An investigation into the cutting efficiency of a novel degradable glass as an alternative to alumina powder in air abrasion cutting of enamel

Objectives

To develop and test the cutting efficiency of a novel degradable glass as an alternative media to alumina powder for air abrasion.

Materials and methods

A zinc-based glass (QMZK2) was designed, produced, and evaluated with a multi-modality imaging analysis. The glass dissolution study was carried out in three acids, using ICP-OES (inductively coupled plasma optical emission spectroscopy) at 5 different time points: 2.5, 5, 10, 60, and 240 min. The cutting efficiency of both materials was tested under the same parameters on slabs of elephant enamel. A stained fissure of a molar tooth was air abraded with the glass and evaluated with X-ray micro-tomography before and after air abrasion.

Results

The particle size distribution of the glass was similar to that of alumina 53 µm but with a slightly greater dispersion of particle size. The shape of the particles was angular, appropriate for cutting purposes. The dissolution study showed that the glass dissolved rapidly in acidic conditions at all time points. Between the two variables, pressure and powder flow, pressure was found to influence the cutting speed to a greater extent than powder flow.

Conclusions

Alumina powder was found to perform significantly better in 4 of the 9 conditions tested on elephant enamel, QMZK2 in one, and no significant differences were found for the rest of the 4 conditions. The QMZK2 seems to offer promising results as an alternative material to alumina.

Clinical relevance.

QMZK2 glass has the potential for replacing aluminum oxide as a degradable material in air abrasion technology.

Root dentine and endodontic instrumentation: cutting edge microscopic imaging

Cutting of the dental hard tissues is an integral part of restorative dentistry. Cutting of the root dentine is also needed in preparation prior to endodontic treatment, with significant commercial investment for the development of flexible cutting instruments based around nickel titanium (NiTi) alloys. This paper describes the evolution of endodontic cutting instruments, both in materials used, e.g. the transition from stainless steel to NiTi, and the design of the actual instruments themselves and their method of activation-by hand or motor driven. We have been examining tooth-cutting interactions microscopically for over 25 years using a variety of microscopic techniques; in particular, video-rate confocal microscopy. This has given a unique insight into how many of the procedures that we take for granted are achieved in clinical practice, by showing microscopic video images of the cutting as it occurs within the tooth. This technology has now been extended to allow imaging of the endodontic instrument and the root canal wall for the first time. We are able to image dentine distortion and crack propagation during endodontic filing of the root canal space. We are also able to visualize the often claimed, but seldom seen action of contemporary endodontic instruments.

Slice-push, formation of grooves and the scale effect in cutting

Three separate aspects of cutting are investigated which complement other papers on the mechanics of separation processes presented at this interdisciplinary Theo Murphy meeting. They apply in all types of cutting whether blades are sharp or blunt, and whether the material being cut is 'hard, stiff and strong' or 'soft, compliant and weak'. The first topic discusses why it is easier to cut when there is motion along (parallel to) the blade as well motion across (perpendicular to) the cutting edge, and the analysis is applied to optimization of blade geometries to produce minimum cutting forces and hence minimum damage to cut surfaces. The second topic concerns cutting with more than one edge with particular application to the formation of grooves in surfaces by hard pointed tools. The mechanics are investigated and applied to the topic of abrasive wear by hard particles. Traditional analyses say that abrasive wear resistance increases monotonically with the hardness of the workpiece, but we show that the fracture toughness of the surface material is also important, and that behaviour is determined by the toughness-to-hardness ratio rather than hardness alone. Scaling forms the third subject. As cutting is a branch of elasto-plastic fracture mechanics, cube-square energy scaling applies in which the important length scale is (ER/k (2)), where E is Young's modulus, R is the fracture toughness and k is the shear yield strength. Whether, in cutting, material is removed as ductile ribbons, as semi-ductile discontinuous chips, or by brittle 'knocking lumps out' is shown to depend on the depth of cut relative to this characteristic length parameter. Scaling in biology is called allometry and its relationship with engineering scaling is discussed. Some speculative predictions are made in relation to the action of teeth on food.

Outside-the-(cavity-prep)-box thinking

Direct placement restorative materials must interface with tooth structures that are often compromised by caries or trauma. The material must seal the interface while providing sufficient strength and wear resistance to assure function of the tooth for, ideally, the lifetime of the patient. Needed are direct restorative materials that are less technique-sensitive than current resin-based composite systems while having improved properties. The ideal material could be successfully used in areas of the world with limited infrastructure. Advances in our understanding of the interface between the restoration adhesive system and the stages of carious dentin can be used to promote remineralization. Application of fracture mechanics to adhesion at the tooth-restoration interface can provide insights for improvement. Research in polymer systems suggests alternatives to current composite resin matrix systems to overcome technique sensitivity, while advances in nano- and mesoparticle reinforcement and alignment in composite systems can increase material strength, toughness, and wear resistance, foreshadowing dental application.