A major concern regarding a custom-milled abutment is the quality of adaptation to the inside of the dental implant. The abutment-implant interface is highly critical to stability of the restoration-abutment-implant complex and long-term peri-implant tissue health.13 An imperfect fit, especially with an internal conical abutment-implant interface design, can cause abutment screw loosening, restoration mobility, and even implant fracture.14 Most dental laboratories cannot afford the expensive high-quality, high-accuracy machining equipment of the implant manufacturer necessary for extremely high-precision machining of the critical implant interface portion of the abutment.
A new solution, a semi-prefabricated abutment blank (DentaSwiss PreFAB-4 Concept, Biodenta, www.biodenta.com), was recently 510K cleared and is available for seven implant systems. The critical implant interface portion of the abutment is milled at the factory with high precision (Figure 1) and the coronal portion can then be custom designed and milled by a clinician or a registered dental lab* with a lower cost/less sophisticated in-house milling machine. The Ti-6Al-4V alloy abutment blank can be rapidly milled in a wet milling system. This approach has the potential to provide the clinician with more control and flexibility and reduce fabrication costs, but little is known about the accuracy of custom-milled abutments on the latest generation of milling machines.
Compact Wet Milling Machine
The compact, fully automated, self-contained tabletop 4-axis CNC milling machine used in this process has a closed liquid cooling system tank so that no external module for pumping and storage is necessary (DentaSwiss DS1300, Biodenta). The integrated water tank contains a filter mat for separating grinding particles from the cooling liquid. A Surface Pro 3 tablet (Microsoft, www.microsoft.com) with provided DentalCAM software has machining strategies specifically designed for grinding ceramics and other dental materials (Figure 1). Equipped with an automatic tool changer for eight tools, the system has two removable changer stations, a three-ceramic block holder, and two PreFAB-4 blank holders for maximum production capability.
The DentaSwiss DS1300 system is ideal for inlays/onlays, crowns, veneers, three-unit bridges, and patient-specific implant abutments fabricated from PMMA, nano-hybrid composites, glass ceramics, lithium disilicate, silicate ceramics, and titanium. Its size and speed make it well suited for chairside milling operations. Despite its high-speed milling capacity, via the CAM software, it carefully mills occlusal screw access holes atraumatically, avoiding introduction of flaws and cracks in the ceramic structure.
The DentaSwiss system integrates all software and hardware components together and provides full-time technical support necessary to maintain continuous operations. Once the semi-prefabricated titanium milling blank is loaded into the milling machine, along with the design files of the patient-specific coronal and subgingival portion of the abutment, the CAM software calculates the milling paths and the DS1300 wet mills the custom abutment. The desktop milling system offers great flexibility in choice of milling materials, as well as the efficiency, control, and cost-effectiveness of in-house same-day milling rather than outsourcing to a remote milling center. Operators can now have complete control of their digital workflow, tailoring it to their individual systems of dental practice or dental laboratory* operations.
Analysis of Milling Accuracy
With all of the promising potential benefits of chairside milling, the accuracy of custom-milled abutments has not been evaluated. The authors set out to determine if the 3D accuracy of custom-milled titanium abutments produced by the system were within acceptable clinical limits.
The ISO developed a standard in 1994 (ISO 5725-1: 1994)15 for the measurement of accuracy comprising two terms:
Trueness: The closeness of agreement between the average value obtained from a large series of test results and the accepted test value.
Precision: The closeness of agreement between independent test results obtained under stipulated conditions.
The purpose of this study was to evaluate the accuracy (trueness and precision) of milled custom abutments fabricated from a semi-prefabricated abutment blank and a new compact 4-axis wet milling machine.16