In 1898, Willoughby Dayton Miller developed his theory on the process of dental caries (tooth decay) entitled 'The Micro-Organisms of the Human Mouth' during his research at the Berlin hospital Charité which is basically still applicable nowadays:
Initially, acid production by micro-organisms causes demineralisation of the dental hard tissues enabling them to enter the tooth.
Thereafter, the bacteria dissolve the organic substance of the hard tissue.
The objective of caries excavation must be to completely eradicate the micro-organisms and thus prevent further destruction of dental tissues and to protect the pulp and thus the body from bacterial infection.
Prior to the introduction of antibiotics, odontogenic abscesses frequently had a fatal outcome.
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While the treatment of superficial enamel caries is relatively easy and does not usually cause problems for the patient, the treatment of dentine caries is more difficult as the odontoblast processes directly connect dentine with the dental pulp.
Dentine caries can be present in 2 different layers.
The superficial, or external, areas are contaminated with bacteria dissolving the dentine's hard tissue and thus damage the collagen matrix in such a way that remineralisation is not possible; therefore, this area must be removed completely.
In the inner, or deeper, part there will be fewer bacteria, sometimes even none at all, so that dissolution of the mineral part can still occur though the collagen matrix will keep its specific ultra-structure. If the bacteria and their metabolites - the major cause for caries - are removed, the deep layer of dentine caries can be remineralised and then left in-situ.
Conventional caries removal
Treatment of dentine caries is usually more painful for the patient then the preparation in enamel because dentine is less mineralised and better innervated.
Therefore, for many patients, conventional caries removal using rose-head burrs (ie the use of milling instruments for the low-speed removal of dentine affected by caries) is associated with unpleasant sensory experiences.
The typical pain during preparation is caused by the opening of dentine tubules and increased by vibration, contact pressure and friction heat in addition to thermal effects of water or air cooling. The sensation of pain often results in the necessity for local anaesthesia which can further frighten the patient.
Because of these problems, alternatives to the classical instruments for preparation have been researched for many years. These alternatives are described as follows:
Mechanical rotating caries excavation
The rose-head burr can be considered to be the classical mechanical instrument for the removal of carious dentine.
According to the German Industrial Standard (DIN) 8580 it cannot be said to be a "drill" (burr) but, more a mill with geometrically arranged cutting blades.
Rose-head burrs are circular, toothed all around (spirally), which, in the case of a motor that turns clockwise, means the dentine is remodelled by the excision of tissue shavings.
The mills are made of stainless steel, tool steel, high-speed steel or carbide.
Under normal circumstances, the rose-head burr is used at rpm of between 500 min-1 and 4500 min-1.
The advantages of the rose-head burr are its
- effective work rate,
- ease of use, and
- low procurement cost.
But on the other hand, there are many disadvantages:
- Noise, vibration, pain.
These are concomitant phenomena that are associated with fear of the dentist by the patient.
- The danger of over-excavation into healthy dentine,
as it is not possible with the rose-head burr to differentiate selectively between healthy dentine and that affected by caries.
- The danger of thermal damage to the pulp
by the use of too high a rotational speed and over-excavation.
As an improvement, some companies offer rose-head burrs with cross-hatched cutting blades to reduce vibration during excavation, e.g. H 1 SE by the company Komet (Brasseler Bros., Lemgo).
SmartPrep instruments (SS White, Lakewood, NJ, USA) are supposed to be able to differentiate during caries excavation between the two layers of carious dentine. This would result in a minimally invasive excavation with the advantage of fewer dentine tubules being cut open and therefore reduced pain.
At first sight, SmartPrep instruments are similar to traditional rose-head burrs; however, they are not manufactured from metal but from a special plastic polymer. The cutting blades are also not aligned spirally but are arranged straight like a row of shovels.
