High-yield overview

A measured difference in limb length is a sign, not a diagnosis — the job is to say whether it is congenital, acquired shortening, or overgrowth, and whether the growth plate is still working

Idiopathic / hemihypertrophy spectrumCommonest cause of a small painless discrepancy in an otherwise well child

Physeal bar after fracture or infectionThe must-not-miss — progressive, deforming, and treatable early

True vs apparentThe discriminator: block test and ASIS-to-medial-malleolus versus umbilicus-to-malleolus

Standing full-length AP (EOS or scanogram) plus bone ageThe next test in almost every case

FOUR MECHANISMS — WHICH LIMB IS ABNORMAL, AND IS THE PHYSIS STILL WORKING?

APPARENT — the bones are equal

PatternTrue lengths equal, apparent lengths unequal. Pelvic obliquity from a fixed hip adduction or abduction contracture, or a structural spinal curve.

TreatmentTreat the contracture or the curve. This never needs a scanogram and never needs an osteotomy - and it is commoner than true shortening.

CONGENITAL SHORTENING — proportional, from birth

PatternPresent since birth, grows as a CONSTANT PERCENTAGE of the normal limb. Fibular hemimelia, PFFD, tibial hemimelia, congenital short femur, DDH.

TreatmentPredict with the multiplier method from a single measurement. Look at the foot and the rays, not just the leg length.

ACQUIRED SHORTENING — the physis has stopped

PatternOnset dated to a fracture, neonatal septic arthritis, tumour treatment or irradiation. The discrepancy ACCELERATES in absolute terms and often angulates.

TreatmentImage the physis for a bar (MRI or CT map). Under about half the physeal area with 2 or more years of growth remaining may be resected; otherwise complete the arrest and equalise.

OVERGROWTH — the LONG limb is the abnormal one

PatternHemihypertrophy, vascular malformation (Klippel-Trenaunay), JIA hyperaemia, post-fracture femoral overgrowth. Compare each limb against the child's own height percentile to see which is the outlier.

TreatmentIn hemihypertrophy the priority is NOT orthopaedic: abdominal surveillance for Wilms tumour and hepatoblastoma, and referral to a clinical geneticist to stratify risk.

Critical Must-Knows

Apparent discrepancy from pelvic obliquity, adduction/abduction contracture or scoliosis is far commoner than true bone shortening — exclude it before imaging.
Discrepancy is a moving target: what matters is not today's number but the projected discrepancy at skeletal maturity, which needs serial measurement plus bone age.
Hemihypertrophy (true overgrowth) mandates abdominal surveillance for Wilms tumour and hepatoblastoma — this is the systemic must-not-miss.
Any unilateral shortening with a painful, warm or stiff joint is septic arthritis sequelae or Perthes until proven otherwise.
A discrepancy of less than 2 cm at maturity is generally not treated; 2-5 cm is epiphysiodesis territory; greater than 5 cm needs lengthening or prosthetic fitting.
Look at the whole limb: femoral, tibial and foot height contributions must be separated because the treatment site differs.

Clinical Pearls

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Ask for the block test first — it converts a vague complaint into a number and separates true from apparent.
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Congenital causes shorten proportionally (a constant percentage); acquired growth arrest produces an accelerating absolute difference.
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Femoral shortening with a bulbous, laterally-facing hip and limited abduction is proximal focal femoral deficiency, not DDH.
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In fibular hemimelia the leg is short AND the foot has fewer rays with an equinovalgus ankle — check the foot, not just the leg.
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Neuromuscular limbs are short because they are unloaded — the shortening is diffuse, thin and osteopenic, not focal.

Recognising the Pattern

Definition. Limb length discrepancy is a difference in the length of the lower limbs, measured from a fixed pelvic landmark to the ground, expressed in centimetres and attributed to a segment: femur, tibia or foot. A measured difference is a sign rather than a diagnosis.

The bedside measurements. They answer two questions: whether the difference is real, and which segment carries it.

