Ceramics For Total Joints

WHAT IS CERAMIC

Ceramic materials used in total joints belong to a class of materials called oxide ceramics. These materials are formed by close packed crystals of oxides of aluminum or zirconium metals. The arrangements of the crystals and the presence of impurities determine the characteristics of the resulting material.

The case of aluminum oxide provides a good example; the aluminum oxide appears in three forms:

Gems:

A large, single, and perfect crystal of aluminum oxide with traces of other metals that give it its color forms the precious ruby and sapphire gems.

Surgical grade ceramic

Very small and very pure crystals of aluminum oxide compressed very close together form the basis of medical grade ceramics.

Pottery

Coarse aluminum oxide crystals mixed together with the clay (silica oxide) and fired form the basis of commercial ceramic materials, such as the china cups on your table (very fragile) or electric insulators (very tough).

The name of the whole group of materials comes just from the Greek term "keramos" which means pottery.

Hopefully you now also understand that the ceramic materials used in total hip prostheses have nothing to do with the pottery used on your dining table. The commercial ceramic products are not only brittle; the body does not tolerate them!

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MEDICAL GRADE CERAMIC

Advantages and risks

Ceramics used for total hip joints surfaces (medical grade ceramics) are solid materials composed of pure crystals of aluminum or zirconium oxides.  Such ceramics are the most chemically and biologically inert of all materials. They are not only inert, they are also stiff, strong, and hard. For example, the only substance harder than aluminum oxide is diamond.

Thus ceramic are very resistant to scratches from the tiny particles that occasionally land between the artificial joint surfaces, be it particles of bone cement or metal. The mechanical law states that the harder the surfaces coupled together are the less wear the coupling system produces. Thus total hip with ceramics surfaces   produces very low rates of wear particles. Ceramics also attract fluid on their surface so that couplings made of ceramic have low friction resistance. 

The main disadvantage of medical ceramic materials is their fragility. The ceramic materials cannot deform under the stress, as can do plastics and metals. When the stress acting on medical ceramic materials exceeds a certain limit, ceramic material bursts, formally explodes in many splinters. See Picture of burst ceramic ball of a failed ceramic total hip.

Such burst fractures of ceramic components of the total hips were observed in the past due to the poor quality of the ceramic material of that time.

However, even the modern third generation medical ceramic is still a fragile material, although it will not suffer the burst fracture. But the rim of the modern ceramic cup may still chip off during assembling with the metallic back up under surgery.

Even modern ceramic materials are very sensitive to asymmetric loading and impingement  by the femoral neck component. Thus, less accurate position of the ceramic cup may increase considerably the wear of the ceramic components in such a total hip joint.

Two kinds of ceramic materials are used in the modern ceramic total hip systems alumina ceramic and zirconia ceramic. Both materials have quite distinct positive sides and also some disadvantages.

 

2A

Alumina ceramic

Modern (third generation) alumina ceramic is composed of very small crystals of aluminum oxide, the impurities make less than 0,5% of the material’s volume. Modern ceramic is a tough and hard material. The smaller the crystals and the purer the material, the more fracture resistant is the final product.

 

Picture:

Old and modern medical grade alumina ceramic under microscope.

Click on the icon for a full size image.

In the old ceramic materials the crystals of aluminum oxides were large, not assembled closely; there were many impurities and voids between them (actually impurities made about 5 % of the ceramic's volume).

These impurities were the weak points for propagation of fracture cracks. The coarse structure and impurities were the cause of the frequent fractures of the old ceramic components.

In the modern ceramic the crystals of the aluminum oxide are very small, very closely packed together, and the impurities are making less than 0,5% of the material’s volume.

The close packing of aluminum oxide crystals is achieved through the so-called HIPing procedure (Hot Isostatic Pressure). The ceramic component is reheated and then subjected to enormous symmetric pressures. The HIPing process extrudes impurities out off the material and packs the crystals very close together.

Every one modern ceramic component is individually stress-tested before it is released on the market. The modern alumina ceramic ball is very tough structure, tougher than the metallic stem on which it is seated, and even more though then the natural thighbone.

