But such calcium phosphate ceramics are not tough enough to replace parts of human skeleton which have to bear heavy weights, for example knee and hip joints. The human hip is like a ball-and-socket joint in which the head ofthe thigh bone acts as a ball and the cup in the pelvis acts as a socket.
Normally, this joint is well lubricated by cartilage, a strong but flexible natural tissue. A disease like arthritis can destroy this harmony, however. It damages the cartilage and even-
Long lasting ceramic hip prosthesis developed byCGCRI
tually two bones of the joint bear upon each other causing stiffness and pain to such an extent that the affected person can no longer lead a normal life. The only remedy is to completely replace the natural joint by an artificial implant.
This is called hip prosthesis.
The conventional hip prosthesis is made of a combination of metallic head and a plastic socket. The metallic head has a stem that can fit into a natural cavity of the thigh bone. Thus, the top of the damaged thigh bone is cut off and replaced by the metallic head, whose lower stem is cemented into the cavity of thigh bone. The problem with this conventional type of joint is that its life is limited. The metal head gets worn out easily due to constant friction with the socket. The friction is considerable since the joint has to bear the entire body weight and even more. It is not usually appreciated that the effective load on the joint is as high as 50-times the body weight during jumping and two and a half times even during normal walk- ing.
It is here that the extreme wear-resistant nature of ceramics comes in handy. If the head of the artificial joint, the ball of
The Central Glass and Ceramics Research Institute
the ball-and -socket joint, could be mad e of a ceramic material, it will last almost for ever.
But the fabrication of the ceramic head with desired properties is a very arduous task. This is because the mechanical properties and strength of the artificial joint are critically important and their control during fabrication is quite complicated. Take for instance the strength and tough- ness of a ceramic, which are governed by particle size.
Smaller the particle size, greater is the toughness because smaller particles can fit together tightly and hence the num- ber of pores in the ceramic is decreased. But sintering, which transforms green body to a ceramic and is basic to ceramic change, increases the particle size. A ceramic scientist has, therefore, to execute a delicate balancing act.
Though the use of hip prosthesis with ceramic heads has been commercialized in many countries of the West, the exact method of preparation remains a closely guarded commer- cial secret. Recently, the method of preparation of ceramic head for hip prosthesis has been perfected in India by scien- tists working at the Central Glass and Ceramic Research Institute (CGCRI), Calcutta. The CGCRI scientists chose high purity, finely powdered aluminium oxide to prepare the ceramic head for the hip prosthesis. Aluminium oxide was
chosen because it is stiffer than steel and harder than granite.
They mixed the powder with an organic binder and applied a very high pressure to the mixture. The solid blocks thus obtained were turned into balls with the help of a machine and then threaded with a screw. Careful sintering followed next. The whole process was so carefully controlled that the density of the ceramic head matched the density of bone samples. This was done to ensure compatibility of the im- plant with the bone. The head was polished for a smooth finish. Next it was fitted to a standard stem called Austin- Moore type stem, already in use in conventional artificial joints.
The estimated wear rate for the ceramic head is 1/300th of the conventional type. Thus, the ceramic hip prosthesis has a long life span. There is no need to reset or replace the implant once it is in position. For younger, more active patients it is a bless'ing in disguise.
Putting spring back into the steps of such unfortunate sufferers is only one gift of materials to mankind. Materials have been the allies of mankind in the journey of evolution.
And the oldest of these faithfuls are ceramics. Their shapes and functions might have been changing over the period, but they have always responded to the pressing needs of mankind in different era. Whether the need relates to shelter, travel, communication, health or entertainment, ceramics have a habit of turning up with new solutions. The journey is still continuing and we shall certainly need the services of such faithfuls in future too.
Things are in the saddle, And ride mankind.
R.W.Emerson
Abrasives: Rough substances used to clean or smoothen uneven surfaces of other materials by scratching or rubbing.
Alternating current: Electric current that reverses its direc- tion of flow alternately at regular intervals of time.
Arthritis: A disease which makes bone joints swollen and painful.
Asbestos: A mineral with long flexible fibres, used as a protection against fire and heat.
Band: A group of closely related energy values for electrons in a solid.
Binders: Solvents added to promote cohesion among par- ticles of powders used to prepare modem ceramics.
Ceramic change: Conversion of clay mixture into a hard stone-like material on heating to red heat.
Collagen: A protein that occurs in connective tissues and bones.
Doping: Intentional addition of impurities to a solid for changing its electrical conductivity.
Ductility: Property of materials which makes them capable of being drawn into wire and hammered into thin plates.
Enzymes: Biochemical catalysts. They speed up biochemical reactions inside living organisms.
Feldspar: Naturally occurring aluminium silicate mineral containing atoms of sodium, potassium, calcium, etc.
Frequency: The number of sound or electromagnetic waves prod uced in a second.
Friction: The force that opposes the sliding of one object over another.
Green body: Shaped pottery or other ceramic articles, dried but not heated.
Green strength: Property due to which clay or powder par- ticles continue to hold on to each other in dried but unheated ceramic articles even in the absence of a binder.
Hydroxyapatite: A calcium phosphate ceramic containing hydroxy group, CaS(P04)3(OH).
Insulator: A material that does not allow electricity or heat to pass through it.
Microwave: Very short electromagnetic wave, say with wavelength between 1 and 30centimeters.
Monoclinic: A crYl>taltype in which three axes are unequal in length and two angles of intersection are right angles.
Ohm: Unit of electrical resistance.
Oxidized: Combinatiqn with oxygen or removal of one or more electrons from an atom, ion or molecule.
Pelvis: A bony girdle formed by hip bones.
Piezoelectricity: Electricity produced by pressing certain crystals.
Prosthesis: A device used as a replacement for the damaged or missing part of the body.
Refractories: Materials undamaged by intense heat.
Shells: Spaces occupied by groups of electrons of equal energy around the nucleus of an atom.
Silicates: Chemical compounds of silicon and oxygen with metals.
Sintering: Formation of a coherent non-porous mass by heat- ing below the melting point of a ceramic preparation.
Tetragonal: A crystal with two of the three axes equal and all axes intersecting at right angles.
Tetrahedron: A geometrical figure with four triangular faces.
Also molecular arrangement in which four atoms symmetri- cally surround. a fifth one. Two adjoining atoms subtend an angle of 109° 28' at the central atom.
Vitrification: Heating of a clay mixture to the point when some of the ingredients melt to form a free flowing paste that binds the clay particles on cooling.