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Hip: J. Daniel, H. Ziaee, C. Pradhan, P. B. Pynsent, and D. J. W. McMinn Blood and urine metal ion levels in young and active patients after Birmingham hip resurfacing arthroplasty: FOUR-YEAR RESULTS OF A PROSPECTIVE LONGITUDINAL STUDY J Bone Joint Surg Br 2007; 89-B: 169-173 [Abstract] [Full text] [PDF]

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Joseph Daniel, Hena Ziaee, Chandra Pradhan, Paul B Pynsent, Derek JW McMinn   (5 July 2007)

Blood and urine metal ion levels in young and active patients after BHR

Pascal-Andre Vendittoli, Muthu Ganapathi, Martin Lavigne   (10 May 2007)

Authors' reply: 5 July 2007
Joseph Daniel, Director of Research The McMinn Centre and the Royal Orthopaedic Hospital, Birmingham B15 3DP, UK, Hena Ziaee, Chandra Pradhan, Paul B Pynsent, Derek JW McMinn Send letter to journal: Re: Authors' reply: josephdaniel{at}mcminncentre.co.uk Joseph Daniel, et al.	Sir, We appreciate the interest of Dr Vendittoli et al in our paper. Our study is the first attempt at understanding in vivo metal release and systemic exposure in patients with modern metal-metal (MM) resurfacings through a prospective longitudinal assessment of the whole blood levels and daily output of cobalt and chromium. Vendittoli et al suggest that we should not have used the inclusion criterion of selecting only two bearing diameters in our study. Including all sizes and genders is commendable provided adequate numbers of patients are recruited to represent every possible bearing diameter and gender; and the temporal trend is assessed in each separately through adequately sized cohorts. Inclusion criteria are designed to reduce confounding variability while allowing the sample to be representative of the larger population under investigation. It has been stated that metal release from surface corrosion is a function of surface area1 and clearly bearings of different diameters will have differing contribution to overall metal ion production. Being ad hoc and unselective with regard to this important variable would lead to unspecified numbers of subjects with each diameter and contribute to statistical confounding. Our choice of the two sizes (50 mm and 54 mm bearings) was based on several observations. First, these two sizes are by far the most commonly used sizes and represent more than twice as many resurfacings as all other sizes put together in our centre (over 70% need one of these two sizes). Second, young men with unilateral hip osteoarthritis managed with resurfacing have been shown2 to be the most active resurfacing group with 92% participating in sport and 62% participating in impact sport. Our purpose was to assess metal release in this extremely active patient group who are the worst case scenario for metal ion release in correctly implanted devices. Furthermore, when attempting to assess timed excretion of metal in urine, patients need to collect urine in a specimen container without spillage or contamination and it is obvious that men are more efficient at complying with this task. Vendittoli et al did not perform urine collection to assess 24 hour production of metal ions thus simplifying their study but reducing the overall scientific value of their work. Moreover, in their study Vendittoli et al instructed their patients not to engage in new, strenuous activities in the week before specimen collection. Being prescriptive with regards to activities in any form has the potential to make patients restrict regular activities, rendering the investigation less representative of the day to day conditions these bearings are subject to in real life, and may reduce the validity of their results. Our patients were asked to live their lives as normal and collect their specimens without any restriction of physical activity at all. It appears to us that Vendittoli et al’s arguments stem from their assumption that we have presented a subset of results selected from a larger cohort. We carefully designed this investigation as a prospective longitudinal study with the stated inclusion criteria, amongst others, of using only two femoral head sizes in men to ensure the study has sufficient power. After informed consent we collected urine and whole blood specimens from all men (who were templated and found likely to need 50 mm or 54 mm femoral components; and consented to participate in this long-term study) pre-operatively but discarded specimens from patients who did not receive the stated head sizes immediately after the operation. In their letter, Vendittoli et al select a subset of results from their publication. This post facto selection is unscientific and does not carry any relevance to the study. From an assessment of blood levels in a large (n = 152) cross-sectional group of men and women with unilateral Birmingham Hip Resurfacings (BHRs) with over five years follow-up we found no significant difference in either cobalt (p > 0.1) or chromium (p > 0.05) when men with 50 mm and 54 mm diameter bearings were compared with all other subjects. Neither was there a significant mean difference nor association between subjects with different bearing diameters or genders. The other possibility is that the design difference between the components used in the two studies account for the differences found in the two centres. Vendittoli et al state that the Durom resurfacing