Volumetric wear study in metal-polyethylene bearing couple, has demonstrated that the wear rate is reduced by the decrease of the diameter of the ball-heads. On the other hand, small ball-heads introduce some limitations: the stability, the subluxation and the dislocation of the prostheses are directly correlated with the diameter and are often cause of failure.

The crosslinked polyethylene, promising lower wear rates, seems to have higher Function Biological Activity (FBA) because of its smaller but more aggressive particles[1]. In ‘70s, the alumina ceramics has been introduced in the hip prostheses due to its high wear resistance and its bulk material and debris biocompatibility. Laboratory test and long terms clinical experience confirmed that the BIOLOX®forte/BIOLOX®forte bearing couple offers a reduction of two orders of magnitude of the linear wear rate (in vivo results 0.005 mm/year) if compared with metal-polyethylene (0,2 mm/year), it does not produce reaction[2] and it has a high reliability (fracture rate = 0,01% )[3]. Moreover, it has been noticed that for each mm of linear wear of the liner, the correspondent penetration of the head reduces the Range Of Motion (ROM) of approximately 7°.

The low friction coefficient of the alumina, reduces the friction torque and the stresses between the bone and the cup[4]. Greater diameters (32 mm and 36 mm) offer different advantages. The ceramic ball-heads of greater dimensions have better mechanical characteristics. The minimal fracture load of BIOLOX®forte ball-heads (L neck) increases from 55 KN of the 28mm ball-heads to 65 KN of the 32 mm until to 90 KN of the 36 mm ones[5]. This superior characteristics allows to increase reliability (fracture rate of 32 mm ball-heads is 0,004%)[3]. The luxation risk is one of the most important parameter for the reliability of prostheses. Bigger diameter increases the luxation distance and consequently decreases the risk of subluxation and dislocation of the prostheses. The ROM is directly correlated to the diameter of the head. With ball-heads of 36 mm it can be caught up a ROM of approximately 136°. This is an advantage for the reliability of the system because it reduces the risk of impingement that is often cause of failure[6,7]. The lengths of the neck (+/− 3,5 mm for 28 mm ball-heads) can be increased. With 32 mm ball-heads there is an excursion of +/− 4 and the increase of the diameter reduces the necessity of a XL neck.

Due to its biocopatibility, low wear characterisitcs and mechanical aspect, ceramic on ceramic bearing couple of bigger diameter seems to be the right solution for long terms results for active and young patients.

Ceramics are used in hip prostheses in approximately 40% of the implants (ce/ce and ce/pe). The increase of the diameters (32 and 36 mm) in order to improve the stability and the Range of Motion of the prostheses is now the topic. Research and development has allowed creating new alumina inserts with smaller out diameter (39 mm for the 32 mm bearing and of 44 mm for the 36 mm ones). The new alumina matrix composite has allowed the realization of ceramic revision ball-heads. This system, made of 28 or 32 mm ball-heads with a titanium slivers (12714 internal cone), will allow applications of the ceramic ball-head on an in situ damaged taper. Beyond S, M and L lengths, will be available also an XL version. A femoral knee component, still in phase of study, has shown advanced resistances of 5, 8 and 15 times the body-weight in different load configurations. Have been carry out some tests in order to estimate the adhesion between the ceramic and the cement of different thickness and have been caught up values of 6,17 MPa (2 mm) and 14,90 MPa (0,7mm).

Alumina and zirconia are known for their general chemical inertness and hardness. These properties are exploited for implant purposes, where they are used as an articulating surface in hip and knee joints. Their ability to be polished to a high surface finish make them an ideal candidate for such wear applications, where they compete against materials such as ultra-high-molecular-weight polyethylene.

Alumina is a highly inert material and resistant to most corrosive environments. The term high alumina ceramics refersr to materials that have a minimal content of 97% of alumina. If there is a 99% minimal percentage of alumina it is called high purity alumina ceramics. In its _ phase (more famous than corundum), characterised by its particular structure and stability, high purity alumina has been being used in orthopaedics since 1970, in the articulations of the hip prostheses.

