach to implant-supported restorations

The Reverse Abutment

Dr Michel Dadi, Tel-Aviv/Israel

The present article describes a new approach to implant-supported restorations (an MIS Ltd. Patent). Two applications are developed: a fabrication technique for semi-customized abutments and a technique to fabricate implant-supported restorations prior to installing the implant.

Fig. 1:
Any abutment can be conceptually divided into three segments: P.C.E., I.C.E. and I.E.

Background

The concept of the Reverse Abutment (R.A.) will be developed based on the following:
1. Any abutment can be conceptually divided into three segments (Fig. 1):
a) Prosthesis connection element (P.C.E.); this is the segment of the abutment connected with the prosthesis.
b) Implant connection element (I.C.E.);this is the segment of the abutment connected with the implant.
c) Intermediary element (I.E.).

Fig. 2:
The I.E. defines the trans -mucosal height and angulation of the abutment.

2. The I.E. defines the transmucosal height and angulation of an abutment, the latter referring to the angle between I.C.E. and P.C.E. (Fig. 2).
3. The different compatible positions of an implant lead only to modifications of the I.E. of the abutment (assuming a single crown fitting on an implant via an abutment and maintained in a predefined position) (Fig. 3).

Fig. 3:
The different compatible positions of the implant lead only to modifications of the I.E. of the abutment.

4. In custom abutments, two of the three segments (P.C.E. and I.E.) are customized.
5. In contrast to currently available abutments,we define as semi-customized an abutment in which one element (the I.E.) can be modified while the other two are prefabricated.

The Reverse Abutment

There are three major techniques of customizing abutments:
1. Milling (starting from a bulky titanium abutment)
2. Physical modelling (creating a model for casting or scanning)
3. Virtual modelling (designing a model in a virtual environment).
This is sometimes a tedious work that always requires a second step of fine adjustment for the production of the temporary prosthesis and the framework of the definitive prosthesis.

Fig. 4:
Complete R.A. kit with transfer guide (1), copings (2 and 4), P.C.E. analog (3), polycarbonate coping (5), P.C.E. (6), and I.C.E. including a connector pin (7).

Fig. 5:
An essential requirement in fabricating an R.A. model is to properly connect the P.C.E. and I.C.E.

Herein we present, for the first time, a semi-customized abutment allowing gingival height and angulation to be customized while retaining the advantages of a standard abutment.We named this structure Reverse Abutment and designed an “R.A. kit” (Fig. 4).This kit includes the P.C.E. and I.C.E. elements as well as various standard components (that are routinely used to manufacture temporary and permanent prostheses) adapted to the R.A. concept.

Fig. 6:
The screw housing is created by positioning the model on the screwdriver.

Laboratory procedures

An essential requirement in fabricating an R.A. model is to properly connect the two prefabricated elements: the P.C.E. is selected from different models, and the connection of the I.C.E. is determined by the implant system used (Fig. 5). The I.C.E. is stabilized on the analog via a connector pin inserted into the element.

Technique:

Fig. 7:
Inserting the implant connection element.

1. The I.C.E. is inserted into the implant analog via the connector pin.
2. The insertion axis is selected and the P.C.E. positioned in accordance with the prospective restoration.
3. The two elements are connected in accordance with the selected relationship.

Before casting, the model obtained is separated from the connector pin and positioned on the screwdriver (Fig. 6), thus creating the housing of the screw.

Fig. 8:
Connecting the two standard connection elements.

A surveyor is recommended for restorations supported by several abutments (Fig. 7); then the I.C.E. is positioned using a transfer guide to which the I.C.E. is accurately fixed (Fig. 8).

Fig. 9:
Titanium Reverse Abutment by Procera.

Models of this type may be scanned to obtain a titanium or zirconium abutment; the Procera system, as used in this specific case, does not require the drilling path to be established on the model (Fig. 9).

Fig. 10:
Two Reverse Abutments cast in gold.

Clinical procedures

The Reverse Abutment concept will invariably result in high-quality restorations (Fig. 10). Other advantages are offered as well.The R.A.models need not be fabricated in the laboratory but can also be created intraorally, using the same methodology. If a temporary abutment is created in this way, the dentist will select a titanium I.C.E. from the various models and a temporary resin for cementation.

Fig. 11:
Polycarbonate basis for duplicating the temporary crown.

