Orthopedics

Metal-Metal Bearing Surfaces in Hip Arthroplasty

Thomas P. Schmalzried, MD

Abstract

There is no mystery regarding the allure of metal–metal bearings: high stability and low wear potential. The special risks associated with these bearings are coming into focus and include a macrophage response to excessive metal particles (metal reactivity) and a lymphocyte-dominated reaction (metal sensitivity). The most common presentation of an adverse local tissue reaction (ALTR) is persistent pain. The incidence of ALTR has not been completely defined, but the risk appears to be increased in resurfacing, in women, and in bilateral cases. The differential diagnosis includes septic and aseptic loosening, and the evaluation should include aspiration. The diagnosis is confirmed by histological examination.

It can be difficult to assess the fixation status of monoblock cobalt chromium acetabular components. Both metal reactivity and metal sensitivity can cause periprosthetic bone resorption (osteolysis). The development of a large effusion may be a sign of metal sensitivity. Ultrasound is a cost-effective diagnostic tool but is operator dependent. A magnetic resonance imaging scan with metal artifact suppression can identify a periprosthetic fluid collection or cystic/solid mass (pseudotumor). Serum or whole blood ion levels elevated to <10 ppb are suggestive of a suboptimal wear mechanism and an increased risk of ALTR.

The indications for revision include component loosening, progressive osteolysis, large effusion or pseudotumor, and unremitting pain. Correction of any identified mechanical aberrations with conversion to a nonmetal–metal bearing is recommended. A well-fixed cobalt chromium substrate (modular stem or modular acetabular shell) can be retained.

The motivation for selecting any arthroplasty technology is the general desire to maximize function and avoid revision surgery. In the Medicare population, the risk of death within 90 days of a revision total hip arthroplasty (THA) is 2.6%, which is 2.6 times higher than that risk following a primary THA.1 There is no mystery regarding the allure of metal-metal bearings: larger diameter bearings have greater stability, and when properly positioned, the wear rate has been documented to be low in vivo for 3 decades.2,3 The special risks associated with these bearings are coming into focus and should be understood relative to the benefits.

Data from a large administrative database indicate that hip instability is the most common indication for revision THA in the United States, and this revision burden has a significant economic impact. Analysis of 51,345 revision THAs performed in the United States between October 1, 2005, and December 31, 2006, revealed that the common causes of revision THA were instability/dislocation (22.5%), mechanical loosening (19.7%), and infection (14.8%).4 Other large administrative database studies indicate that the risk of a primary total hip dislocation in the first 6 months following primary THA is 3.9%5 and that the risk of dislocation is significantly higher for low-volume surgeons.6 Long-term data indicate that the risk of dislocation increases with time to 7% at 25 years and that the risk of dislocation is higher in women and people older than 70 years.7

Hips with a larger bearing diameter have a significantly lower risk of dislocation, with the greatest benefit being associated with the posterior approach.8 Laboratory simulations indicate that femoral heads >32 mm provided greater range of motion and virtually complete elimination of component-to-component impingement. A significant increase in both flexion before dislocation and displacement between the femoral head and the acetabulum to produce dislocation occurred with heads >32 mm.9

Retrieval studies of first-generation metal–metal bearings with long in vivo service lives consistently demonstrate low wear of the bearing.10-12 Autopsy retrieval analyses of hips with metal–metal bearings that functioned well for 30 years demonstrated low wear with no evidence of adverse local or distant tissue reactions. When the desired tribological conditions are met, metal–metal bearings…

Abstract

There is no mystery regarding the allure of metal–metal bearings: high stability and low wear potential. The special risks associated with these bearings are coming into focus and include a macrophage response to excessive metal particles (metal reactivity) and a lymphocyte-dominated reaction (metal sensitivity). The most common presentation of an adverse local tissue reaction (ALTR) is persistent pain. The incidence of ALTR has not been completely defined, but the risk appears to be increased in resurfacing, in women, and in bilateral cases. The differential diagnosis includes septic and aseptic loosening, and the evaluation should include aspiration. The diagnosis is confirmed by histological examination.

It can be difficult to assess the fixation status of monoblock cobalt chromium acetabular components. Both metal reactivity and metal sensitivity can cause periprosthetic bone resorption (osteolysis). The development of a large effusion may be a sign of metal sensitivity. Ultrasound is a cost-effective diagnostic tool but is operator dependent. A magnetic resonance imaging scan with metal artifact suppression can identify a periprosthetic fluid collection or cystic/solid mass (pseudotumor). Serum or whole blood ion levels elevated to <10 ppb are suggestive of a suboptimal wear mechanism and an increased risk of ALTR.

