To The Editor:
I read with interest the recent review by Schmalzried and Callaghan,
entitled "Current Concepts Review. Wear in Total Hip and Knee Replacements"
(81-A: 115-136, Jan. 1999).
In my opinion, one of the most important points raised by the
authors concerns the propensity of data that are published linking
clinical rates of wear, and indeed survivability, to a denominator of
time. The authors stress that "the wear of a prosthetic hip or knee
is a function of use or the number of cycles and not a function
of time in situ." The authors go on to report on investigations
they have performed, indicating that "a forty-fivefold difference
in rates of wear as well as rates that are more than 3.5 times the average
can be accounted for by differences in an individual's activity."
How many times have we read in published papers, and particularly
in manufacturers' advertisements: "x hip has a survivability of
y% at z years"? Surely, this is misleading, and, while it may well
be true in some instances, it is scientifically superficial in that
it fails to address the critical issues of activity level, intensity
of activity, and joint-loading. Should we not be reporting implant
survivability based on estimates of the number of cycles that the
joint has undergone rather than as a measure of time? I accept the
difficulties that this may present, but it would be far more scientifically meaningful.
If we were to approach a car-tire manufacturer and ask how long
its tires would last, we would probably not get an answer to the question,
but, if we did, I imagine the answer would be based upon the number
of miles the car was driven rather than the length of time the tires were
used.
With increasing patient expectations and with the current debates
that are being heard on treatment options for younger, more active
patients (whose hips will undergo considerably more cycles in a
year than those of the average patient), is it not now time to hear
these arguments and recognize their validity?
Ian H. Paling, B.Met., M.Met.
Cyclamen Lodge, Private Road, Rodborough Common
Stroud, Glos GL5 5BT, United Kingdom
T. P. Schmalzried and J. J. Callaghan reply:
We thank Mr. Paling for his supportive comments regarding our
position that the wear of a prosthetic hip or knee should be evaluated
based on the amount of use, or the number of cycles, and not the
time in situ. The traditional method of reporting wear as a function
of time exists because of convenience, not accuracy. It is much easier
to determine the amount of time that a prosthesis has been in service
than it is to assess the amount of use of the prosthesis. There
has been some work1, however,
indicating that office assessments of patient activity are related
to quantitative measures of activity and can add value to our outcome
assessments.
With the current focus of both science and industry on polyethylene
wear, it is high time to hear these arguments and recognize their
validity.
Thomas P. Schmalzried, M.D.
John J. Callaghan, M.D.
Corresponding author: Thomas P. Schmalzried, M.D.
Joint Replacement Institute, 2400 South Flower Street Los Angeles,
California 90007