W&HのSynea Power Edition(サイネア パワーエディション)による技術革新と問題解決

花崗岩のような硬い材料を切削するため、回転器具を使うという概念は、少なくとも6,000年以上前からありました。紀元前25,000年前には、貝殻を壊さずに切るため、私たちの祖先は手持ちの石器を使っていました。これらの器具は一般的に火打ち石でできていました。穴を開けたい対象物に押し当て、低速かつ比較的強いトルクで前後に回転させて使用しました。 現代のパワフルなハンドピースは、高トルクかつ回転速度変更の原理を発展させ、ジルコニアのような遥かに硬い材料を正確に切断できるようになりました。

修復材料に関する技術は急速に発展しており、器具の開発にも革新的な動きが見られます。ジルコニアのような耐久性のある材料は、本物の歯の審美性と機能を模倣する可能性を、益々高めています。このような堅牢な材料に伴い、回転切削に関する技術の進歩が必要とされています。なぜなら、破折した歯冠を除去するためには、破壊的な分解が最も安全で外傷の少ない選択肢であり続けるからです。

The use of zirconia in restorative dentistry

Zirconia, ZrO2 zirconium dioxide, has become a popular material in restorative dentistry due to its biocompatibility, high fracture toughness and radiopacity. Dental zirconia is more commonly a modified yttria (Y2O3) tetragonal zirconia polycrystal (Y-TZP), because it has greater mechanical properties and better tear resistance than other ceramics. Various formulas are currently in development to continue to improve on aesthetics as well as durability.

Despite the benefits of zirconia, restoration failure due to crown fracture remains a possibility. In a recent survey, the difficulty associated with its removal or replacement was reported by practitioners to be one of the main disadvantages of using zirconia as a prosthetic material.

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 ranks at 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 required 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 an increased 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. In tests, an intrapulpal temperature increase of 5.5℃ for 10 seconds was shown to cause irreversible damage to pulp tissues. 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, this can cause the water to heat up, which can potentially hurt the patient and/or handpiece operator, so techniques to reduce overheating and improve cooling have had to be innovative. 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 not only reduces the risk of thermal damage, it has also been shown to improve cutting performance and keep burs cleaner.

The cutting edge

W&H has been a global leader in the development and manufacture of medical technology products since 1890. Their new Synea Power Edition has been specifically designed to manage the hardest 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.

Innovation in one area of technology often requires other technologies to advance rapidly in oder to keep up. As long as material sciences continue to develop to deliver greater restoration robustness, innovation in handpieces designed to support new clinical needs will undoubtedly also grow.

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