The polymer used to make SmartPrep instruments has a Knoop hardness index of 50 and was developed under the constraint that it should be harder than carious dentine (Knoop hardness index of 0 to 30) yet softer than healthy dentine (Knoop hardness index of 70 to 90). Therefore, the SmartPrep should be able to remove carious dentine selectively, while leaving healthy dentine intact.
The plastic cutting blades wear away immediately when they come in contact with harder material and become unusable. The instruments are therefore self-limiting and intended for single use only.
SmartPrep instruments are available in the three ISO sizes 010, 014 and 018. A SmartPrep instrument is used in slow moving angle pieces at rpm of between 500 min-1 and 800 min-1.
With traditional rose-head burrs, the lesion is excavated following primary preparation from the peripheral area towards the centre. In the case of SmartPrep instruments, it is precisely the other way round. In order to avoid contact with the enamel, the lesion is excavated from the peripheral towards the central area following primary preparation. The diseased dentine should be excised in layers using a light application of force.
In comparison to rose-head burrs, adhesion coefficients of various dentine adhesives were sometimes significantly lower at the surface of the dentine when SmartPrep instruments had been used for excavation. The reason for this may lie in the insufficient removal of caries by the polymer instrument as the adhesion coefficients of dentine adhesives to healthy dentine are significantly higher than those to carious dentine. In-house controlled tests have shown, that SmartPrep was not always able to completely remove carious dentine (Dammaschke et al. 2005). Histologically, significant carious remnants were often found in extracted teeth in comparison to those excavated using the rose-head burr. Additionally, the layer of residual caries remnants was, in comparison to the rose-head burr, significantly thicker. With regard to the time used in excavation, no significant differences could be found between the two instrument types. In the meantime, the company SS White has changed the configuration of the cutting blades. Currently the instrument is being sold under the name SmartBur. To date, further information has not been made available.
Mechanical non-rotating caries excavation
The basic shape of these traditional, proven instruments extends from spoon-like dentine to plate-shaped pulp excavators. Most excavators have diametrically opposed, double ends with one anti-clockwise and one clockwise cutting tool section, whereby the rake angle may changed from one to three times. In cross section the sharp-edged arc of a circle can usually be seen.
To work efficiently, these cutting blades must be re-sharpened over and again.
One of the great disadvantages of excavators made of carbide is that these cannot be re-sharpened as a result of their method of production. Consequently, the use of excavators cannot be recommended wholeheartedly. Excavators are suited in particular to the removal of softened dentine, whereby they remove it in layers and leave healthy dentine fundamentally alone.
Excavation is a relatively long-winded process.
The advantages of the excavator are:
- the low procurement cost and
- the simple and gentle ease of use.
There is no vibration and no thermal irritation. However, even here pain can be caused as a result of pressure especially on the odontoblast processes.
It was possible to show that in comparison with the rose-head burr adequate eradication of caries
can be achieved with manual instruments as well.
Air abrasion devices
The historical development of air abrasion devices in the US can be traced back to R. B. Black (1945). Because of technical problems and the advent of turbine and high-speed angled instruments, air abrasion devices were not able to penetrate the dental market.
Air abrasion devices operate using aluminium oxide particles between 27 µm and 50 µm in diameter and at a pressure of between 7 and 11 atm. The particles leave the jet nozzle at a speed of approximately 2,000 km/h. In so doing, 4,300,000 particles leave the machine every second. Air abrasion is particularly effective in removing hard material, while soft tissue slows the particles down and thus absorbs the kinetic energy.
Cavities made using air abrasion are round and uneven with unclear and frayed preparation boundaries,
so that the cavity has to be reworked using traditional rotating instruments in order to achieve a classic preparation. As adhesive filling technology has developed, using in part semi-liquid, flowable composite materials, some manufacturers have propagated a renaissance of the kinetic preparation method. Since 1992 and 1994, second and third generation machines have been on the market.