BIOLOX^®forte (commercially available since 1994) is high purity alumina (ca 99.7 %) with a small percentage of magnesium oxide (MgO). Approximately 50 years ago, MgO was introduced during the sintering phase of alumina because it was discovered that a small amount of this additive prevented the increase in grains of alumina during the sintering process. It was therefore possible to have a more homogenous and dense microstructure; both characteristics directly correlated with the mechanical resistance. The suffix forte derives from the increased mechanical characteristic and continuous optimisation of the fabrication technology.

One of the main factors involved in wear reduction is the characteristic molecular structure of alumina. Its superficial layer is composed of oxygen atoms that create a residual electric power which interacts with polarized molecules of the lubricant, tying it to the surface by strong Van der Waals ties. Therefore the presence of a fluid film that reduces the coefficient of clutch between the two surfaces involved during the articulation is guaranteed.

The colour of alumina components varies. Originally it is ivory, but it can easily become brown after sterilization with gamma beams that interact with the free valences introduced by the MgO. This change in colour does not change the mechanical characteristics. Currently the systems are completely modular and allow a wide choice of couplings. In 1984 and subsequently in 1995, the introduction of ISO norms for the production of ceramics ball-heads and inserts and the concept of conical fixation has provided higher reliability.

Today, the alumina BIOLOX^®forte components are prepared in clean-rooms, sintered with high quality control processes, laser marked and accurately inspected and tested. The tolerances between ceramics (ball-heads and inserts) and metallic parts (taper and metal shell) are fundamental for increasing implant reliability. It is important to control and validate the stems and cups which the ceramic parts are applied on. Correct assembly and the respect of the compatibilities between parts (angle, material, producer) guarantee the longevity of the implants.

Actually, in the orthopaedic field, alumina is mainly used in standard applications of hip prostheses. Ball-heads of 22 mm in diameter, lengths of neck type XL, and the knee prostheses are not possible because the mechanical characteristics of alumina do not allow for the elevated stress values requested for these special applications.

Between 1975 and 1977, it was discovered that the strength and toughness of alumina could endure a remarkable increment by developing composites with oxide of zirconium (zirconia). In zirconia, during the phase of cooling from temperatures over 1170°C, the grains go through a change of phase (from tetragonal to monoclica), with an increase of 3% of volume. At ambient temperatures the monoclica phase is stable. This transformation is martensitic, with energy absorption, and involves a heat-proof change of the symmetry of the structure. In the case of dispersed grains of zirconia in the alumina matrix, the transformation absorbs the energy of the crack and the strength of the ceramics increases. With the use of yttria (Y₂O₃) to stabilise the zirconia the problem of the structure defects can be resolved.

A percentage of zirconia stabilized with yttria (Y-TZP) was introduced in the alumina matrix and other mixed oxides to counterbalance the reduction of the hardness caused by particles of zirconia and to create lengthened particles during the sintering.

All these studies have been used to create the new ceramic BIOLOX^®delta. Tests of biocompatibility in agreement with norms EN 30993 have been carried out, so that implants can be made of these new composite ceramics.

Since 1970, more than 3,500,000 ball-heads and 350,000 inserts of alumina BIOLOX^® have been implanted. Owing to the grain size, currently reduced to values under 2 μm, the value of the mechanical resistance has been raised to about 580 MPa. The increase in the mechanical characteristics, the new shapes and the conical fixation have reduced the risk of fracture of the BIOLOX^®forte ball-heads and inserts to around 0.01% (Ø28 mm), maintaining the excellent tribology and wear characteristics. Many laboratory tests and clinical cases have shown that the wear rate of the alumina-alumina bearing complex is extremely low (0.001 mm/year). If compared with metal-polyethylene (0.2 mm/year) it shows a drastic reduction of particles of debris and therefore of the osteolysis problem

BIOLOX^®delta has a bending strength of around 1000 MPa, which is more than double that of the alumina ISO (400 MPa). In the minimum fracture load test, ball-heads of 28 mm Ø millimeter (neck L) have achieved values of around 100 KN, well beyond the 46 KN requested by the FDA. Multiple cycles of sterilisation in autoclaves have demonstrated that the the mechanical and tribological characteristics of BIOLOX^®delta are not altered.

