Leg Length Inequality: Prevention/Treatment

The “Direct Method”: Using a Tape Measure

A tape measure can be used to assess both true and apparent leg lengths. In the estimation of true leg lengths, the tape measures the distance from the ASIS to the medial malleolus; this reflects the actual length of the extremity. The range of error of this measurement has been reported to be 0.5 to 1.0 cm. This is also known as the direct method of measuring leg length ( Fig. 109.1 ). However, difficulty in identifying bony prominences, particularly in obese patients, and angular deformities in the lower limb can contribute to error when this clinical measurement tool is used.

Apparent leg lengths are measured from the umbilicus to each medial malleolus. When a patient has apparent leg length inequality, despite true leg lengths being equal, the patient feels that one limb is longer, and the measurement from umbilicus to medial malleolus on that side is longer. Apparent leg length measurement takes into account contractures around the hip and lumbar spine pathology, both of which can lead to pelvic obliquity. An abduction contracture of the hip, which is common in the early postoperative period after hip arthroplasty, results in an apparent leg length discrepancy on the involved side, but an adduction contracture of the hip has the opposite effect ( Fig. 109.2 ).

Eichler described several potential sources of error when measurements are obtained using a tape measure, including poor reproducibility of finding the ASIS and abduction or adduction contractures of the hip. Beattie and colleagues reported on the reliability of tape measure method (TMM) measurements. They found low reliability (interclass correlation coefficient [ICC] of 0.668; ICC is a descriptive statistic that describes how strongly units in the same group resemble each other) when measurements were obtained by different examiners (interobserver variability). However, when the mean value of paired measurements taken by the same observer was used, intraobserver variability was much lower (ICC, 0.910). Investigators concluded that the reliability of tape measure measurement improved when the mean value of repeated measurements by the same observer were compared with measurements by other observers.

The Indirect Method: the “Block Test”

The use of blocks placed underneath the patient's foot is known as the indirect clinical method of assessing leg length inequality. It is helpful in differentiating fixed from flexible pelvis obliquities. Causes of fixed pelvic obliquities include lumbosacral fusion and severe lumbar degenerative scoliosis as well as previous pelvic fracture. The surgeon can inspect and palpate the spinal curvature and pelvic tilt; then, the pelvis should be leveled by placing a series of blocks under the short leg until the spinal curvature and pelvic tilt disappear ( Fig. 109.3 ). If the pelvic tilt corrects, then the pelvic obliquity is flexible; if not, the patient has a fixed pelvic obliquity and will be unable to compensate for any change in leg lengths. Regardless of whether the patient has a fixed or flexible pelvic obliquity, the height of the block that the patient identifies as making the patient feel equal indicates the amount of length discrepancy that the surgeon may consider correcting.

Hanada and colleagues studied the reliability and validity of measuring leg length inequality using the iliac crest palpation and block correction method in 34 healthy volunteers with simulated leg length inequality; they compared clinical observations with those obtained using standing anteroposterior (AP) pelvis radiographs. Iliac crest palpation underestimated the leg length discrepancy by a mean of 4 mm. Similar findings have been reported by other authors.