Because of high dust pollution, Beetke et al. (1998) report that a clinical study reviewing preparations using air abrasion was abandoned. Horiguchi et al. (1998) were able to show in their in-vitro studies on experimentally reproduced dental caries that selective caries removal with a specially ground polycarbonate plastic appeared to be possible. The hardness of the polycarbonate particles (Vickers hardness index 40 to 50, particle dimension 338 µm) lies slightly below that of healthy dentine (Vickers hardness index 70). Only this approach seems to make selective caries excavation possible without the impairment of healthy dental hard substance (a line that is being pursued by the SmartPrep/SmartBur system as well). Aluminium oxide particles, which are normally used, are considerably harder (Vickers hardness index 2,000 to 2,300) and simply stick into softened dentine without having any visible effect.
There are many serious disadvantages to air abrasion:
- lack of tactile sensitivity,
- visual impairment due to dust,
- scratching of the dental glaze,
- dust inhalation by the patient and dental personnel etc.
- Caries cannot be prepared or excavated from undercuts.
Modern air abrasion devices may be able to remove dental discolourations, cleanse fissures and pits prior to sealing and roughen surfaces in preparation for an application of a composite adhesive agent, but they are unable to attain a quality standard comparable to that which can be achieved with conventional cavity preparation techniques
Similarly it is not possible either to talk of effective caries removal, as the powder particles show no effect on softened dentine, whereby hard tissue is easily cut away by the kinetic energy
As early as the 1950s, various ultrasonically driven systems for dental preparation were introduced. Recently, oscillating instruments have aroused serious interest once again where minimally invasive treatments are concerned.
The Sonicflex System (KaVo, Biberach, Germany), a new development of the sonically abrasive preparation process, will be examined more closely below.
Sonicflex is a modified airscaler, which is connected to the dentist’s unit via a turbine connection. Oscillation occurs in the sonic range below 6.5 kHz. For preparation, unilaterally diamond-embedded instruments e.g. in a hemispherical, torpedo or angled shape are available. The average grain size is 40 µm. These instruments are mainly intended for preparation and not for excavation, even if their use for such is not unthinkable. Specifically for caries excavation, ball-shaped diamond-embedded instruments (diameter 0.8 mm and 1.2 mm) as well as carbide instruments have been developed (diameter 1.0 mm and 1.4 mm) (cariex D and cariex TC, KaVo, Biberach).
Dentists' experiences or scientific test results with regard to the effectiveness of these instruments are, however, not yet available.
The advantage of these ultrasonically driven excavation instruments is obviously the minimally invasive character of the way they function.
The unpleasant noise during treatment as well as the hitherto lack of test results with regard to their effectiveness have to count as disadvantages. Banerjee et al. (2000) examined the five different excavation methods (rose-head burr, air abrasion, ultrasonic abrasion, Carisolv and excavator) by means of a confocal laser scanning microscope with regard to their effectiveness ("excavation result") and measured the elapsed time ("duration"). As far as excavation was concerned, ultrasonic abrasion performed worst of all. However, it should be noted that, for the ultrasonic abrasion by means of a Sonicflex, diamond-embedded preparation instruments and not the attachments specially developed for excavation purposes were tested.
- Banerjee A, Kidd EAM, Watson TF (2000), In vitro evaluation of five alternative methods of carious dentine excavation, Caries Res 34: 144-150
- Beetke E, Slowikowski S, Meißner M (1998), Pulverstrahlgeräte Möglichkeiten und Grenzen, ZWP 4 (Heft 6), 134-142 (1998)
- Black RB (1945), Technique for nonmechanical preparation of cavities and prophylaxis, J Am Dent Assoc 32, 955-965
- Dammaschke T, Rodenberg T, Schäfer E, Ott K (2005), Vergleich der Effektivität von SmartPrep mit herkömmlichen Rosenbohrern bei der Exkavation der Dentinkaries, Dtsch Zahnärztl Z 60:203-6
- Horiguchi S, Yamada T, Inokoshi S, Tagami J, (1998), Selective caries removal with air abrasion, Oper Dent 23:236-43