Block test — standing with the knees straight, blocks of known thickness go under the short side until the iliac crests are level, and the block height is the functional discrepancy. It includes foot height and is the single most reproducible bedside measurement.
True length — ASIS to the ipsilateral medial malleolus, with the pelvis square and both limbs in identical positions, compared side to side.
Apparent length — xiphisternum or umbilicus to the medial malleolus. Unequal apparent lengths with equal true lengths mean pelvic obliquity from a contracture or from a spinal curve.
Galeazzi test — supine, hips and knees flexed, feet flat. A lower knee indicates tibial shortening; a knee lying more posterior indicates femoral shortening.

The film. Radiographic confirmation is a standing full-length AP of both limbs, by EOS or a calibrated scanogram, with a ruler in the field, the patient weight-bearing, the patellae facing forward and a block under the short side to level the pelvis. Position and calibration are part of the measurement, because an externally rotated femur projects shorter and an uncalibrated film exaggerates. Ask for the ruler before believing the number.

Standing full-length AP radiograph of both lower limbs with software alignment overlays and a total length reported for each leg. — The film the topic is built on: a standing full-length AP of both limbs with the hip centres marked, the knee and ankle joint lines drawn, mechanical axis lines running to the ankles, and a radiopaque calibration sphere between the thighs. The software prints a total length for each leg (right 47.56 cm, left 48.37 cm) beside the proximal and distal joint angles, so length and alignment are read off one image.Credit: VanAken TD et al., Cureus 2024 (CC BY 4.0).

Knee-centred AP radiograph with the femoral and tibial mechanical axis lines and the LDFA, MPTA and JLCA constructions labelled. — The angular measurements the long film carries, drawn at the knee: the femoral and tibial mechanical axes (fMA, tMA), the distal femoral and proximal tibial joint lines, and arcs labelled LDFA, MPTA and JLCA (joint-line convergence). These separate femoral from tibial shortening, and angular deformity from true loss of length.Credit: Fontalis A et al., Journal of Bone and Joint Surgery (Am) 2024 (CC BY 4.0).

Localising the shortening. Femoral and tibial segments are measured separately against the ruler, and the angles on the same film say whether the problem is length or alignment. On a supine AP pelvis the articulotrochanteric distance, compared side to side, separates a proximal femoral growth disturbance from shortening of the whole limb.

Supine AP pelvic radiograph with the articulotrochanteric distance measured on each side. — The articulotrochanteric distance drawn on both hips of a supine AP pelvis: a long line down each femoral shaft crossed by two transverse lines, one at the level of the top of the femoral head and one at trochanteric level. A single screw projects over one femoral head-neck junction and a shield covers the midline, neither of which prevents the side-to-side comparison.Credit: Kim S et al., Scientific Reports 2022 (CC BY 4.0).

Mimics and false positives. Each of these gives a difference that is not bone length.

Pelvic obliquity from a fixed adduction contracture (cerebral palsy, post-septic hip) — corrects only partially on the block test, and the true lengths are equal
Scoliosis with a lumbosacral hemivertebra — the pelvis is obliquely set within the spine
Knee or hip flexion contracture — shortens the limb functionally without bone shortening; check with the limbs extended
Genu valgum or varum — angular deformity shortens the limb geometrically while the bone lengths may be equal
Equinus contracture — a fixed equinus foot lengthens the functional limb and may mask a true shortening

AP radiograph of a young child's pelvis and lower limbs showing bowed femora and tibiae with flared metaphyses. — Angular deformity read on the same kind of film: an AP of a young child's pelvis and lower limbs, physes open at the knees and ankles, both femora and tibiae bowed outward and the metaphyses beside the knees and ankles widened and flared. Angular deformity of this kind shortens the limb geometrically while the bone lengths may be equal, which is why the film has to run from pelvis to feet and each segment be measured on its own.Credit: Robbins CA, Children 2021 (CC BY 4.0).

