To The Editor:
I read with great interest the article "Effects of Preheating of Hip
Prostheses on the Stem-Cement Interface" (2003;85:421-7), by Iesaka et
al. I would like to congratulate the authors on an excellent paper, but as we
recently finished a similar study on the preheating of total knee arthroplasty
components, I wish to raise some questions relating to this article.
First, I wonder if the authors have any data on the exact bone temperature
in the femoral shaft during total hip replacement. In a paper involving
temperatures, a reference value to recreate the real in vivo bone temperature
is necessary.
The authors used 37°C as the standard body (and therefore bone)
temperature. In our study, we found that the body temperature during surgery
(mean, 35.8°C) was lower than the standard 37°C and that the local
bone temperature was even lower. We measured the in vivo bone interface
temperature prior to cementing and for twenty-four hours postoperatively. We
found that the mean intra-articular temperature during total knee arthroplasty
was 26°C and not 37°C.
There are, of course, many technical differences between total hip and
total knee arthroplasty, and therefore we would not extrapolate our results to
the hip. For example, during total knee arthroplasty, a tourniquet is usually
used, the bone is directly exposed to cold air (laminar airflow), and pulsed
lavage is used on an open surface. During total hip arthroplasty, the femoral
shaft is less exposed to air and blood perfusion of the bone remains. However,
exposure of the bone during surgery and the use of pulsed lavage will reduce
the bone temperature. Therefore, the mean temperature in the shaft will
probably be lower than the physiologic 37°C.
Second, the authors state that porosity was associated with the cooler
surface of either the stem or the bone since polymerization is initiated at
the warmer surface. In this study, the sawbones were heated in a water bath to
37°C and in one subgroup the femoral component was also preheated to
37°C. I therefore notice no temperature difference between the two
conditions. How do the authors explain the finding that the porosity reduction
at the cement-metal interface was similar to that in the groups in which the
stem was preheated to 44°C and 50°C? Why should the cement
polymerization start first at the cement-metal interface and not at the
bone-cement interface in this subgroup since there is no temperature
difference?
In vivo, however, I guess that preheating components to 37°C could be
sufficient since the real bone temperature is probably lower. This gives us a
certain degree of freedom during the handling of the preheated components,
perhaps with less thermal cell damage to the bone.
We chose 37°C as the representative body temperature because it is used
for most in vitro laboratory studies on bone cement. However, factors such as
perianesthetic thermoregulation, patient age, room temperature, use of metal
tools, pulsed lavage, and introduction of cement dough that is at room
temperature would produce a lower temperature at the bone interface. As Dr.
Thienpont indicates, in the case of total knee arthroplasty, the temperature
would be even lower because of tourniquet use, more exposure of prepared bone
surfaces to room air, and open pulsed lavage. This would not affect the
conclusions of our study as polymerization (and reduced porosity) is initiated
at the hotter surface of the interfaces. However, it probably would reduce the
temperature at the bone-cement interface and increase cement-setting time.
We performed a few experiments with tibial components at 37°C and found
that preheating of the thin metal trays during application resulted in minimal
porosity reduction. This implies that a higher temperature would be required
for these components. It is reasonable to expect that preheating the tibial
and femoral stems used in knee arthroplasty will yield results similar to
those seen with the preheated hip stems in our study.
Because of the higher thermal conductivity and heat capacity of metals
compared with bone, the femoral stem can maintain a higher temperature over
time and can provide more energy to the cement dough than the femur does. This
explains why the polymerization is probably initiated at the stem-cement
interface rather than the cement-bone interface when the temperatures of the
stem and femur are the same, as in our experiments. Theoretically, interfacial
porosity reduction will be accomplished even when polymerization is initiated
simultaneously at both interfaces, isolating porosity between the
interfaces.