Oxidized Zirconium Bearing Surfaces

The Way Forward In Hip Arthroplasty?

Author: S. Chatterjee MBBS MRCS MSc
With a contribution from F.Haddad BSc MCh (Orth) FRCS (Orth)

What Is Oxidized Zirconium?

Oxidized zirconium (Oxinium™ - Smith & Nephew) is a product of new hybrid technology which seeks to combine the properties of metals and ceramics into a single substance. Specifically, it is an oxidized zirconium surface layer on underlying zirconium metal. The underlying base metal is a zirconium-niobium alloy and the surface layer is oxidized zirconium (aka zirconia) which is one of the common ceramic bearing surfaces used in orthopaedics. However what is unusual about this hybrid technology is that unlike previous attempts to combine material properties such as hydroxyapatite on stainless steel femoral stems, this oxidized zirconium is not an applied coating to the underlying zirconium metal but rather a change is the structure of the zirconium itself at its surface. A typical component is first forged or machined from a wrought alloy of zirconium (97.5%) and niobium (2.5%). The component is then heated in air at about 500°C for about 3 hours. This process is known as thermal diffusion and the subsequent oxidation of the alloy results in the zirconia surface layer. The layer of zirconia formed upon the metal alloy is about 5µm thick. The hardness (resistance to deformity) of the zirconia is about 14-15 GPa. This is in contrast to the hardness of the underlying alloy which is about 3 GPa.

Do we need it?

To answer this question, we require a brief review of the currently available bearing materials for lower limb joint arthroplasty and their shortcomings. 1) Metal on Polyethylene Bearings
These bearings, heavily popularised in the 80’s and 90’s, continue to perform well particularly after the introduction of ultra high molecular weight polyethylene. However the demographics of the population has changed since. With the increasing demand for arthroplasty in younger patients the wear characteristics of this coupling may not provide the best answer. The problem lies with the polyethylene. Ultra high molecular weight polyethylene (UHMWPE) although superior to standard polyethylene, suffered from its sterilisation methods. Sterilisation with radiation can cause breakage of chains and is even worse if done in air. Alternative sterilisation methods with ethylene oxide or sterilisation in an inert gas all proved to be detrimental in terms of decreasing the cross links within the polyethylene. The emergence of highly crossed linked polyethylene (HCLPE) is a significant improvement upon UHMWPE(5). Radiostereometric analysis of HCLPE suggests after 30 million cycles, there is much less wear (30-70%) than with UHMWPE. However even with the improved material characteristics, it is debatable as to whether acetabular components of reduced thickness, to allow larger femoral heads, will have the sufficient toughness required. This area is still rapidly developing with even newer forms of polyethylene in the making. 2) Ceramic on Ceramic
Their main advantages are their hardness and resistance to surface scratching and pitting. They are also biologically very well tolerated and excite little tissue reaction. However it is their brittleness and risk of catastrophic failure which remains a concern. The quality of ceramic manufacture has improved dramatically with a reduction in grain size leading to a more uniform structure but nevertheless with repetitive impact and point loading situations such as impingement, the coupling is not very forgiving. Ceramics generate far less wear particles in comparison with polyethylene but any wear particles produced can lead to third body wear production, and a brittle fracture is a catastrophe which greatly complicates a revision situation. Thus it is their fracture toughness which remains a concern. Squeaking of the joint, although a well discussed complication, is uncommon but is a result of a breakdown in fluid film lubrication resulting in pistoning motions between the ball and the liner. These pistoning movements may also play a part in the development of stripe wear which is seen within the cup where the ball repeatedly pistons over the same area during the gait cycle. 3) Ceramic on Polyethylene
The results of zirconia on polyethylene are well documented as a poor coupling2 on account of the zirconia changing its crystalline structure to less wear resistant form during manufacture. This was countered by adding yttrium oxide to stabilise the structure. But despite this addition there is little data to substantiate the theoretical reduction in wear particles generated with ceramic on polyethylene as compared with metal on polyethylene. The same is not true of alumina on polyethylene bearings. However although data for the alumina on polyethylene bearings is much better than for zirconia3, there does not seem to be widespread interest in these bearings. Some surgeons maintain that there is insufficient data at the moment to determine its superiority over other bearings. 4) Metal on Metal
There has been a recent resurgence of interest in these couplings following the improvement in metallurgy and manufacture techniques. Cobalt chromium alloys now have smaller grain sizes with improved hardness and wear characteristics. This means that acetabular components can be made with a smaller surface thickness without sacrificing toughness and being over susceptible to wear. This allows surgeons to use larger femoral heads which lends to increased stability of the arthroplasty. This also allows the manufacture of large yet thin diameter sockets which can accommodate the femoral shell as used in hip resurfacing techniques. Although the number of wear particles produced in these bearings are high in number because the size of the particles is in the 0.015-0.12µm range they tend to produce a lesser reaction than polyethylene wear particles which are in the 0.2-7µm range. There are several concerns with this type of bearing. One is the uncertain implications of metal ion accumulation. Whilst it is well documented that there are increased metal ion levels in the serum, spleen and kidney there are only postulations of a cancer or other health risk. There is also the phenomenon of ALVAL (aseptic, lymphocytic vasculitis and associated lesions) which results from exposure to metal ions. Whilst still being extensively researched it appears to be a type IV cell mediated hypersensitivity reaction with vascular endothelial swelling which can result in massive soft tissue destruction sometimes with surprisingly little in terms of external symptoms. Why this occurs precisely and to whom is not yet well characterised. Interestingly though results so far indicate that perimenopausal women fare considerably worse in comparison to men with metal on metal articulations. Women who have received metal on metal arthroplasties have roughly twice the failure rate as compared with men.

