All Total Hip Replacement Patients Should Receive Metal-on-Polyethylene Bearings: Counterpoint

Sanket Diwanji M.S (Ortho)

Pascal-André Vendittoli M.D., MSc, FRCSC
Montreal, QC

Sir John Charnley pioneered the use of metal-on-polyethylene (M-PE) bearing surfaces for total hip arthroplasty (THA) in the 1960s and these have been improving the quality of life (QOL) in arthritic patients ever since. When their application was extended to young and active patients, M-PE bearings had earlier and higher rates of failure. Osteolysis associated with bearing wear was identified as a culprit in early failures1. The incidence of osteolysis in most series beyond ten years and in a few series of less than ten years exceeds that of sepsis, dislocation, fatal pulmonary embolization, nerve damage and other complications2. Periprosthetic fractures are also attributed, in part, to femoral osteolysis3-5. 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 loosening6.

 

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 five years. However, clinical follow-up of less than five years has little value, as the evolution of radiographic evidence is a slow process in periprosthetic osteolysis2. Even though absolute wear is decreased with HCPE, the particles generated are biologically active and have the potential of inducing osteolysis7. 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 polyethylene8. Recently-published data indicate excessive volumetric wear with 36- to 40-mm heads and HCPE liners9. 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 and these are attributed to decreased plasticity at the fracture tip10.

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 wear11-13. COC bearing 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 cycles14. 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 as compared with M-PE.

In the early years, osteolytic lesions were reported after MOM implants with low on high carbon content, which are now recognized as detrimental factors15,16. 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 levels17,18 and good to excellent mid-term clinical results with rare osteolytic lesions17,19-21. 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 components22-24, new acetabular porous coatings23, new neck sleeves for leg length adjustment and various modular femoral head designs25,26. Early failures in of these many cases are attributable to rapid introduction of untested implants on the market. Biological response like pseudotumour and metal allergic reaction seems to occur in abnormally high wear situations either due to poor implant positioning or design. In our own Canadian MOM hip resurfacing experience, only three cases of pseudotumour out of 3400 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 198927,28. More recently, monoblock acetabular component have been introduced. These thin acetabular 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, roofing, etc., 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 M-PE in a randomized trial29. 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: 1) M-PE wear, 2) elevated metal ions and hypersensitivity to MOM, 3) 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. M-PE bearings may not be the best option for patients with 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 (i.e. patients with known metal allergies, women of child-bearing age, and patients with chronic renal failure). M-PE should be limited to older patients or ones with low physical activity. We strongly believe that not all total hip replacement patients should receive M-PE bearings.

References

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