Methods for assessing leg length discrepancy

Abstract

The use of accurate and reliable clinical and imaging modalities for quantifying leg-length discrepancy (LLD) is vital for planning appropriate treatment. While there are several methods for assessing LLD, we questioned how these compared. We therefore evaluated the reliability and accuracy of the different methods and explored the advantages and limitations of each method. Based on a systematic literature search, we identified 42 articles dealing with various assessment tools for measuring LLD. Clinical methods such as use of a tape measure and standing blocks were noted as useful screening tools, but not as accurate as imaging modalities. While several studies noted that the scanogram provided reliable measurements with minimal magnification, a full-length standing AP computed radiograph (teleoroentgenogram) is a more comprehensive assessment technique, with similar costs at less radiation exposure. We recommend use of a CT scanogram, especially the lateral scout view in patients with flexion deformities at the knee. Newer modalities such as MRI are promising but need further investigation before being routinely employed for assessment of LLD.

Level of evidence: Level IV, diagnostic study. See the Guidelines for Authors for a complete description of levels of evidence.

Fig. 1

A flow diagram outlines the search criteria and methodology employed that lead to the 42 pertinent articles on methods for assessing leg length discrepancy.

Fig. 2

A “direct” measurement using a tape measure can be utilized to measure the “true” leg length from the anterior superior iliac spine (ASIS) to the medial malleolus. The “apparent” leg length is measured from the umbilicus to the medial malleolus. Reprinted with permission from Morrissy RT, Weinstein SL, eds. Lovell and Winter’s Pediatric Orthopedics. Philadelphia: Lippincott Williams & Wilkins; 2006 [29].

Fig. 3

Placing blocks of known height beneath the heel of the short leg to level the pelvis allows “indirect” measurement of leg length discrepancy. This method is slightly more reliable and accurate than use of the tape measure. Reprinted with permission from Morrissy RT, Weinstein SL, eds. Lovell and Winter’s Pediatric Orthopedics. Philadelphia: Lippincott Williams & Wilkins; 2006 [29].

Fig. 4A–C

(A) An orthoroentgenogram utilizes three radiographic exposures centered over the hip, knee and ankle joints in order to minimize magnification error. A single large cassette is placed under the patient who remains laying still between the three exposures. Reprinted with permission from Morrissy RT, Weinstein SL, eds. Lovell and Winter’s Pediatric Orthopedics. Philadelphia: Lippincott Williams & Wilkins; 2006 [29]. (B) The scanogram technique also utilizes three radiographic exposures, one each centered over the hip, knee and ankle joint in order to minimize magnification error. The patient remains supine next to a calibrated ruler and unlike the orthoroentgenogram, the standard length radiographic cassette is moved for the three exposures. Reprinted with permission from Morrissy RT, Weinstein SL, eds. Lovell and Winter’s Pediatric Orthopedics. Philadelphia: Lippincott Williams & Wilkins; 2006 [29]. (C) A teleoroentgenogram consists of a single long cassette placed behind the patient, while the xray beam is centered over the knee joint. It is preferable to do this study with the patient standing. While this technique is subject to magnification, less radiation exposure and opportunity to comprehensively assess the entire extremity for underlying etiology and deformity analysis makes this imaging tool an attractive option for detailed assessment of leg length discrepancy. Reprinted with permission from Morrissy RT, Weinstein SL, eds. Lovell and Winter’s Pediatric Orthopedics. Philadelphia: Lippincott Williams & Wilkins; 2006 [29].

Fig. 5A–B

(A) Standing AP radiograph of the lower extremity (modified teleoroentgenogram) performed using computed radiography on a young child with a congenital shortening of the tibia of approximately 4.5 cm. This radiograph is made with the child standing on a appropriate height lift under the short leg to level the pelvis. Besides assessing leg length discrepancy, along with length of the whole leg (W) as well as femur (F) and tibia (T), this imaging modality can be used to measure mechanical axis deviation (MAD) and joint orientation angles around the knee. (B) The modified scanogram of the same child as shown in A performed using computed radiography. Unlike a teleoroentgenogram, this imaging modality requires three radiographic exposures; one each centered over the hip, knee and ankle joints. Although a scanogram has less magnification error compared to a teleoroentgenogram, the scanogram is performed supine, is typically associated with greater radiation exposure, does not allow visualization of the entire length of the femur (F) and tibia (T) and fails to account for any shortening related to the foot. Reprinted with permission from the Journal of Bone and Joint Surgery, Inc., from Sabharwal S, Zhao C, McKeon JJ, McClemens E, Edgar M, Behrens F. Computed radiographic measurement of limb-length discrepancy. Full-length standing anteroposterior radiograph compared with scanogram. J Bone Joint Surg Am. 2006;88:2243–2251 [38].

Fig. 6

An AP CT scanogram of an adult patient following surgical treatment of fractures of the pelvis and right femoral shaft demonstrates a mild (2 mm) LLD in the femoral segment (courtesy of Dr Mark C. Reilly).

Fig. 7

A standing full-length computed radiograph (modified teleoroentgenogram) of a 14 year old patient following right sided tibial lengthening for a 6 cm LLD. Note the use of a midline ruler and magnification markers adjacent to the right hip, knee and ankle joints to decrease the magnification error in measuring the residual LLD in this child. Use of a small lift under the right leg to level the pelvis may also have been useful.