Electronic Letters to:

Hip: A. J. Hart, S. Sabah, J. Henckel, A. Lewis, J. Cobb, B. Sampson, A. Mitchell, and J. A. Skinner The painful metal-on-metal hip resurfacing J Bone Joint Surg Br 2009; 91-B: 738-744 [Abstract] [Full text] [PDF]

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Electronic letters published:

Cup orientation calculation and "safe zone"

Brian Derbyshire, Wigan, UK   (25 September 2009)

Measuring component position – a complex issue

Simon S Jameson, Tony V. F. Nargol   (3 August 2009)

Molybdenum in metal-on-metal arthroplasty

B. Michael Wroblewski, Wrightington Hospital WN6 9EP   (2 July 2009)

Cup orientation calculation and "safe zone" 25 September 2009
Brian Derbyshire, Senior Research Fellow Wrightington Hospital, Wigan, UK Send letter to journal: Re: Cup orientation calculation and "safe zone" Brian.Derbyshire{at}wwl.nhs.uk Brian Derbyshire, et al.	Sir, This is an interesting paper which adds to our understanding of problematic hip resurfacing arthroplasty. However, the presentation of results in figure 1 is misleading. The authors carried out CT scans on 16 of the 26 patients in order to assess the orientation of the cups. These orientation measurements were presented in a chart of anatomical version versus anatomical inclination in figure 1. In the same chart, the authors inserted a box representing the “safe zone” for inclination and anteversion settings suggested by Lewinnek et al.1 The problem is that Lewinnek et al determined that “safe zone” from measurements of radiographic version and inclination. It is possible to convert from anatomical to radiographic version/inclination using basic trigonometric formulae:2 radincl = tan-1(tan(anatincl) . cos(anatvers)) radvers = tan-1(tan(anatvers) . sin(radincl)) I re-plotted figure 1 in terms of the radiographic orientations (the graph cannot be included in this letter). In the new format, twice as many points (six) were within the “safe zone” (rounding to the nearest degree). None of the cups was anteverted more than 40°, and the cup that was originally retroverted by 47° in figure 1, had a retroversion of 29°. It may well be important that ten of the 16 cups had an inclination of less than 50°, whereas, in figure 1, ten cups had an inclination greater than 50°. B. Derbyshire, Senior Research Fellow, Wrightington Hospital, Wigan, UK. 1. Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg [Am] 1978;60-A:217-20. 2. Murray DW. The definition and measurement of acetabular orientation. J Bone Joint Surg [Br] 1993;75-B:228-32.
Measuring component position – a complex issue 3 August 2009
Simon S Jameson, Specialty Registrar, Trauma & Orthopaedics University Hospital of North Tees, Tony V. F. Nargol Send letter to journal: Re: Measuring component position – a complex issue simonjameson{at}doctors.org.uk Simon S Jameson, et al.	Sir, We read this paper with interest. We would like to thank the authors for their valuable contribution to the ongoing investigation of the painful metal-on-metal (MoM) hip arthroplasty. However, we would also like to raise a number of issues. Whilst we agree that CT with 3D reconstruction is an excellent tool for the investigation of component position, methods used in this study are neither widely available nor proven to be cost effective. Crucially, this investigation has one specific drawback. If implant wear is affected by component position, should imaging not be performed in the weight bearing position? Due to the effects of pelvic tilt, cups that appear acceptably positioned while supine may be grossly misaligned when standing. It has been shown that radiographic anteversion may significantly change depending on patient position.1,2 There is growing evidence that interpretation of cup position from plain standing radiographs is accurate,3,4 although we acknowledge that poor quality films may reduce accuracy.5 Interestingly, after highlighting the inadequacies of plain radiographs, Hart et al state that the “safe zone of Lewinneck”6 appears to be a good guide for the placement of MoM resurfacing cups. Lewinneck’s measurements, derived from plain radiographs, were not referenced from the bony landmarks of the anterior pelvic plane.7 Therefore, we feel it is inappropriate to compare cup orientations in these two studies. Finally, the authors state that an acceptable upper limit for inclination is 50°, in terms of metal ion generation. This arbitrary limit appears to be derived from results of plain film studies by the same authors.8 Although generic limits are notionally useful, cup positioning must be device- and component size-specific.9-12 S.S. Jameson, MBBS, MRCS(Eng), Specialty Registrar, Trauma & Orthopaedics, T.V.F. Nargol, MBBS, FRCS(Tr&Orth), University Hospital of North Tees, Stockton, UK. 1. Eddine TA, Migaud H, Chantelot C, et al. Variations of pelvic anteversion in the lying and standing positions: analysis of 24 control subjects and implications for CT measurement of position of a prosthetic cup. Surg Radiol Anat 2001;23:105-10. 2. Babisch JW, Layher F, Amiot LP. The rationale for tilt-adjusted acetabular cup navigation. J Bone Joint Surg [Am] 2008;90-A:357-65. 3. Langton DJ, Sprowson AP, Mahadeva D, et al. Cup anteversion in hip resurfacing: validation of EBRA and the presentation of a simple clinical grading system. J Arthroplasty 2009;in press. 4. Malviya A, Lingard EA, Malik A, Bowman R, Holland JP. Hip flexion after Birmingham Hip Resurfacing: role of cup anteversion, anterior femoral head-neck offset, and head-neck ratio. J Arthroplasty 2009 Mar 12 [Epub ahead of print]. 5. Goergen TG, Resnick D. Evaluation of acetabular anteversion following total hip arthroplasty: necessity of proper centring. Br J Radiol 1975;48:259-60. 6. Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg [Am] 1978;60-A:217-20. 7. Murray DW. The definition and measurement of acetabular orientation. J Bone Joint Surg [Br] 1993;75-B:228-32. 8. Hart AJ, Buddhdev P, Winship P, et al. Cup inclination angle of greater than 50 degrees increases whole blood concentrations of cobalt and chromium ions after metal-on-metal hip resurfacing. Hip Int 2008;18:212-19. 9. Langton DJ, Jameson SS, Joyce TJ, Webb J, Nargol AV. The effect of component size and orientation on the concentrations of metal ions after resurfacing arthroplasty of the hip. J Bone Joint Surg (Br] 2008;90-B:1143-51. 10. Vendittoli PA, Mottard S, Roy AG, Dupont C, Lavigne M. 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-B:441-8. 11. Langton DJ, Sprowson AP, Joyce TJ, et al. Blood metal ion concentrations post hip resurfacing arthroplasty: a comparison of the Articular Surface Replacement and Birmingham Hip Resurfacing devices. J Bone J Surg [Br] 2009; in press. 12. De Haan R, Pattyn C, Gill HS, et al. Correlation between inclination of the acetabular component and metal ion levels in metal-on-metal hip resurfacing replacement. J Bone Joint Surg [Br] 2008;90-B:1291-7.
Molybdenum in metal-on-metal arthroplasty 2 July 2009
B. Michael Wroblewski, Director Charnley Research Institute, Wrightington Hospital WN6 9EP Send letter to journal: Re: Molybdenum in metal-on-metal arthroplasty bmwhipdr{at}hotmail.com B. Michael Wroblewski, et al.	Sir, I read this paper with interest. The very detailed study makes no mention of Molybdenum. In fact most of the studies on the subject seem to avoid this element. Although it forms only a small proportion of the alloy it has a significant function in a number of processes in living organisms. Information, if available, could shed some light on the problems reported with metal-on-metal arthroplasties. B.M. Wroblewski, Director, Charnley Research Institute, Wrightington Hospital, Wigan, UK.