Sir,
We appreciate Drs Vendittoli and Lavigne’s interest in our paper. In
response to the concerns raised by them we would like to make the
following comments.
Patient-Related Factors.
Inclusion criteria were clearly stated in our paper and are as
follows. Well-functioning unilateral Metasul total hip replacements (THRs) or Birmingham hip replacements (BHRs) in patients with no other
metal devices in the body, and who were at the appropriate time points planned
for metal level measurement, were included. Diagnoses other than
osteoarthritis and patients who lived abroad or with impairment of renal function were excluded. The measurement times were spread
out over this extensive time period in order to cover the largest possible
duration (between one year and four to six years). Earlier studies
predict a run-in peak wear around one year and steady state wear
in later years. The mean duration from operation was similar in
the groups with the two different bearing sizes.
There are bound to be minor variations in the acetabular inclination
angles in any group of arthroplasty patients. Vendittoli and Lavigne quote
the report by Brodner et al to highlight the effect of acetabular
inclination angle on metal ion release. In that study no correlation could
be found between inclination angles and cobalt or chromium levels and no
significant differences were found between the median metal ion levels in
groups of patients with the greatest, lowest and intermediate inclination
angles. They found three outliers in terms of serum metal ions (with serum
levels ranging up to 26.8 µg/l for cobalt and 33.6 µg/l for chromium) who had
abnormally high inclination angles. In our study there were no
outliers with abnormally high blood levels in either group. This
observation rules out the suggestion that there may be individual metal
ion variations as a result of steep inclination angles in one device group
or the other.
In our own unpublished work we have data which shows that
neither height, weight, body mass index, patient activity nor a
combination of these factors, nor inclination angle of the cup, nor age show any
association with metal ion output in urine or metal levels in blood.
Implant-Related Factors.
Resurfacings are bound to be of larger
diameter than 28 mm hip replacements. The 28 mm bearing is the most
frequently used THR in our centre and is one of the most commonly used
bearing diameters in THRs everywhere. The 50 mm and 54 mm bearings are the most
frequently used BHR bearings, therefore these sizes were chosen.
Differences in the actual diametral clearances are inevitable when
bearings of different diameters are used, even if these bearings have been
designed to provide the same geometric configuration. A bearing with a smaller diameter will always have a lower clearance. For instance, in the design of
the Metasul bearing, Schmidt et al1 proposed a clearance of 200 µm for
bearings with a diameter of between 37 mm and 42mm and appropriate smaller
clearances for bearings with a diameter of 28 mm. Campbell2 reported clearances in retrieved
28mm Metasul bearings to be in the range of 100 µm to 150 µm. The clearances
of the 50 mm and 54 mm BHR bearings have comparable geometric configuration
to the clearance of the 28 mm Metasul bearings. For surface roughness3 both the BHR and the Metasul fall in the same
range and therefore this factor is unlikely to have affected the results.
Our study does not claim to compare the tribology of one system with another. Vendittoli and Lavigne conclude that
the "larger than average component sizes in the SRA group… might have
selectively reduced the metal ion level of the SRA group and compensated
the tribologic properties of the Metasul 28 mm implant favourably versus
the BHR". In doing so they are assuming that the hip simulator finding
(that larger bearing diameters wear less) always works in an identical manner in vivo (i.e. bearings of a larger diameter generate less metal
ions). However, in the only published report4 on the effect of bearing
diameter on metal ion levels, it was shown that bearings of a larger diameter generate more metal ions. This contradicts the simulator results. We
conducted our study in order to verify or disprove the information
in that report.
In order to overcome one very obvious problem in the earlier
publication (i.e. metallurgic confounding factors) we chose to perform the study with systems that are both made of high carbon cobalt chrome
alloy, while retaining similar bearing diameters as used in the earlier
report. We highlighted in our paper that Metasul is of the forged
variety and the BHR as-cast, however, no carbide-depleting late-stage heat treatments were performed on either device.
