Limb-Length Discrepancy after Total Hip Arthroplasty: Avoidance and Treatment

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CHAPTER SYNOPSIS

The primary goals of total hip arthroplasty are to relieve pain and restore function. To achieve this reproducibly, initial implant stability, restoration of proper hip biomechanics, and equalized limb lengths must be achieved. Limb-length discrepancy is a common cause of patient dissatisfaction and litigation after total hip arthroplasty. The boundary between acceptable and unacceptable levels of limb-length discrepancy still remains undefined, but lengthening of more than 1.5 cm generally is associated with complaints. This chapter systematically describes the steps involved in the avoidance and treatment of limb-length discrepancy.

IMPORTANT POINTS

1

Clinical evaluation
2

Types of limb-length discrepancy
3

Radiographs and templating
4

Intraoperative techniques to assess for limb-length discrepancy
5

Postoperative limb-length discrepancy
6

Treatment of limb-length discrepancy
7

Future trends.

CLINICAL/SURGICAL PEARLS

1

A thorough preoperative evaluation should seek to identify patients at risk for developing postoperative limb-length discrepancy. Various intraoperative techniques have been described to assess limb-length discrepancy. The preoperative clinical and templated findings must be correlated during surgery.

CLINICAL/SURGICAL PITFALLS

1

Failure to optimize positioning of the femoral and acetabular components during total hip arthroplasty is one of the most important factors leading to the development of limb-length discrepancy. In general, more than 1.5 cm of lengthening has been associated with backache, awkward gait, nerve palsy, and dislocation.

VIDEO AVAILABLE

“Intraoperative Assessment of Limb-Length Discrepancy during Total Hip Arthroplasty” (3:50).

INTRODUCTION

The primary goals of total hip arthroplasty (THA) are to relieve pain and restore function. To accomplish this reproducibly, the reconstructive surgeon must achieve initial implant stability, restore proper hip biomechanics, and equalize limb lengths. Limb-length discrepancy (LLD) is one of the most common causes of patient dissatisfaction after THA and also has been associated with problems such as nerve palsy, low back pain, abnormal gait, instability, aseptic loosening and, occasionally, revision surgery. As such, it also is one of the most common reasons for litigation against orthopedic surgeons.

Appropriate history and physical examination, radiographic evaluation, preoperative templating, and intraoperative measurements can be used to identify patients at risk for developing LLD. Although some lengthening (less than 0.5 cm) of the operative limb in THA may, in fact, be necessary and advisable, methods are available to minimize the risk of overlengthening. If an unacceptable LLD occurs postoperatively, it must be recognized and treated appropriately.

PREOPERATIVE CLINICAL EVALUATION

When dealing with an LLD, the discrepancy must be determined to be true (real) or functional (apparent). A functional discrepancy may be the result a variety of causes, such as an abduction contracture at the hip or a spinal deformity, which can cause a pelvic obliquity. Functional length is measured from xiphisternum or umbilicus to medial malleolus. True length is measured from the anterior superior iliac spine to the medial malleolus and reflects the actual length of the extremity. Quite often a combination of the two discrepancies causes a small, true LLD to lead to a large, functional LLD. Patients’ expectations after the surgery also must be understood; they should be made aware of the realistic possible outcomes after surgery, especially regarding LLD. Thus recognizing the functional discrepancy is important because this is what the patient perceives. This should be documented and the patient should be counseled that the goal of the surgery is to attempt to equalize the true length and not necessarily the functional length.

Certain patients are at risk for developing postoperative LLD: those with coxa vara, dysplastic hips, protrusio, flexion contractures, uncorrectable pelvic tilts, spinal deformities, neuromuscular disorders, hypermobile joints, shorter patients, and those who have a preexisting longer limb. Preoperative discrepancies can be assessed clinically by placing graduated blocks under the shorter leg until the patient is level and feels balanced. Clinical evaluation should include assessment for any rotational and angular deformities, scoliosis, pelvic obliquity, and joint mobility. These patients also should be counseled preoperatively about their increased risk and their potential need for postoperative shoe lifts; this should be adequately documented in their medical records. Finally, clinical discrepancies must be confirmed by radiographic evaluation.

PREOPERATIVE RADIOGRAPHIC EVALUATION

Preoperative radiographs are important in planning the type and size of the prosthesis and in deciding the position and the orientation of the components, with the aim of restoring the biomechanics of the hip in terms of the center of rotation, the offset, and the limb length. Various methods are used for radiographically evaluating a true LLD in primary THA; most involve measuring from an identifiable point on the pelvis to a fixed point on the femur.

