Screening standards needed for blood metal ion concentrations in metal-on-metal hip resurfacing, THR

Editor’s note:

As knowledge of the performance and outcomes of hip arthroplasty with metal-on-metal bearings increases, the guest commentaries below and here revisit the many issues surrounding this type of arthroplasty. We hope these opinion pieces will provide important information to help readers arrive at their own answers to the many outstanding questions regarding these treatments.

Confusion surrounds the interpretation of blood metal ion concentrations following implantation of metal-on-metal hip prostheses. Over the last 4 years, the University Hospital of North Tees in Stockton-on-Tees, England, together with Newcastle University in Newcastle upon Tyne, England, has carried out an on-going investigation into failed metal-on-metal hips.

The first points arrived at from the investigation are practical and apply to both metal-on-metal (MoM) resurfacing and MoM total hip replacement (THR) procedures:

1. The units used to report blood metal ion concentrations should be standardized and universal. Most studies use micrograms per liter (µg/L), which is identical to parts per billion (ppb). Other studies and laboratories use molar values which require a formula to convert to µg/L. This creates unnecessary confusion for patients and surgeons. We therefore suggest standardizing units to µg/L.
2. Chromium measurement appears to be unreliable at lower concentrations and should not be used for comparison of devices. Chromium should not be used to make clinical decisions in the absence of a concomitant increase in cobalt concentrations. This variability in median chromium concentrations can be seen in numerous publications.
3. When screening patients to identify abnormally wearing implants, either whole blood or the serum fraction can be tested.

Management of hip resurfacings

David J. Langton

These points derived from the study pertain to managing patients who have undergone hip resurfacings.

1. Any symptoms demand investigation in the presence of normal radiographs, normal blood metal ion concentrations and normal inflammatory markers.
2. There is a highly significant linear relationship between whole blood/serum chromium or cobalt concentrations and wear rates of retrieved resurfacing components. Our retrieval analysis has shown that unilateral components without obvious excessive wear rates are associated with whole blood/serum cobalt concentrations less than 2 µg/L and this is consistent with median levels reported in multiple studies. Cobalt concentrations between 2 µg/L and 5 µg/L should not be acted upon in the absence of symptoms, but should be considered when planning future management. Cobalt concentrations greater than or equal to 5 µg/L are indicative of increased articular wear with a specificity of 100% and a sensitivity of 93%. This level of 5 µg/L ties in very well with recently published clinical data. A cobalt concentration greater than 10 µg/L should always be regarded as highly abnormal. Sixty percent of our asymptomatic patients with cobalt levels greater than 10 µg/L went on to experience failure of their resurfacings within 3 years of the blood test.

Inspection of a retrieved taper with a light microscope revealed unworn surfaces and imprint of the trunnion causing outward bulging of the taper. Images: Langton DJ

A cobalt concentration greater than 20 µg/L is now an indication at our center to offer revision to the patient. The justification for this approach is that wear rates are approaching the equivalent volumetric wear of polyethylene joints and will not reduce over time if the patient remains at their normal level of mobility. Gross blackening of tissues has been encountered in every revision case of ours involving patients with cobalt greater than 20 µg/L (n=37); in 31 of these cases (84%) there was femoral and/or acetabular osteolysis often requiring grafting. In 9 patients with osteolysis, pain was either absent or only described as “mild.” We recommend CT for evaluation of bony integrity with concentrations as high as this.

36-mm and greater MoM head

Patients with large diameter MoM THR should be assessed carefully. As with resurfacings, symptoms demand investigation in the presence of normal radiographs, normal metal ion concentrations and normal inflammatory markers.

The guidelines above would still apply were it not for the presence of the extra metallic interface at the taper junction of these devices. We have commonly seen patients with severe tissue destruction with normally functioning THR bearing surfaces. In these cases, severe damage to the taper junction was identified. A number of reports have shown that the U.K. Medicines and Healthcare products Regulatory Agency (MHRA) metal ion interpretation guidance is inadequate for THRs. At the time these guidelines were issued the significance of the modular junction was not fully appreciated. Metal debris generated from tapers appears to readily stimulate a negative immune cascade leading to extensive soft tissue and bony destruction. Crucially, unit for unit, taper debris appears to be more damaging than bearing surface debris. It takes a total wear rate of approximately 3 mm³ per year for blood or serum cobalt levels to rise to 5 µg/L. A total volumetric loss of 3 mm³ from a taper indicates extensive taper damage and is more than sufficient a total dose of taper debris to lead to moderate to severe soft tissue destruction. In other words, a significant number of patients with failing tapers are likely to have cobalt levels even lower than 5 µg/L. This is an absolutely critical message. If a THR patient has a blood cobalt of 3µ/L and the bearing surface is functioning perfectly well, it may well be a cause for concern.

The areas in red represent wear of the internal surface of the taper analyzed using a coordinate measuring machine.

