these cavity shapes can be slightly over-tapered as most of the retention is gained from the cement lute. however, for indirect restorations using gold then the preparation shape must have parallel walls as most of the retention is gained from the cavity shape. the process of preparation and cementation of an inlay or onlay is usually carried out over two appointments, with the preparation being carried out during the first appointment and cementation at the second. after the tooth has been prepared at the first visit a putty and wash impression should be taken of the prepared tooth to be sent to the laboratory for fabrication of the indirect restoration. also, a provisional restoration is made using material such as protemp, to protect the tooth from sensitivity, loss of vitality, bacterial contamination, to prevent over-eruption of the prepared tooth and also maintains dental aesthetics for the time being. for tooth preparation, firstly start with occlusal reduction which depending on the restorative material being used can range from 0.5mm-2mm. the best instrument to use for this is a high-speed diamond fissure bur and the reduction should follow the inclination of the cusps and grooves as this will allow the preservation of more tooth tissue. a functional occlusal bevel should be created on the occlusal loading cusp of molar teeth by holding the bur at a 45o angle to the occlusal surface. this is the buccal cusp for mandibular teeth and the palatal cusp for maxillary teeth. a high-speed tapered diamond bur has the most convenient shape to prepare the buccal, lingual and proximal reduction of the tooth. in the majority of clinical situations an inlay preparation is being made from a tooth that already has a class ii restoration and is being prepared to protect the tooth. the restoration is firstly removed and then the cavity can be converted by ensuring any undercuts are eliminated from the preparation. there are two ways this can be done, either by blocking the undercuts out with an adhesive restorative material or by removing tooth tissue to create the divergent cavity needed. fabrication methods there are a few methods of fabricating inlays and onlays, depending on the restorative material used. indirect restorative technique the first common step is always to take an impression of the tooth preparation – either by scanning it using an intraoral scanner or by taking a conventional impression using polyvinyl siloxane. table adapted from a study done by hopp et al. type of restorative material examples of system available on mmarket fabrication method feldspathic porcelain lost wax technique leucite reinforced porcelain ips empress 2 / ips e.max by ivoclar vivadent lost wax technique cad-cam alumina core porcelain inceram by vita zahnfabrik- germany slip-casting technique lithium disilicate porcelain duceram plus by dentsply degussa lost wax technique cad-cam the details of the fabrication methods are as follows: cad-cam cad-cam (aka computer aided design-computer aided manufacture) is a fabrication method which aids the production of dental restorations e.g. crowns, bridges, inlays and onlays. it is possible for these indirect restorations to be provided in one visit. this works by first taking an intra-oral scan to produce a 3d virtual impression. the 3d image is then sent to the chosen software programme, available software programmes include cerec acquisition center system (sirona dental systems, charlotte, n.c.) or the e4d dentist system (d4d technologies, richardson, texas). it is possible for the 3d image to originate from conventional casts which have been poured up in stone/plaster. the model is scanned and the 3d image is uploaded onto the software. next, the restoration can be designed, by the software programme, on the virtual casts. once confirmed, the milling process can begin. the milling process uses pre-fabricated blocks of restorative material, e.g. lithium disilicate or composite reinforced ceramic, to produce the end product. finally, the restoration is sintered/glazed chair side. lost wax technique there are two ways in which wax can be used to produce cast metal/ceramic indirect restorations, these are known, firstly, as the indirect wax technique using type 1 inlay wax and, secondly, the direct wax technique using type 2 inlay wax. for technique 1, a wax pattern is designed on the die from the cast impressions and for technique 2 the wax is packed into the tooth preparation in the mouth and adapted the shape of the cavity. inlay wax is chosen due to its brittleness – it breaks upon removal from undercut of a cavity, either on the die or in the mouth. in this case, this is a beneficial character as it helps us to identify the presence of an undercut which then can be removed. once the wax pattern is constructed the fabrication of this into the final restoration can begin, a method known as the lost wax technique is used. the wax is embedded into an investment material with a sprue former – this forms a passage for molten metal to be poured through into the cylinder. the investment material must produce enough expansion to compensate for shrinkage of the metal on solidification and should be slightly porous to allow for dissipation of released gases. the container, or casting cylinder, is then placed in a furnace to burn out the wax and what is left is a hollow shape ready for molten metal to be poured into. the metal can be melted using either gas and compressed air, gas and oxygen or electric arc. casting methods include the use of steam pressure or a centrifugal system. gold is an example of a material that can be used to produce inlays/onlays. once cast a layer of oxides are present on the surface, these can be removed by placing the restoration in an ultrasonic bath for 10 minutes. this removes the oxides along with any remnants of investment material. possible faults at casting: subsurface nodules – poor investment causing air bubbles which become filled with molten metal ridges – moving casting cylinder before investment sets fins – heated too quickly roughness – overheating causing investment to disintegrate foreign bodies distorted casting – faulty impressions or casts, failure to invest wax once immediately removed from the die or insufficient heating of mould porosity – presence of voids due to uncompensated alloy contraction and absorption of mould gases slip casting technique this method is only applicable