Is the ceramic hard/hard bearing in total hip replacements the (only) right solution?

Wolfhart Rieger, Stefan Leyen, Stefan Köbel

communication complète

Metoxit AG, CH-8240 Thayngen

Saphirwerk Industrieprodukte AG, CH-2555 Brügg


Since 1970, the use of ceramics in orthopaedics has been growing constantly. Ceramics in orthopaedics are today as common as metals and polymers. In France, the combination of ceramic and metal ball heads with PE inserts (including X-linked PE) is by far the largest part of the market and rates at 70%; ceramic/ceramic bearings make up 25% and metal/metal 5% of all total hip replacements. For a certain period, after the introduction of high strength materials such as Delta, the impression was that hard/hard bearings were taking over command.

On the contrary, Metoxit sees an increased demand for ceramic ball heads to be used in combination with PE and X-link PE. In addition, demand for the long prevailing ball head diameter 28 mm is decreasing, while 32 mm diameter taking the lead, and larger diameters (36 mm and larger) coming up.

What are the reasons for this change of paradigm, as we may call it?

There are several potential reasons to be mentioned:

  1. The risk of fracture is, notwithstanding all other potential influences, decreasing with increasing diameter of ball head and/or acetabular cup.

  2. Articulating partners: The development of X-link PE, available in various qualities, has demonstrated that the wear rates can come close to those of hard/hard couplings. PEEK, another artificial resin material, seems to show into the same direction. Soft materials of this kind have, apparently, high potential of further development. Recent research shows that even metal vs. ceramic can be of potential benefit.

  3. All ceramic materials behave well against soft partners: Alumina, in use since 1970;

Zirconia, widely applied between 1986 and 2001, grounded due to faulty workmanship of one producer; Zirconia Toughened Alumina (ZTA, Delta), introduced 2000; Alumina Toughened Zirconia (ATZ), introduced 2006, the material with the highest strength. Due to the soft character of the acetabular cup material, edge contact of ball head and cup is of no negative consequence.

  1. Risk statistics: The figures for hard/soft coupling are constant and equivalent for leading producers; the rate being one in 5000 to one in 8000 THR’s. For hard/hard, the initial figures of 1998, 1 in 10’000, have been to over-optimistic. With the increasing number of hard/hard couplings implanted, recent figures are considerably higher than 1 in 5000.

  2. Resurfacing: Resurfacing has found great interest in recent years. However, this method will require a design with large diameters and thin material shells. This will increase the risk of fracture.

  3. Other influences: Luxation, edge contact and third particle contact present a higher risk in hard/hard than in hard/soft systems.

  4. Special attention has to be given to the influence of hard (and soft) particles in the interface between ceramic cone opening and metal trunnion. Measurements have shown that the load bearing capacity of the ball head can be reduced by up to 90% even by small blood droplets and/or soft tissue in the interface.

  5. Age of patient: The average age for first implantation is 57 years , with tendency towards younger age. With expected life times of implantation of 20 years and more for alumina vs. PE, the need for hard/hard is not obvious, even for middle aged and active persons.

  6. Cost factor: It is obvious that a hard/soft solution is beneficial for the patient from the point of view of cost of implantation.

 

Based on these arguments, we conclude:

  • There is currently a co-existence of hard/hard and hard/soft couplings in THR articulations. A tendency is observed towards improved hard/soft bearings.

  • High strength ceramic materials such as Alumina Toughened Zirconia (ATZ) and Zirconia Toughened Alumina (ZTA, Biolox® Delta) can be of additional benefit as compared to alumina, i.e. reducing the fracture rate in the case of third particles at the ceramic/metal trunnion interface.

  • The increase in the perfection of ball head machining with figures of ball sphericity of better than 0,1 μm, ball diameter within plus/minus 3 microns, and ball surface parameter Ra better than 0,02 μm will decrease the wear rate of the partner material (PE, X-link PE, PEEK) considerably.

  • The tendency to larger diameters is not contributing in this context, but assures higher mobility and more comfort for the patient.

  • Combinations of these hard and soft materials, especially with larger diameters, point in the direction of low wear, low risk, low cost.

  • In summary, with today’s hard ceramic/soft bearings, the majority of patients will have a long lasting, safe and affordable THR.

 

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