Last Updated on May 26, 2025
Leg length measurement is a basic part of orthopedic clinical examination in spine and lower limb pathologies, helping to identify discrepancies that may affect gait, posture, and long-term joint health.
Determining whether a difference in limb length is true (structural) or apparent (functional or compensated) is crucial for diagnosing and managing accurately.
Understanding the difference between true and apparent leg length helps clinicians pinpoint the source of limb inequality, recognize compensatory mechanisms, and plan appropriate interventions. In summary, the limb length measurement is used to find leg length discrepancy if any.
Clinical Importance of Leg Length Measurement
Accurate leg length measurement is essential in for diagnosing discrepancies that can affect gait, cause pain, and predispose to long-term joint and spinal problems. It is particularly important in patients with suspected limb shortening following trauma, childhood disease, surgery, or developmental disorders.
The goals of measuring leg length are [1]
- Identifying true versus apparent limb length discrepancies
- Assessing the need for intervention
- Deciding on surgical vs non-surgical management
- Monitoring progression over time
True and Apparent Leg Length [1]
True Leg Length
True leg length is the anatomical measurement of the limb from a fixed bony landmark on the pelvis—the anterior superior iliac spine (ASIS)—to the tip of the medial malleolus at the ankle.
This measurement is only meaningful when both lower limbs are placed in identical positions and the pelvis is squared (aligned without tilt).
Why use ASIS?
While the ideal measurement is from the center of the femoral head, this point is not easily palpable in clinical practice, so the ASIS is used as a practical, reliable reference.
The identical position of the lower limb and pelvis squaring is very important because ASIS, though a substitute point for the center of the femoral head, is outside the limb, and any change in the position can affect the length.
Just to emphasize further, any change in limb position does not affect the limb length, had the upper reference point been the head of the femur, and that would allow us to measure it in any position of the limb.
However, from ASIS, the limb length measurement will differ if the limb is lying straight or in abduction, the latter being shorter as the foot moves closer to ASIS while ASIS stays there. Similarly, pelvis squaring is required because pelvis tilt can itself change the position of ASIS, leading to an error in measurement.
Key point: True leg length reflects the actual length of the bones (femur and tibia) and is not influenced by pelvic tilt, joint contractures, or compensatory posture.
Apparent Leg Length
Apparent leg length is measured from a central body landmark (such as the umbilicus or xiphisternum) to the tip of the medial malleolus. This measurement is taken with the patient lying in a natural, resting position, without squaring the pelvis or aligning the limbs identically.
Clinical significance: Apparent shortening may “mask” or “magnify” the true bony difference if pelvic or spinal compensation is present. Apparent length is measured in a similar way with the following differences
Key point: Apparent leg length includes not only true bony shortening but also the way the body masks the shortening. That could be pelvic obliquity, joint contractures, or soft tissue abnormalities.
Why Measure Both True and Apparent Length
Comparing true and apparent leg length helps distinguish structural (bony) discrepancies from those due to pelvic or soft tissue factors. This distinction is crucial for accurate diagnosis and effective management.
For finding the apparent and true shortening, both the limbs are measured separately and the difference of the measurements is calculated.
Apparent shortening means the difference between the apparent leg lengths of two limbs.
True shortening means the difference between two true lengths.
Compensation Mechanisms in Limb Shortening
When there is a limb length discrepancy, the body often adapts by developing compensatory mechanisms to maintain balance during standing and walking. These compensations can influence clinical findings and affect the apparent length of the limb.
Common compensation mechanisms include: [2]
- Pelvic Tilt: The pelvis tilts downward on the side of the shortened limb to help bring the foot closer to the ground.
- Equinus Foot: The shorter limb may adopt a toe-down (equinus) posture, allowing the toes to reach the floor.
- Flexion of the Longer Limb: The longer limb may flex at the hip and knee to help equalize limb lengths during gait.
- Spinal Curvature (Compensatory Scoliosis): The lumbar spine may develop a curve to help the limb gain length. The curve generally has convexity towards the shorter limb.
These adaptations can mask the extent of the true discrepancy.
Techniques for Leg Length Measurement
The two primary methods are the standing block method and the supine tape method.
Block Method
It is the best for ambulatory patients who can stand and helps assess true limb length discrepancy under weight-bearing conditions.
Steps:
- Ask the patient to stand upright, barefoot, with feet parallel.
- Observe for compensatory mechanisms (pelvic tilt, equinus foot, flexion of contralateral limb). Usually, the patient compensates for shortening by abducting the leg.
- Correct any compensation by positioning the trunk erect
- Place flat blocks of known thickness beneath the shorter limb’s foot until both ASIS (anterior superior iliac spines) are at the same horizontal level.