Saying it out loud in a viva. "This is a standing full-length AP radiograph of both lower limbs in a skeletally immature patient. There is a limb length discrepancy of approximately X centimetres, short on the [side]. Measuring segments, the femoral difference is X and the tibial difference is Y, so the shortening is predominantly [femoral/tibial]. The mechanical axis passes [medial/lateral] to the knee centre. I would want the clinical block test, a bone age film, and serial measurements before committing to a projection."

Traps That Lose Marks

Measuring apparent length and calling it true. An umbilicus-to-malleolus figure quoted as a true length carries the pelvic obliquity inside it. A fixed adduction contracture makes a normal limb look short, and an abduction contracture makes it look long.

Quoting today's number as the answer. A 2 cm discrepancy in a 4-year-old with a complete physeal arrest will become far greater, while the same 2 cm in a 14-year-old girl at Risser 4 will barely change. Pair every measurement with a bone age and the growth remaining.

Missing the tumour behind hemihypertrophy. Isolated hemihyperplasia and the Beckwith-Wiedemann spectrum carry an embryonal tumour risk, and the correct answer is three-monthly surveillance, not a shoe raise. The detail that scores is that the two tests run for different durations: abdominal ultrasound to age 7 for Wilms tumour and serum alpha-fetoprotein to age 4 for hepatoblastoma.

Forgetting the foot and the knee. Total limb length includes calcaneal and talar height, so a short limb with a normal scanogram of femur and tibia may be a hindfoot problem. The knee may also be held flexed by a contracture, which is why the segments are measured separately.

Next Investigation

What each test changes. Imaging follows the mechanism, and each test below earns its place by changing something: which bone is operated on, when it is operated on, or whether the priority is orthopaedic at all.

Choosing the Right Test

Key Point

Radiation matters: a full-length scanogram carries a real gonadal dose and these children are measured repeatedly for a decade. Use EOS where available, shield, and do not order films at intervals shorter than 6 months without a reason.

Standing radiograph of the pelvis and both femurs with a graduated ruler between the legs; both hips are replaced, one femur carries a long lateral plate and the left limb is longer. — Quantifying a discrepancy after surgery: a standing film of the pelvis and both femurs with a graduated ruler down the midline, both hips replaced and one femur carrying a long lateral plate down to just above the knee. The two knee joint lines sit at different levels against that ruler — a postoperative discrepancy of 3.5 cm, with lengthening on the left. Standing full-length films remain the standard for quantifying discrepancy after surgery.Credit: Patient safety in surgery (CC BY) via Open-i / PMC

Axial CT slices at hip, knee and ankle level with paired transverse measurements, beside a full-length AP radiograph and a list of the measured lines. — Measuring at joint level without magnification: axial CT through the femoral heads (A), the knees (B) and the ankles (C), each level measured on both sides (44 against 43.2 mm, 82.8 against 82.7 mm, 62.5 against 61 mm), with the corresponding full-length film (D) and the six measured lines listed again in centimetres (E). This is the cross-sectional equivalent of the standing film when the child cannot stand.Credit: VanAken TD et al., Cureus 2024 (CC BY 4.0).

Four-panel AP radiograph set of an adult with a hip replacement on one side, hip centres circled and long lines drawn from each hip centre down to the ankle. — Scanogram technique one: circles fitted to both hip centres on the AP pelvis with a line joining them (a), the two vertical lines carried down through the tibiae to the ankles (b), and a related construction on the full-length films (c, d). The inter-teardrop to tibial plafond distance is the reference being measured here, and it survives a prosthetic hip and a tilted pelvis better than a line taken from the femoral head.Credit: Le BTN et al., Skeletal Radiology 2025 (CC BY 4.0).

Full-length AP radiograph with hip-to-ankle lines, magnified at the hips and at the ankles, in a patient with a hip replacement on one side. — Scanogram technique two: the full-length film with a line down each limb from hip centre to ankle (a), magnified at the femoral heads (b) and at the ankle joint centres (c) so the endpoints can be checked. The acetabular apex to tibial plafond distance is the alternative reference the same study validates against the inter-teardrop method, so the report has to state which one was used.Credit: Le BTN et al., Skeletal Radiology 2025 (CC BY 4.0).