Alumina ceramic ball must sustain 60 times the average patient’s weight Metallic femoral stem must sustain 15 times the average patient’s weight

Thighbone 15 times the average patient’s weight (The average patient’s weight is 77 kg)

The ceramic components should also endure fatigue strain. The certification requires that the ceramic boll should endure repeated loadings of 18.5 times the average patient’s weight, whereas the requirement for the metallic stem is three times less – it must endure "only" 6.5 times the average patient’s weight.

The disadvantage of the modern alumina ceramic is lower toughness in spite of the impressive test figures. Thus the material engineers developed the

Zirconia Toughened Alumina (ZTA) ceramic.

Compared to pure Alumina ceramic, the ZTA has superior strength and resistance to wear. The ZTA is commercially available under the name Biolox Delta. This ceramic contains about 75% of alumina and the rest are zirconium, Yttrium and chrome oxides. Several manufacturers use this ZTA ceramic in their total hips. The pure Aluminum medical ceramic. produced by the same manufacturer (Ceram Tex) has the name Biolox Forte. As you see on this picture, there are many possible combinations of these two forms of Alumina ceramic.

Possible combinations of Biolox ceramics (Click on the icon for the full size picture)

Overall the ZTA  ceramic combinations in microseparation simulator studies consistently showed lower wear than the alumina used as historical control. This was in contradistinction to ytrria-stabilized zirconia balls that showed increased wear compared to the control. Thus the wear performance of ZTA implants in the laboratory was quite different to zirconia implants and appeared superior to the alumina as the historical control. The superior strength and wear resistance of ZTA ceramics may become advantageous in sub-optimal clinical cases that may have had the risk of implant fracture or abnormally high wear.(Clarke 2006)

References:

Clarke et al: 11th Biolox R Symposium, Rome 2006)

Kay J : Engineering Materials 2005;Vols 284-6: 979 -82

3A

Zirconia ceramic

Zirconia ceramic is one of the highest-strength ceramics suitable for medical use. It is two to three times stronger material than alumina ceramic. Thus one can make the femoral balls out of this material that are smaller (22 mm diameter) than the balls made out of alumina ceramic (28 - 32 mm). The surface of the zirconia ball can be made smoother than the surface of the ball made out of the alumina ceramic; the wear produced by the zirconia ball coupled with polyethylene cup is only half as large as the wear produced by alumina ceramic ball in identical coupling. These characteristics apply, however, only to ceramics made from the tetragonal crystals of zirconia.

 

Whereas the high strength and low wear made the zirconia ceramic is so attractive for constructers of total hips, the instability of zirconia is a big and not well-understood problem. Ceramic made out of the strong tetragonal crystals may spontaneously transform in other crystalloid forms. The ceramic consisting of these other crystalloid form is weak, rough, and fragile.

Thus, zirconia ceramic must be "stabilized" by addition of oxide of another metal, yttrium. The whole process is not well understood and this may be the cause of the many fractures of zirconia ceramic balls reported in the literature.

	Recent studies demonstrate that zirconia ceramic ages in the body’s temperature and the surface of the zirconia ball’s surface roughens. On the left there is a picture (scanning electrone microscope) of the surface of a Zirconia ball retrieved after few years in the body . You can see several craters on the surface of the zirconia ball instead of a plain even surface.
Rough surface of Zirconia ball (From Clarke: Current status of Zirconia, J Bone Joint Surg- Am 2003, 85-A Supplement 4: 73 - 84.)

This is also the reason why in the current total hip systems the zirconia balls are always coupled with a polyethylene cup and not with a ceramic cup. Laboratory experiments demonstrated namely that the wear of this zirconia on zirconia system might be very high with aging of ceramic (Clarke 2003).

3B

Oxinium materials for total hips.

Through a special technology one can create a thin layer of zirconium oxide on the surface of the solid zirconium metal.

Zirconium is a strong and biocompatible metal similar to titanium.

