CERAMIC-ON-CERAMIC ARTICULATION IN THA

Abstract

Late aseptic loosening of total hip arthroplasty (THA) components due to wear debris especially sub-micron Polyethylene induced osteolysis has been identified to be the major cause for revision. Therefore, the use of wear resistant designs and materials is imperative for the long-term success of articulating implants. One of the most promising articulations for THA regarding extremely low wear is the hard-on-hard Alumina/Alumina combination with a long history in orthopaedics accumulating to now 30 years of experience.

Alumina Ceramic: Aluminum-Oxide Ceramic (Alumina, Al2O3, ISO 6474) is an extremely hard material that can only be scratched and also machined and polished by diamond. It has excellent mechanical properties but is brittle as are most ceramic materials and can, therefore, not be used for highly stressed implants like an artificial hip joint stem. Due to its ionic structure it is hydrophilic allowing liquids to bound to and lubricate its surface. Alumina’s hardness allows a polishing to a low roughness, which is entirely inverse and therefore able to create lubricating “pools.”

Density, purity and grain size in combination with an optimised manufacturing process are crucial for the final properties of Alumina and have been sub-optimal at the early pioneering times. Also the design has to be adapted to the critical properties of this ceramic.

The biological activity of Alumina is graded as bio-inert and no direct osteointegration is to be expected, also proven by clinical experience. Alpha Alumina is bio-stable and practically insoluble in the body environment. Therefore, ageing or any systemic reaction in the human body with this ceramic is of no concern.

First Generation Alumina Ceramic: Boutin has introduced Alumina ceramic components for articulation with itself for THA in 1970, followed by Mittelmeier and other surgeons. First reports of its combination with UHMWPE cups date back to 1972. More than one million Alumina heads have been worldwide implanted since then. Problems regarding fracture of the head were mostly encountered with collared heads of one design and the cup (especially if placed at > 50 deg abduction position). Run-away wear in case of edge contact have been reported with these first generation implants.

A comprehensive retrieval study using a systematic analysis of two different designs confirmed that Alumina/ Alumina couples have a low clinical wear rate and identified the main risk factors that can lead to early failure of such devices. The wear rate of those historic Alumina / Alumina articulations is in the range of less than 5 microns per year. This is one to two orders of magnitude less than for any articulation with Polyethylene cups. Reaction to Alumina wear particles, mostly encountered on catastrophic failure of the historic implant designs is sparse and mostly benign.

Third Generation Alumina Ceramic: Significant improvements in material properties and quality have been made since its introduction. The third generation of Alumina has been introduced in 1994 and was a further evolution regarding material properties, manufacturing, quality control and design. Refinement of purity, grain size and manufacturing results in improved fracture and also wear resistance, which decreased from low to almost immeasurable values. With the evolution of this new generation ceramic the problems of the past have been successfully addressed. Following improvements are characteristic for a third generation Alumina:

• Improved purity and reduced grain size (Figure 1) for better properties

• Improved density, HIP for enhanced toughness and bending strength

• Less stress raisers due to laser marking instead of mechanical engraving

• Optimised head internal geometry

• Rounded and polished rim for risk reduction of runaway wear on sub-luxation

• Improved safety due to 100% proof test on heads and inserts

These improvements result in a significant increase in mechanical properties. The risk of head fracture has been reduced for this 3rd generation Alumina ceramic heads from 1 per 500 (0.2%) to 1 per 25 000 (0.025%).

The wear resistance of the couple Alumina/Alumina has also been enhanced which was measured in simulator tests to be around 1 micron per year. Tribological investigation involved a series of screening, pendulum and anatomical hip simulator tests with actual Alumina/ Alumina components in respect to the effect of clearance and cup angle (45° & 60°) in a series of tests for up to 5 million cycles. Adverse testing conditions for Alumina e.g. dilution of lubricant, dry and water, high load in swing phase, stop-start, etc in ascending aggressiveness (each at 1 million cycles) have also been investigated. No significant difference in wear volume was found comparing clearance or cup angle for all components tested. A new simulator test set up using a microseparation mode during every single cycle was also run for 5 million cycles combining Alumina from one single manufacturer and also mixing Alumina’s from different manufacturers. The wear in all cases was low and lower than for the first generation Alumina’s.

A series of implantations with Alumina/Alumina articulation has been performed since November 1996 according to a prospective multicentric FDA IDE protocol comparing the same implant with CoCr metal heads/PE combinations. Short-term results demonstrate no early complications with this third generation Alumina/Alumina articulation if implanted correctly. The benefit of the dramatically reduced wear rate will show only after longer-term follow-up.

Conclusion: Alumina/Alumina articulation demonstrates the lowest wear rate of all available material combinations for THA in the laboratory as well as in clinical praxis. The bulk material as well as its particles is bio-inert, therefore, giving no concern for adverse biological reactions.

Problems with earlier designs of Alumina/Alumina articulation have been successfully addressed by taking the identified risk parameters into consideration. Components of the third generation Alumina ceramic and design have been extensively tested. All components pass the mechanical tests. Alumina heads and liners of the third generation in the size and under the conditions tested are safe and efficient. Their wear rate even under the influence of adverse condition is minimal.

Further clinical trials parallel the application of this superior articulation couple in an innovative and modern design.Alumina/Alumina is, therefore, the material combination of choice for the active patient with high life expectancy.