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Hip: R. De Haan, P. A. Campbell, E. P. Su, and K. A. De Smet Revision of metal-on-metal resurfacing arthroplasty of the hip: THE INFLUENCE OF MALPOSITIONING OF THE COMPONENTS J Bone Joint Surg Br 2008; 90-B: 1158-1163 [Abstract] [Full text] [PDF]

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Authors' reply:

KOEN A DE SMET, Patricia Campbell   (13 November 2008)

Should we always revise with malpositioning?

Shiraz A Sabah, Johann Henckel, John Skinner and Alister J. Hart   (16 October 2008)

Authors' reply: 13 November 2008
KOEN A DE SMET, orthopaedic surgeon director AMC Gent Belgium, Patricia Campbell Send letter to journal: Re: Authors' reply: koen.desmet{at}skynet.be KOEN A DE SMET, et al.	Sir, We thank SA Sabah et al for their interest in our paper. In answer to their questions we would like to state the following: 1. The authors suggest that component "malpositioning" is the main reason to undertake revision surgery. The detailed results of the Harris Hip Score are: HHS number of patients >90 : 2 80-90 : 8 70-80 : 16 60-70 : 10 50-60 : 1 40-50 : 2 min:40, max:96, median:73. The three fractures were not included in the HHS scores. Four of these revisions were done primarily on x-ray findings of non-acceptable cup position and/or raised metal-ion levels. In addition, early stage revisions (less than six months) were performed on two malpositioned cups on the basis of x-ray findings of abduction angles of 68° and 74°. There is growing evidence that high serum metal ion levels ( >20µg/l) can be an indicator for early revision surgery because of the ongoing process of joint tissue damage (described in this article as enlarged bursae, or by the term "pseudotumour" by others such as Pandit et al 20081). We urge early revision because of the concern of pseudotumour with more complications after the revision surgery. When the consequences of delayed revision are explained, the patients understand very well. 2. The authors state that the optimum acetabular positioning is at 40° of abduction (30° to 50°) and 20° of anteversion (15° to 25°). Answer: Malpositioning of the cup is often not only an abduction problem, the angle of anteversion may also be wrong. The cup with 32° of abduction had 37° of anteversion, which is malpositioned. 3. We also note that 12 patients are reported to have a periprosthetic lesion. Answer: We believe the mechanism is the production of excessive debris by the malpositioned components, and the subsequent soft tissue reaction to the debris, that leads to periprosthetic lesions. It is the soft tissue reaction or pressure of the reactive fluid that causes the pain. 4. Assessment of component position in this series is made using plain radiographs. Answer: It is clear that x-ray assessment of the cup is inferior compared with CT analysis. To perform CT analysis of resurfacing cups, special imaging protocols and experienced personnel are needed. These are not available in our radiology department. 5. Finally, the authors mention "high rates of wear". Answer: We did not include specific ion data in our article as this information was included in two other papers by our group, namely R. De Haan et al2 and K. De Smet et al.3 In these papers, we have shown that patients with malpositioned components had a higher risk of outlier serum ion levels, i.e. three or more times the standard deviation of the group. For example, a 17-year-old female patient with a HHS of 96 and with a cup abduction angle of 71°, had serum chromium and cobalt levels of 93 µg/L and 94 µg/L respectively. The maximum total wear depth of the bearings as measured by the co-ordinate measuring machine was 246 microns after three years in vivo. K.A. De Smet, MD, ANCA Medical Centre, Gent, Belgium. P. Campbell, Orthopaedic Hospital/ UCLA, Los Angeles, CA, USA. 1. 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-B:847-51. 2. 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 and Joint Surg [Br] 2008;90-B:1291-7. 3. De Smet K, De Haan R, Calistri A, et al. Metal ion measurement as a diagnostic tool to identify problems with metal-on-metal hip resurfacing. J Bone and Joint Surg [Am] 2008;90-A:(Suppl IV)202-208.
Should we always revise with malpositioning? 16 October 2008
Shiraz A Sabah, Medical Student, Imperial College Implant Retrieval Centre, Imperial College Healthcare NHS Trust, Charing Cross Campus, London., Johann Henckel, John Skinner and Alister J. Hart Send letter to journal: Re: Should we always revise with malpositioning? shiraz.sabah{at}imperial.ac.uk Shiraz A Sabah, et al.	Sir, We were delighted to read this article which has provoked debate on the indications for revision of hip resurfacing prostheses. The authors suggest that component "malpositioning" is the main reason to undertake revision surgery. They imply that even when it occurs in the absence of clinical symptoms it is sufficient indication to operate. However, from their report we are unclear as to the proportion of patients revised for “malpositioning” whilst asymptomatic. On the one hand, we notice that 90% of their study population presented with pain and 69% had mechanical symptoms, whilst on the other we see that one patient was revised with a Harris Hip Score (HHS) of 96. We ask if the authors could share the number of patients that they revised with HHS>90 ("excellent") and the process of consent in this group. We see that the authors recognise that malpositioning may result in only “mild or tolerable pain” and wonder whether the authors could clarify whether they would support revision in these cases. The authors state that the optimum acetabular positioning is at 40° of abduction (30° to 50°) and 20° of anteversion (15° to 25°). Any cup lying outside this range is said to be malpositioned. However, the results presented in this report suggest that the reverse is not true – components within the suggested range are not necessarily well positioned. We note that the “insufficient abduction” group has at its upper extreme a patient with 32° inclination – a value within the suggested range. We also note that 12 patients are reported to have a periprosthetic lesion. We are curious as to how malpositioning has been determined to be the cause of pain in these patients and not soft tissue disease (though we acknowledge that the former is implicated as a cause of the latter). Assessment of component position in this series is made using plain radiographs. These are widely recognised to have limitations. We wonder if the authors would consider 3-dimensional computed tomography a more appropriate investigation given its greater accuracy and the emphasis they would place on measurement results in the decision to revise. Finally, the authors mention “high rates of wear” and “high levels of serum ions”. We would welcome quantification of these statements by the authors and further information as to how they are important in the decision to revise. S.A. Sabah, Medical Student, J. Henckel, J. Skinner, A.J. Hart, Implant Retrieval Centre, Imperial College Healthcare NHS Trust, Charing Cross Campus, London, UK.