Correction of the deformity is suggested by the iliac spines being at the same level.
A schematic representation is given in the following diagram.
Supine Position Method
This method compares apparent and true limb lengths, helps discover deformities, and provides better control over deformity correction for the measurements. [3]
Steps:
- Position the patient supine on a firm surface. Firm is important as compensation may be concealed on other surfaces.
- Square the pelvis so both ASIS are at the same level (if possible**).
- Place both lower limbs in equivalent positions—if not possible due to fixed deformity, adjust the contralateral limb to match.
- Measure the distance from the ASIS to the tip of the medial malleolus on each side using a non-stretch tape.
- Record and compare measurements to determine any discrepancies.
- For better results, the landmarks should be marked before the measurement.
** Some fixed deformities will not allow this. See below the challenge of fixed hip deformities section.
Challenge of Squaring Pelvis in Fixed Hip Deformities
One of the challenges of measuring limb length in the supine position is the inability to have a square pelvis in fixed hip deformities.
When the hip is fixed in abduction, the pelvis is tilted towards the affected side to align the leg. This makes the normal leg appear shortened and the affected leg lengthened.
The opposite occurs with an adduction deformity.
How to check for deformity? [4]
- If both anterosuperior iliac spines or ASIS are at the same level and the pelvis is square, there is no adduction or abduction deformity
- If the ASIS on the affected side is higher, a fixed adduction deformity is present.
- If the ASIS on the affected side is lower, a fixed abduction deformity is present.
Now, if there is a fixed deformity, either adduction or abduction, you cannot square the pelvis by moving the affected limb. Therefore, the two limbs must be placed in comparable positions relative to the pelvis.
[This is necessary as our upper reference point is external. If we could measure somehow from the head of the femur, we would not need the squaring. The following diagram would explain this. This point has been discussed above.]
Thus, if one limb is adducted and cannot be brought out to the neutral position, the other limb must be adducted through a corresponding angle by crossing it over the first limb before the measurements are taken.
This maneuver makes both limbs in comparable positions, and the values arrived at would remove any fallacy.
Therefore, in adduction deformity, adduct the other limb through the same degree, and in abduction deformity, abduct the other limb through the same degree.
For flexion and extension deformities, the same level of flexion or extension is desirable. (Most of the cases have flexion deformities, so the sound limb is placed in similar flexion.
Interpretation of Findings
Comparing true and apparent leg length measurements provides insight into the nature and cause of limb length discrepancy.
Key Points for Interpretation:
- True shortening is the difference in bony length between the two limbs, measured from ASIS to medial malleolus with the pelvis squared and limbs in identical positions.
- Apparent shortening includes the effects of pelvic tilt, joint contractures, or compensatory mechanisms, measured with the pelvis and limbs in their natural positions.
Clinical Scenarios
- True shortening is equal to apparent shortening:
- There is no compensation
- The discrepancy is entirely due to a difference in bony length.
- True shortening is greater than apparent shortening:
Part of the shortening is compensated for by pelvic tilt, equinus foot, or flexion of the opposite limb. - True shortening is less than apparent shortening:
Indicates the presence of a fixed adduction deformity, which makes the apparent shortening seem greater than the true (bony) shortening.
Localizing the Discrepancy: Femoral or Tibial?
After confirming a leg length discrepancy, the next step is to determine whether the difference arises from the femur, tibia, or both. The following tests help localize the site of shortening.
Segmental Measurements
- Thigh Length: Measure from the anterior superior iliac spine (ASIS) to the knee joint line (usually the medial joint line).
- Leg (Tibia) Length: Measure from the knee joint line to the tip of the medial malleolus.
By comparing these segmental lengths side to side, the shortening can be located to one or both bones.
Galleazzi’s (Allis’) Test
It is a gross test to find out if the shortening is in the femur or tibia.
The patient is supine with the hips flexed to 45º and the knees flexed up to 90º. Place the malleoli together(the test is inaccurate if you are unable to do so).
Assess the position of the knees.
- When one knee projects farther forward, the problem lies with the femur
- When one knee is higher than the other, the tibia is the culprit
Where in the Femur?
Tibial discrepancies are fairly straightforward to investigate.
However, the same is not true for the femur because of its deep-seated anatomy in the hip region.
If femoral shortening is detected, further tests help localize whether it is supratrochanteric (proximal to the greater trochanter) or infratrochanteric (below the trochanter). [4]
Shortening above the trochanter may be caused by the destruction of the femoral head or acetabulum or both, a dislocated hip, coxa vara deformity of a malunited intertrochanteric fracture.
