The Case Against MOP Bearings for All THA Patients

The Case Against MOP Bearings for All THA Patients

In the 1960s, Sir John Charnley pioneered the use of metal-on-polyethylene (MOP) bearing surfaces for total hip arthroplasty (THA), which have been improving the quality of life (QOL) in patients with arthritis ever since. When their application was extended to young and active patients, MOP bearings had earlier and higher rates of failure.

Osteolysis associated with bearing wear was identified as a culprit in early failures. The incidence of osteolysis in most series beyond 10 years and in a few series of less than 10 years exceeds that of sepsis, dislocation, fatal pulmonary embolization, nerve damage, and other complications. Periprosthetic fractures are also attributed, in part, to femoral osteolysis.3-5 Apart from causing osteolysis, polyethylene wear per se decreases range of motion (ROM) due to femoral head penetration into the polyethylene. Polyethylene wear also causes increased rates of late dislocation and heightened torsional forces on the implant-bone interface resulting in mechanical loosening.

Newer bearing options have been introduced to tackle the problem of wear in hip arthroplasty. Highly cross-linked polyethylene (HCPE) has reduced the wear rate in laboratory and clinical studies with follow-up of up to 5 years. However, clinical follow-up of less than 5 years has little value, as the evolution of radiographic evidence is a slow process in periprosthetic osteolysis. Even though absolute wear is decreased with HCPE, the particles generated are biologically active and have the potential of inducing osteolysis.

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Investigation of HCPE performance in the presence of third-body wear or rough counter surface (femoral head’s surface roughness of more than 0.9 µm) has established that cross-linking does not provide any advantage and wear is greater than with conventional polyethylene. Recently published data indicate excessive volumetric wear with 36- to 40-mm heads and HCPE liners. These authors urge caution with larger femoral heads in young and active patients and in those with low risk of dislocation. Cross-linking alters the material properties of polyethylene, which has evoked concerns about loss of fracture toughness and the brittle nature of HCPE. Baker and colleagues have demonstrated that true stress at breakpoint and resistance to crack propagation are inversely related to cross-linking radiation dose; these are attributed to decreased plasticity at the fracture tip.10

The science of tribology has shown that hard-on-hard bearings — metal-on-metal (MOM) and ceramic-on-ceramic (COC) components — with small clearance, good surface finish and optimal implant sphericity minimize wear.11-13 COC bearings demonstrated excellent resistance to surface damage, such as scratching, by their high-level hardness and offer the lowest wear rate of all available bearings. In MOM bearings, after an initial run-in period, the wear rate declines to below 1 µm/million cycles.14 They are also known to self-polish with usage, a phenomenon unique to these bearings. Thus, in terms of wear, these hard-on-hard bearings present a clear potential for wear reduction and longer survivorship when compared with MOP.

In the early years, osteolytic lesions were reported after MOM implants with low-on-high carbon content, which are now recognized as detrimental factors.15,16 On the other hand, some implants made of high-on-high carbon content CrCo (for example, 28-mm Metasul, Zimmer, USA) have consistently presented a low wear rate, low serum ion levels,17,18 and good to excellent mid-term clinical results with rare osteolytic lesions.17,19-21

Recent problems encountered with MOM (early failures, metal allergy, and pseudotumour) have raised concerns in the orthopaedic community. We must differentiate bearing-related problems from fixation and implant positioning issues. Many large-diameter MOM bearings have been introduced recently with new implant designs, such as thin monoblock acetabular component,22-24 new acetabular porous coatings,23 new neck sleeves for leg length adjustment, and various modular femoral head designs.25,26 Early failures in of these many cases are attributable to rapid introduction of untested implants on the market. Biological response such as pseudotumour and metal allergic reaction seems to occur in abnormally high wear situations, either due to poor implant positioning or design.

In our experience, only three cases of pseudotumour out of 3,400 hips were found (AAOS 2010: A survey on the incidence of pseudotumours with MOM hip resurfacing in Canadian academic centres). We are not aware of any published pseudotumour cases with Metasul 28 mm, which has been in use for more than 20 years. Thus, not all MOM bearings perform similarly.