At the Ranawat Orthopedic Center, New York, the preoperative evaluation includes five radiographs: a low anteroposterior (AP) view of the pelvis with the proximal one third of the femur, an AP view of the affected hip, a false-profile view of the hip, and AP and lateral views of the lumbosacral spine. Obtaining the AP view of the pelvis with the hips internally rotated approximately 10 to 15 degrees is important to calculate the offset accurately. The teardrop is the most consistent landmark on the AP view of the pelvis, and any LLD is measured off the inter-teardrop line to a fixed point on the lesser trochanter, usually the most medial point. Alternatively, a line drawn between the inferior aspect of both ischium or the superior margin of both greater trochanters also may be used as a reference. In addition, the distance from the top of the lesser trochanter to the center of the femoral head is measured, and every attempt is made to reproduce this length during surgery. The effect of magnification must be considered during these measurements. A scanogram may be needed if the LLD is attributable to extraarticular causes distally in the extremity (e.g., healed fractures in the distal femur or tibia).

The next step is to template the implants. In cases of unilateral hip disease, the goal is to match the opposite leg in terms of limb length and offset; thus the templating should be done on the normal side, provided that the socket would reproduce the normal hip center. The acetabulum is templated for the approximate component size and position and to determine its effect on the center of rotation. The acetabular template is placed just lateral to the lateral edge of the teardrop at a 45-degree angle. Ideally, the cup should be completely covered by bone and span the distance between the teardrop and the superolateral margin of the acetabulum ( Fig. 26-1, A ). In most cases this will restore the anatomic center of rotation. Templating of the femoral canal is done to determine the size of the component to be selected, its position relative to the lesser trochanter, the level of the neck osteotomy, and the selection of the head length to reproduce both leg length and femoral offset (see Fig. 26-1, B ). The component size that best accomplishes this is chosen. All the preoperative sizes that are templated should be noted and correlated with the operative findings.

Based on preoperative templating, Eggli et al planned the correct type of prosthesis in 98% of their cases; the agreement between the planned and actually used component was 92% for the femur and 90% for the acetabulum, the mean LLD was 0.2 ± 0.1 cm, and more than 80% of the intraoperative difficulties had been anticipated. Similarly, modifying the Muller method, Woolson et al calculated the amount of bone resection on the preoperative radiographs and applied this to correct the LLD during surgery. The average postoperative LLD was 1 mm (range, −20 to +22 mm) in a series of 351 patients with 408 THAs, with 306 (86%) patients having an LLD of 6 mm or less.

INTRAOPERATIVE MEASUREMENT OF LEG-LENGTH DISCREPANCY

Because determining limb length intraoperatively relies on identifying anatomic landmarks, patient positioning and its assessment before prepping and draping is crucial. The relative position of the knees and feet with symmetrical flexion of the hips and knees can provide an idea about the starting leg length relation. The superior side on the lateral position usually would appear slightly shorter because of the adduction. On the basis of these landmarks, any LLD may be assessed intraoperatively, although its accuracy depends on the patient’s position, the limb’s position, the effect of the drapes, and accessibility to the knees and feet. Moreover, correct patient positioning is extremely important if external acetabular guides are used.

Several intraoperative methods have been described to minimize LLD after THA. A variety of measuring calipers have been described in which one end articulates with a pin, screw, or spike anchored into the pelvis, and a stylus references off a mark on the greater trochanter. The accuracy of all these methods may be affected by the inherent variability of the leg’s position when the measurements are made. Pin bending and dislodgement of a pin are frequent problems.

AUTHORS’ PREFERRED METHOD

The authors prefer the method described by Ranawat et al. This technique has been described using the posterior approach. After initial dissection and release of the short external rotators, the inferior capsule is incised at the 6 o’clock position to expose the posteroinferior lip of the acetabulum. A <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='9/16′>9/16
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-inch Steinmann pin is inserted into the posterior infracotyloid groove ( Fig. 26-2 ). This represents the bony groove inferior to the posteroinferior lip of the acetabulum. The advantage of using this landmark is the close proximity of the pin to the center of rotation of the hip. The pin initially is placed at an angle of approximately 60 degrees until it touches the ischium; it is then made vertical and allowed to slide along the bone into the infracotyloid groove. Keeping the pin vertical and viewing it end-on from above, a mark on the greater trochanter is made before hip dislocation (see Fig. 26-2, B ). The hip is then dislocated. The center of the femoral head is marked with electrocautery, and the distance between the center and the lesser trochanter is noted and compared with the calculation made during preoperative planning ( Fig. 26-3, A ). The neck resection is now completed based on preoperative templating.