to sintered alumina core porcelain. firstly, a sub-structure made of alumina powder and modelling fluid is built on the special die. w the die is then sintered by firing with the sub-structure, causing the fluid to be absorbed by the die and the alumina powder become more tightly packed. following sintering, the outer surface of the sub-structure is painted with lanthanum aluminosilicate glass powder. the sub-structure is porous and therefore allows infiltration of the glass powder when fired again. further strengthening of the material can be done by applying zirconium oxide. direct restorative technique by using this technique, an impression of the tooth preparation is not required. instead, the tooth preparation is coated with a layer of separating material such as glycerin first. then, a composite restoration is built up directly on the preparation, allowing it to take the shape of the cavity. the restoration is then light-cured in the tooth before being removed from the tooth to be further light-cured. prior to cementation of the restoration onto the prepared the tooth, the layer of separating materials needs to be removed to ensure effective bonding between the tooth and the restoration. this technique is only applicable when composite is used as the restorative material. inlays and onlays made from ceramic or metal alloy require laboratory work and therefore can only be fabricated using indirect restorative techniques as mentioned in the previous section. survival rates a study done by rippe et al. has shown that ceramic inlays produced by the different methods, via indirect restorative techniques aforementioned, have similar longevity. materials gold onlay and inlay gold the use of gold as a restorative material for the production of inlays and onlays is fading due to the increase in usage of more aesthetically pleasing tooth coloured materials. gold has many advantages as a restorative material, including high strength and ductility, making it ideal to withstand the masticatory forces put upon the teeth. it is strong, ductile, can be cast accurately and not abrasive to opposing dentition. this property allows gold to be used in thinner cross sections, meaning less tooth tissue needs to be removed during tooth preparation compared to other restorative materials, to achieve the same strength. the tooth preparation needs near parallel wall and an absence of undercut, hence less occlusal and axial reduction. when the patient is not concerned with aesthetics, such as in posterior teeth, gold can provide the properties needed. ceramic ceramic materials began being used in restorative dentistry in the 1900s. ceramic offers a more aesthetically pleasing restoration colour than previous gold and amalgam restorations. it is aesthetic and has high wear resistance. however, ceramic used as a restorative material without metal reinforcement have reduced strength and are more prone to failure. this is because conventional ceramics have a higher fracture risk and fractures can propagate easily under cyclical loading causing marginal ridge or bulk fracture. to compensate for this, ceramic is placed in a thicker cross-section of at least 2mm for stress bearing areas. this reduces flexure under loading and prevents crack formation. ceramic can also be abrasive to opposing tooth surfaces upon mastication. more tapered walls are needed in tooth preparation. undercuts can be eliminated by further tooth preparation or be blocked out with an adhesive tooth - coloured material. composite resin composite is widely used in dentistry as a direct and indirect restorative material. it comes in different compositions, variable in content and size of filler particles. composite inlays and onlays offer great aesthetics, as a combination of different shades and opacities can be used in a layering technique, equalling or surpassing the aesthetically pleasing all-ceramic restoration. it can be easily repaired or modified as composite can bond to existing material. using composite as a direct restoration can have a relatively high polymerisation shrinkage, but this can be avoided by using a laboratory indirect composite restoration. due to the more in-depth curing method, using heat, pressure or strong light, this can have a lower polymerisation shrinkage. lab made composite has reduction in polymerisation shrinkage as there is higher degree of curing in lab as compared to chair-side curing. however, fewer reactive resin groups are available to bond to the resin luting cement indicating lower accuracy in fitting compared to the other materials. however, using this indirect laboratory method demands more skill and time, and is more destructive as tooth preparation is needed prior to taking an impression. compared to ceramic and gold inlays and onlays, composite can provide similar advantages, but a comparison of the longevity of composite is unknown. metal-ceramic metal-ceramic inlays were developed to see if the aesthetic advantages of an all-ceramic inlay restoration could be replicated, whilst improving the strength and stability of the restoration. a study showed that the fracture resistance of all-ceramic inlays was greater than that of these metal-ceramic inlays. it went on further to find that it was the taper of the inlay preparation that affected the fracture resistance more so than the choice of restoration material. cementation inlays and onlays are cemented in the mouth using adhesive resin luting cements. these materials are placed in the inlay/ onlay and placed onto the prepared tooth. once the onlay/ inlay is seated, the viscosity of the adhesive resin luting cement decreases. this makes placing the inlay/onlay easier and reduces the chances of breakage. luting cements can have a low modulus or a high modulus. low modulus luting cements are more flexible and withstand more force without failing. if a high modulus cement is used to cement an inlay or onlay, the restoration will not be able to transfer stresses properly into the cavity walls. with a low modulus; the cement will absorb deformations under loading and limit the pressure transmitted to surrounding tooth structures. composite inlays are cured by polymerisation shrinkage. usually the composite will have been ‘super cured’, which means cured using temperature, pressure or light. this makes the inlay sturdier and improves wear. however, the disadvantage of super curing is that it leaves less