We believe that MOM implants are a good option for THA in young and active patients. MOM bearing acetabular components can be modular or monoblock. Modular inserts can be used with good track record acetabular components and allow insertion of screws to supplement primary fixation. Very good clinical results (low ions, absence of osteolysis, no pseudotumour and favourable revision rate) have been reported with the original Metasul 28 mm since its introduction in 1989.27,28

More recently, monoblock acetabular components have been introduced. These thin components allow anatomical bearing diameter for hip resurfacing or large-diameter head THA. Large-diameter heads eliminate joint stability problems and permit unrestricted postoperative ROM with a return to demanding sports activities or occupations, such as plumbing, policing, and roofing, a major benefit for patient QOL. Also, patients with very small acetabulums (less than 48 mm) can receive hard-on-hard bearings and large-diameter heads with MOM bearings. Otherwise, cemented polyethylene bearings with 22- or 28-mm metal head would have to be used. However, as mentioned earlier, some of these large diameter head THA implants presented early failures that can be attributed to introduction of deficient designs (new porous coating, corrosion at the neck sleeve, etc) and not related to the MOM bearing itself.23-26

COC bearings have shown significantly lower wear rates than MOP in a randomized trial.29 But as with MOM implants, the European experience with these bearings was largely disregarded, and this led to multiple failures due to design modifications. Recent problems with new alumina implants in metal-back inserts30,31 or polyethylene sandwich32,33 have been clearly documented. These design problems should not diminish the performance of well-functioning COC bearing implants. For the moment, COC bearings do not allow for monoblock acetabular implants and thus limit head diameter to 32 or 36 mm in sockets exceeding 48-52 mm in most instances.

Each bearing surface has its limitations:

  • MOP, wear
  • MOM, elevated metal ions and hypersensitivity
  • COC, bearing fracture and squeaking

However, all bearings do not perform alike, with good and bad performers being found in each group. Bearing selection for THA should be based on individual patient requirements rather than one-size-fits-all. MOP bearings may not be the best option for patients with a very small acetabulum (less than 50 mm), young patients with high physical demands, good candidates for hip resurfacing, and subjects with increased postoperative dislocation risk. COC bearings could be an attractive alternative when MOM bearings are contraindicated (ie, patients with known metal allergies, women of child-bearing age, and patients with chronic renal failure). MOP should be limited to older patients or those with low physical activity. We strongly believe that not all total hip replacement patients should receive MOP bearings.