Bryant’s triangle
A perpendicular line is dropped from the anterior superior iliac spine onto the bed in a square pelvis.
From the tip of the greater trochanter, another perpendicular line is dropped onto the first line.
The length between the greater trochanter and ASIS is measured. Relative shortening on one side indicates that the femur is displaced upwards. Bryant triangle is not helpful in bilateral pathologies..
Nelatons line
Nelaton’s line connects the anterior superior iliac spine and the ischial tuberosity. In cases of supra-trochanteric shortening, the trochanter will be proximal to this line.
in normal conditions, the greater trochanter tip is below this line.
Shoemaker’s line
- In supine position, a line joining ASIS and tip of the greater trochanter is extended on the abdomen on both sides.
- Normally, these lines meet in the midline above the umbilicus.
- Proximal migration of the greater trochanter, the lines will meet on the opposite side of the abdomen, and below the umbilicus.
- Bilateral condition – the lines will meet at or near the midline but below the umbilicus.
Chiene’s line or parallelogram
In supine position, in a normal hip, the line connecting both ASIS and the line connecting both greater trochanters are parallel.
The lines would converge on the side of the proximal migration of the trochanter, indicating supratrochanteric shortening.
Limitations in Leg Length Measurement
Several factors can affect the accuracy and interpretation of leg length measurements. Being aware of these pitfalls helps clinicians avoid common errors.
- Incorrect Patient Positioning: Failing to square the pelvis or place limbs in identical positions can lead to false readings.
- Inaccurate Identification of Landmarks: Difficulty locating the ASIS or medial malleolus, especially in obese or muscular patients, can result in measurement errors.
- Unrecognized Contractures or Deformities: Fixed adduction or abduction deformities may mask or exaggerate the true discrepancy if not accounted for during measurement.
- Apparent vs True Discrepancy Confusion: Not distinguishing between structural shortening and functional/apparent shortening (due to pelvic tilt or compensation) can mislead diagnosis and management.
- Bilateral Abnormalities: In cases where both limbs are affected, comparison may be less reliable.
- Indirect Measurement: Clinical (surface) measurements are a proxy for true bony length and may be affected by soft tissue thickness, muscle bulk, or scarring.
Imaging-Based Measurement of Leg Length
In addition to the limitations listed above, clinical methods are less accurate for smaller discrepancies (<5 mm).
Imaging, in contrast, can provide more accurate and objective measurements, can detect clinically non-evident shortening, and is essential for surgical planning. [3]
Imaging is used when
- Clinical measurements are inconclusive
- Precise quantification is needed for surgical planning
- Assessing complex deformities or rotational issues.
The following modalities are commonly used
Radiographic Scanogram:
It utilizes standard X-rays to measure limb lengths and is considered the gold standard with minimal magnification errors. However, it requires proper patient positioning and may not account for rotational deformities.
It is the first natural choice for most patients.
CT Scanogram:
This employs computed tomography (CT), and results are almost comparable to radiographic scanogram, though some studies have reported them to be slightly less precise.
It also carries a higher radiation exposure.
Its advantage lies in providing cross-sectional images, useful in complex cases or when rotational assessment is needed, and can be considered in cases where detailed bone assessment is required.
MRI Scanogram:
It uses magnetic resonance imaging and is slightly less accurate than radiographic methods. It has no radiation exposure but is costly. But it is highly suited where radiation exposure is a concern, like children or pregnant women. [5]
EOS Imaging System
It is a low-dose, biplanar X-ray system that provides 3D reconstructions and carries simultaneous evaluation of spine and lower limbs, beneficial in scoliosis or complex deformities
The radiation exposure is lower than traditional X-rays.
References
- Martin HD, Palmer IJ. History and physical examination of the hip: the basics. Curr Rev Musculoskelet Med. 2013 Sep;6(3):219-25. [PubMed]
- Cummings G, Scholz JP, Barnes K. The effect of imposed leg length difference on pelvic bone symmetry. Spine (Phila Pa 1976). 1993. [Pubmed]
- Sabharwal S, Kumar A. Methods for assessing leg length discrepancy. Clin Orthop Relat Res. 2008 Dec;466(12):2910-22. [PubMed]ClinicalGate. (2015, September 4). The hip region. Retrieved May 26, 2025, from https://clinicalgate.com/the-hip-region/
- Leitzes AH, Potter HG, Amaral T, Marx RG, Lyman S, Widmann RF. Reliability and accuracy of MRI scanogram in the evaluation of limb length discrepancy. J Pediatr Orthop. 2005 Nov-Dec;25(6):747-9. [PubMed]
