The Brånemark system : new developments in prosthetic components

Ever since we have been using implants for the replacement of teeth, the indications for their use have extended to include all types of tooth loss. In order to meet the requirements of all these clinical situations, prosthetic components have been developed which can be classified under two categories:

- standard components;

- adjustable components.

The standard components are easy to use but are only applicable for use in favourable situations; when the axis of the implant is ideally aligned to the axis of the prosthesis, the emergence profile is compatible with the diameter of the implant and when there is a favourable peri-implant environment (^{fig. 1} and ²).

In difficult clinical situations, when the axis of the implant diverges from that of the prosthesis, an unusual emergence profile, an unfavourable peri-implant environment, a complex aesthetic problem, then adjustable components are not able to meet the clinical requirements (^{fig. 3} and ⁴). When standard abutments cannot be used, it becomes necessary to devise specially constructed abutments. This type is made in the laboratory by casting directly on the implant using the lost wax technique. A machined gold core is fitted to the head of the implant being used and the abutment is fabricated by over-casting with a gold alloy (^{fig. 5}, ⁶, ⁷ and ⁸).

This method of constructing purpose-built abutments enables us to deal with certain unfavourable clinical situations but there are some disadvantages:

- the need to use a lost wax technique;

- the use of gold alloys;

- the lack of an antirotation device.

According to the studies of ^{Abrahamsson et al. (1998)} on the reactions of the peri-implant mucosa to different types of abutment, alumina and titanium were the materials that provided the best epithelium-connective tissue junction. Since this study, other materials have been tested, such as gold alloys and glazed dental porcelain. Not only do they not permit the formation of an attachment around the abutment but they also bring about a persistent inflammation with recession and bone loss (^{fig. 9}). In 1993, ^{Prestipino and Ingberg (1993a} and ^b) reported the use of an alumina component which can be machined in a prosthetic laboratory using diamond burs in a turbine handpiece: the Ceradapt^® abutment. This component can be made from a prefabricated cylinder of pure alumina baked to its final density (^{fig. 10}, ¹¹, ¹², ¹³, ¹⁴ and ¹⁵).

Advantages:

- it is aesthetic because it is constructed from alumina;

- it is bio-compatible; the alumina allows the formation of a gingival attachment;

- the component is shaped in the laboratory and is therefore adapted to the gingival anatomy of the patient.

The cervical margins of the restoration conform exactly to the gingival margin;

- the emergence profile can be modified by the addition of ceramic;

- the abutment incorporates an antirotation device which allows it to be screwed on to the implant at 32 ncm.

Disadvantages:

- an impression must be taken of the implant in order to produce a working model for machining the abutment;

- the abutment must be tried in and, if necessary, adjusted prior to making the prosthesis;

- the process of machining the abutment in the laboratory is a relatively difficult and lengthy procedure. The abutment is somewhat fragile and there is a significant risk of fracture during the machining and try-in;

- there is only a limited possibility of making good the axis of the implant or of modifying the emergence profile;

- the fitting of the abutment on the implant in the mouth and its tightening with the antirotation device are delicate operations;

- the abutment is only available in a single shade;

- the emergence profile can be modified by adding ceramic but the reaction of the gingiva to glazed porcelain is not as good as with alumina (^{fig. 16}, ¹⁷, ¹⁸, ¹⁹ and ²⁰).

Procera^® System

The Procera^® system is a computer assisted design and manufacturing process that allows the production of prosthetic components in titanium or alumina (single jacket crowns, implant abutments, bridge pontics).

The first CAD/CAM studies were carried out by ^{Duret (1992)}, ^{Mörmann et al. (1989)} and ^{Rekow (1993)}. It was ^{Andersson et al. (1989}, ¹⁹⁹³, ¹⁹⁹⁶, ¹⁹⁹⁸⁾, ^{Bergman et al. (1990)} and ^{Oden et al. (1998)} who perfected the Procera^® technique. An impression of the implant must be taken in order to produce a working model with imitation gingivae. A 3D image of the abutment is produced by scanning the model. The data is sent via a modem to the production unit in Sweden (^{fig. 21} and ²²). The process of fabrication begins with the recording of the digital data from the scanned model or wax mock up to create a virtual prosthesis. Then a machine-tool, guided by the software, mills the piece. The production of the prosthetic components involves:

- a recording tool (3D scanner);

- software to create the recording;

- a machine to mill the prosthesis.