Four full-length AP radiographs labelled A to D across growth, two of them with a graduated ruler and with implants at the medial distal femora and distal tibiae. — The series that decides treatment: four full-length APs across growth (A to D), the middle two shot with a graduated ruler along the edge of the film and carrying small implants at the medial distal femur and the distal tibia on both sides. The knock-kneed alignment of A is less marked in B and C, and by D the implants have gone and the legs are substantially straighter. Films like these are what a multiplier projection is anchored to.Credit: Vandergaag ID et al., JBMR Plus 2026 (CC BY 4.0).

The Differential

Reading the differential by mechanism. Four mechanisms cover almost all of it, and each sends the workup somewhere different: the bones may be equal with the pelvis tilted, the short limb may have been short since birth, a physis may have stopped, or the long limb may be the abnormal one.

Apparent shortening. The true lengths are equal and the pelvis is not level, from a fixed hip adduction or abduction contracture or from a structural spinal curve. It is commoner than true bone shortening, and it needs neither a scanogram nor an osteotomy: the contracture or the curve is what gets treated.

Congenital shortening. Present since birth and growing as a constant percentage of the normal limb — fibular hemimelia, tibial hemimelia, proximal focal femoral deficiency, congenital short femur, DDH. Because the ratio holds, the mature discrepancy can be predicted from a single measurement by the multiplier method.

In fibular hemimelia the leg is short and the foot has fewer rays with an equinovalgus ankle, the tibia bows anteromedially, and the femur is often mildly short as well
Congenital short femur shortens the femur overall, with hypoplasia of the lateral condyle and a shallow intercondylar notch
The tibial hemimelia spectrum runs as far as femoral bifurcation with agenesis of the tibia and the patella

Acquired shortening. The onset dates to a fracture, neonatal septic arthritis, tumour treatment or irradiation, and once a physis has stopped the discrepancy accelerates in absolute terms and often angulates. The physis is imaged to size and locate a bar, and the size of that bar decides whether it can be resected or the arrest is completed. Any unilateral shortening with a painful, warm or stiff joint is septic arthritis sequelae or Perthes until proven otherwise.

Overgrowth. Here the long limb is the abnormal one — hemihypertrophy, vascular malformation such as Klippel-Trenaunay, JIA hyperaemia, post-fracture femoral overgrowth — so each limb is compared against the child's own height percentile to decide which is the outlier. In hemihypertrophy the priority is not orthopaedic: the abdominal surveillance is arranged first, and the child is referred to a clinical geneticist to stratify the risk.

The neuromuscular limb belongs with the short-side causes but has no culprit physis. It is short because it is unloaded, so the shortening is diffuse rather than focal.

Physeal bar after fracture (Salter-Harris III-V, distal femur, proximal tibia)

Typical age / setting: Any age, months to years after a documented injury
Discriminating feature: Progressive shortening WITH angular deformity converging toward the bar; Park-Harris growth arrest line is oblique and converges on the bar rather than parallel to the physis
What confirms it: MRI physeal mapping (fat-suppressed 3D) or CT to size and locate the bar as a percentage of physeal area

Post-septic physeal destruction (neonatal osteomyelitis / septic arthritis of hip or knee)

Typical age / setting: Sequel of neonatal or infant sepsis, presenting at ages 2-10
Discriminating feature: Severe shortening far out of proportion to the history, often with femoral head loss and a stiff painless hip; frequently a history of NICU line sepsis
What confirms it: Radiographs showing absent or deformed capital femoral epiphysis; MRI for residual physis

Hemihypertrophy (isolated or Beckwith-Wiedemann)

