Screw-retained, digitally designed implant-supported restorations

implant restorationMichael Norton explores the new Primescan intraoral scanner for delivery of screw-retained, digitally designed implant-supported restorations.

Computer assisted design/computer assisted milling, so-called CAD/CAM technology, has been adopted in dentistry for the fabrication of restorations such as inlays, crowns and prosthetic frameworks. There is increasing evidence for their accuracy of fit and their longevity. This has been shown to supersede that achieved with more conventional techniques (Wittneben et al, 2009; Gonzalo et al, 2009a; Gonzalo et al, 2009b; Kohorst et al, 2011; Kapos et al, 2009).

In addition, the application of digital intraoral scans for impressions continues to progress apace. Mounting evidence shows that the accuracy of these scans is approximating that of dimensionally stable silicone or polyether impressions (Haddadi, Bahrami and Isidor, 2009; Ferrini et al, 2019), with increased speed and comfort for the patient (Yuzbasioglu et al, 2014). 

However, only recently have studies begun to report that their accuracy is superior for implant dentistry.  Although, there is significant variation when comparing one scanner with another (Vandeweghe et al, 2017; Chiu et al, 2020). 

Given the various tissues that need to be captured, not least enamel, dentine, mucosa, and a huge variety of prosthetic materials, again intraoral scanners have been tested and shown to deliver accurate results (Mennito et al, 2019).

In addition to the above, recent articles on the use of CAD/CAM for fabrication of abutments have demonstrated remarkably stable mechanical and biological stability (Parpaiola et al, 2013; Parpaiola et al, 2020). It is inevitable then that implant companies and clinicians should look to tie the digital workflow together and develop implants that benefit from all this technology.

A new generation

In 2009, I became lead clinician in the development of the new EV generation of the Astra Tech implant (Figure 1). This implant is designed to benefit specifically from digital technologies. With a ‘one-position only’ index that is site specific and ensures that Atlantis scan FLOs (Feature Locating Objects) and subsequent CAD/CAM abutments can only locate spatially in the one required position. 

The following case exemplifies the benefits of this implant when used in tandem with the new Primescan intraoral scanner and Atlantis Custombase abutment system for a seamless and superlative screw-retained implant-supported restoration.

Case study

  • Figures 2 to 7: Vertical fracture of the upper right second premolar. The split root and deep subgingival was removed to allow for immediate implant placement and temporisation

In the case example, a patient presented with a vertical fracture of her upper right second premolar (Figures 2-7). 

The root that was split obliquely and deep subgingival was removed to allow for immediate implant placement and temporisation, using a stock abutment and hand-fabricated immediate temporary crown. 

After six months of healing (three months delayed due to COVID-19 lockdown), the implant stability was assessed using resonance frequency and the ISQ had increased from a baseline of 66/65 to 83/83 as depicted in Figure 8 in the Osstell Connect data. 

It is worthy of note that the peak insertion torque was only 7Ncm and reinforces the data published by the author about primary stability, immediate temporisation and the need to redefine how it is assessed (Norton, 2013; Norton, 2017).

Latest technology

Primescan (Figure 9) is the latest scanner to enter the market. It incorporates all the very latest technology an intraoral scanner has to offer. 

With a high precision, smart pixel sensor, Primescan captures images at high speed, accuracy and dynamic depth. This allows greater than one million 3D points to be captured per second with up to 50,000 images consolidated per second and up to a depth of 20mm. 

Images (such as those shown in Figures 10 and 11) can be captured in just minutes. The bite map is produced instantaneously. From these scans, models can be printed that have, in my experience, been of the highest dimensional accuracy. 

This is evidenced by the fact that, to date, there has been little if any need to adjust either contact points or the occlusion in a total of 25 implant-supported crowns fabricated using this technology (Figures 12 and 13). 

Zirconia crowns are fabricated on Atlantis Custombase abutments, which are designed using the Virtual Abutment Design (VAD) software. 

This software centres on a philosophical belief. That the morphology of an abutment should ideally be determined by the morphology of the restoration it is intended to support. The emergence profile is determined by the form of the soft tissue morphology. This is developed by the temporary crown and accurately captured in the intraoral scan (Figures 14 and 15). 

The need to contour the sub-mucosal portion of an abutment is critical. Not only to emergence profile and soft tissue health but to also establish a good cervical anatomy. One that enhances aesthetics and also reduces the risk for food trapping issues (Figure 16). 

  • Figures 10 and 11: Intraoral images captured using the Primescan

The shape

To this end, the software utilises three classic anatomical shapes:

  • Triangular
  • Ovoid
  • Square.

The shape is determined by tooth position (ovoid for premolars, for instance). As well as four options to allow for increasing support of the soft tissues. 

In an effort to move away from restorations that are cemented intraorally, the Custombase abutment provides for an aesthetically deep subgingival margin but with a zirconia crown. This can be bonded to the abutment in the laboratory for screw retention. 

Added to this, it is also possible to use the new Atlantis re-angulating screw, if needs be. This ensures the optimal position for the screw access hole. 

In addition, this screw offers a much smaller head. Hence, a much smaller access hole is required. This yields a superior aesthetic outcome once filled with composite as it becomes virtually invisible (Figures 17-19).

  • Figure 16: Contouring the sub-mucosal portion of an abutment is critical

Summary

CAD/CAM abutments have been in use for many years. With documentary evidence for excellent biological and mechanical outcomes (Parpaiola et al, 2020), the introduction of Primescan and the Astra Tech EV implant appear to square the circle, allowing for a digital workflow that delivers a completely digitally designed and milled abutment (titanium or zirconia) and zirconia crown that is truly site specific. 

These restorations represent the pinnacle of implant-supported prostheses and. In my opinion they offer the best hope for long-term biological and mechanical stability. As well as an excellent aesthetic outcome that is unsurpassed in my 30 years of placing and restoring implants.  


For references please contact [email protected].

This article first appeared in Implant Dentistry Today. Read the latest issue of Implant Dentistry Today here.

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