Tooth crowns can only resist the high loads of chewing for a certain amount of time. The most frequent cause of failure is that crowns become loosened from the stump. Researchers from the Laser Zentrum Hannover (Laser Center Hanover) and the Goethe University Frankfurt Dental Clinic have discovered how to improve the adherence of ceramic crowns. To accomplish this they create groove-like micro structures on the inside of the crown with a femtosecond laser thus increasing the effective bonding surface of the crown and cement. In this way the friction-grip can be increased by 30%. Material ablation from hardened dental ceramics (HIP-ZrO₂) has proven to be very efficient. Together with the fact that today CAD/CAM systems have also established themselves in the areas of dentistry and dental technology, the idea was formed to use femtosecond lasers not only for the micro structuring of the crown inner surfaces but also for producing complete, complex crowns from the hardened ceramics.

Necessary prerequisites for this project were the development of a high-power industrial femtosecond laser source (P>30W, T<500fs) along with a highly dynamic machining station. Each crown is made individually and represents a 3D-workpiece with a complex free form geometry that must be accessible to the focused laser beam from all sides. For this the workpiece requires the use of a 5-axes positioning system with a rotary and swivel axis. Due to the special requirements of the target dentistry and dental technology market, the unit should be as small and compact as possible.

The realization of the complete production process, starting from scanning and digitalization of the dental impressions up to production and micro structuring of the ceramic crowns by means of ultrashort laser pulses, took place within the framework of the Federal German Ministry of Education and Research (BMBF) joint project FORCERAMUS (reference number 13N8556). The joint project was successfully completed in 2009. The development work was split amongst the following partners:

• University of Frankfurt Dental Clinic (project coordination, material ablation strategy, CAD/CAM Module)
• POLYTEC GmbH (3D-scanner, system integration, validation)
• LZH (seed oscillator for the fs industrial laser source, process parameters for the laser preparation of HIP ZrO₂ and micro structuring)
• JENOPTIK L.O.S. GmbH (development and construction of an industrial 5-axes fs-laser demonstrator)
• KUGLER GmbH (development and construction of a 5-axes laser machining station for crown production).

The construction of the laser machining station with its 5 motion axes (XYZ plus Y-slide integrated rotary-swivel unit that carries the workpiece) is marked by the extraordinary requirements regarding compactness (the machine must pass through a normal interior door), axes dynamics (linear axis acceleration up to 100 m/s², angular acceleration of the rotary axis up to 18000°/s²) and a machining accuracy of ±20µm. 

In order to be able to ensure the demanded manufacturing accuracy and extreme process dynamics of the numerically controlled axes, special measures had to be taken to increase the structural rigidity and to suppress jerk effects (lightweight construction, dissipation of transferred recoil in damped mass-spring modules, s-shaped acceleration ramps). All axes have direct drive, incremental measuring systems and precision bearings with ceramic balls.

Figure 1 shows a view into the working area of the machine and displays the arrangement of the axes. The laser beam comes from the right and the focusing lens moves in the X-Z-plane. The ceramic workpiece is held on a pin that is clamped in the rotary axis, which is mounted to the swivel axis at a 90° angle. The swivel axis is integrated into the Y-slide, which moves horizontally and holds the workpiece surface in focus. Two integrated measuring systems (a 5-way tactile probe and a swingable video microscope) serve for the automated referencing of the workpiece as well as the laser focus.

Figure 2 shows the complete laser machining system. The housing corresponds to all safety requirements. The working area has its own enclosure in which the fine dust produced during machining is vacuumed off and filtered out. This is necessary because the dust contains a high proportion of micro and nano-particles. This fully protects the health of the service personnel.
The processing time for a single HIP zirconia (ZrO₂) crown amounts to approx. 30 min. This opens a completely new method to dentistry and dental technology for the efficient, economical production of all-ceramic dental replacements.

Thus in the future all-ceramic crowns will also provide you with a powerful bite.

By Dr. rer. nat. Klaus Baier
Technical Management R&D
klaus.baier@kugler-precision.com

See also Research.