To The Editor:
In regard to the Commentary "Design Issues in Clinical
Studies of the in Vivo Volumetric Wear Rate of Polyethylene Bearing
Components" (82-A: 281-287, Feb. 2000), by Dr. Gladius
Lewis, I wish to point out the following:
(1) We clearly stated in our report that "taking one million
cycles [is] roughly equivalent to one year of clinical
use" for the joint simulator studies1.
The wide variations were clearly documented in our pedometer studies2.
(2) Dr. Lewis states that "the ages of the patients were
not reported" in our study. I direct his attention to the
details in Table I of our publication.
(3) In their study of wear measurements, Livermore et al. clearly
state: "The radiographic measurements were made as described
by Griffith et al."3 The
method attributed to Livermore is thus not original to him.
(4) Since a radiograph is usually a magnification of the real
image, Dr. Lewis is incorrect in stating that "all measurements
are corrected by multiplying by the magnification factor";
surely, they should be divided by the magnification factor.
(5) The suggestion that prospective, randomized wear studies
should be carried out is admirable. To fail to acknowledge or to
misinterpret published evidence is not.
G. Lewis replies:
I appreciate the succinct manner in which Mr. Wroblewski has
stated his five points concerning my Commentary, and I shall thus
attempt to reply in similar fashion.
As for the first two points, it is true that in the report by
Wroblewski et al.1, the authors
(a) explicitly stated the assumption of one million cycles on the
joint simulator "as roughly equivalent to one year of clinical
use," and (b) provided the patients’ ages. I apologize
for the errors on these points in my Commentary. Having said that,
I should state that I believe that the thrust of the general theses
related to these points is unaffected by these errors. These theses
are that, in many reports, important pertinent data are not stated,
and the issue of the impact of patient age and activity level on
the amount of walking is not taken into account when estimating
clinical wear rates of articular components.
As for the third point, it is true that, in their original report,
Livermore et al.3 stated: "The
radiographic measurements were made as described by Griffith et
al." However, in current orthopaedic literature, this method
of obtaining the clinical wear rate of acetabular components is
commonly referred to as the "Livermore method"4-9. Thus, in my Commentary, I followed
what appears to be the convention.
With regard to the fourth point, I defined the magnification
factor in the Commentary thus: "Second, a magnification
factor is calculated as the ratio of the actual diameter of the
femoral head to its apparent radiographic diameter." This
definition is the same given by Livermore et al.3 except
that I termed mine "magnification factor" while
Livermore et al. termed theirs "correction factor." In
their report, Livermore et al. stated: "The factor to correct
for magnification (that is, the factor by which the observed dimension
on the radiograph must be multiplied to obtain the real dimension)
is calculated by the formula: correction factor = known
diameter of the implant/apparent radiographic diameter." In
other words, the formula that arises from my definition is: the
dimension on the radiograph times the magnification factor equals
the real dimension. By this definition, all magnification factors
are less than one. For example, a magnification factor of 0.01 means
that what is seen on the radiograph has been increased 100-fold
relative to the real dimension.
My response to the last point is encapsulated in my response
to the first two points; namely, it is hoped that the reader would
focus on all general theses presented in the Commentary.