The indications for revision include component loosening, progressive osteolysis, large effusion or pseudotumor, and unremitting pain. Correction of any identified mechanical aberrations with conversion to a nonmetal–metal bearing is recommended. A well-fixed cobalt chromium substrate (modular stem or modular acetabular shell) can be retained.

The motivation for selecting any arthroplasty technology is the general desire to maximize function and avoid revision surgery. In the Medicare population, the risk of death within 90 days of a revision total hip arthroplasty (THA) is 2.6%, which is 2.6 times higher than that risk following a primary THA.1 There is no mystery regarding the allure of metal-metal bearings: larger diameter bearings have greater stability, and when properly positioned, the wear rate has been documented to be low in vivo for 3 decades.2,3 The special risks associated with these bearings are coming into focus and should be understood relative to the benefits.

Hip Stability

Data from a large administrative database indicate that hip instability is the most common indication for revision THA in the United States, and this revision burden has a significant economic impact. Analysis of 51,345 revision THAs performed in the United States between October 1, 2005, and December 31, 2006, revealed that the common causes of revision THA were instability/dislocation (22.5%), mechanical loosening (19.7%), and infection (14.8%).4 Other large administrative database studies indicate that the risk of a primary total hip dislocation in the first 6 months following primary THA is 3.9%5 and that the risk of dislocation is significantly higher for low-volume surgeons.6 Long-term data indicate that the risk of dislocation increases with time to 7% at 25 years and that the risk of dislocation is higher in women and people older than 70 years.7

Hips with a larger bearing diameter have a significantly lower risk of dislocation, with the greatest benefit being associated with the posterior approach.8 Laboratory simulations indicate that femoral heads >32 mm provided greater range of motion and virtually complete elimination of component-to-component impingement. A significant increase in both flexion before dislocation and displacement between the femoral head and the acetabulum to produce dislocation occurred with heads >32 mm.9

Low Wear

Retrieval studies of first-generation metal–metal bearings with long in vivo service lives consistently demonstrate low wear of the bearing.10-12 Autopsy retrieval analyses of hips with metal–metal bearings that functioned well for 30 years demonstrated low wear with no evidence of adverse local or distant tissue reactions. When the desired tribological conditions are met, metal–metal bearings are well tolerated and can function for 3 decades with little wear.2,3

All metal–metal bearings are not created equal; however, consensus exists from theoretical and laboratory analyses on the formula for reducing the wear of a metal–metal bearing. Initial running-in wear decreases as the bearing diameter increases and/or the clearance decreases. Because the subsequent steady-state wear rate is less, running-in wear contributes significantly to the total volume of metal wear, even over long periods of time.13,14 Lower clearance has been associated with lower ion levels in vivo.15 All other factors being equal (eg, roundness and smoothness), larger bearings with smaller clearances will have less wear.13,14 Given that the material has high carbon content, metallurgy has little effect on bearing wear. Low-carbon materials exhibit higher wear than high-carbon materials. There is little difference in the wear of high-carbon wrought or cast materials.16

Special Risks

Adverse local tissue reactions (ALTRs) can occur in association with prostheses made of cobalt chromium alloys.17,18 This term, ALTR, generically includes all adverse responses, as different mechanical and biological processes are potentially at play. Metal reactivity is characterized by high wear of the bearing with a predominantly foreign-body (macrophage) inflammatory response. Excessive metal wear particles within the size range incite a macrophage response. A lateral opening angle >55° and/or excessive combined anteversion predisposes to edge loading and high wear with the production of relatively large metal particles. Circulating cobalt and chromium ion levels are typically elevated to .10 ppb.19 To increase bearing surface contact and achieve the intended tribology, the lateral opening angle should be 40° to 45° and the combined anteversion (acetabular1femoral) 20° to 30°.20

Metal sensitivity is characterized by a predominantly lymphocytic (immune) response or aseptic lymphocyte-dominated vasculitis-associated lesion with or without evidence of excessive metal–metal bearing wear. Metal wear particles are rare on light microscopic examination of periprosthetic tissues. This is thought to be a reaction to metal ions with development of metal–protein complexes (haptens) that stimulate an immune reaction. There may be an effusion or a cystic/solid mass, a so-called pseudotumor. Such reactions are not new or unique to metal–metal bearings and have been previously reported in association with cobalt chromium corrosion products from a modular head–morse taper connection.21 It is important to keep in mind that there are sources of particles and ions other than the bearing.