The temporary superstructure will be made from a prefabricated polycarbonate shell compatible with the P.C.E., which is a simple way of obtaining a high-quality fit on non-standard abutments (Fig. 11).

Immediate loading is, however, the application where the Reverse Abutment concept really comes into its own. There are two possible scenarios:
1. Surgery was programmed (Simplant, NobelGuide, Med 3D…). While everything may appear perfect in the planning stage, there will always be some discrepancy between virtual and actual implant positions. It is the surgeon’s task to correct this imprecision by intraoral relining of the temporary restoration. This can be perfectly accomplished by delivering prefabricated polycarbonate copings intraorally on R.A.s that were previously manufactured in the laboratory.
2. Surgery was programmed through the use of a standard surgical guide causing a greater degree of imprecision, thus making it impossible to fabricate abutments like in the previous scenario. However, the technician can still prepare the restoration by following a protocol that will be covered in detail in the following section (while the case we selected involved a single crown, the same principles will also apply to multiple crowns).

Fig.12:
Stone cast including a P.C.E. analogue.

Protocol using Reverse Abutments for Immediate Loading

Laboratory procedures
The dental technician prepares the model (Fig. 12) and crown. He also fabricates a jig to hold the crown in its reference position (Fig. 13) and a surgical guide (Fig. 14).

Fig. 13:
Jig maintaining the crown.

The P.C.E. analogue on the stone cast is positioned as planned by the surgeon. Then the components supplied in the kit are used to fabricate the crown.

The surgical guide is fabricated on the same cast.

Fig. 14:
guide including a transfer guide.

Its purpose is twofold:
1. Accurate positioning of the P.C.E. planning of the incision.
2. Indicating the approximate position of the implant head.
The surgical guide is fabricated with the transfer guide positioned accurately on the P.C.E. analog after being perforated to allow for calibrated drilling.

Fig. 15:
The surgical guide is positioned and intraorally stabilized.

Clinical procedures

By positioning and stabilizing the surgical guide intraorally (Fig. 15), we obtain a printed circle on the gingiva that helps with the design of the incision.
The first drilling is then done through the transfer guide which indicates where the head of the implant must be centered approximately on the bone.

Fig. 16:
I.C.E. placed on the implant.

At this point, regular surgery is resumed until the implant is installed.

Once the final crown has been fabricated and the implant installed, the last step will be to fabricate the R.A. connecting the prosthesis to the implant.
This can be accomplished in two alternative ways:

1. Creating the R.A. model intraorally:
a) Placing the I.C.E. on the implant, preferably

Fig. 17:
P.C.E. placed into the prefabricated crown.

under radiographic visualization (Fig. 16).
b) Placing the P.C.E. accurately into the transfer guide, or into the prefabricated crown maintained in place with the prefabricated jig (Fig. 17).

Fig. 18:
Model of Reverse Abutment, obtained intraorally.

c) Connecting the I.C.E. to the P.C.E. with a burnout acrylic resin (Fig. 18).
d) Disassembling the R.A. model once the acrylic resin has set and the I.E. has been optimized, followed by creating the screw housing.

Fig. 19:
Index impression of implant.

2. Taking an index impression of the implant (Fig. 19) and fabricating the R.A. in the laboratory (as described under “Laboratory Procedures/ Technique”).

Note that the first technique will create a unique model while the second technique offers advantages in terms of simplicity and reproducibility.

Fig. 20:
The R.A made in the same session

The R.A. model obtained may be considered temporary in nature.
Another option would be to obtain the cast R.A. on which the previously created prosthesis fit perfectly several hours after the surgery (Figs.20 and 21).

Fig. 21:
The crown made before surgery fits on a R.A .

Conclusions
The R.A. concept was developed in response to the theoretical question whether permanent or temporary dental restorations can be fabricated prior to placing the implants and without knowing their exact positions beforehand.
Our research has answered this question and has led to the Reverse Abutment concept as a novel type of “semi-customized” abutment. This new laboratory and clinical procedure rests on the realization that permanent or temporary restorations can be fabricated prior to installing the implant(s) in the presence of abutments whose P.C.E. and I.C.E. are prefabricated while the I.E. can be modified.We believe that both technicians and clinicians will benefit from this novel concept.

This Case Study - Reverse Abutment is by:

Contact Address

Dr Michel Dadi, DMD
9, Schlomtsion Amalka
Tel-Aviv 62267
ISRAEL
dadimic@zahav.net.il

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