Internal tapered implant systems prove challenging

The current Stoneglass range of customised abutments is compatible with 46 implant interfaces. In addition, our customised implant superstructure range is compatible with 64 implant interfaces. These are two of many facts about the new range our engineers are very proud of. Our ability to react to requests and add compatibility for new interfaces has been part of our success in recent years and allowed this range to grow quickly. However, providing compatible products for the emerging internal tapered implant systems presented new challenges, in particular, extending our Customised Zirconia Abutment range.

The traditional experience of the dental technician is typically the starting point for product development at Stoneglass. In this case, the first step in the project was to identify shortfalls and problems with similar products that are already in the market. Working closely with dental technicians and dental surgeons, we quickly identified one serious concern regarding existing products fracturing. Abutments were failing at the extremity of the taper and the engaging hex.

We then had to identify the failure mode of the abutments. Investigations in the laboratory and surgery showed the problem was occurring when the abutment was being removed from the analog model or patient's implant. The tapered surface between the abutment and implant internal is intended to ensure a seal between the components and even though there is a relatively large taper angle, there is a "morse taper" effect which locks the abutment in place. This prevents the abutment from being extracted easily and the tendency is to lever the abutment to break the tapered seal and then remove it. The result is a localized bending force applied to the tip of the abutment. To make matters worse, the wall thickness of the abutment at the tip is extremely thin, resulting in the fracture of the abutment.

To provide a solution, it was essential we did not limit the function of the tapered seal; therefore the geometry of the abutment engaging the implant restricted any design changes. We chose to use a titanium intermediate adapter that would engage the implant and provide an alternative platform for the Zirconia abutment. Titanium provides the flexibility and the "foregiveness" in the material that Zirconia does not.

In designing the adapter and the resulting zirconia abutment interface, there were three main challenges:

1. Obtaining accurate measurements of the implant interface onto which to engage. As a starting point, the accurate measurement of the implant interface is required. Our in-house optical measuring machine allows magnification of components up to 360x and measurements to be taken with an accuracy of 2 microns.

2. Design the abutment interface for manufacturing and ensure it provides smooth insertion and minimal rotation of the final abutment. The design of the interface must ensure the abutment fits correctly. To avoid rotation, an interference fit is ideal; however this will make it difficult, if not impossible for the abutment to be inserted into the implant and extremely difficult for it to be removed. We must also take into account the tolerances involved in the manufacture of the implant, the adapter and the customised abutment. Fortunately we could draw on our previous experience. Stoneglass has implemented into its product range a design of interface that allows the abutment to be easily located and inserted by the dentist and then engages to give zero rotation. Furthermore, this design caters for the tolerances in the manufacture of the components that make up the finished assembly, including the relatively large tolerances in milling of the customised abutment.

3. Ensuring the design process of the customised abutment is not complicated. To ensure the design process is no more complicated for the technician, the customised abutment can be designed as if it were a one-piece abutment. This can be done either as a wax-up abutment or using the 3Shape Virtual Abutment designer software. All the technician need be concerned with is the design of the abutment. Our Stoneglass SDS Link software allows technicians to send the order direct to Stoneglass. When the order is received, our manufacturing software automatically adjusts the design to cater for the adapter.

Once the design is complete, the parameters are fed into the adaptive manufacturing system developed by Stoneglass. This system allows new products to be fed quickly into the production facility and gives us the ability to test new products and make necessary adjustments with relative ease.

A thorough pre-production testing phase is carried out prior to the release of any new product. All our development work is done with real cases sent by our clients. This ensures we receive direct feedback from dental technicians and dentists on the performance of our products and allows us to do any fine tuning to the final product. This also gives us the confidence that our production system can handle the range of custom designs we may receive.

Finally the new product is released to our clients. The SDS Link communicates directly with the Stoneglass server and as soon as we update the product list database, our clients have immediate access to them.

This project is another example of how we combine Tradition with Technology in the development of new products.

Acknowledgement

We must thank Brian Walters of Dento Genics in Sydney for his invaluable input and expertise through the development of our new range. It is an integral part of the Stoneglass development process to involve the technician, hands-on. By involving Brian and the dentist throughout the development cycle, it ensures we can deliver solutions that meet expectations and solve real problems at the bench. After all, the bench is where Stoneglass started.

About the author

Darren Littlefair is Chief Technical Officer at Stoneglass Industries.