The search for durable, low wear bearing couples for total hip arthroplasty has gained momentum in recent years, stimulated by the successful results of some metal on metal implants,1 the development of and precise insertion of more durable ceramic components,2 and advances in the production of highly cross-linked polyethylene,3 including fortification with vitamin E.4
Within the discussion relating to metal on metal bearings, concerns have persisted because of metal ion production. Particulate debris from metal on metal articulations may result in local soft tissue reactions (some of which may be spectacular and carry provocative nomenclature).5 Chromosomal changes including aneuploidy and failure of telomerase reaction have been noted, as well as potential toxicity to many organ systems,6 and there is persisting concern relating to the hexavalent form of chromium leading to reduced CD8 lymphocyte counts and the possibility of oncogenesis.7 The last of these concerns relates specifically to haemopoietic neoplasia against a background of lower lifetime risk of cancer.8
Recent evidence suggests that the increased use of metal on metal bearings is a factor in both Europe and North America.9
The possibility that lower wear might result from a ceramic head articulating with a metallic acetabular liner has attracted considerable interest. Much of the lubrication which occurs in a metal on metal bearing (with larger head sizes) is mixed phase lubrication, and little is known about the potential consequences of a ceramic on metal (CoM) articulation. Consequently, the work by Isaac et al10 is of considerable importance. In their clinical study, they measured whole blood metal ion levels (chromium is largely intracellular) and found that chromium levels at 12 months were significantly lower with the use of ceramic on metal bearings compared with metal on metal bearings. Since blood metal ion levels seem likely to be a good surrogate marker for wear,11 these findings alone may be of considerable importance in future.
There are major efforts underway to reduce stem size in young patients undergoing total hip arthroplasty, and to conserve proximal femoral bone stock.12 A durable ceramic on metal articulation is an attractive concept, and it overcomes the concerns relating to impingement damage to ceramic acetabular liners.13
Isaac et al10 were also able to examine explanted heads and cups and then review them in a hip joint simulator. The wear rates were the same as in vivo, and an order of magnitude less than reported for metal on metal bearings. Material transfer was visible on ceramic heads, but this appeared to have no relevance in relation to wear.
The observations relating to blood chromium levels and wear rates following material transfer in this study are novel, but it remains to be seen whether the bearing couple described achieves increasing and universal acceptance, or whether hitherto unknown disadvantages will become apparent with time. The authors acknowledge outliers in their series related to malpositioned implants. Accelerated wear is a feature of incorrectly-inserted metal on metal resurfacing devices, and it is not known whether the associated periarticular effects may be equally significant with CoM bearings. Since younger patients are likely to be selected for such procedures, there will be considerable interest in ideal diametric clearance, wear debris analysis, particle size, local tissue histology, patterns of osteolysis, bone conservation and implant survivorship. There is some early evidence that particle size may be relatively constant from MoM and CoM bearings,14 but much remains unknown. Nevertheless, the authors have followed their early ground-breaking work with a useful clinical study and provided much food for thought.15
1. Treacy RBC, McBryde CW, Pynsent PB. Birmingham hip resurfacing arthroplasty: A minimum follow-up of five years. J Bone Joint Surg [Br] 2005;87-B:167-70.
2. Sugano N, Nishii T, Miki H, Yoshikawa H, Sato Y, Tamura S. Mid-term results of cementless total hip replacement using a ceramic-on-ceramic bearing with and without computer navigation. J Bone Joint Surg [Br] 2007;89-B:455-60.
3. McCalden RW, MacDonald SJ, Rorabeck CH, Bourne RB, Chess DG, Charron KD. Wear rate of highly cross-linked polyethylene in total hip arthroplasty: a randomized controlled trial. J Bone Joint Surg [Am] 2009;91-A 773-82.
4. Maher SA, Hidaka C, Cunningham ME, Rodeo SA. What's new in orthopaedic research. J Bone Joint Surg [Am] 2008;90-A:1800-8.
5. Pandit H, Glyn-Jones S, McLardy-Smith P, et al. Pseudotumours associated with metal-on-metal hip resurfacings. J Bone Joint Surg [Br] 2008;90-B:847-51.
6. Keegan GM, Learmonth ID, Case CP. Orthopaedic metals and their potential toxicity in the arthroplasty patient: A review of current knowledge and future strategies. J Bone Joint Surg [Br] 2007 89-B:567-73.
7. Afolaranmi GA, Meek RMD, Grant MH. Release of chromium from orthopaedic arthroplasties. Open Orthop J 2008;2:10-18.
8. Visuri TI, Pukkala E, Pulkkinen P, Paavolainen P. Cancer incidence and causes of death among total hip replacement patients: a review based on Nordic cohorts with a special emphasis on metal-on-metal bearings. Proc Inst Mech Eng H 2006;220:399-407.
9. Bozic KJ, Kurtz S, Lau E, et al. The epidemiology of bearing surface usage in total hip arthroplasty in the United States. J Bone Joint Surg [Am] 2009;91-A:1614-20.
10. Isaac G, Brockett C, Breckon A et al. Ceramic-on-metal bearings in total hip replacement: whole blood metal ion levels and analysis of retrieved components. J Bone Joint Surg [Br] 2009,91-B:in press.
11. Shimmin A, Beaulé PE, Campbell P. Metal-on-metal hip resurfacing arthroplasty. J Bone Joint Surg [Am] 2008;90-A:637-54.
12. Learmonth ID. Conservative stems in total hip replacement. Hip International 2009:in press.
13. Malik A, Maheshwari A, Dorr LD. Impingement with total hip replacement. J Bone Joint Surg [Am] 2007;89-A:1832-42.
14. Brown C, Williams S, Tipper JL, Fisher J, Ingham E. Characterisation of wear particles produced by metal on metal and ceramic on metal hip prostheses under standard and microseparation simulation. J Mater Sci Mater Med 2007;18:819-27.
15. Williams S, Schepers A, Isaac G, et al. The 2007 Otto Aufranc Award. Ceramic-on-metal hip arthroplasties: a comparative in vitro and in vivo study. Clin Orthop 2007;465:23-32.
Spencer RF, MD, FRCS
Weston General Hospital, Weston-super-Mare and the Avon Orthopaedic Centre, Bristol, United Kingdom