For dental restoration in cases of caries, burrs, sometimes colloquially referred to as drills, are used in every dental practice. These instruments are also used in dental surgery, especially for jaw surgery.
What is a milling machine?
Burrs, sometimes colloquially referred to as drills, are used in every dental practice for the restoration of teeth affected by caries. A milling cutter is defined as a tool in which a material is removed by means of a rotating movement or a fixed body is brought into a certain shape. Milling is closely related to drilling. However, while drilling only involves working in one direction, milling takes place in all spatial positions, i.e. three-dimensionally. The rotating tool with wedge-shaped cutting edges is moved to the stationary object to be machined. The cutting edges penetrate the object and remove material. This material removal is determined by the desired penetration depth and feed rate. In dentistry, milling cutters made of hard metals are mainly used. Carbide is a sintered mixture of hard material, mostly tungsten carbide, and a binder. These materials are characterized by very high hardness and wear resistance and are also easy to sterilize. Alternatively, steel burs or, especially in oral surgery, burs tipped with natural diamonds are used. When using carbide or diamond burs, it should be noted that these materials are extremely brittle and can break quickly. Therefore, care must be taken to minimize vibration when working on the diseased tooth.
Shapes, types and styles
The most commonly used burr in dentistry is the ball burr. Thanks to its spherical shape, this instrument is universally applicable; it is mainly used to remove carious areas on or in the tooth. A special cutting geometry ensures fast and gentle material removal in this type. As a rule, the cutting edges are also toothed, which ensures quiet and vibration-free working. As work can be carried out with minimum pressure, thermally induced traumatization of the tooth root is avoided. Milling machines with elongated heads are used for root therapy. These periodontal instruments rotate at extremely high speeds (8000 – 12000 min-1) and are used to clean or smooth exposed root canals. These burs are also used for root surfaces in deep pockets or in hard-to-reach tooth spaces. Finishing burs are used for smoothing the surfaces of the tooth. These burs can have different geometries, round or conical shaped attachments are the rule here. The cutting edges of the finishers are designed less for material removal than for smoothing surface roughness. As a result, the teeth here are narrower and the cutting angles are smaller than on cutters designed to remove defective tooth material. Another milling cutter is the crown cutter. This instrument, which has a very special cutting geometry, is designed for machining non-precious alloys, i.e. removing amalgam fillings or crowns. In oral surgery, the Lindemann burr, named after its inventor August Lindemann, is used to cut through bone or teeth. This burr is a hybrid of a milling instrument and a saw, and its special shape, which tapers toward the tip, makes it particularly gentle to use during surgery.
Structure and mode of operation
Regardless of its shape and intended use, a burr always consists of a handle (“shank”) and the interchangeable burr attachment. In the case of the handle, a distinction is made between the turbine and the dental contra-angle handpiece. The turbine is operated by compressed air and reaches speeds of up to 450,000 rpm, but has a lower torque than the motor-driven contra-angle handpieces. The milling attachment is distinguished between the neck and the head. The head refers to the part of the cutter that works directly on the tooth, while the neck is an extension between the head and the shank. This extension maintains a distance between the head and the shaft so that the instrument can be easily inserted into the mouth. Dental milling machines operate in high speed ranges between 2000 and 20,000 revolutions per minute. These speeds mean that there are always several cutting edges in operation, which enables gentle treatment of the diseased tooth.
Medical and health benefits
The use of milling machines has revolutionized dentistry. Until the end of the 18th century, the only way to treat a diseased tooth was to remove it. This was by no means done in a way that was gentle on the patient. So-called “tooth breakers” often levered the teeth out of the jaw without anesthesia and stopped the bleeding by applying red-hot irons. The New York dentist John Greenwood then developed the world’s first drill in 1790 – hand-operated and using his grandmother’s spinning wheel as a drive. A lot has changed in milling machines since then. After the electric drill was developed at the end of the 1880s, dentistry took a quantum leap, especially in the area of rotational speeds. What was initially 500 min-1 has now become 450,000 min-1, which is what made low-pain tooth removal possible in the first place. Diseased teeth not only look unsightly and can cause enormous pain, but if left untreated, infections in the jaw area in particular can quickly lead to sepsis, which can be life-threatening. In earlier times, many an unhealed root infection led to an early death. Missing teeth also cause jaw misalignment and speech problems. The use of milling machines in dentistry has reduced the suffering of millions of people to a tolerable level and largely removed the horror of dental treatment.
