Aluminium Oxides

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ALUMINIUM OXIDES (ALUMINA) & ZIRCONIA

(1) Main source of high purity Alumina (Aluminium Oxide) is bauxite and native corundum.

(2) The commonly available a-alumina can be prepared by calcining alumina trihydrate resulting in calcinedalumina.

(3) The American Society for Testing and Materials (ASTM) specifies 99.5% pure alumina and less than 0.1 %

of combined Si02 and alkali oxides (mostly Na20) for implant use.

(4) Natural alumina is known as sapphire or ruby (depending on the types of impurities which gives rise to

color)

(5) The single crystal form of alumina has been used successfully to make implants. Alpha Alumina (a-Al203)

has a hexagonal close-packed structure (a = 0.4758 nm and c = 1.2991 nm).

(6) Single crystal alumina can be made by feeding fine alumina powders onto the surface of a seed crystalwhich is slowly withdrawn from an electric arc or oxyhydrogen flame as the fused powder builds up.

(7) The strength of polycrystalline alumina depends on porosity and grain size.

(8) Generally smaller the grains and porosity, the higher the resulting strength.

(9) Alumina is quite a hard material, in general.

(10) The hardness varies from 2000 kg/mm2

(19.6 GPa) to 3000 kg/mm2

(29.4 GPa).

(11) The high hardness is accompanied by low friction and wear.

(12) Therefore, we use it as joint replacement material.

(13) Alumina has been used in the area of orthopaedic for more than a quarter of a century. Single crystal

Alumina has been used in orthopaedic and dental surgery for almost 20 years.

(14) Orthopaedic uses consist of hip and knee joints, tibial plate, femur shaft, shoulders, radius, vertebra,

leg strengthening spacer and ankle joint prostheses. The hip prostheses consist of square or cylindrical


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shaped alumina socket, the latter with an outer screw profile, for cement free anchorage to the bone.

An alumina ball is attached to a metal femoral stem by aid of self-locking tapers. The stem itself is

implanted with PMMA cement, though recently cement free prostheses have developed. Different

combinations of sockets, screws and balls made of Alumina are used. Proper alignment and orientationof sockets is very important in the prostheses, otherwise wear or loosening of socket takes place.

(15) Alumina finds applications in dentistry as well as reconstructive maxillofacial surgery to cover the

bone defects.

(16) Porous alumina is also used in teeth roots.

(17) A relatively thin fibrous tissue capsule is formed around Alumina which is a good indication of tissue

compatibility. The exact nature of tissue capsule form depends upon the type of the material used, i.e.

solid, perforated, porous and the order of thickness.

(18) Alumina is the implant material of choice for highly loaded applications in surgery when sphericalgliding contact is possible.

(19) Surface finish, small grain size, biomechanically correct design, exact implant technique and an

excellent manufacturing technology are the important prerequisites for success of Alumina implants.

ZIRCONIA (Zr02)

Zirconia (Zr02) have been tried for application in fabricating implants. Zirconia is called fake diamond or

cubic Zirconia since it has a high refractive index (as does diamond) and some Zirconia single crystals can

be made gem grade. The cubic structure of Zirconia is shown in figure .


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Q Oxygen 0 Zirconium

Fig. ; Cubic structure of zirconia that belongs to the fluorite structure.

Pure Zirconia can be obtained from chemical conversion of Zircon (ZrSi04) which is an abundant mineral

deposit, It undergoes a large volume change during phase changes at high temperature in pure form;

therefore a dopant oxide such as Y203 is used to stabilize the high temperature (cubic) phase. We have

used 6 mol % Y203 as dopant to make Zirconia for implantation in bone. Zirconia produced in this manner isreferred as the partially stabilized Zirconia. At room temperature, Zirconia has monoclinic crystal structure.

Upon heating, it transforms to a tetragonal phase at 1000C to 1100C and the cubic phase at around

2000C. Yttrium Oxide (Y203) stabilizes the tetragonal phase so that upon cooling, the tetragonal crystals

made of Zr02-Y203 can be maintained in metastable state and not transformed in a monoclinic structure. Its

modulus is half of Alumina. Bending strength is two to three times greater than the Alumina. Fracture

toughness is two times greater. Increased mechanical properties may allow for smaller diameter femoral

heads to be used. The wear resistance is a function of the fine grain size, lack of surface roughness and

residual compressive stresses induced by transformation back to a monoclinic system.

Zirconia is highly biocompatible, as are other ceramics and can be made in the form of large implants such

as the femoral head and acetabular cup in total hip joint replacement.

The improved mechanical properties of Yttria stabilized Zirconia ceramics (Y-TZP) combined with excellent


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biocompatibility and wear properties make this material the best choice for the new generation of

orthopaedic prosthesis. The phase diagram is shown in figure 4.5. The other is higher fracture strength and

toughness due to the phase transformation toughening process. The increased fracture is due to a phase

transformation that operates by arresting the propagation of cracks. Today over 1,50,000 Zirconia Ceramic

hip joint heads have been implanted in Europe and US. E.g. (W-TZP) Zirconia ceramics are employed to

develop new shoulder prosthesis, replacing the conventional materials. The functional shapes of the

shoulder heads are similar to those of hip joints as finally hydroxyapatite coated Y-TZP dental implants are

in use without failure for more than 10 years.

retention of a metastable state tetragonal structure at ambient temperature. During the crack

propagation, the tetragonal particles in the crack tip region undergo phase transformation, increasing its

volume, which sets up a compressive field surrounding the particles and closes the crack opening, resulting

in a stronger material. The process is similar to the precipitation of a tetragonal structure in cubic grains.Other applications include knee prosthesis, because of specific high stress in knee prosthesis Y-TZP (Yttria

stabilized Zirconia) ceramics should be preferable to Alumina ceramics for knee femoral components, the

toughened mechanical properties of Y-TZP leading to improved safety and reliability testing has to be done

and it will be successful within next 5 to 10 years of upmost significance for orthopaedics applications in

the long term aging of Zirconia ceramics.

Indeed long term stability of physical properties is required to provide long implantation times.

Zr02 is a gold-colored silicate of zirconium. Zircon is a mineral (Baddeleyite) found in igneous and

sedimentary rocks and occurring in tetragonal crystals colored yellow, brown or red depending on

impurities. The Zircon is first chlorinated to form ZrCl4 in a fluidized bed reactor in the presence of

petroleum coke. A second chlorination is required for high quality Zirconium. It is precipitated with either

hydroxides or sulfates, then calcined to its oxide. The Zirconia is partially stabilized above 1700C in the


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cubic phase, which results in large grain sizes (50 - 70 jim). When it is cooled, a phase transformation takes

place and tetragonal precipitates can be formed in the cubic matrix when combined with cubic and

tetragonal phase results in enhanced mechanical properties.

Aluminium Oxides

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