We read with great interest the article "Surgical Treatment of
Limb-Length Discrepancy Following Total Hip Arthroplasty"
(2003;85:2310-7), by Parvizi et
al. The study retrospectively reviewed the cases of twenty-one patients who
had had revision total hip replacement for limb-length discrepancy. According
to the authors, in six patients (Cases 2, 6, 9, 12, 13, and 20), the primary
problem leading to limb-length inequality was excessive anteversion or
retroversion of the acetabular component. In these patients, there was no
obvious longitudinal malalignment of the cup or of the femoral component
(Table I). All of these six patients underwent revision of the acetabular
component alone, and the limb lengths equalized in four patients and the
discrepancy decreased in the other two.
The authors described two different categories of limb-length discrepancy.
The six patients mentioned above obviously belong to the second category, in
which instability of the total hip replacement was possibly noted
intraoperatively (primarily as a result of wrong version of the acetabular
cup) and the surgeon tried to improve the soft-tissue restraints by increasing
the neck length or offset of the femoral stem. We therefore feel that,
although these cases had wrong version of the cup, the lengthening was more
likely due to a longer neck or offset of the femoral component.
It is obvious that a pure version problem of the acetabular component can
result in a rotational malalignment of the leg. However, it is difficult to
understand how true limb-length inequality can be caused by incorrect version
of the acetabular cup alone. We therefore cannot understand how it is possible
to correct up to 4 cm of lengthening (a longitudinal deformity) by altering
the acetabular component version (a deformity in the horizontal plane).
Over the years, our understanding of the causes and treatment of
limb-length discrepancy after hip replacement has evolved. The relationship
between acetabular component orientation and limb lengthening has been the
most difficult for us to appreciate. It is clear that Mr. Theruvil and Mr.
Kapoor have given a great deal of thought to their interpretation of our
paper. Perhaps the following will provide further insight into this issue.
An excessively anteverted or retroverted acetabular component will create a
situation of intraoperative hip instability. Should a surgeon not appreciate
the source of this instability, the usual solution is to restore hip stability
through modification of the femoral component (by lengthening the neck,
inserting the femoral component in a more proximal position, or increasing the
offset). Our interpretation of the resultant limb lengthening is that the
underlying source of the problem is the acetabular component position and that
the femoral component choices are a secondary result of the primary problem.
At this point, the situation becomes even more involved. Many times, the
secondary attempts at obtaining stability through modification of the femoral
component seem to correct stability intraoperatively but postoperatively the
patient continues to experience symptoms of hip instability (subluxation and
pain). To reduce these problems, the patient reflexively tilts the pelvis and,
in most cases, maintains the muscles on that side in a contracted state
(essentially creating an abduction contracture), resulting in an increase in
the apparent length of the limb. Once the primary problem of cup version is
corrected, both the real and the apparent lengthening disappear; thus, a
substantial amount of correction of the limb lengthening can be achieved. It
must be emphasized that we were not treating a radiographic limb-length
discrepancy but rather a limb lengthening that was clinically apparent to the
patient.