To The Editor:
In their article "Axial Alignment of the Lower Extremity
in Chinese Adults" (2000;82:1603-8), Tang et al. indicated a
need for establishment of a so-called "norm" for
southern Chinese adults with respect to identification of any differences
between white and Chinese asymptomatic adults in the axial alignment
of the lower extremity.
Tang et al. defined axial alignment with use of weight-bearing
radiographs of the entire lower limb, including the hips and ankles.
The mechanical axis, hip center, midpoints for the knee (defined as
the center point of the femoral condyles and the center of the tibial
plateau), and center of the tibial plafond were used as reference
points. The mechanical axis of the femur was defined as the line
joining the center of the femoral head to the center of the knee,
and that of the tibia was defined as the line from the center of
the knee to the center of the ankle.
The authors specifically described the transverse axis of the
knee as the tangent to the femoral condyles, and they defined angle
A as the medial angle formed by the mechanical axes of the femur
and the tibia and angle B as the inferolateral angle formed by the
transverse axis of the knee and the mechanical axis of the tibia.
Two added features of femoral anatomy were measured: femoral
anatomical axis I, defined as a line drawn from the center of the
femoral shaft to the center of the knee, and the femoral shaft center
II, located at the middle of the medullary canal at a level 10 cm
proximal to the knee-joint surface.
But these definitions do not encompass all of the constituents
of axial alignment. The individual contributions of the articular
components of the joint (medial or lateral cartilage changes) and/or
any contribution of joint laxity in addition to any contribution
of the alignment of the tibial plateau are needed1.
In a study of axial lower-limb alignment in healthy volunteers
and osteoarthritic Canadian adults, my colleagues and I included
both younger and older age-groups of both sexes and we used standardized
radiographs2. We found that alignment
features in the asymptomatic adults were similar to those found
in the study by Moreland et al.3,
with respect to the hip-knee-ankle angle, and were also comparable
with the findings for angle A in the study by Tang et al.
Also similarly to Moreland et al., we found a varus inclination
of the joint surfaces of 3° to 4°. We arrived at this assessment
directly from measurement of the condylar hip angle, the tibial
plateau-ankle angle, and the angle between the joint surfaces; the
total of these three angles determined the hip-knee-ankle angle1.
The study by Tang et al. is limited by the lack of data on the
contribution of articular components and/or that of the tibial
plateau-ankle angle to the alignment of the lower extremity. Thus,
one cannot know if the varus alignment in the Chinese women is due
to tibia vara or to narrowing of the medial joint space. Nor is
it clear from the data available how much of the varus may relate
to varus of the femur (a feature identified as prominent in our
osteoarthritic population2).
In their discussion, the authors suggested that the varus identified
may warrant greater external rotation of the femoral component in
total knee arthroplasty to maintain a balanced flexion-extension
gap. But without defining the articular components in their study group,
this is only speculation, which, if used as a routine guide for
implant positioning, may lead to errors. In this regard we recommend
that the femoral transverse-epicondylar line be the basis for femoral
component positioning4, as it
avoids reference to any femoral condylar geometry that may, of itself, be
abnormal in the arthritic knee.
W.M. Tang, Y.H. Zhu, and K.Y. Chiu reply:
We appreciate the interest in our article expressed by Dr. Cooke.
The method he used to define lower-limb alignment is a very refined
one that can generate very informative results1.
The required equipment is, however, not widely available. In our
study, we aimed to establish a database of measurements of lower-limb
alignment in Chinese adults and to identify the differences between
Chinese and Caucasiansin this
regard. To allow for a direct comparison of results, we elected
to repeat the method used by Moreland et al.3 because
it is simple to carry out and it uses measurements more closely resembling
those that we make regularly in our preoperative planning for reconstructive
knee surgery. We do not think that the lack of data on the contribution
of each individual component to the varus alignment has, in any
way, compromised the practical value of our study.
Total knee arthroplasty is much more than a piece of carpentry
work. Following any one single rule or recommendation and disregarding
the others will invariably lead to surgical error. It is because
of the normal medial inclination of the tibial plateau3 that, when a tibial cut is made perpendicular
to the tibia, lateral laxity will result when the knee is in flexion.
To compensate for this lateral ligamentous laxity, Insall recommended
a lateral rotation of the femoral component so as to produce the
correct soft-tissue tensioning on both sides, that is, a rectangular
flexion gap5. Therefore, placement
of the femoral component should achieve a fine balance between the
tibial bone cut and soft-tissue tensioning as well as take into
account the distal femoral anatomy. Observing only the femoral transepicondylar
axis is not foolproof6. In our
article, we did not recommend a routine 5° external rotation of
the femoral component, although in our clinical experience with
total knee arthroplasty in Chinese patients, a higher degree of external
rotation is usually needed to produce a rectangular flexion gap
when the tibial cut is perpendicular to the tibial shaft.