Tips and Pearls
Metal-on-metal hip resurfacing and large metal-on-metal bearing total hip replacements were popular because of their low wear rates and the advantages of large diameter bearings without a significant increase in acetabular bone loss. There has been increasing concern about the safety of some resurfacing implants in certain patients, particularly in the presence of pain.1 The exact mechanisms which cause soft tissue reactions leading to pain, aseptic lymphocytic vasculitis-associated lesions (ALVAL), and pseudotumors are not fully understood but hypersensitivity and wear debris cytotoxicity etiologies have been reported.2 The incidence of adverse metal reactions in metal-on-metal resurfacings is reported ranging from 0.3% to 3.4%.3–6 In this article we describe a salvage revision technique used in a patient with painful metal-on-metal hip replacement.
A 60-year-old lady was referred from another institution to the senior author with a painful metal-on-metal bearing left total hip replacement. She had a soft tissue swelling laterally combined with back and leg pain. C-reactive protein and erythrocyte sedimentation rate were 8 and 29, respectively. Magnetic resonance imaging with metal artefact reduction sequence demonstrated localized lymphadenopathy but no pseudotumor. Her preoperative metal ions were cobalt 4.1 ppb and chromium 4.2 ppb (normal Co<0.59 ppb and Cr<2.08 ppb). Because of her rising metal ions and, more significantly, her pain, the decision was made to revise the prosthesis. As there was difficulty gaining sufficient acetabular stability in the primary operation, as reported by the referring surgeon, the 58 mm outside diameter, uncemented, porous backed metal cup was cemented into the pelvis (Fig. 1). Removing a large, well-fixed cemented shell would potentially involve significant bone loss and morbidity, not to mention the difficulties of achieving a stable reconstruction in a relatively young patient who is becoming increasingly disabled. Options for a revision procedure included using a cement-in-cement cup, highly porous metal cup with augments, impaction and structural bone allograft, or custom-made hemipelvic replacement.7
We investigated the option of cementing a polyethylene liner into the existing metal cup to negotiate this challenging dilemma. Cementing a polyethylene liner into an acetabular shell,8 cementing metal-backed components into a stable metal shell,9 or cementing into an existing cage are techniques that have been used previously with good reports. However, locking mechanism failure, significant polyethylene wear, or malpositioning of the metal shell have also been reported in these cases. To date there are no reports of cementing a polyethylene cup into a low-friction large diameter metal bearing, ending the release of metal ions from the bearing surface while limiting bone loss.
Unlike the surface of cages and metal-backed acetabular sockets in which the liner mechanism has failed, the surface of a metal-on-metal cup is a polished, low-friction cobalt chrome surface.
We tested the pull-out strength of this technique before implantation in the patient. A metal cup was rigidly fixed to a stable frame into which a polyethylene liner was cemented in (Palacos, Zimmer, Warsaw, IN). This was mounted on a calibrated Zwick/Roell Z005 (Zwick Testing Machines Ltd, Leominster, UK) force-controlled hydraulic tension, compression frame accurate to 0.1 N (Fig. 2). The first test involved cementing the polyethylene cup into the pristine, low-friction CoCr cup with 1 mm cement mantle. An increasing tensile force was then applied at 10 mm/min (Fig. 3). The pull-out strength was 1179 N until dissociation occurred with failure at the cement CoCr–bearing-cement interface.
The CoCr surface was subsequently roughened with a carbide burr aiming for multiple concentric ridges 1 mm deep. The test was repeated using the same variables achieving a pull-out strength of 3500 N (Fig. 4). The dissociation occurred at the cement polyethylene interface suggesting that our method of burring the CoCr cup provided superior pull-out resistance. This is comparable to published data regarding pull-out strengths of similar salvage procedures10 and justified proceeding with the method clinically.
At the time of surgery the implant was revised with a cemented-in 50 mm outside diameter, 36 mm Marathon liner (Depuy, Johnson and Johnson, Warsaw, IN). The minimum cement mantle was at least 1 mm but greater in the ridges reproduced within the existing 52 mm inside diameter cup. A 36 mm ceramic head was applied to the existing femoral stem. Tissue samples excised at time of surgery were histologically consistent with ALVAL. There was no evidence of infection.
Cobalt and chromium ions at 3 months reduced to 0.5 and 1.4 ppb, respectively, within the normal range. Hip score improved from 23/48 to 28/48. At 2 years’ follow-up there was no radiographic evidence of loosening (Fig. 5), she has returned to work, and her function is limited by her native right hip.
Metal-on-metal bearing hip arthroplasty and resurfacing are subject to increased controversy and have a higher revision rate than the alternative bearing prostheses.11 This case report highlights a previously undocumented revision technique in a well-fixed CoCr bearing acetabular component. The long-term success of this new technique is unknown, but it is likely to be comparable to published results on revisions cementing polyethylene into existing acetabular shells. Published series are small but up to 90% survival is reported at 6 years.12,13 Clearly, if fixation fails at the polyethylene-cement or cement-CoCr interface, subsequent revision is technically no more demanding than originally expected and potentially easier if the original cup becomes loose. Patients are potentially at a greater risk of dislocation than their original large bearing metal-on-metal hip, but it is not known whether the dislocation rate is greater than any other 36 mm head revision. This technique allows for a small cement mantle to accommodate the polyethylene thickness without a dramatically reduced head size, which may be associated with early failure.
Biomechanical testing was used to assess if roughening of polished cobalt chrome surface would provide adequate fixation. The complex and dynamic physiological loads in the acetabulum are not reproduced by this simple test. Compressive and torsional forces predominate but tensile and lever-out forces do occur. Further biomechanical tests would need to be performed fully recommending this method in this small population.
The above case demonstrates a potential salvage operation available to the revision surgeon in the case of certain, well-fixed metal-on-metal bearing cups even if they are uncemented and well osseointegrated. The method may also be of benefit to the well-fixed uncemented metal-on-metal cup warranting revision because of ALVAL reaction, raised metal ions, or patient concern about the potential implications of their metal-on-metal hip. This relatively simple procedure preserves bone stock, has a reduced morbidity, and ameliorates the production of metal ions from an existing metal-on-metal bearing. The long-term clinical results are unknown but likely to be similar to other cemented polyethylene salvage revision techniques.
The authors thank Mark Harrison and Jay Meswania in the John Scales Bio-medical Engineering department for designing and making the testing apparatus. They also thank Pam Coward and Christine Bows for their help in following up the patient.
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Keywords:© 2014 by Lippincott Williams & Wilkins
metal-on-metal; hip arthroplasty; acetabular revision