metal alloys are available and are likely to gain fairly wide use as alternatives for those patients with known or suspected nickel allergy.

Standards for these base metal alloys place considerable emphasis on composition and the relationship of composition to known or potential biological hazards. Manufacturers of both groups of alloys are required to place warnings on the packaging of materials relating to the presence of more than 1% nickel or other potentially hazardous element in Co/Cr alloys (ISO 6871-1:1996) or a warning that the alloy contains nickel or Ni/Cr alloys (ISO 6871-2:1996) and in the latter case a further warning if the alloy contains more than 0.02% beryllium or other hazardous element. ISO 6871-2:1996 also outlines guidelines for the handling of alloys containing beryllium, including the need for adequate ventilation and the use of protective clothing and equipment. An amendment to ISO 6871 (parts 1 and 2) now limits the permitted level of beryllium to less than 0.02%.

Titanium and alloys of this metal with, for example, vanadium are known to have favourable biocompatibility and are likely to become more widely used for dental applications in the future. At the present time the use of these materials is mainly restricted to preformed implants.

8.7 Metals and alloys for implants

Implants offer an alternative method of treatment for the replacement of missing teeth which can be used instead of dentures or fixed bridges. They generally have a structure which enables one part of the implant to be located beneath the oral soft tissues (mucosa) such that it can be stabilized by resting on the bone or by being embedded in the bone. The other part of the implant structure protrudes through the mucosa to provide a structure suitable for supporting a denture, crown or bridge. The requirements of implant materials encompass biocompatibility, acceptable stability in the medium-long term, acceptable function and ease of manufacture. Biocompatibility and stability are often seen as closely related in that some materials are known to encourage bone growth which produces a very intimate interface between bone and implant which helps to stabilize the latter. Function primarily depends upon the rigidity of the implant structure. This in turn is related to the dimensions and the modulus of elasticity of the material from which the implant is manufac-

tured. The use of high modulus materials enables implants of smaller cross-sectional bulk to be used.

Dental implants are normally classified according to the way in which they are stabilized. The three most common types are: subperiosteal, blade-vent endosseous and osseointegrated. Subperiosteal implants consist of an open framework of cast alloy which rests on top of the bony ridge but beneath the mucosa. The variable geometry of the bony surface means that each implant must be fabricated individually. Cast cobalt–chromium alloys are most commonly used for these applications. The very high modulus of elasticity of these materials (Table 8.1) combined with reasonable castability are the main factors affecting this choice. Attempts have been made to improve the biocompatibility of the alloys by using hydroxyapatite coatings. Subperiosteal implants have now been superseded largely by osseointegrated implants embedded in bone. Their major cause of failure was that the oral mucosa grew to cover the whole of the surface of the implant, ‘externalizing’ it. Obviously the risks of infection in such a situation are high.

Blade-vent endosseous implants involve the use of a design in which one end of the implant (the blade) is embedded into the bone whilst the other end protrudes through the mucosa into the oral cavity. These implants are normally constructed from titanium which has excellent biocompatibility, although this characteristic cannot be used to the greatest effect in blade-vent implants because the implant has insufficient time to stabilize within the bone before it is placed under load. Also the techniques for preparation of the bony socket tend to be less well controlled. A critical factor for success in osseointegration is careful bone surgery to minimize bone damage associated with heating. This necessitates the use of very low cutting speeds and copious cooling of the cutting instruments. Unlike the subperiosteal implants, the blade-vent variety are commercially available in standardized shapes and sizes and casting is not required. The clinical success or failure of the blade-vent implants is a controversial issue. The most common cause of failure is due to the implant becoming loose.

More recently a combination of improved awareness of the clinical techniques that minimize trauma to bone and soft tissues during implant insertion and the development of biocompatible implant materials have led to the current

generation of clinically and commercially successful implant designs.

Fundamentally the clinical technique is characterised by careful planning of the site and orientation of the implant (or fixture as the intraosseous element is often termed), to ensure that there is an adequate quantity of bone in the relevant position in the jaw. If there is not adequate bone present then it is possible to place a bone graft to increase the amount of bone present. The surgical technique is designed to minimize the surgical damage to bone. This is associated predominantly with excessive heating of the bone during preparation of the implant site; thus all drilling procedures are carried out at low drill speeds with copious cooling of the burs with normal saline. Some implant manufacturers produce drills and taps which have internal cooling to ensure adequate cooling of the bone at the end of the cutting tip of the drill. The current generation of dental implants are all root form (i.e. cylindrical in nature) and usually have a screw thread to hold them in place while the healing process occurs.

