Commentary & Perspective

Commentary & Perspective on
"Surgical Treatment of Limb-Length Discrepancy Following Total Hip Arthroplasty"
by Javad Parvizi, MD, et al.

Commentary & Perspective by
Douglas D.R. Naudie, MD, and Charles A. Engh, Sr., MD*,
Anderson Orthopaedic Research Institute, Alexandria, Virginia

Limb-length discrepancy is a potential complication following total hip arthroplasty. However, the true prevalence of postoperative limb-length discrepancy is difficult to quantify because of marked variation in reporting methods and in the interpretation of its clinical significance. Nonetheless, there are occasional patients who express dissatisfaction because of an apparent lengthening or shortening of the leg following total hip arthroplasty.

True postoperative limb-length discrepancies are usually the result of technical errors regarding bone resection and component sizing during surgery. Most such limb-length discrepancies can be prevented by conducting a proper preoperative physical examination, by performing careful preoperative templating from standardized radiographs, and by taking intraoperative measurements of limb-length differences and offset with rulers or calipers. During surgery, it is also important to consider the soft-tissue tension across the hip joint and to not increase limb length in hopes of increasing the stability of the hip joint.

When faced with a patient who has symptomatic postoperative limb-length discrepancy, it is important first to determine if the limb-length discrepancy is a true discrepancy or an apparent discrepancy secondary to a flexion or abduction contracture of the hip. In many cases, patients have a postoperative abduction contracture of the hip, which gives them a pelvic obliquity, resulting in an operated leg that seems too long. When these patients stand with the feet close together, the contralateral normal hip will be in an adducted position and will feel shorter than the leg on the operated side. However, when asked to stand with the feet widely separated so that both hips are equally abducted, the pelvis becomes level and the legs then seem equal in length. In most cases, I can explain to the patient the reason for the apparent difference in limb length. Patients then understand why they should not wear a shoe lift on the contralateral side, and it becomes possible for me to plan for them an appropriate program of hip abductor muscle stretching. Usually within several months, the leg on operated side no longer seems too long.

When a true limb-length discrepancy exists because of overlengthening, it is important to determine the amount of true lengthening. This can be accomplished with the use of osseous landmarks or blocks under the foot. It is also important to discuss with the patient the reason for dissatisfaction and his or her expectations of treatment. It is unlikely that a half-inch discrepancy would result in any symptoms, and I would not reoperate for a difference of this amount or less.

Revision to correct a substantial postoperative limb-length discrepancy is seldom indicated because the procedure is fraught with the possibility of postoperative hip instability. As a result, although patient dissatisfaction may be great, surgery to correct these true overlengthenings are not frequently performed. In fact, Parvizi and associates report that less than one-half of 1% of total hip surgery performed at the Jefferson Hospital was for revision for a true overlengthening. Parvizi et al. also separate true overlengthenings into those with and without problems secondary to the overlengthening, such as back pain, hip pain, sciatica, or recurrent dislocation. They correctly point out that overlengthening that is not associated with one of these secondary problems should almost always be treated nonoperatively by placing a shoe lift on the contralateral side.

When I do consider performing revision surgery for a true lengthening, I try to determine whether there is a reasonable probability that I will be able to relieve the secondary problem. When the secondary problem is simply postoperative back pain, I prefer to try to correct the true limb-length difference by using a shoe lift to level the pelvis. If the back pain is purely mechanical, a shoe lift beneath the opposite foot to level the pelvis should relieve it.

Sciatica that occurs after total hip replacement and is caused by overlengthening is more difficult to assess. Because I am often uncertain that shortening by revision will eliminate this sciatica, I seldom revise for this reason. If I were to consider it, I would carefully evaluate the patient with magnetic resonance imaging of the spine and, possibly, with electromyography.

Postoperative hip pain and stiffness is another problem that potentially could be related to overlengthening of the leg, but usually is not. More likely causes for early postoperative hip pain include heterotopic ossification, infection, and component loosening. To rule out these causes, I would carefully evaluate serial radiographs. I would also obtain a complete blood-cell count and determine the level of C-reactive protein and the erythrocyte sedimentation rate, and I would probably aspirate fluid from the hip.

When overlengthening of the limb is associated with postoperative dislocation, the possibility exists that the operating surgeon may not have recognized the real cause of the hip-joint instability until after he or she inserted the components. The surgeon may then have attempted to solve the problem by using a different ball length to increase the neck length of the already implanted femoral prosthesis in hopes of gaining stability at the expense of an increased limb length. My approach to this scenario would include looking for a correctable cause. I would obtain multiple radiographs in various views, including a cross-table lateral view of the femur, to try to identify any component malposition (particularly of the cup). I would also consider fluoroscopy of the hip to look for any unrecognized extra-articular osseous impingement problems. I would also examine the patient's other joints, looking for excessive ligamentous laxity.

Finally, if I were strongly considering revision surgery, I would try to determine how difficult it would be to revise well-fixed components and how much bone stock would be lost by removing one or both of the components. I would also be sure to ascertain that the increased joint laxity that might occur when I revised the components would not leave the patient with an unstable hip.

Revision surgery with removal of well-fixed components is inevitably associated with bone loss. The extent of the bone loss would of course depend on the positioning of the components to be revised, particularly the acetabular component. Porous-coated acetabular components that are well fixed against the inner pelvic wall at the depth of the acetabular fossa cannot be removed without damaging the medial cortical wall. In general, therefore, I prefer to shorten the leg by revising the femoral side, not the acetabular side.

I would first select a new femoral prosthesis that I could insert into the medullary canal at a lower neck-resection level, and I would change the neck-shaft angle of the prosthesis from 135° to 120° to try to improve stability by increasing the femoral prosthetic offset. I would also attempt to increase stability by using a larger femoral head size. Although well-fixed, extensively porous-coated stems and well-fixed, well-cemented stems can represent difficult revision problems, I believe that there is less bone-stock reserve in the pelvis than there is in the femur.

Fortunately, in the past five to ten years, implant manufacturers have developed a variety of ways to improve hip stability without overlengthening a limb. These include decreasing the neck diameter of the femoral prosthesis and increasing the femoral head size. This combination improves the range of motion in the hip and reduces the chance of dislocation resulting from femoral-neck impingement against the rim of the acetabulum. Femoral prostheses have also been developed with decreased neck-shaft angles. In addition, improved capture devices for the femoral head have been developed. These improvements to components and technique help eliminate some of the uncertainty in revision procedures. As we surgeons develop more confidence in the outcomes, we may become more inclined to reoperate for overlengthening problems.

*The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. One of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Johnson and Johnson). No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.