Telling True From Apparent Leg Length Discrepancy
Unequal leg length can cause a limp, back pain and a compensatory spinal curve, and a shoe lift for an apparent discrepancy from a hip contracture leaves the real problem untreated.
Key terminology
- Iliac crest
- This curved upper rim of the hip bone is easy to feel under the skin. The block test levels the two crests to read the discrepancy.
- Pelvic obliquity
- This is a sideways tilt of the pelvis so one side sits higher than the other. It makes legs of equal bone length look unequal.
- Anterior superior iliac spine (ASIS)
- This bony point is the front end of the iliac crest. It is the upper landmark for measuring true leg length with a tape.
- Medial malleolus
- This bump on the inner ankle is the lower end of the tibia. It is the lower landmark for both tape measurements.
- Adduction contracture
- The hip is stuck drawn toward the midline and cannot reach neutral. The pelvis tilts up on that side to keep the legs parallel.
- Full-length standing AP radiograph
- This single film shows both legs from hip to ankle with the patient standing. It measures length and the mechanical axis together.
- Scanogram
- This study takes separate exposures centered over the hip, knee and ankle beside a ruler. It limits magnification when measuring bone length.
Takeaways
- True LLD is a real difference in bone length, and apparent LLD comes from a tilted pelvis. True (structural) LLD means the femur or tibia is physically shorter or longer. Apparent (functional) LLD has normal bones and comes from pelvic obliquity due to joint or muscle contracture, soft-tissue tightness, ligamentous laxity or axial malalignment. One iliac crest sits higher and the legs are pulled into different apparent lengths (Applebaum et al, Clin Orthop Surg 2021;13:127-134, PMID 34094002).
- Measure true length from the anterior superior iliac spine (ASIS) and apparent length from the umbilicus. Both readings end at the medial malleolus. The ASIS is part of the pelvis, so that reading tracks bone length. The umbilicus sits on the trunk, so a pelvic tilt shows up in that reading. Put both legs in the same position first, since hip flexion or abduction shortens the ASIS reading. (Applebaum et al, Clin Orthop Surg 2021;13:127-134, PMID 34094002; Anonymous, The Hospital 1910, PMID 29822036).
- A hip adduction contracture makes the leg look short, and an abduction contracture makes it look long. To keep a fixed adducted leg parallel to the other, the pelvis tilts up on that side and the leg looks shorter. A fixed abducted leg drops the pelvis on that side and the leg looks longer. The tilt can bring a compensatory lumbar curve (functional scoliosis) (Anonymous, The Hospital 1910, PMID 29822036; Applebaum et al, Clin Orthop Surg 2021;13:127-134, PMID 34094002).
- Apparent LLD is treated by correcting the posture, and true LLD by changing bone length. The postural part resolves when the contracture or pelvic tilt is corrected, so physical therapy is used for the functional component. Only the true part needs a shoe lift or a length procedure. A lift for a purely apparent LLD leaves the hip contracture untreated (Anonymous, The Hospital 1910, PMID 29822036; Applebaum et al, Clin Orthop Surg 2021;13:127-134, PMID 34094002).
- A full-length standing AP radiograph is the first imaging study for LLD. In 111 patients it matched the scanogram closely (r = 0.96, mean difference 0.5 cm) and also shows the mechanical axis, at lower cost and radiation. It magnifies limb length by about 4.6%. A CT scanogram with a lateral scout helps when a knee flexion deformity distorts the AP film (Sabharwal et al, J Bone Joint Surg Am 2006;88:2243-2251, PMID 17015603; Sabharwal and Kumar, Clin Orthop Relat Res 2008;466:2910-2922, PMID 18836788).
- Nearly everyone has a small LLD, and it is rarely significant below about 2 cm. On accurate radiographs, 90% of people had an anatomic LLD with a mean of 5.2 mm. Most do not appear clinically significant until about 20 mm. Smaller differences are often an asymptomatic normal variant, and the cutoff is debated (Knutson, Chiropr Osteopat 2005;13:11, PMID 16026625; Applebaum et al, Clin Orthop Surg 2021;13:127-134, PMID 34094002; Gurney, Gait Posture 2002;15:195-206, PMID 11869914).
- A single tape reading is unreliable, so average repeated measurements. Against radiographs, agreement (ICC) rose from 0.68 for one tape reading to 0.79 for the mean of 2, in a small study of 19 subjects. Tape was weakest in healthy subjects (ICC 0.36), whose differences were small. Tape precision is about 5 mm at best (Beattie et al, Phys Ther 1990;70:150-157, PMID 2304973; Applebaum et al, Clin Orthop Surg 2021;13:127-134, PMID 34094002).
- The standing block test is the usual bedside alternative to the tape, and reviews disagree on which is better. Blocks go under the short leg until both iliac crests are level, and their total height is the LLD. One systematic review rated the block test most useful, and a review in hip arthroplasty found the tape valid and the blocks not. (Alfuth et al, PLoS One 2021;16:e0261457, PMID 34928991; Pettit et al, J Arthroplasty 2022;37:2507-2516, PMID 35605764).