implant produces higher metal ion blood levels in women compared with men and higher blood levels in small implants compared with large. Also they note that the radial clearance on the Durom is 75µm. Since it is not otherwise stated, we have assumed that this clearance is held constant across the range of sizes. The BHR, on the other hand, has a decreasing clearance with decreasing head diameter with the ratio of diameter to clearance held constant across the range. The fact that the blood metal ions with the BHR do not differ in women compared with men and across the range of head sizes may merely reflect a constant lubrication regimen across the range of BHR sizes. With a constant clearance across the range of Durom head sizes, clearly the lubrication regimen will be less favourable in smaller sizes and this design difference may account for our differing perspectives. In conclusion, we reiterate our assertion that our longitudinal metal ion study is carefully designed to minimise confounding factors and monitor in vivo metal ion release on a continuing time scale following implantation of a particular metal-metal resurfacing and forms a valid baseline for future studies. J. DANIEL, FRCS, Director of Research, H. ZIAEE, BSc(Hons), C. PRADHAN, FRCS, P.B. PYNSENT, PhD, D.J.W. MCMINN, FRCS, The McMinn Centre and the Royal Orthopaedic Hospital, Birmingham, UK. 1. Black J. Does corrosion matter? J Bone Joint Surg [Br] 1988;70-B:517-20. 2. Daniel J, Pynsent PB, McMinn DJ. Metal-on-metal resurfacing of the hip in patients under the age of 55 years with osteoarthritis. J Bone Joint Surg [Br] 2004;86-B:177-84.
Blood and urine metal ion levels in young and active patients after BHR 10 May 2007
Pascal-Andre Vendittoli, Assistant professor of surgery Maisonneuve-Rosemont Hospital, Department of surgery: Montreal University, Muthu Ganapathi, Martin Lavigne Send letter to journal: Re: Blood and urine metal ion levels in young and active patients after BHR pa.vendittoli{at}videotron.ca Pascal-Andre Vendittoli, et al.	Sir, We read this paper with interest. This paper presents the pre- and post- operative metal ion measurements of 26 patients who have undergone Birmingham Hip Resurfacing with 50 mm or 54 mm femoral components. While we acknowledge the science behind the measurement method used by the authors (urine and whole blood with HR-ICPMS), we would be interested in the authors' comments regarding the following issues. It is unclear to the readership whether or not this cohort of 26 patients is representative of the entire cohort of patients who received metal ion analyses. If this is a subset of the cohort which only involves 50 and 54mm femoral components then this may not be representative and should not be considered as a baseline upon which other metal ion studies should reference themselves for comparative purposes. In selecting only 50 mm and 54 mm components they have probably included mainly, if not exclusively, men. Gender ratio is not reported in the paper. Very rarely, women will need these implant sizes (acetabular components between 56 mm to 62 mm). Gender and bearing diameter has a significant impact on patients' metal ion levels. Recently, we reported Chromium and Cobalt metal ion levels with the Durom (Zimmer, Warsaw, USA) in whole blood.1 Our mean chromium and cobalt levels in whole blood for the whole group (men and women, all sizes) was 1.6 ug/L of chromium and 0.7 ug/L. When splitting the group by gender, mean levels at one year were lower in men: 1.5 ug/L versus 1.9 ug/L for chromium and 0.6 ug/L versus 0.8 ug/L for cobalt (p=0.05). Using a similar selection process as Daniel et al in their paper (limiting the results to 50 mm and 54 mm femoral components), the mean metal ion levels in our patients (24 men and 1 women) were 1.4 ug/L of chromium and 0.6 ug/L of cobalt in whole blood at one year (25% reduction versus our whole cohort results). As a comparison, Daniel et al reported in their paper, 2.4 ug/L of chromium and 1.3 ug/L of cobalt with the Birmingham Hip Resurfacing. As the study was a prospective study where pre-operative whole blood was collected (before the implant size was known) they could have reported the results of the whole cohort (all implant sizes). By doing so, they could have provided the readers a real baseline for further comparison. While assessing the performance of a new implant or technique, the results should be presented for the whole cohort where the innovation will be used. In the case of hip resurfacing, males and females with good femoral bone stock, significant life expectancy, no metal allergy, unimpaired renal function and with suitable anatomy for the procedure are examples. Selecting subgroups based on diagnosis, implant size, or gender may not reflect the real performance of the presented device or technique. Hence we feel that the authors' conclusion that their results provide a baseline for comparison with other implants is not justified. For the moment, we can only use their data for comparison with patients with similar component diameters (50 mm to 54 mm). P-A. VENDITTOLI, MSc FRCS, Assistant Professor of Surgery, M. GANAPATHI, FRCS, M. LAVIGNE, MD FRCS, Maisonneuve-Rosemont Hospital, Department of Surgery, Montreal University, Montreal, Canada. 1. P.-A. Vendittoli, S. Mottard, A. G. Roy, C. Dupont, M. Lavigne. 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.