We fully agree that a systematic review of prospective double blinded
randomised controlled trials(RCT) provides more robust evidence than a
retrospective cohort study. However, the
question of whether an RCT is the gold standard study design or even the
best study design in surgical and orthopaedic practice continues to be
debated.5-7 We do not agree that a retrospective study has no
place in scientific enquiry. A retrospective cross-sectional study gives a
snapshot of the temporal trends in metal ion exposure and are particularly
useful for guidance on study design and in sample size and power
determination of a longitudinal study. Metal ion levels exhibit time-related changes following hip arthroplasty. A cross-sectional study
provides the basis to decide the critical time intervals when measurements of metal levels
should be made in a prospective longitudinal trial. We
appreciate that Lavigne and Vendittoli are involved in a longitudinal
study and that the five to six year results are unlikely to be available for another two
to three years. We too are involved in such a study and are
awaiting the medium term results in that study before publication.
The preliminary work on metal ions by Lavigne et al8,9 has shown
that in addition to cobalt and chromium, their resurfacing device (Durom)
releases elevated levels of titanium, evidently from the titanium plasma
spray cementless fixation surface of the cup. Studies10,11 have shown that
titanium alloy total hip replacements produce up to a five-fold increase in
chromosomal aneuploidy as compared with control subjects and cobalt-chrome
THRs which produce up to a 2.5-fold increase in aneuploidy and a 3.5-fold
increase in chromosomal translocations. It is known that both titanium12
and the elements in cobalt-chrome13 alloy also lead to hypersensitivity.
The possible synergistic effects of elevated levels of the constituent
elements of cobalt-chrome and titanium in the same patient are still
unknown and are a matter of great concern. The BHR, with an integral porous surface, eliminates that element of possible adverse synergism
between titanium and cobalt-chrome.
J. Daniel, FRCS,
H.Ziaee, BSc(Hons),
C. Pradhan, FRCS,
D.J.W. McMinn, FRCS,
The McMinn Centre,
Birmingham, UK.
1. Schmidt M, Weber H, Schon R. Cobalt chromium molybdenum metal
combination for modular hip prostheses. Clin Orthop 1996;(329 Suppl):S35-47.
2. Campbell P, Shen FW, McKellop H. Biologic and tribologic
considerations of alternative bearing surfaces. Clin Orthop 2004;418:98-111.
3. Pynsent WB, Band T. Smith and Nephew Product Reports N162.002 and
N165.001.
4. Clarke MT, Lee PTH, Arora A, Villar RN. Levels of metal ions after
small and large diameter metal-on-metal hip arthroplasty. J Bone Joint Surg [Br] 2003;85-B:913-17.
5. McCulloch P, Taylor I, Sasako M, Lovett B, Griffin D. Randomised
trials in surgery: problems and possible solutions. BMJ 2002;324:1448-51.
6. Hartling L, McAlister FA, Rowe BH, et al. Challenges in systematic reviews of therapeutic devices and
procedures. Ann Intern Med 2005;142:1100-11.
7. Amstutz HC. Innovations in design and technology. The story of hip
arthroplasty. Clin Orthop 2000;378:23-30.
8. Lavigne M et al. Early results of a RCT comparing conventional and
resurfacing total hip arthroplasty. International Symposium on Resurfacing
of the Hip Joint. Zurich, April 2005.
9. Lavigne M, Vendittoli PA. Early results of a randomized controlled
trial of hip resurfacing and total hip arthroplasty. OSTEOLOGIE - OSTEOLOGY. Interdisciplinary Journal for Bone and Joint Studies. 2005;14(suppl 2):80-83.
10. Doherty AT, Howell RT, Ellis LA, et al. Increased chromosome translocations and aneuploidy in
peripheral blood lymphocytes of patients having revision arthroplasty of
the hip. J Bone Joint Surg [Br] 2001;83-B:1075-81.
11. Ladon D, Doherty A, Newson R, et al. Changes
in metal levels and chromosome aberrations in the peripheral blood of
patients after metal-on-metal hip arthroplasty. J Arthroplasty 2004;19(8 Suppl 3):78-83.
12. Lalor PA, Revell PA, Gray AB, et al.
Sensitivity to titanium. A cause of implant failure? J Bone Joint Surg [Br] 1991;73-B:25-8.
13. Willert HG, Buchhorn GH, Fayyazi A, et al. Metal-on-metal bearings and hypersensitivity in patients with
artificial hip joints. A clinical and histomorphological study. J Bone
Joint Surg [Am] 2005;87-A:28-36.
eLetters: ![[Read eLetter]](/icons/misc/sectionDOWN.gif){kind=icon}