What are the advantages of oxidized zirconium over conventional bearings?

i) In vitro testing
The results from Smith & Nephew’s in vitro testing are impressive(4). In an abrasion test with a cement pin against separate discs of cobalt chrome and oxidized zirconium, after 10 million cycles, the wear tracks were more shallow, the volumetric wear 4,900 times less and the surface was 160 times smoother in the oxidized zirconium. With respect to hardness, the ceramic surface was about 15GPa reducing to about 3GPa with the underlying zirconium-niobium alloy. This compares favourably with cobalt chrome which has a hardness of about 5GPa. Oxide adhesion tests showed that even after creating a defect in the ceramic surface layer down to the zirconium and undergoing 10 million cycles of rubbing with cement pins, there was no evidence of ceramic delamination or additional damage and catastrophic failure did not result. Fatigue strength testing showed an equivalent profile to cobalt chrome. In adhesive wear tests, compared to cobalt chrome, oxidized zirconium has a lower coefficient of friction and therefore less wear against both UHMWPE and cartilage. ii) Hip Arthroplasty
The oxidized zirconium ball has a lower generation of wear particles with UHMWPE in comparison with stainless steel and cobalt chromium heads. This reduction is even greater with HCLPE and this particular coupling has the lowest wear rate of any articulation. The ball is also more resistant to scratching than cobalt chrome heads and has a lower surface roughness than cobalt chrome. The problems of brittle fracture, squeaking and stripe wear seen with pure ceramics are not a feature of oxidized zirconium. It therefore seems to have similar tribology to a ceramic, but without its disadvantages, combining high wear resistance and high toughness. It should be mentioned that Smith and Nephew have a wide range of head and liner options to partner every cup compared with ceramic implants. This allows more accurate restoration of the offset and leg length. iii) Nickel Hypersensitivity
Oxidized zirconium can be used in patients exhibiting nickel sensitivity safely as there is no traceable nickel in this material.

What concerns have been raised?

Theoretically, there are several concerns with this relatively new material. i) Could the surface layer delaminate?
The abrupt difference in material hardness (15GPa to 3GPa) could lead to delamination of the surface layer due to build up of internal stresses at the transitional area. Should this occur then the tribologic result could be a large amount of metallic wear. But the in vitro data as mentioned above suggests that it does not occur but we await in vivo data to confirm this. ii) Are the reduced levels of the wear particles generated clinically significant?
A standard cobalt chrome femoral component for hip with UHMWPE produces around 38mm3/106 cycles. Although it is agreed that the wear particles generated by oxinium heads with UHMWPE in hip arthroplasty (21mm3/106 cycles)are less than with cobalt chrome, an even greater reduction in particle wear numbers (undetectable) could be achieved with HCLPE and either a cobalt chrome or oxidized zirconium head(5). Thus it is suggested the new oxinium technology may not be the only option.However, more critical analysis of these results indicate that roughening of the cobalt chrome head significantly increases the wear particles generated accelerating polyethylene wear. So the superior resistance to roughening of oxidized zirconium may yet improve the longevity of the implant in vivo. iii) Is there any clinical in vivo data?
No rigorous clinical trials are yet published but Mr Fares Haddad (Consultant Hip Surgeon at University College Hospital, London) has this comment about some preliminary clincal data he has available. “There is currently a multi centre study ongoing in the United Kingdom which has recruited almost 400 patients. In that study Oxidized Zirconium and Cobalt chrome have been matched with standard polyethylene and highly crosslinked polyethylene to compare respective wear rates with 32mm heads. The preliminary data is very promising with almost no wear in the Oxidized zirconium on XLPE articulation. Our studies thus far where Oxidized zirconium heads have been implanted in the first stage of two stage revisions and in revisions that have subsequently failed / retrievals would suggest that there is very little head damage and even were there have been dislocations the head damage is out of the standard bearing area of the hip and has not led to significant polyethylene damage or wear. The explanted heads have been put through simulators and still show excellent wear properties.”

Conclusions

Oxidized zirconium is a new and elegant technology. In my opinion it does offer mechanical advantages over other bearing surfaces in total hip arthroplasty in the objective of reducing wear and thus is an important step forward. However as always technology is on the move and concurrent developments in polyethylene technology and acetabular buffers are equally exciting. As always the proof in all new technologies are in the clincal trials and we await the full data from the trials to become available but certainly initial impressions are promising.

References

1. A novel method of cross-linking ultra-high-molecular-weight polyethylene to improve wear, reduce oxidation, and retain mechanical properties. Muratoglu O.K., Bragdon C.R., O’Connor D.O., Jasty M., Harris W.H. Journal of Arthroplasty, 2001, 16, p. 149.
2. Poor eight-year survival of cemented zirconia-polyethylene total hip replacements. J. Allain, S. Le Mouel, D. Goutallier, M. C. Voisin J Bone Joint Surg [Br] 1999;81-B:835-42.
3. Ceramic-on-polyethylene bearing surfaces in total hip arthroplasty. Seventeen to twenty-one-year results. Urban JA, Garvin KL, Boese CK, Bryson L, Pedersen DR, Callaghan JJ, Miller RK. J Bone Joint Surg Am. 2001 Nov;83-A(11):1688-94
4. Smith & Nephew Website: www.oxinium.co.uk – downloaded August 21/8/08
5. Reduced wear with oxidized zirconium femoral heads. Victoria Good, Michael Ries, Robert L. Barrack, Kirstin Widding, Gordon Hunter and Dan Heuer. The Journal of Bone and Joint Surgery – Supplement 4, 2003, 85A, p. 105.

Absolutely no contributions or assistance have been sought or provided from Smith & Nephew. This is an independent article with no conflicts of interest.