In 1980, Dobbs
introduced the statistical technique of survivorship analysis in
the setting of total hip arthroplasty. We expanded upon the need
for survivorship analysis in this setting in 1986
, and today survivorship analysis is the standard technique for
any long-term statistical analysis of the results of total hip arthroplasty.
As with any statistical technique, however, a proper evaluation
can only be performed after accounting for all of the factors that
may have contributed to implant failure.
This issue of
contains an article by Dr. Crowther and Dr. Lachiewicz that discusses
the excellent survival of a porous-coated acetabular component in
patients who were less than fifty years old at the time of surgery.
The results of total hip arthroplasty in younger patients are important because,
in the literature published both prior to and following the use
of survivorship analysis, patient age at the time of surgery has
always been one of the most important prognostic factors in the
prediction of early revision of the prosthesis. The question still
remains, however, as to why patient age is so important. As there has
never been a convincing biological explanation for this association,
the assumption has always been that patient activity was the real
variable that accounted for most of the effect of age on the durability
of the prosthesis.
This supposed relationship between patient activity and prosthetic
survival is further supported by the fact that the duration of prosthetic
survival can vary widely among patients of the same age. Thus, an
evaluation of patient activity, even in younger populations, should
be considered a crucial step when comparing prostheses. A very inactive
patient might well have a prosthesis that lasts more than twenty
years, whereas a very active patient with the same prosthesis might
need a revision within the first ten years. Thus, measurements of
patient activity are crucial for a proper understanding and evaluation
of the results of any long-term study of total hip arthroplasty.
Even though all of the patients in the study by Crowther and
Lachiewicz were young, they clearly had different activity levels.
Twelve (27%) of the forty-four patients had a bilateral implant,
twenty-seven (61%) were female, and seven (16%) had undergone additional
procedures for reasons other than acetabular loosening. The activity
levels of these patients were undoubtedly heterogeneous. How could
the results of this study be compared with those of a hypothetical
study of unilateral total hip arthroplasty in which 70% of the patients were
male and 20% participated in some type of sports activity?
One of the major problems in the comparison of observational
studies today is that there is no good existing measure of activity
that can be easily used to evaluate patients over time. Since the
early 1980s, surgeons at the University of California at Los Angeles
have routinely estimated patient activity at every clinical follow-up
visit with use of a 10-point scale
. Unfortunately, although this scale is helpful, it has not proven
to be precise enough to account for the relationship between age
and revision. Kilgus et al.
evaluated the effect of sports activity following total hip arthroplasty
and found that there was some correlation between participation
in impact sports and early revision, but the subdivision of patients
clearly was not refined enough. Other authors who have evaluated
the relationship between activity and revision have reported varying
. Recently, Zahiri and colleagues
found a strong correlation between the number of steps per day
as recorded with use of a pedometer and the level of patient activity
as assessed with other methods. However, since patient activity
is not static, any attempt to incorporate patient activity into
an analysis of total hip arthroplasty must be flexible enough to
allow for multiple readings over time. Future evaluation of new bearing
materials and existing or new implant designs will be easier and
more efficient if there is an assessment of both loading cycles
Thus, it is clear that a more precise and formal method of evaluating
patient activity will be needed in order for investigators to make
fair comparisons between the results of procedures performed with
different prostheses or under different surgical conditions. In
order for investigators to more objectively evaluate patient activity
over time, some type of validated patient questionnaire will be
needed to more accurately adjust for patient activity in any statistical
analysis. If such an evaluation of patient activity were available
at several time-points, that variable could be used in a time-dependent
way when comparing different prostheses, especially with reference
to different patient populations.
The quantification of patient activity is a challenging goal
that involves at least two steps: (1) collection of relevant data
regarding all aspects of patient activity (including the nature
of the activity and the frequency, duration, and intensity of participation)
and (2) a summary score that reflects the relative weight of all
of the different aspects of activity.
In the development of such a questionnaire, the various aspects
of activity must be considered. Besides professional and social
activities, recreational activities (including sports) seem to account
for an increasingly important part of patient activity following
total hip arthroplasty. In 1994, Wright et al.
found that patients rated the resumption of such activities as
their highest priority after the elimination of daytime pain and
the return to normal walking function.
The development of a validated questionnaire will allow for more
frequent patient input regarding their activity level. Today, the
use of such questionnaires is facilitated by modern means of communication
such as the Internet. The availability of the Internet allows for
an easier data-collection process, which will be necessary if we are
to obtain patient data over time as will be required. Without these
tools, we will continue to compare apples and oranges in the observational
studies that are the norm of the orthopaedic literature.