Placement of an implant usually involves two surgical procedures, with the fixture being buried in bone at the first procedure with soft tissue closure to allow for bone healing and integration of the fixture into bone. A second procedure is then undertaken some time later (3 months in the mandible and 6 months in the maxilla) when the fixture is identified and the element of the implant that passes through the gingival tissues (the trans-

mucosal element) is attached to the top of the fixture. There is now a trend for ‘immediate loading’ of implants where the trans-mucosal element is attached to the fixture during the first procedure. Great care needs to be taken not to apply excessive loads to the fixture when doing this as this would prevent integration of the implant to bone. The objective of this process is to allow osseointegration to occur. This is a biological state where the bone of the mandible or maxilla grows into physiological contact with the implant itself effectively ankylosing the titanium fixture into place. The healing process is also characterised by the formation of a tight epithelial cuff around the head of the fixture or the transmucosal element with cellular attachment between the epithelium and the metal of the fixture. This pattern of attachment between implant and bone/epithelium is in contrast to that seen with a tooth which is effectively independently ‘sprung’ within its socket with support from the periodontal tissues (Fig 8.1). This difference can lead to complications if restorative structures are supported partially by teeth and partially by implants. This scenario should be avoided if possible. Obviously the material that is used to manufacture the implant is critical to this pattern of success. Modern implantology is only possible because of the biocompatibility of commercially pure (cp) titanium or a titanium/aluminium/ vanadium (Ti-6Al-4V) alloy. The majority of commercial implants use cp titanium. Titanium

is both relatively light and has adequate physical strength for the purpose. As with any metal it can be subject to heat treatment post-manufacture to optimize the grain configuration, giving a microstructure with a small α grain size of less than 20 μm and a well dispersed β phase with a small α-β interfacial zone. Such structures are best able to resist cyclical fatigue and crack propagation.

There have been numerous attempts to improve the pattern of attachment of bone and epithelium to the implant. These include the application of thin layers of bioactive ceramic onto the surface of the fixture, comparisons between screw threaded and smooth implant designs and surface treatments of the titanium surface itself.

Bioactive glass-coated implants had good initial success rates, but there proved to be a problem with breakdown at the ceramic–metal bond resulting in premature failure of the fixture.

The current evidence would suggest that threaded fixtures are more successful than smooth surface implants, probably because the threaded structures have greater initial stability in bone and hence are more likely to integrate well.

Finally the current evidence would suggest that some modification of the surface of the titanium to produce a textured surface will improve shortterm and long-term stability of the fixture. This roughening can be achieved by machining the surface of the implant or by sandblasting. These textured surfaces generate greater interfacial fracture strength with bone giving better stress transfer between bone and the implant. Roughening of the surface of the implant or the addition of a screw thread will weaken the fixture itself but the reduced strength is more than compensated by their improved clinical performance.

8.8 Suggested further reading

Keller, J.C. (1999) Physical and biological characteristics of implant materials. Adv. Dent. Res. 13, 5.

Moffa, J.P. (1983) Alternative dental casting alloys.

Dent. Clin. North. Am. 27, 733.

Wataha, J.C. (2002) Alloys for prosthodontic restorations. J. Prosthet. Dent. 87, 351.

Wataha, J.C. (2000) Biocompatibility of dental casting alloys: a review. J. Prosthet. Dent. 83, 223.

9.1 Introduction

Previous chapters have dealt with wax-pattern materials, investment materials and casting alloys. This chapter describes how the investment mould is formed and how the wax pattern is replaced by the alloy using a casting process. One of several methods can be used for casting depending upon the alloy which is to be used.

There are several faults which can occur in alloy castings, most of which can be traced to incorrect selection of materials or faulty technique.

9.2 Investment mould

The various components of a typical investment mould are illustrated in Fig. 9.1.

The mould cavity is formed by allowing the investment to set around the wax pattern and associated sprue former and spruce base (crucible). After allowing the investment material to set, the sprue base and former are removed and the wax pattern ‘burnt out’ to leave the completed mould cavity. The choice of investment material depends on the type of alloy which is to be cast. The casting ring liner serves a dual purpose. It forms a relatively pliable lining to the inner surface of the rigid metal casting ring. This allows almost unrestricted setting expansion and thermal expansion of the investment. In addition, its thermal insulating properties ensure that the investment mould does not cool rapidly and contract after removal from the ‘burn out’ oven.

The temperature to which the investment mould is heated during burn out deserves special mention since this controls the thermal expansion of the investment. For gold alloys, either a slow burn out at 450ºC or a more rapid burn out at 700ºC is commonly used with gypsum-bonded investments. For Ni/Cr alloys a temperature in the range 700–

900ºC is normal, whilst for Co/Cr alloys a burn out temperature of 1000ºC is typical.

Heating of the investment mould should be carried out at a rate which enables steam and other gases to be liberated without cracking the mould. Also it is important that the temperature to which the mould is heated is adequate to enable thermal expansion and inversion to occur and that this temperature is not allowed to fall significantly before casting begins. Casting into a quartz-con- taining investment mould should be carried out with a mould temperature above 650ºC if adequate expansion due to thermal expansion and inversion is to be achieved. This requires that the mould be heated to about 750ºC to allow for cooling which may occur before casting commences. For cristobalite-containing investment moulds the mould temperature should be above 350ºC at the time of casting and this requires heating to about 450ºC to allow for cooling effects.

The balance between the molten alloy temperature and mould temperature is important in terms of producing a complete and accurate casting with a fine grain structure. The alloy should be hot enough to ensure that it is fully molten and remains so during casting into the mould, but should not be so hot that it begins to oxidize or that crystallization is delayed when it reaches the extremities of the mould cavity or causes damaging interactions with the mould walls. The mould temperature should be great enough to ensure complete expansion of the mould and to prevent premature crystallization leading to incomplete filling of the mould by alloy, but not great enough for crystallization to be delayed for so long that a coarse grain structure forms.

Factors such as length and diameter of the sprue and the distance of the mould cavity from the base