Screen Shot 2014-07-31 at 8.34.47 PMKate Tilley, third-year, investigates the effects of using metals in prostheses.  

Research published by the Lancet has provided ‘unequivocal evidence’ that metal on metal stemmed prostheses are associated with a high rate of failure, particularly among the younger generation, women, and patients with larger implants. Since the 1960’s, metal on metal hip replacements have been used to provide treat- ment for symptomatic os- teoarthritis of the hip using a cobalt-chrome alloy as a stemmed replacement [1]. Osteoarthritis is becoming increasingly prevalent, with the hip being the second most common large joint to be affected [2]. Osteoarthritis is an important cause of pain and disability in the older generation with roughly 5 percent of people over 60 having the disorder [3].

 

Screen Shot 2014-07-31 at 8.36.12 PMIt is thought that an aging population and obesity is positively correlated with the prevalence of osteoarthritis. This will be accompanied with an increase in the number of total hip replacement (THR) operations. THR involves removing a dam- aged hip joint and replac- ing it with an artificial one known as a prosthesis. The prosthetic parts are either cemented by securing to healthy bone using glue, or uncemented, where the prosthesis is made from permeable material where the bone can grow into it, holding it in place. In addition to metal on metal THRs, there are several other materials employed instead such as ceramic and plastic [4].

THR is extremely common and has been described as ‘the operation of the 20th century’. It revolutionised management of elderly patients with significant morbidity and restored quality of life with good long term results. However, there are increasing numbers of active and young patient that undergo THR. They have different expectations of management and quality of life, which need to be accommodated [5].

The main concern with long term THR is failure, resulting in revision surgery. In England and Wales, it is estimated that 5% of THR’s are revised within 7 years [6]. The majority of THR failure is caused by aseptic loosening due to polyethylene wear [7], followed by dislocation affecting approximately 4% of THR’s within 6 months [8].

In order to address these problems, alternative bearing surfaces have been investigated to reduce wear and to produce larger head sizes. Metal on metal bearing surfaces have been reported to be highly resistant to wear and Healthcare Regulatory Agency (MHRA) raised alarm in the UK by advising patients, who have had large metal on metal THRs, to be monitored for life. Evidence showed that patients with failed metal on metal hip arthroplasty had higher concentrations of serum cobalt and chromium. This is caused by the ball and socket of the hip joint rubbing together, which result in the release of metal debris.

Elevated serum metals can enter the bloodstream, and seep into surrounding tissue to cause inflammation, destroy muscle and bone. Therefore, the MHRA has suggested that all patients in the UK with large head hip replacements should have blood tests to check for high metal concentrations. They may also require MRI scans if there are raised levels of metal ions or if patients show adverse symptoms. [13]

Although our understanding of the local and systemic pathological effects of raised metal ion concentrations are limited, analysis of the National Joint Registry has provided critical information about the high failure rates of metal on metal hip implants, particularly in the female population. Failure was related to head diameter with larger heads failing earlier. MHRA recommends that all patients with metal on metal THR’s have regular follow up to ensure there are no further complications in relation to increased metal ion concentrations. However, further research is required to understand the physiological consequences of exposure to orthopaedic metals.


References

  1. Daniel J, Pynsent PB, McMinn DJ. Metal-on-metal resurfacing of the hip in patients under the age of 55 years with osteoarthritis. J Bone Joint Surg Br 2004 Mar;86(2):177-184.
  2. Scott D. Osteoarthritis of the hip. Am Fam Physician 2010 Feb 15;81(4):444.
  3. Guillemin F, Rat AC, Mazieres B, Pouchot J, Fautrel B, Euller-Ziegler L, et al. Prevalence of symptomatic hip and knee osteoarthritis: a two-phase population-based survey. Osteoarthritis Cartilage 2011 Nov;19(11):1314-1322.
  4. Hip Replacement. 2012; Available at: http://www.nhs.uk/Conditions/ Hip-replacement/Pages/How-it-is-performed.aspx.
  5. Learmonth ID, Young C, Rorabeck C. The operation of the century: total hip replacement. The Lancet ;370(9597):1508-1519.
  6. National Joint Registry for England and Wales. 8th annual report 2011. http://www.njrcentre.org.uk/NjrCentre/Portals/0/Documents/ NJR%208th%20Annual%20Report%202011.pdf (accessed Jan 31, 2012).
  7. Howard JL, Kremers HM, Loechler YA, et al. Comparative survival of uncemented acetabular components following primary total hip arthroplasty. J Bone Joint Surg Am 2011; 93: 1597–604.
  8. Blom AW, Rogers M, Taylor AH, Pattison G, Whitehouse S, Bannister GC. Dislocation following total hip replacement: the Avon Orthopae- dic Centre experience. Ann R Coll Surg Engl 2008; 90: 658–62.
  9. Smith SL, Dowson D, Goldsmith AA. The eff ect of femoral head diam- eter upon lubrication and wear of metal-on-metal total hip replace- ments. Proc Inst Mech Eng H 2001; 215: 161–70.
  10. Keegan GM, Learmonth ID, Case CP. A systematic comparison of the actual, potential and theoretical health effects of cobalt and chrome exposures from industry and surgical implants. Crit Rev Toxicol 2008; 38: 645–74.
  11. Langton DJ, Jameson SS, Joyce TJ, et al. Accelerating failure of the ASR total hip replacement. J Bone Joint Surg Br 2011; 93: 1011–16.
  12. Smith AJ, Dieppe P, Vernon K, Porter M, Blom AW. Failure rates
    of stemmed metal-on-metal hip replacements: analysis of data from the National Joint Registry of England and Wales. The Lancet ;379(9822):1199-1204.
  13. Newton AW, Ranganath L, Armstrong C, Peter V, Roberts NB. Differ- ential distribution of cobalt, chromium, and nickel between whole blood, plasma and urine in patients after metal-on-metal (mom) hip arthroplasty. Journal of Orthopaedic Research 2012:n/a-n/a.
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