A wide range of occurrences of ALTR has been reported in cohort studies: from >1 in 100 to <1 in 1000 cases. The factors contributing to this range of incidence are under investigation and include component manufacturing and positioning variables, as well as patient-related variables.18 Evidence suggests that the risk is higher in resurfacing, in women, and in bilateral cases.22 This seems to suggest a biological basis, but mechanical factors have not been thoroughly investigated and cannot be ruled out.

The most common presenting symptom of an ALTR to a metal–metal bearing is persistent pain, most commonly in the groin.23 The differential diagnosis includes septic and aseptic loosening. It can be difficult to assess the fixation status of monoblock cobalt chromium acetabular components. Both metal reactivity and metal sensitivity can cause periprosthetic bone resorption (osteolysis). The development of a large effusion may be a sign of metal sensitivity. Ultrasound is a cost-effective diagnostic tool but is operator dependent. A magnetic resonance imaging scan with metal artifact suppression can identify a periprosthetic fluid collection or cystic/solid mass (pseudotumor). Serum or whole blood ion levels elevated to >10 ppb are suggestive of a suboptimal wear mechanism and an increased risk of an ALTR.24

The erythrocyte sedimentation rate and C-reactive protein may or may not be elevated. As with any painful arthroplasty, an aspiration is advised to exclude infection.25 The joint fluid characteristics (eg, appearance, cell count and differential, protein and glucose concentration, pH, and viscosity) associated with metal reactivity or metal sensitivity are being defined. The fluid associated with a sensitivity reaction is low viscosity, yellowish and cloudy, and has a low cell count. Histological examination can identify the elements of metal reactivity and metal sensitivity, but the role of a biopsy has not yet been established. The characteristic histological features of a sensitivity reaction are diffuse and perivascular infiltrates of T and B lymphocytes and plasma cells, high endothelial venules, massive fibrin exudation, accumulation of macrophages with droplike inclusions, and infiltrates of eosinophilic granulocytes and necrosis.26

The indications for revision include component loosening, progressive osteolysis, large effusion or pseudotumor, and unremitting pain. Correction of any identified mechanical aberrations with conversion to a nonmetal–metal bearing is recommended. A well-fixed cobalt chromium substrate (modular stem or modular acetabular shell) can be retained.

