The painful metal-on-metal hip resurfacing study about high metal ions 2009
Link **A. J. Hart, FRCSG(Orth), Clinical Senior Lecturer & Honorary Consultant Orthopaedic Surgeon1; S. Sabah, BSc, Medical Student1; J. Henckel, MRCS, Clinical Research Fellow & Specialist Registrar in Orthopaedics1; A. Lewis, FRCS(Orth), Consultant Orthopaedic Surgeon1; J. Cobb, FRCS, Professor of Orthopaedic Surgery1; B. Sampson, MRSC, CChem, Director of Supraregional Trace Element Laboratory1; A. Mitchell, FRCR, Consultant Musculoskeletal Radiologist1; and J. A. Skinner, FRCS(Orth), Consultant Orthopaedic Surgeon2 1 Department of Radiology Imperial College, Charing Cross Hospital, Fulham Palace Road, London W6 8RF, UK. 2 Department of Orthopaedics Royal National Orthopaedic Hospital, Brockley Hill, Stanmore, Middlesex, HA7 4LP, UK.Abstract: We carried out metal artefact-reduction MRI, three-dimensional CT measurement of the position of the component and inductively-coupled plasma mass spectrometry analysis of cobalt and chromium levels in whole blood on 26 patients with unexplained pain following metal-on-metal resurfacing arthroplasty. MRI showed periprosthetic lesions around 16 hips, with 14 collections of fluid and two soft-tissue masses. The lesions were seen in both men and women and in symptomatic and asymptomatic hips. Using three-dimensional CT, the median inclination of the acetabular component was found to be 55° and its positioning was outside the Lewinnek safe zone in 13 of 16 cases. Using inductively-coupled plasma mass spectrometry, the levels of blood metal ions tended to be higher in painful compared with well-functioning metal-on-metal hips. These three clinically useful investigations can help to determine the cause of failure of the implant, predict the need for future revision and aid the choice of revision prostheses.…We included the first 26 consecutive, consenting patients (9 men with a mean age of 52.3 years (33 to 63), 17 women with a mean age of 52 years (38 to 70) at primary operation; 18 unilateral, 8 bilateral)) who presented with unexplained, painful MOM resurfacings from February 2008. Ethical approval had been granted by the appropriate committee. All patients had previously undergone a traditional hip assessment, comprising a clinical history and examination, blood infection screen and examination of serial pelvic radiographs… Results …All the angles of the acetabular component were expressed as the anatomical angle. There was a median inclination of 55° (39° to 78°) and version of 31° (48° to 47°). These values were within the safe zone of Lewinnek et al18 in only three of 16 patients. All the acetabular components were anteverted except one who was retroverted 47°. He described pain on hip flexion which was restricted to 60°. At operation the hip was found to be subluxing during flexion due to anterior impingement. Ten patients had inclination of the component greater than 50°, the threshold for a clinically detectable increase in whole-blood metal ion levels… Discussion …We found three clinically relevant results. First, 13 of the 16 painful MOM hip resurfacings that had undergone CT scanning were associated with positions of the component outside the safe zone of Lewinnek as measured by 3D CT.18 Secondly, the patients had higher blood metal levels than those with well-functioning prostheses.14–17 Thirdly, metal artefact reduction sequences MRI demonstrated the presence of periprosthetic masses in 14 of the 26 painful MOM hips… …Currently, most patients with a painful MOM hip are assessed by the same diagnostic algorithms as used for the painful non-MOM hip. These include the taking of a clinical history and examination followed by a review of serial plain radiographs, blood inflammatory markers and hip aspirates. Isotope bone scanning to determine loosening and infection10 and CT to determine the position of the component, particularly in a case of dislocation, are occasionally used. However, hip resurfacing appears to have specific modes of failure which are not detectable using these protocols. These may be attributable to two main causes, namely high rates of wear with high local levels of Co and Cr and an adverse response to the metal wear debris… …The position of the component may influence the risk of failure of a MOM hip because suboptimal acetabular version may cause impingement, and inclination angles greater than 50° may cause high rates of wear. The risk of impingement is greater for a hip resurfacing than a THR because the head-neck ratio is reduced. High inclination angles in MOM hips can increase the blood levels of metal wear debris up to 150 times greater than the median blood levels in patients with well-functioning MOM hips. Unfortunately, assessment of the position of the component of large-diameter MOM hips is difficult in practice because the large metal head obscures edges of the acetabular component on plain radiography and axial CT. Appropriate CT protocols and validated 3D measurement software are needed to overcome these difficulties and to minimise the radiation dose. We have demonstrated these in our protocol… ..The use of blood metal levels as biomarkers of wear rate is an exciting possibility for monitoring a MOM hip. There is debate on the relationship between the wear rate and blood metal levels.20 Recent reports have shown that patients with bilateral MOM hips have 1.5 times the level of Co and Cr in blood than those with unilateral MOM hips21 and that removal of a MOM hip results in a rapid fall in blood metal levels.22 In our patients the levels of Co were significantly greater than in those with well-functioning MOM hips and this suggests that painful hip resurfacings have greater wear rates. However, the levels of Cr were barely raised in patients with unilateral MOM hips and therefore analysis of blood metal ions as a screening test for painful MOM hips is likely to be sensitive but not specific. As seen from our ranges, for 100% sensitivity the threshold for blood metal levels would need to be set at around 0.5 ppb for unilateral and 2.0 ppb for bilateral resurfacings. However, within our control population we noted that some patients had extremely high blood metal levels and therefore, even at 50 ppb, the test was not 100% specific for painful MOM hips. Our control data suggest that, with a threshold of 15 ppb, the test would have a specificity of around 98%. We have begun prospective metal ion and clinical monitoring of all patients with high levels to verify the value of this test… |