This article is sponsored content brought to you by W&H.
Innovation and problem-solving
The concept of using rotary tools to cut hard materials, such as granite, is at least 6,000 years old. As early as 25,000BCE our ancestors were using handheld lithic borers to cut shells without breaking them. These tools were generally made of flint and were pressed against the object to be perforated and then twisted back and forth at low speed and relatively high torque. Powerful modern handpieces have elevated the principle of variable speed and high torque to enable precise cuts into much harder materials like zirconia.
Restorative material science is developing quickly, driving innovation in the development of tools. Durable materials like zirconia increasingly have the potential to emulate the aesthetic and function of real teeth. With these robust materials has come a need for advances in the technology around rotary cutting.
The use of zirconia in restorative dentistry
Zirconia, ZrO2 zirconium dioxide, has become a popular material in restorative dentistry for its biocompatibility, high fracture toughness and radiopacity. Dental zirconia is more often a modified yttria (Y2O3) tetragonal zirconia polycrystal (Y-TZP), because it has greater mechanical properties and better tear resistance than other ceramics.
Material science is a crucial aspect of drilling science, and different materials are described in terms of hardness using the Mohs Hardness Scale. Zirconia is 8.8; around the same hardness as tungsten carbide, which is a commonly used material in burs. Diamond is 10 on the Mohs Scale, and super coarse diamond burs have been found to be more effective at cutting zirconia than others, although the greater force needed to cut zirconia still results in frequent bur wear.
Friction and heat
Balancing the relationship between torque and speed, while managing the effects of friction, is an enormous challenge when cutting very hard materials. Thermal damage to patients is a greater risk when working intraorally, due to the very high temperatures generated in the process.
The bur in a modern handpiece can rotate at speeds of 200,000–400,000 rpm, and the associated friction can heat the target material to 240°C before cooling. To mitigate against thermal damage to patients, high-speed handpieces have incorporated a function to spray cooled water, usually so that water flows through the tool and is sprayed onto the surface through holes in the head during cutting.
The heat generated within the handpiece itself is higher when cutting hard material. The faster the coolant flow rate, the more effectively it will maintain a lower temperature. The normal spray rate is 15ml/min. Increasing this to 25ml/min reduces the risk of thermal damage, and has also been shown to improve cutting performance and keep burs cleaner.
The cutting edge
W&H’s new Synea Power Edition has been specifically designed to manage the hardest new materials used in dentistry today including zirconia. Cutting efficiency is maximised by the optimum ratio of torque and speed, making it the optimal solution for advanced ceramic material like zirconia. Its superior cooling capabilities allow a flow rate of over 50ml/min – more than triple the normal spray rate – ensuring greater safety and improved durability.
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