With the Procera^® system it is possible to produce:

- abutments in alumina;

- abutments in titanium;

- three-unit bridges in alumina;

- bridge abutments in titanium (All-in-One^®);

- single jacket crowns in alumina.

Alumina abutments

The abutment is milled digitally from a single ceramic block which is similar to Ceradapt^® but of different sizes. Therefore, adjustment of its axis and emergence profile is more likely to be possible. The Procera^® system also allows the production of ceramo-ceramic jacket crowns (^{fig. 23}).

Advantages:

- compared with Ceradapt^® abutments, the shaping is not undertaken with a bur and turbine handpiece in a prosthetic laboratory by grinding but by digital machining in a production unit;

- the cervical margins and the emergence profile are defined during the scanning procedure but modifiable at the clinical try-in stage;

- the abutment possesses an internal counter-rotation system that allows it to be screwed onto the implant at 32 ncm (^{fig. 24}, ²⁵, ²⁶, ²⁷ and ²⁸).

Titanium abutment

The abutment is milled digitally from a single block of titanium that has a hexagon that fits the head of the implant. This abutment has an internal counter-rotation device that allows it to be screwed on to the implant at 45 ncm (large platform type) (^{fig. 29} and ³⁰). Unlike the standard abutment (CeraOne^®) it is possible to create an emergence profile which on the one hand resembles the morphology of the teeth being replaced and on the other hand incorporates a cervical shoulder that supports the ceramic element. The latter can be in ceramo-ceramic (^{fig. 31} and ³²).

Procera^® ceramic bridge

The Procera^® system can be used for small (three element) bridges (^{fig. 33}, ³⁴, ³⁵, ³⁶ and ³⁷). However, it should be noted that up to the present time, there has been no study to evaluate the long term success of these all-ceramic Procera^® bridges.

All-in-One^® bridge pontic (fig. 38, 39, 40, 41, 42, 43, 44 and 45)

The Procera^® system allows the production of titanium pontics without the need for recourse to casting by the lost wax technique. The laboratory technician makes a resin model of the final pontic. This model is scanned by laser and the data transmitted to the production unit. The pontic is then milled from a single block of titanium. It is possible to mount resin teeth on the pontic. There have been attempts to bond porcelain to titanium but we lack studies of this procedure.

This technique is indicated for full or partial prostheses.

Advantages:

- an industrial procedure replaces the casting stage;

- improved accuracy;

- the need for gold cores is obviated;

- the pontic is very light.

Disadvantages:

- it is necessary to produce a resin model with the risk of generating dimensional errors, compared with having a master model;

- ceramic bonded to titanium is still an experimental technique.

Discussion

If modifiable components are better able to meet the periodontal, morphological, mechanical and aesthetic requirements of implant-supportable prostheses, it is necessary to take into account the increase in the number of stages in the construction of the prosthesis:

- impression of the implants;

- construction of a model with imitation gingivae;

- construction of a wax or resin model in the laboratory;

- scanning of the model;

- creation of a small resin key in order to facilitate the positioning of the abutment in the mouth;

- try-in of the abutment (local anaesthesia is often necessary);

- adjustments to the abutments in the mouth are difficult. They must be done in the laboratory.

Conclusion

Over the last few years, the development of prosthetic components have proceeded in two directions:

- custommade abutments;

- industrialisation of the fabrication process.

These two evolutionary directions are seen to improve the consistency and predictability of the aesthetic results and the quality of fit of the prostheses to their implants.

Surgical procedures: Dr Franck Renouard. Prosthetic laboratories: Al-Zr and Nicolas Millière.

Demande de tirés à part

Jean-Michel GONZALEZ et Philippe RAJZBAUM, 57-61, rue du Président-Wilson, 92300 Levallois-Perret - FRANCE.

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