What causes taper damage?

We have performed volumetric wear analysis of 180 MoM THRs. The majority of these components were manufactured by DePuy, but this may well be a reflection of the past preference of the local hospital for DePuy products. The patterns of damage identified on these retrieved components are strikingly similar, irrespective of manufacturer, presence/absence of taper adaptor, hollow or solid femoral component or clearance of the bearings. Taper damage can occur in well-positioned low wearing components. Bearing diameter appears to be important, but more important still is the site of the taper engagement level and the distance of that site from the bearing surface. In other words, the root cause of the problem appears to be the large lever arm acting on the taper junction rather than head size per se. The lever arm is increased by the following:

• Bearing diameter;
• Varus neck shaft angle of the femoral stem; and
• Taper engagement level (caused by taper angle mismatch and increasing head offset).

There may be an added torsional effect caused by the friction generated by the large diameter bearing surfaces. This, however, remains to be proven.

Surgeons should not implant large diameter MoM heads until there is further clarification on this subject. Large diameter ceramic-on-ceramic bearings may be afforded protection from this phenomenon due to the taper engagement level being closer to the center of rotation of the joint. Also, the lower coefficient of friction with ceramic-on-ceramic bearings may reduce torsional effects if this is proved to be a significant factor. However, surgeons should re-evaluate their use of large diameter THR heads, irrespective of the bearing surfaces, in this period of uncertainty.

References:

• Bolland BJ, Culliford DJ, Langton DJ, et al. High failure rates with a large-diameter hybrid metal-on-metal total hip replacement: clinical, radiological and retrieval analysis. J Bone Joint Surg Br. 2011;93(5):608-615.
• Daniel J, Ziaee H, Pradhan C, McMinn DJ. Six-year results of a prospective study of metal ion levels in young patients with metal-on-metal hip resurfacings. J Bone Joint Surg Br. 2009;91:176-179.
• Daniel J, Ziaee H, Pynsent PB, McMinn DJW. The validity of serum levels as a surrogate measure of systemic exposure to metal ions in hip replacement. J Bone Joint Surg Br. 2007;89:736-771.
• De Smet K, De Haan R, Calistri C, et al. Metal ion measurement as a diagnostic tool to identify problems with metal-on-metal hip resurfacing. J Bone Joint Surg Am. 2008;90:202-208.
• Hart AJ, Sabah SA, Bandi AS, et al. Sensitivity and specificity of blood cobalt and chromium metal ions for predicting failure of metal-on-metal hip replacement. J Bone Joint Surg Br. 2011;93(10):1308-1313.
• Kim PR, Beaulé PE, Dunbar M, et al. Cobalt and chromium levels in blood and urine following hip resurfacing arthroplasty with the Conserve Plus implant. J Bone Joint Surg Am. 2011;93 Suppl 2:107-117.
• Langton DJ, Jameson SS, Joyce TJ, et al. Accelerating failure rate of the ASR total hip replacement. J Bone Joint Surg Br. 2011;93(8):1011-1016.
• Langton DJ, Jameson SS, Joyce TJ, Nargol AVF. Asymptomatic patients with increased blood metal ion levels following metal on metal hip resurfacing arthroplasty: Is failure imminent? British Hip Society Annual Meeting 2010.
• Langton DJ, Joyce TJ, Jameson SS, et al. Adverse reaction to metal debris following hip resurfacing: the influence of component type, orientation and volumetric wear. J Bone Joint Surg Br. 2011;93(2):164-171.
• Langton DJ, Joyce TJ, Mangat N, et al. Reducing metal ion release following hip resurfacing arthroplasty. Orthop Clin North Am. 2011;42(2):169-180, viii.
• Pandit H, Glyn-Jones S, McLardy-Smith P, et al. Pseudotumours associated with metal-on-metal hip resurfacings. J Bone Joint Surg Br. 2008;90:847-851.
• Smolders JM, Bisseling P, Hol A, et al. Metal ion interpretation in resurfacing versus conventional hip arthroplasty and in whole blood versus serum. How should we interpret metal ion data. Hip Int. 2011; 21(5):587-595.
• Vendittoli PA, Mottard S, Roy AG, et al. Chromium and cobalt ion release following the Durom high carbon content, forged metal-on-metal surface replacement of the hip. J Bone Joint Surg Br. 2007;89:441-448.
• www.britishhipsociety.com/pdfs/BHS_MOM_THR.pdf.

• David J. Langton, MRCS, can be reached at the Joint Replacement Unit, University Hospital of North Tees, Hardwick, Stockton-on-Tees, TS19 8PE, England; email: djlangton22@doctors.org.uk.
• Disclosure: Langton has received payment for talks given at DePuy and Finsbury Orthopaedics conferences, received travel reimbursement from DePuy, Finsbury Orthopaedics, Smith & Nephew and Wright Medical, and is an unpaid consultant for Wright Medical.