Typical age / setting: Recognised in infancy or early childhood
Discriminating feature: The LONG side is abnormal — asymmetry involves soft tissue, hemiface, tongue or trunk, not just the limb; asymmetry is proportional and non-progressive in ratio
What confirms it: Clinical asymmetry across body segments plus tumour surveillance ultrasound; genetic testing for 11p15 imprinting defect

Neoplasia adjacent to the physis (osteosarcoma, Ewing, or post-irradiation / post-chemotherapy arrest)

Typical age / setting: Ages 8-18 with pain, or a survivor of childhood cancer
Discriminating feature: Night pain, a metaphyseal lesion with periosteal reaction, or shortening dating precisely from radiotherapy or endoprosthesis
What confirms it: MRI of the whole bone; oncological history and radiotherapy field records

Idiopathic / constitutional discrepancy

Typical age / setting: Well child, any age, discrepancy typically less than 2 cm
Discriminating feature: Diagnosis of exclusion: no deformity, no dysmorphism, symmetric segments, ratio of shortening constant on serial measurement
What confirms it: Serial scanograms plus bone age showing a stable percentage difference; no imaging abnormality

Developmental dysplasia of the hip (dislocated / high-riding)

Typical age / setting: Infant to walking age; unilateral dislocation
Discriminating feature: Apparent rather than true shortening, with limited abduction, asymmetric thigh creases and a positive Galeazzi at the KNEE level
What confirms it: Ultrasound before 4-6 months; AP pelvis thereafter with broken Shenton line and proximal femoral migration

Perthes disease and other avascular necrosis of the capital epiphysis

Typical age / setting: Boys aged 4-8, painful limp
Discriminating feature: Modest shortening (usually less than 2 cm) with coxa magna, coxa breva and loss of hip internal rotation and abduction
What confirms it: AP and frog-lateral pelvis showing fragmentation and subchondral fracture; MRI if early

Cerebral palsy / hemiplegia and other neuromuscular disease

Typical age / setting: Spastic hemiplegia, walking age onwards
Discriminating feature: The affected limb is short, THIN and osteopenic overall, with equinus and reduced foot size — disuse affects the whole limb, not one physis
What confirms it: Neurological examination; radiographs show slender diaphyses and generalised osteopenia on the affected side

Fibular hemimelia (postaxial longitudinal deficiency)

Typical age / setting: Recognised at birth or in infancy
Discriminating feature: Short tibia with anteromedial bowing and a skin dimple, absent or short fibula, equinovalgus ball-and-socket ankle and missing lateral rays of the foot
What confirms it: AP and lateral tibia and foot radiographs counting rays and assessing the ankle mortise

Proximal focal femoral deficiency / congenital short femur

Typical age / setting: Birth
Discriminating feature: The whole shortening is femoral, the thigh is short bulbous and held in flexion, abduction and external rotation, and the knee often has cruciate deficiency
What confirms it: AP pelvis and femur assessing acetabulum and proximal femoral segment; Aitken classification; MRI in infants for cartilaginous continuity

Hyperaemic overgrowth after a diaphyseal femoral fracture

Typical age / setting: Ages 2-10, in the 1-2 years after a healed femoral shaft fracture
Discriminating feature: The previously FRACTURED side is longer, typically by up to about 1-2 cm, and the overgrowth plateaus by about 2 years post-injury
What confirms it: Serial scanograms showing overgrowth that stabilises; no physeal abnormality

Vascular malformation or arteriovenous fistula (Klippel-Trenaunay, Parkes-Weber)

Typical age / setting: Childhood, with a port-wine stain or varicosities
Discriminating feature: Overgrowth of the limb with a cutaneous vascular stain and lateral venous anomaly; warmth and bruit in high-flow lesions
What confirms it: Duplex ultrasound and contrast MR angiography of the limb

Juvenile idiopathic arthritis (mono/oligoarticular knee)

Typical age / setting: Girls aged 2-8 with a chronically swollen knee
Discriminating feature: The AFFECTED side is LONGER early (chronic synovitis hyperaemia accelerates the physis), then shorter if the physis closes prematurely; look for accompanying uveitis
What confirms it: Knee radiographs showing epiphyseal overgrowth and squared patella; MRI synovitis; ANA and slit lamp