One manufacturer exploited this technology and produced total hip and total knee components made out of this material composite.

Total hip system (Oxinium total hip system) has the femoral head made out of Oxinium that articulates with a polyethylene cup. See the chapter Ceramic total hips.

The femoral ball is first made from zirconium metal (actually from zirconium alloy with other metals such as Niobium). The surface of this metallic component is then oxidized: the metallic components surface is heated and then subjected to oxygen gas that diffuses into the surface of the metal. This process creates a thin and durable layer of zirconium oxide on the surface of the metal. The finished product thus combines the benefits of metals and ceramics. It offers superior wear resistance on its surface whereas the zirconium metal itself, with characteristics close to titanium, is a material without the risk of brittle fracture.  Note that  oxidized zirconium is black.

OXINIUM TM femoral ball On this picture you see A - the finished, highly polished Oxinium femoral ball. A little rectangle shows the area of cross section in B. B - cross section through the surface of the cup: It shows from at the top  a thin layer of black zirconium oxide (Oxinium TM)     successively changing into  a compact zirconium alloy material of the femoral ball (yellow) bellow. C - an artist's view of diffusion of oxygen molecules into the surface of the zirconium ball. You see from the top :  the oxygen atmosphere  with oxygen molecules (blue) diffusing into the surface of the zirconium ball - middle layer: a layer of gray molecules of oxidized zirconium ( zirconium oxide) on the surface of the ball - lowermost: zirconium alloy molecules (yellow) with occasional oxygen molecules.

Production of Oxinium (TM) femoral ball

This is a new technology, see also the manufacturer's website(www.strongasanox.com).  The manufacturer maintained in 2001 that the zirconium oxide is not an externally applied coating but rather a transformation of the original metal surface into zirconium -oxide ceramic...Previous testing has demonstrated that this oxide has excellent cohesion and adhesion..." (Spector 2001)

This statement is in recent years (2006-7) called in question by some surgeons.  There are appearing reports about failure of the oxinium femoral balls. The problem is namely that the Zirconium metal used for production of Oxinium femoral balls is about twice as soft as the metal backing sleeve (made from Cobalt Chrome alloy) that encloses the polyethylene cup.

With a hip dislocation, the Oxinium cup comes into contact with the hard rim of the metallic sleeve: the result is deep scratch on the ball surface. Dislocation of a total hip is usually managed by reposition on the emergency room - a nuisance not needing operation. For patients with Oxinium total hip the surgeon must open the total hip, remove and replace the scratched ball with all risks and problems that follow such a revision operation. (Evangelista 2007)

Some surgeons now say that they will not use the Oxinium total hip until this problem is solved.

Hardening of the total joint surfaces by diffusion of gases is nothing new in the history of total joints. Nitrogen diffusion was once used for hardening of titanium made ball components; the laboratory results were splendid, the clinical results were a fiasco.

Reference: Evangelista GT et al.: Surface damage to an Oxinium femoral head... J Bone Joint Surg-Br 2007; 89-B: 535 - 7.

Spector  M et al:: J Bone Joint Surg-Am 2001, 83-A Supplem 2 Part 2: 80 - 6.

 

3C

Hydroxyapatite coating

See also the chapter Cemented and cementless TH / Hydroxyapatite

This is another ceramic substance used in the total joint prostheses.

In normal bone tissue, the collagen fibers are interspersed with crystals of hydroxyapatite. Synthetically produced hydroxyapatite is used as thin coating on the porous surfaces of the cementless total hip prostheses. The purpose is to enhance the ingrowth of the bone tissue into the surface of the hip prosthesis.

Chemically. hydroxyapatite (HA) used for coating purposes is a calcium phosphate. Its composition and crystallographic structure are similar to those of the bone mineral phase. The chemical formula of this material is Ca₁₀ (PO4)₆(OH)₂ .  

