Response to Dr Theruvil:

Over the years, our understanding of the causes and treatment of leg length discrepancy after hip replacement has evolved. The relationship between acetabular component orientation and leg lengthening has been the most difficult for me to appreciate. It is clear that Drs. Theruvil and 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 the femoral component (either by lengthening the neck, by inserting the femoral component in a more proximal position, or by increasing the offset). Our interpretation of the resultant leg 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 stability through the femoral component seems to correct intraoperative stability, but post-operatively the patient continues to experience symptoms of hip instability (subluxation, 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). By doing so, an increase in the apparent lengthening of the leg is established. Once the primary problem of cup version is corrected, both the real and the apparent lengthening disappear – thus a substanttial amount of correction of the leg lengthening discrepancy can be achieved. It must be emphasized that the authors are not treating a radiographic leg length discrepancy, but rather, a leg lengthening clinically apparent to the patient.

Response to Dr. Avci:

Our response to Dr. Avci’s concerns parallel in many respects our comments to Dr. Theruvil. In response to his first concern, indeed the wording of Figure 1-A is confusing. Dr. Avci is correct in noting the accidental misplacement of the short line in that figure. However, the limb length difference mentioned in the explanation for the figure refers not to the radiographically measured difference, but rather to the clinically measured difference (the technique of measurement is discussed in the text of the paper). In this patient, the radiographic difference was less than the leg lengthening perceived by the patient and measured to be 3.5cm. clinically.

As discussed in our response to Dr. Theruvil, the compensations made by the patient to minimize the symptoms of instability caused by position of the acetabular component can lead to a substantial magnification of the actual radiographically measured difference in leg length. By repositioning the acetabular component in the anatomically correct (version, abduction, and vertical) position, complete correction of the leg length difference was achieved without changing the femoral component. It should be noted that the center of the femoral head position of the replaced hip with respect to the tip of the greater trochanter is virtually identical to that of the un-replaced opposite hip (suggesting that the femoral component is not the source of the problem).

Figures 2A and 2B represent a patient who has no difference in clinical leg length after the revision operation. Certainly the exposure of the figures leaves something to be desired, but our interpretation of the radiographs (based on close measurement of the true radiographs) suggests that the center of the replaced femoral head is identical in position with respect to the greater trochanter as is the unreplaced opposite femoral head. After placing the acetabular component in a more anatomic position, a favorable soft tissue balancing of the hip was restored, leg equalization was achieved, and therefore no femoral component revision was considered.

We have achieved considerable success in treating this difficult problem based on the detailed analysis of radiographs and patients as described in the text of our manuscript. It continues to be our experience that the acetabular component is the primary culprit, but not the only one. Dr. Avci’s experience with this difficult problem seems to be somewhat different. A detailed publication of his experience would be a welcome contribution to the literature.

The article by Parvizi et al, "Surgical Treatment of Limb-Length Discrepancy Following Total Hip Arthroplasty ",deals with an important complication of total hip arthroplasty, but in looking at their the figures, I believe some of the cases may have been misinterpreted.

Figure 1A belongs to case 7. As seen on the X-ray, an ischial line was drawn for the measurement of limb-length discrepancy, but the difference in leg length should be measured from the most distal aspect of the trochanter minor of the right femur as was done on the left side. When this is done, the leg length discrepancy is less than one centimeter, which is negligible. The legend states that the leg length discrepancy was corrected by acetabular revision but on the post-op X-ray, the center of the femoral head does not seem to be changed and a shorter femoral neck corrected the negligible leg length discrepancy. Hence, this does not seem to be a case to be included in this study.

Figure 2 (Case 14) also needs clarification. When we look at the pre-op X-ray, the center of the both femoral heads seem to be at the same level, so the comment of the authors that the acetabular component was too low is not justified. On the other hand, the femoral component seems to be placed very proud on the femur. Thus on the post-op X-ray we see a proud femoral component and a high seated acetabular component and again an apparent length discrepancy that is contrary to the legend. Why didn’t the authors consider revising the femoral component in this patient?

The authors claim to have corrected limb-length discrepancy by acetabular revision alone in fifteen hips and by femoral revision alone in only three hips.In my practice, most of the leg length inequalities I see originate from problems attributable to the femoral component.

Sincerely,

To The Editor

We read with great interest the article “Surgical treatment of limb- length discrepancy following total hip arthroplasty” (2003; 85:2310-7), by Parvizi J et a1. The study retrospectively reviewed 21 patients who had revision total hip replacements for limb length discrepancy. According to the authors, in 6 patients (Cases 2,6,9,12,13 and 20), the primary problem leading to limb length inequality was either excessive anteversion or retroversion of the acetabular components. In these patients, there was no obvious longitudinal malalignment of the cup or of the femoral component (Table 1). All of these 6 cases underwent revision of acetabular component alone and the limb lengths equalised in 4 patients and the discrepancy decreased in the two others.

The authors describe two different categories of limb length discrepancy. The above six patients obviously belong to the second category, where instability of the total hip replacement was possibly noted intra-operatively(primarily due to 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 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. But it is difficult to understand how true limb length inequality can be caused by incorrect version of the acetabular cup alone. We cannot therefore 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).

Yours sincerely

Bipin Theruvil MS, FRCS

Vikas Kapoor MS, FRCS (Tr & Orth) 13/12/2003

CORRESPONDING AUTHOR Mr B Theruvil 10 Cedar walk Winchester SO22 5EU UK Phone: 0044 7968 122 145

References

1. Parvizi, Sharkey PF, Bissett GA, Rothman RH, Hozack WJ. Surgical treatment of limb-length discrepancy following total hip arthroplasty. J.Bone Joint Surg.Am. 2003; 85-A:2310-2317