Reprinted with permission from the Summer 2010 issue of COA Bulletin

References

  1. Willert HG, Bertram H, Buchhorn GH. Osteolysis in alloarthroplasty of the hip. The role of ultra-high molecular weight polyethylene wear particles. Clin Orthop Relat Res 1990;(258):95-107.
  2. Harris WH. Wear and periprosthetic osteolysis: the problem. Clin Orthop Relat Res 2001;(393):66-70.
  3. Charnley J, Halley DK. Rate of wear in total hip replacement. Clin Orthop Relat Res 1975;(112):170-9.
  4. Clarke IC. Wear in total hip replacement. Hip arthroplasty, ed. A. HC. 1991, New York: Churchill Livingstone 535-554.
  5. Yamaguchi M, Bauer TW, Hashimoto Y. Deformation of the acetabular polyethylene liner and the backside gap. J Arthroplasty 1999;14(4):464-9.
  6. Wroblewski BM. Wear and loosening of the socket in the Charnley low-friction arthroplasty. Orthop Clin North Am 1988;19(3):627-30.
  7. Ingram JH, Stone M, Ingham E. Effect of crosslinking on biological activity of UHMWPE wear debris. Trans Orthop Res Soc 2003; 28:1439.
  8. McKellop H, DiMaio W, Lancaster JG. Wear of gammacrosslinked polyethylene acetabular cups against  roughened femoral balls. Clin Orthop Relat Res 1999;(369):73-82.
  9. Lachiewicz PF, H.D., Soileau ES et a, Femoral head size and wear of highly cross linked polyethylene at 5 to 8 years. Clin Orthop Relat Res 2009;467(12):3290-6.
  10. Baker DA, Bellare A, Pruitt L. The effects of degree of crosslinking on the fatigue crack initiation and propagation resistance of orthopedic-grade polyethylene. J Biomed Mater Res A 2003;66(1):146-54.
  11. Dowson D, et al. A hip joint simulator study of the performance of metal-on-metal joints: Part II: design. J Arthroplasty 2004;19(8 Suppl 3):124-30.
  12. Dowson D, et al. 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):118-23.
  13. Rieker CB, et al. Influence of the clearance on in-vitro tribology of large diameter metal-on-metal articulations pertaining to resurfacing hip implants. Orthop Clin North Am 2005;36(2):135-42, vii.
  14. McMinn DJ, Daniel J, Ziaee H. Controversial topics in orthopaedics: metal-on-metal. Ann R Coll Surg Engl 2005;87(6):411-5.
  15. Park YS, et al. Early osteolysis following second-generation metal-on-metal hip replacement. J Bone Joint Surg Am 2005;87(7):1515-21.
  16. Milosev I, Pisot V, Campbell P. Serum levels of cobalt and chromium in patients with Sikomet metal-metal total hip replacements. J Orthop Res 2005;23(3):526-35.
  17. Brodner W. et al. Serum cobalt levels after metal-on-metal total hip arthroplasty. J Bone Joint Surg Am 2003;85-A(11):2168-73.
  18. Vendittoli PA, et al. Randomised controlled trial comparing two methods of acetabular cup positioning during total hip arthroplasty. Hip Int 2007;17(3):137-42.
  19. Sieber HP, Rieker CB, Kottig P. Analysis of 118 second-generation metal-on-metal retrieved hip implants. J Bone Joint Surg Br 1999;81(1):46-50.
  20. Delaunay C. Can metal-on-metal bearings improve the longevity of total hip prostheses?. Rev Chir Orthop Reparatrice Appar Mot 2005;91(1):70-8.
  21. Najjar D, et al. Mechanisms of damage to metal-on-polyethylene articulating surfaces of total hip prostheses: influence of intra-articulate migration of metallic debris. Rev Chir Orthop Reparatrice Appar Mot 2004;90(8):732-40.
  22. Berton C, et al. The Durom large diameter head acetabular component: early results with a large-diameter metal-on-metal bearing. J Bone Joint Surg Br;92(2):202-8.
  23. Long, W.T., et al., Failure of the Durom Metasul acetabular component. Clin Orthop Relat Res 2010;468(2):400-5.
  24. Langton DJ, et al. Early failure of metal-on-metal bearings in hip resurfacing and large-diameter total hip replacement: a consequence of excess wear. J Bone Joint Surg Br 2010;92(1):38-46.
  25. Garbuz DS, et al. The John Charnley Award: Metal-on-metal hip resurfacing versus large-diameter head metal-on-metal total hip arthroplasty: a randomized clinical trial. Clin Orthop Relat Res 2010;468(2):318-25.
  26. Vendittoli P-A, Amzica T, Roy A, et al. Metal ion release with large-diameter metal-on-metal hip arthroplasty. J Arthroplasty 2011;26(2):282-8.
  27. Delaunay C. [Can metal-on-metal bearings improve the longevity of total hip prostheses?] Rev Chir Orthop Reparatrice Appar Mot 2005;91(1):70
  28. Migaud H, Jobin A, Chantelot C, Giraud F, Laffargue P, Duquennoy A. Cementless metal-on-metal hip arthroplasty in patients less than 50 years of age: comparison with a matched control group using ceramic-on-polyethylene after a minimum 5-year follow-up. J Arthroplasty 2004;19(8 Suppl 3):23
  29. Vendittoli PA, Girrard J., Lavigne M, et al. Comparison of alumina-alumina to metal-polyethylene bearing surfaces in THA : A randomized study with 4- to 9- years follow-up. Acta Orthop Belg 2007;73:468-477.
  30. Miller AN, et al. Incidence of ceramic liner malseating in Trident acetabular shell. Clin Orthop Relat Res 2009;467(6):1552-6.
  31. Langdown AJ, Pickard RJ, Hobbs CM, et al. Incomplete seating of the liner with the Trident acetabular system: a cause for concern? J Bone Joint Surg 2007;89(3):291-5.
  32. Milosev I, et al. Dissociation of the metal inlay from the polyethylene liner in an uncemented threaded cup. Arch Orthop Trauma Surg 2005;125(2):134-41.
  33. Kircher J, et al. Extremely high fracture rate of a modular acetabular component with a sandwich polyethylene ceramic insertion for THA: a preliminary report. Arch Orthop Trauma Surg 2009;129(9):1145-50.
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