On the basis of these results, BIOLOX^®forte can be considered a reliable alternative to other materials in standard applications and the new alumina composite BIOLOX^®delta will allow the realization of medical ceramics devices, already in the study phase, such as knee prosthesis, 22-mm ball-heads and thinner wall-thickness of inserts, which could not be developed up to now with the available ceramic materials.

Biomaterials improve the quality of life for an ever increasing number of people each year. The range of applications is vast and includes such things as joint and limb replacements, artificial arteries and skin, contact lenses and dentures.

Ceramic biomaterials can be divided roughly into three main types governed by their in vivo behaviour and tissue response. In broad terms, there are the bioresorbable ceramics (b-tricalciumphosphate), bioreactive (hydroxyapatite, fluorapatite and bioglass) and bioinert (alumina, zirconia and pyrolytic carbon). The resorbable ones are incorporated into the surrounding tissue, or may even dissolve completely over a period of time. The bioreactive ones, like hydroxyapatite (used for coatings on metallic pins), encourage bonding to surrounding tissue with, for example, new bone growth being stimulated. The bionert ceramics are mostly used for structural components. Alumina and Zirconia are known for their general chemical inertness and hardness. These properties are exploited for implant purposes, where they are used as an articulating surface in hip and knee joints. Their ability to be polished to a high surface finish make them an ideal candidate for this wear application, where they operate against materials such as ultra high molecular weight polyethylene (UHMWPE).

Alumina is a highly inert material and resistant to most corrosive environments, including the highly dynamic environment that is the human body. Under physiological conditions, it is classed as nearly inert, with evidence of any response from surrounding tissues and remaining essentially unchanged after many tyears of service. However, the body does recognise it as a foreign material and does attempt to isolate it by forming a layer of non adherent fibrous tissue around the implant where possible. Porous alumina may also be used to replace large sections of bone that have been removed for reasons such as cancer.

Alumina has been used in dental applications. Specifically, it has been used for tooth replacements. The term high alumina ceramics is referred to materials that have the minimal content of 97% of alumina. If the percentage of minimal alumina is of 99% it is called high purity alumina ceramics. In its α phase (better famous like corundum), characterized from its particular structure and stability, the high purity alumina is used in orthopaedics, in the articulations of the hip and knee prostheses.

From more than 30 years, the alumina has been successfully used. Today, more than 3,5 million of ball-heads e and 350 thousand of inserts of alumina BIOLOXA8 have been implanted confirming, in clinical use, the characteristics of low wear and biocompatibility that has allowed to reduce the problems of osteolisis induced by the polyethylene. The increase of the mechanical characteristics, the new shapes and the conical fixation have raised the reliability of the ball-heads and inserts of alumina.

The BIOLOXA8forte (in commerce from 1994) is an high purity alumina (ca 99,7 %) with a small percentage of magnesium oxide (MgO). Approximately 50 years ago, magnesium oxide was introduced in the phase of sintering of the alumina, because it was discovered that a small amount of this additive prevented the increase of grains of alumina during the sintering process. It was therefore possible to have a more homogenous and dense microstructure, both characteristic directly correlated with the mechanical resistance. The suffix ‘forte’ derives from the increased mechanical characteristic caught up with the continuous optimization of the fabrication technology.

Many laboratory tests and clinical cases have shown that the wear rate of alumina-alumina bearing coupling is extremely low (0.001 mm/year). If compared with metal-polyethylene (0,2 mm/year), it evidences the drastic reduction of particles of debris and therefore of the osteolysis problem.