Skeletal dysplasia or bone lesion (Ollier disease, multiple hereditary exostoses, fibrous dysplasia, neurofibromatosis type 1)

Typical age / setting: Childhood, often with multiple lesions
Discriminating feature: Discrepancy accompanied by a matrix-containing or expansile lesion abutting the physis, or anterolateral tibial bowing in NF1; often bilateral but asymmetric
What confirms it: Skeletal survey; characteristic lucent columns (Ollier), sessile exostoses, or ground-glass matrix

Cause	Typical age / setting	Discriminating feature	What confirms it
Physeal bar after fracture (Salter-Harris III-V, distal femur, proximal tibia)	Any age, months to years after a documented injury	Progressive shortening WITH angular deformity converging toward the bar; Park-Harris growth arrest line is oblique and converges on the bar rather than parallel to the physis	MRI physeal mapping (fat-suppressed 3D) or CT to size and locate the bar as a percentage of physeal area
Post-septic physeal destruction (neonatal osteomyelitis / septic arthritis of hip or knee)	Sequel of neonatal or infant sepsis, presenting at ages 2-10	Severe shortening far out of proportion to the history, often with femoral head loss and a stiff painless hip; frequently a history of NICU line sepsis	Radiographs showing absent or deformed capital femoral epiphysis; MRI for residual physis
Hemihypertrophy (isolated or Beckwith-Wiedemann)	Recognised in infancy or early childhood	The LONG side is abnormal — asymmetry involves soft tissue, hemiface, tongue or trunk, not just the limb; asymmetry is proportional and non-progressive in ratio	Clinical asymmetry across body segments plus tumour surveillance ultrasound; genetic testing for 11p15 imprinting defect
Neoplasia adjacent to the physis (osteosarcoma, Ewing, or post-irradiation / post-chemotherapy arrest)	Ages 8-18 with pain, or a survivor of childhood cancer	Night pain, a metaphyseal lesion with periosteal reaction, or shortening dating precisely from radiotherapy or endoprosthesis	MRI of the whole bone; oncological history and radiotherapy field records
Idiopathic / constitutional discrepancy	Well child, any age, discrepancy typically less than 2 cm	Diagnosis of exclusion: no deformity, no dysmorphism, symmetric segments, ratio of shortening constant on serial measurement	Serial scanograms plus bone age showing a stable percentage difference; no imaging abnormality
Developmental dysplasia of the hip (dislocated / high-riding)	Infant to walking age; unilateral dislocation	Apparent rather than true shortening, with limited abduction, asymmetric thigh creases and a positive Galeazzi at the KNEE level	Ultrasound before 4-6 months; AP pelvis thereafter with broken Shenton line and proximal femoral migration
Perthes disease and other avascular necrosis of the capital epiphysis	Boys aged 4-8, painful limp	Modest shortening (usually less than 2 cm) with coxa magna, coxa breva and loss of hip internal rotation and abduction	AP and frog-lateral pelvis showing fragmentation and subchondral fracture; MRI if early
Cerebral palsy / hemiplegia and other neuromuscular disease	Spastic hemiplegia, walking age onwards	The affected limb is short, THIN and osteopenic overall, with equinus and reduced foot size — disuse affects the whole limb, not one physis	Neurological examination; radiographs show slender diaphyses and generalised osteopenia on the affected side
Fibular hemimelia (postaxial longitudinal deficiency)	Recognised at birth or in infancy	Short tibia with anteromedial bowing and a skin dimple, absent or short fibula, equinovalgus ball-and-socket ankle and missing lateral rays of the foot	AP and lateral tibia and foot radiographs counting rays and assessing the ankle mortise
Proximal focal femoral deficiency / congenital short femur	Birth	The whole shortening is femoral, the thigh is short bulbous and held in flexion, abduction and external rotation, and the knee often has cruciate deficiency	AP pelvis and femur assessing acetabulum and proximal femoral segment; Aitken classification; MRI in infants for cartilaginous continuity
Hyperaemic overgrowth after a diaphyseal femoral fracture	Ages 2-10, in the 1-2 years after a healed femoral shaft fracture	The previously FRACTURED side is longer, typically by up to about 1-2 cm, and the overgrowth plateaus by about 2 years post-injury	Serial scanograms showing overgrowth that stabilises; no physeal abnormality
Vascular malformation or arteriovenous fistula (Klippel-Trenaunay, Parkes-Weber)	Childhood, with a port-wine stain or varicosities	Overgrowth of the limb with a cutaneous vascular stain and lateral venous anomaly; warmth and bruit in high-flow lesions	Duplex ultrasound and contrast MR angiography of the limb
Juvenile idiopathic arthritis (mono/oligoarticular knee)	Girls aged 2-8 with a chronically swollen knee	The AFFECTED side is LONGER early (chronic synovitis hyperaemia accelerates the physis), then shorter if the physis closes prematurely; look for accompanying uveitis	Knee radiographs showing epiphyseal overgrowth and squared patella; MRI synovitis; ANA and slit lamp
Skeletal dysplasia or bone lesion (Ollier disease, multiple hereditary exostoses, fibrous dysplasia, neurofibromatosis type 1)	Childhood, often with multiple lesions	Discrepancy accompanied by a matrix-containing or expansile lesion abutting the physis, or anterolateral tibial bowing in NF1; often bilateral but asymmetric	Skeletal survey; characteristic lucent columns (Ollier), sessile exostoses, or ground-glass matrix