 

A  The picture shows the complex spatial organization of the hydroxyapatite crystal. This crystal has biological properties for which it is described as “living crystal”. It attracts the bone forming cells, osteoblasts, to its surface. B  Microscopic studies demonstrated that bone forming cells (osteoblasts) adhere to the HA crystals coating the surface of the total joints already few weeks after implantation. This process fixes firmly the total joint to its bone bed. This picture shows an osteoblast cell (pink color) adhering to the hydroxyapatite coating (green color).

 In this way the total joint device is soon enveloped by a newly formed bone tissue and fixed to the bone bed.  

This picture shows a cross section through a thighbone which had the shaft component of a total hip placed into the marrow cavity (animal experiment). After some months there formed sufficient new bone tissue that filled entirely the previous bone marrow cavity (black arrows). The shaft component (B) in the marrow cavity is now embedded by the newly formed bone (A). The thin layer of hydroxyapatite coating is still visible on the surface of the shaft as a thin white line (C).

 The picture shows the apparatus used for plasma spraying of HA.  

The application of the hydroxyapatite coating onto the surface of the total joint is done by a plasma spray technique. In principle the hydroxyapatite particles (B) - white powder - are injected into a chemically inert gas stream raised to high temperature and speed. In this hot gas stream (A)  the HA particles melt and are then projected onto a previously treated metal surface of the total joint where they adhere and form a continuous surface thick some tenth of millimeter. . (Bioland 1988, old facts still valid 2010 – and perfect pictures!)

 Many HA coatings contain only about 70% of HA crystals and the rest is calcium phosphate, whereas other manufacturers (Howmedica) claim that they use pure (100%) hydroxyapatite substance. It is difficult to tell whether this change in chemical composition of the coating may influence the long term stability of the total hip device.

In the 1980’s, when the HA coating was introduced on the market, there appeared many short term observations, made on small patient groups that claimed excellent results with the HA coated total hips. In later years, when a long term observation became available, many authors are claiming that HA coating is not an advantage but a risk factor for loosening of the total hip devices (Lazarinis S et al: Acta Orthopaedica 2009, 80 (6), x-x)

 

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Ceramic for total knees:

Use of ceramic for total knee joints is hampered by two facts:

First:  The total knee joints have not congruent joint surfaces. Thus, in a total knee joint with both joint surfaces made from ceramic materials, there would appear large localized stresses that would destroy components made from the contemporary ceramics.

Second: It is as yet difficult to fabricate such a large yet thin ceramic component as is the form of the femoral component that would sustain the stresses that occur during walking in the knee.

One solution that appeared recently is the Oxinium total knee prosthesis: The femoral component of this total knee is made from the metal Zirconium, which is highly biocompatible and will sustain the localized stresses that occur in the knee.

The surface of this component is then oxidized: the metallic components surface is heated and then subjected to oxygen gas that diffuses into the surface of the metal. This process creates a thin and durable layer of zirconium oxide on the surface of the metal. The finished product thus combines the benefits of metals and ceramics. It offers superior wear resistance on its surface whereas the metal itself, with characteristics close to titanium, is a material without the risk of brittle fracture. Note that the oxidized zirconium is black

Oxinium made femoral component of a total knee

The first results with the cemented Oxinium total knees were positive (Laskin 2003) whereas the cementless Oxinium total knees ended in a catastrophe well contained by the manufacturer.

Japanese surgeons use, however, components made from alumina ceramic in some of their total knee systems (Akagi 2000).  The details of these materials are unknown to me. The results show that these components do not sustain fractures. The authors, however, point out that these characteristics are applicable to the (small) Japanese population only.

 

References: 

Akagi et al:  J Bone Joint Surg-Am, 2000; 82-A:1626-33  

Clarke IC et al.: J Bone Joint Surg-Am 2003; 85-A Suppl 4: 73 – 84

Good V et al.: J Bone Joint Surg-Am 2003; 85-A Suppl 4: 105 – 110

Heisel Ch et al.: J Bone Joint Surg-Am 2003; 85-A: 1366 - 79

Laskin RS. : An oxidized Zr ceramic surfaced femoral component for total knee arthroplasty. Clin Orthop. 2003 Nov; (416): 191-6