One of the main factors that the reduction of the wear rate involves is the characteristic molecular structure of alumina. Its superficial layer is composed of oxygen atoms that create a residual electric power which interacts with polarized molecules of the lubricant, binding it to the surface by strong Van der Waals ties. It is therefore guaranteed the presence of a fluid film that reduces the coefficient of clutch between the two surfaces involved during the articulation.

The colour of alumina components is subjected to variations. Originally it is ivory, but it can easy stretch to the brown after sterilization with gamma beams that interact with the free valences introduced by the MgO. This change of colour does not induce changes of the mechanical characteristics.

Currently the systems are completely modular and allow a wide choice of couplings. Ceramic acetabulum has been abandoned and replaced by ceramic inserts. In 1984 and subsequently in 1995, the introduction of ISO standards for the production of ceramics ball-heads and inserts and the concept of conical fixation has allowed to catch up higher reliability. The third generation of alumina has reduced the complications rates to values around 0.01% (for the 28 mm ball-heads and inserts), maintaining the excellent tribology and wear characteristics.

Today, the alumina BIOLOXA8forte components are prepared in clean-room, sintered with high quality control processes, marked by laser and accurately inspected and tested. The dimension of grains of the microstructure, currently reduced to inferior values of 2 B5m, has allowed to raise the value of the mechanical resistance of about 45% (580 Mpa) of the value requested by ISO standard (400 Mpa). The tolerances between ceramics (ball-heads and inserts) and metallic parts (taper and metal shell) are fundamental for lengthening the implant reliability. It is important to control and certificate the stems and cups which the ceramic parts are applied on. Correct assembling and the respect of the compatibilities between parts (angle, material, producer) guarantee the longevity of the implants.

Actually, in the orthopaedic field, the alumina application is mainly used in standard applications of the hip prostheses. Ball-heads of 22 milimeters of diameter, lengths of neck type XL, and the knee prostheses are not possible because of the mechanical characteristics of alumina not allowing to catch up the elevate stress values requested for these special applications.

Between 1975 and 1977, the first studies issued that the strenght of alumina could be reinforced by the introduction of ceramic oxides. It was discovered that the strenght and toughness of alumina could endure a remarkable increment through the realization of composites with oxide of zirconium (zirconia). In the zirconia, during the phase of cooling from temperatures over 1170A1C, the grains endure a change of phase (from tetragonal to monoclinic), with an increase of 3% of volume. At ambient temperature the phase monoclina is stable. This transformation is martensitic, with energy absorption, and involves a heat-proof change of the simmetry of the structure. In the case of dispersed grains of zirconia in the alumina matrix, the transformation absorbs the energy of the crack and the tenacity of the ceramics increases. The Yttria (Y2O3) use, as stabilizing of the zirconia, has allowed to exceed the problem of the defects of the structure. It was introduced a percentage of zirconia stabilized with yttria (Y-TZP) in the alumina matrix and other mixed oxides to counterbalance the reduction of the hardness caused by particles of zirconia and to create lengthened particles during the sintering.

All this studies have been used to create the new ceramics BIOLOXA8delta. Tests of biocompatibility in agreement with norms EN 30993 have been carried out allowing the implants of these new composite ceramics. The BIOLOXA8delta has a bending strenght around 1000 MPa, that is more than the double of the alumina standard (400 MPa). In the minimum fracture load test, ball-heads of 28 mm AF millimeter (neck L) have caught up values around 100 KN, very beyond the 46 KN requested by the FDA.

Multiple cycles of sterilization in autoclaves have demonstrated that the BIOLOXA8delta does not endure alterations of the mechanical and tribological characteristics.

On the basis of these results, BIOLOXA8delta will allow the realization of medical ceramics devices, already in study phase, like knee prosthesis, 22 mm ball-heads, thinner wallthickness of inserts, whose realization was not possible with the ceramic materials up to now available.