Three radiographs of a child's distal femur: crossed wires within a cast, then AP and lateral views after removal, with arrows on the distal femoral cortex and physis. — Where an acquired arrest starts: a distal femoral physeal injury in a child, held with crossed wires inside a cast (a), then the AP (b) and lateral (c) after the wires are out. Red arrows mark the distal femoral cortex and white arrows in (b) point up at the physis itself — the structure whose behaviour over the next few years decides whether this child returns with a discrepancy.Credit: Jauniškytė E et al., Children 2026 (CC BY 4.0).

Coronal and sagittal MRI of a skeletally immature knee in which the central proximal tibial physis is interrupted by bone bridging it. — Physeal mapping in the child suspected of an arrest: coronal (left) and sagittal (right) MRI of an immature knee. The distal femoral physis runs as an intact dark undulating line, while the central part of the proximal tibial physis is interrupted with bone appearing to cross it — the bar as a focal bridge across the growth plate. Sizing it as a percentage of physeal area, and calling it central or peripheral, is what the sequence is for.Credit: Sarrami M et al., Journal of Orthopaedics 2025 (CC BY 4.0).

Five AP knee radiographs labelled a to e, each with a graduated radiopaque ruler between the legs and one distal femur coarse and expanded. — Serial films after a physeal injury: five AP knee views (a to e), each shot with a graduated radiopaque ruler between the legs, the distal femur on one side coarse and expanded throughout. The interval between the films, rather than any single film, is what shows how a discrepancy is behaving.Credit: Jauniškytė E et al., Children 2026 (CC BY 4.0).

AP radiograph of both lower legs labelled LEFT and RIGHT; the right tibia is short and bowed with no fibula visible and the foot deviated laterally. — Fibular hemimelia on one AP of both lower legs, labelled LEFT and RIGHT: on the right the tibia is shorter and curved, no fibula is visible beside it at any level including the ankle, and the foot is turned so that the forefoot points laterally. The two ankles sit at different heights, and that is where the lost length is read off.Credit: Shadi M et al., Journal of Pediatric Orthopaedics 2025 (CC BY 4.0).

