Cutting burs are rotary instruments used in various fields, including dentistry and manufacturing, for shaping, grinding, or removing hard materials. The material used to make the bur’s head is the primary factor determining its performance, longevity, and optimal application. The three main categories of materials used for these instruments are Tungsten Carbide, Steel, and Diamond.
Tungsten Carbide: The Standard for Precision Cutting
Tungsten carbide burs are considered the industry standard for rapid and precision cutting due to their exceptional hardness, which is three times greater than steel. The bur head is composed of tungsten powder mixed with cobalt. This mixture is compacted and then sintered to create a material that is second only to diamond in hardness.
The manufacturing process involves soldering the tungsten carbide head onto a stainless steel shank and then grinding the head into shape. This grinding creates sharp, blade-like cutting edges, known as flutes, which are positioned for efficient material removal. The flutes slice or shave away the material rather than grinding it down.
The wear resistance of tungsten carbide allows the cutting edges to remain sharp longer, making it ideal for high-speed applications. These burs are primarily used for bulk material removal, such as preparing a tooth cavity or sectioning a tooth for extraction. Speed and durability make tungsten carbide the preferred choice for cutting through hard structures like enamel and old metal restorations.
High-Speed Steel and Carbon Steel Burs
Steel burs, made from materials like stainless steel, were historically significant and are still used in specific, limited applications. These instruments are notably softer and more flexible than their tungsten carbide counterparts. This difference in hardness means they dull much faster, especially when used on extremely hard materials like tooth enamel.
The relative softness of steel burs makes them unsuitable for the high-speed friction grip handpieces used for hard tissue removal. They are used with slower-speed handpieces, operating at rotational speeds below 25,000 revolutions per minute. Steel burs excel in applications where a gentler approach is needed, such as the removal of softer carious dentin without damaging the underlying pulp.
They are also a cost-effective option for procedures that do not require the extreme hardness of carbide or diamond, such as trimming soft acrylic materials in a laboratory setting. However, their tendency to rust and their lower resistance to wear have largely relegated them to specific, low-force procedures in modern practice.
Diamond Burs: Abrasive vs. Bladed Cutting
Diamond burs function on a fundamentally different principle than the bladed cutting action of carbide and steel burs. Instead of slicing, diamond burs operate through abrasion, grinding the material away. They are manufactured by bonding natural or synthetic diamond particles to a stainless steel shank.
This bonding is often achieved through an electroplating process, where metal is deposited onto the shank, entrapping the diamond particles. The size of these embedded diamond particles, referred to as grit, dictates the bur’s function. Coarse grit burs, with larger particles, are used for rapid, gross material reduction, such as the initial preparation of a tooth for a crown.
Conversely, fine and extra-fine grit diamond burs are employed for the final stages of a procedure, such as smoothing, finishing, and polishing a prepared surface. These finer burs remove less material but create a much smoother finish, which is necessary when working on materials like ceramics or for delicate aesthetic adjustments to enamel.
Matching Material Properties to Clinical Application
Tungsten carbide burs are selected when the objective is controlled, efficient material removal at high speed, relying on their sharp flutes to cut through dense tissue like enamel and dentin. They are the go-to for opening cavities and bulk reduction because of their fast cutting rate.
Diamond burs are chosen when a smooth, precise surface finish is the priority, which is achieved through their abrasive action. The grit size allows for a graded approach, starting with coarse for initial shaping and finishing with fine for a polished result, often used on the hardest surface tissues and restorative materials.
Steel burs are limited to lower rotational speeds and are preferred for excavating softer materials, like decayed dentin, where the goal is to remove the soft tissue without unnecessary pressure or heat generation. This material selection ensures the bur is optimally suited for the tissue and the desired outcome.