December 2015
Volume 11, Issue 12

The Desktop Dental Lab

Investigating a new option for chairside milling of custom abutments

John A. Sorensen, DMD, PhD, FACP | Hongseok An, DDS, MSD

With the recent advent of a compact tabletop precision wet CNC milling machine, dentistry has reached a major milestone in the development of a completely digital workflow for single-tooth implant treatment. This new system brings revolutionary change to in-office dental technology in terms of flexibility, affordability, and control, enabling virtually any dental practice or laboratory* the capability of wet milling ceramics, titanium, or polymethyl methacrylate (PMMA).

Combined with a new semi-prefabricated titanium abutment milling blank, the operator can now fabricate equal in quality to an implant manufacturer. These systems complete the loop in providing the clinician or laboratory technician* control over the entire dental implant therapy process from surgical planning and surgical guidance to provisionalization and definitive prosthesis fabrication.

The synergy between modern materials and digital technology has yielded increased milling rates and accelerated processing times, facilitating chairside indirect procedures for both conventional and implant prosthodontics. The desktop milling system even makes same-day definitive implant prostheses feasible and practical for any practice or laboratory.*

This article introduces the next generation of compact wet milling machines and semi-prefabricated titanium abutment milling blanks and reports on a validation study analyzing the 3-dimensional (3D) accuracy of custom-milled abutments fabricated with these systems. A clinical demonstration of how these transformative innovations enable the implementation of a completely digital workflow concept for dental implant treatment from diagnosis to definitive restoration is illustrated.

A Completely Digital Workflow for Single-Tooth Implants

Three phases are involved in the digital workflow process—acquisition, computer-assisted planning and design, and computer-assisted machining. Research has shown the accuracy of intraoral scanning technologies for acquisition of the tooth preparation and soft tissues,1 full-arch preparations,2,3 and single-tooth implants with scan bodies4 to be at least equal to conventional elastomeric impressions. In a study comparing single-tooth implant digital versus analog impressions, 80% of subjects favored the digital impression, and digital impressions took about half the time of analog impressions.5

Radiographic imaging and treatment planning software has revolutionized 3D site assessment, planning of surgical implant placement, and fabrication technologies such as 3D printing or milling of surgical guides. Now by combining cone-beam computed tomography (CBCT) radiography and intraoral scans, the provisional crown or custom healing abutment can be milled out in advance from a gradient PMMA material. A recent study compared the physical properties of provisional restorations fabricated from milled PMMA blocks with manually fabricated resin.6 The researchers concluded that CAD/CAM block restorations had superior color stability, lower water sorption, higher wear resistance, higher surface hardness, and significantly higher fracture resistance. For definitive prostheses fabrication, the shape of the developed soft tissue site and a scan body representing the position of the implant can be scanned intraorally, a virtual model created, and either a hybrid abutment crown or custom abutment and crown designed and milled.

New Semi-Prefabricated Milling Blank

Titanium alloy has been a popular material of choice for custom abutments due to its compatibility with titanium implants as well as ease and rapidity of milling. While stock abutments allow use of in-office inventory at a lower cost, they can result in restoration cemented margin placement many millimeters below the crest of the gingiva. With the anatomical features of a curved ridge, the greatest potential problem area is at the interproximal zone.

This has potentially deleterious effects on peri-implant tissue health, particularly if the crown must be cemented directly in the mouth. With a 4 to 5 mm subgingival margin, it is virtually impossible to remove the excess cement, risking serious clinical consequences as severe as implant failure.7-10 Wilson11 reported that the majority of severe peri-implantitis incidents occurred at 3 years post-restoration cementation. Studies have shown a higher rate of peri-implantitis when using stock abutments versus custom-designed abutments due to the location of the margins with stock abutments.12

The control facilitated by a custom-designed abutment with cementation margins just below the crest of the gingiva optimizes long-term peri-implant health and significantly reduces the incidence of soft tissue complications.12 Ideal margin placement also allows the dentist and hygienist to better evaluate and monitor the implant restoration.

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