References

  1. Mahomed NN, Barrett JA, Katz JN, et al. Rates and outcomes of primary and revision total hip replacement in the United States Medicare population. J Bone Joint Surg Am. 2003; 85(1):27-32.
  2. Campbell P, Urban RM, Catelas I, Skipor AK, Schmalzried TP. Autopsy analysis thirty years after metal-on-metal total hip replacement. A case report. J Bone Joint Surg Am. 2003; 85(11):2218-2222.
  3. Clarke MT, Darrah C, Stewart T, Ingham E, Fisher J, Nolan JF. Long-term clinical, radiological and histopathological follow-up of a well-fixed McKee-Farrar metal-on-metal total hip arthroplasty. J Arthroplasty. 2005; 20(4):542-546.
  4. Bozic KJ, Kurtz SM, Lau E, Ong K, Vail TP, Berry DJ. The epidemiology of revision total hip arthroplasty in the United States. J Bone Joint Surg Am. 2009; 91(1):128-133.
  5. Phillips CB, Barrett JA, Losina E, et al. Incidence rates of dislocation, pulmonary embolism, and deep infection during the first six months after elective total hip replacement. J Bone Joint Surg Am. 2003; 85(1):20-26.
  6. Katz JN, Losina E, Barrett J, et al. Association between hospital and surgeon procedure volume and outcomes of total hip replacement in the United States Medicare population. J Bone Joint Surg Am. 2001; 83(11):1622-1629.
  7. Berry DJ, von Knoch M, Schleck CD, Harmsen WS. The cumulative long-term risk of dislocation after primary Charnley total hip arthroplasty. J Bone Joint Surg Am. 2004; 86(1):9-14.
  8. Berry DJ, von Knoch M, Schleck CD, Harmsen WS. Effect of femoral head diameter and operative approach on risk of dislocation after primary total hip arthroplasty. J Bone Joint Surg Am. 2005; 87(11):2456-2463.
  9. Burroughs BR, Hallstrom B, Golladay GJ, Hoeffel D, Harris WH. Range of motion and stability in total hip arthroplasty with 28-, 32-, 38-, and 44-mm femoral head sizes. J Arthroplasty. 2005; 20(1):11-19.
  10. Schmalzried TP, Peters PC, Maurer BT, Bragdon CR, Harris WH. Long-duration metal-on-metal total hip arthroplasties with low wear of the articulating surfaces. J Arthroplasty. 1996; 11(3):322-331.
  11. McKellop H, Park SH, Chiesa R, et al. In vivo wear of three types of metal on metal hip prostheses during two decades of use. Clin Orthop Relat Res. 1996; (329 Suppl):S128-140.
  12. Howie DW, McCalden RW, Nawana NS, Costi K, Pearcy MJ, Subramanian C. The long-term wear of retrieved McKee-Farrar metal-on-metal total hip prostheses. J Arthroplasty. 2005; 20(3):350-357.
  13. Chan FW, Bobyn JD, Medley JB, Krygier JJ, Tanzer M. The Otto Aufranc Award. Wear and lubrication of metal-on-metal hip implants. Clin Orthop Relat Res. 1999; (369):10-24.
  14. Dowson D, Hardaker C, Flett M, Isaac GH. A hip joint simulator study of the performance of metal-on-metal joints: Part II: design. J Arthroplasty. 2004; 19(8 Suppl 3):124-130.
  15. Ziaee HK, Daniel J, Pradhan C, McMinn D. Metal ion levels in low clearance hip resurfacings. Poster presented at: 54th Annual Meeting of the Orthopaedic Research Society; March 2-5, 2008; San Francisco, California.
  16. Dowson D, Hardaker C, Flett M, Isaac GH. A hip joint simulator study of the performance of metal-on-metal joints: Part I: the role of materials. J Arthroplasty. 2004; 19(8 Suppl 3):118-123.
  17. Hallab N, Merritt K, Jacobs JJ. Metal sensitivity in patients with orthopaedic implants. J Bone Joint Surg Am. 2001; 83(3):428-436.
  18. Jacobs JJ, Hallab NJ. Loosening and osteolysis associated with metal-on-metal bearings: A local effect of metal hypersensitivity? J Bone Joint Surg Am. 2006; 88(6):1171-1172.
  19. De Haan R, Pattyn C, Gill HS, Murray DW, Campbell PA, De Smet K. Correlation between inclination of the acetabular component and metal ion levels in metal-on-metal hip resurfacing replacement. J Bone Joint Surg Br. 2008; 90(10):1291-1297.
  20. Langton DJ, Jameson SS, Joyce TJ, Webb J, Nargol AV. The effect of component size and orientation on the concentrations of metal ions after resurfacing arthroplasty of the hip. J Bone Joint Surg Br. 2008; 90(9):1143-1151.
  21. Svensson O, Mathiesen EB, Reinholt FP, Blomgren G. Formation of a fulminant soft-tissue pseudotumor after uncemented hip arthroplasty. A case report. J Bone Joint Surg Am. 1988; 70(8):1238-1242.
  22. 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(7):847-851.
  23. Campbell P, Shimmin A, Walter L, Solomon M. Metal sensitivity as a cause of groin pain in metal-on-metal hip resurfacing. J Arthroplasty. 2008; 23(7):1080-1085.
  24. De Smet K, De Haan R, Calistri A, et al. Metal ion measurement as a diagnostic tool to identify problems with metal-on-metal hip resurfacing. J Bone Joint Surg Am. 2008; (90 Suppl 4):202-208.
  25. Mikhael MM, Hanssen AD, Sierra RJ. Failure of metal-on-metal total hip arthroplasty mimicking hip infection. A report of two cases. J Bone Joint Surg Am. 2009; 91(2):443-446.
  26. Willert HG, Buchhorn GH, Fayyazi A, et al. Metal-on-metal bearings and hypersensitivity in patients with artificial hip joints. A clinical and histomorphological study. J Bone Joint Surg Am. 2005; 87(1):28-36.

Aurthor

Dr Schmalzried is from the Joint Replacement Institute at St Vincent Medical Center, Los Angeles, California.

Dr Schmalzried receives royalties from DePuy.

Presented at Current Concepts in Joint Replacement 2008 Winter Meeting; December 10-13, 2008; Orlando, Florida.

Correspondence should be addressed to: Thomas P. Schmalzried, MD, Joint Replacement Institute at St Vincent Medical Center, 2200 W Third St, Ste 400, Los Angeles, CA 90057.

DOI: 10.3928/01477447-20090728-06

10.3928/01477447-20090728-06

Sign up to receive

Journal E-contents