AP radiographs of both lower limbs and of the right femur with an axis line drawn, beside a line diagram of the bowed femur. — Congenital short femur: (A) both limbs on one AP with an R marker, the right femur bowed and irregular through the shaft; (B) the right femur alone with a dotted axis line and a length readout in millimetres; (C) a line drawing of the same limb. Shortening of this kind holds its percentage as the child grows, which is what makes it predictable.Credit: Aldzhamal N et al., Case Reports in Orthopedics 2026 (CC BY 4.0).

Two infant radiographs: an AP pelvis with an arrow on a shallow acetabulum and an arrowhead on the rounded upper end of the short right femur, and an AP of both lower limbs marked R with an arrowhead and an asterisk on the malformed right femur. — Proximal focal femoral deficiency at eight months: (A) the AP pelvis, arrow on a shallow, sloping acetabular margin on the right and arrowhead on the rounded upper end of the short right femur lying lateral to it, the left hip better formed; (B) both limbs with the R marker, arrowhead on the short malformed proximal right femoral segment and an asterisk beside the right femoral shaft, the right knee lying higher than the left. The femur carries the shortening, and this bulbous, laterally facing hip is PFFD rather than DDH.Credit: Llanes AC et al., Diagnostics 2025 (CC BY 4.0).

Two radiographs of an infant lower limb; the distal femur is broad and forked and a single long bone lies in the lower leg. — The severe end of the tibial hemimelia spectrum in two panels: in (A) the distal femur is broad and forked into two prongs, and in both (A) and (B) a single long bone occupies the lower leg. No ossified patella is visible in either panel.Credit: Kim JB et al., Pediatric Reports 2025 (CC BY 4.0).

Supine AP radiograph of an infant's pelvis and lower limbs; the limb marked L has a diminutive femur, no identifiable tibia or fibula below it and a foot of three digital rays. — Read from a single supine film, foot included: on the side marked L the arrowhead sits on a small ossified femur in the proximal thigh, the arrow on the lower limb where no tibia or fibula can be identified, and the foot has three digital rays angled inferolaterally. The other limb has a normally formed femur, tibia and fibula.Credit: Llanes AC et al., Diagnostics 2025 (CC BY 4.0).

AP and lateral radiographs of a child's tibia labelled A1 and A2, with lines along the proximal and distal shaft marking the apex of the bow. — Congenital bowing of the tibia measured at presentation: the medial bow on the AP (A1) and the posterior bow on the lateral (A2), a line along the proximal shaft and another along the distal shaft crossing at the apex of the bow. The plane it bows in matters: anterolateral bowing warns of neurofibromatosis and pseudarthrosis, while posteromedial bowing remodels and leaves shortening behind.Credit: Kamath S et al., Journal of Orthopaedic Surgery and Research 2025 (CC BY 4.0).

Paired AP pelvic radiographs: an arthritic hip before surgery, and the same pelvis after total hip arthroplasty with leg-length measurement lines drawn. — The adult version of the same problem: (A) the preoperative AP pelvis of a 50-year-old woman with severe right hip pain and restricted movement due to osteoarthritis, with a limb-length inequality of 14.6 mm; (B) the same pelvis after total hip arthroplasty, a horizontal reference line drawn across the lower pelvis and a length measured down from it on each side, reading the same figure on both. Degenerative hip disease is a common acquired cause of apparent and true discrepancy.Credit: Journal of orthopaedic surgery and research (CC BY) via Open-i / PMC

Evidence

Abdominal tumour incidence in syndromic and isolated hemihyperplasia — the basis for surveillance

III

Dempsey-Robertson M, Wilkes D, Stall A, Bush P • Journal of Pediatric Orthopedics (2012)

Key Findings:

Isolated (idiopathic) hemihyperplasia carried a 1.2% abdominal tumour incidence (3 of 250 children: one adrenal carcinoma, two Wilms tumours) - real, but lower than historically quoted.
Syndromic hemihypertrophy carried a higher risk (1 of 10 with Beckwith-Wiedemann syndrome developed a hepatoblastoma, 10%) - so the syndromic child and the isolated overgrowth are not the same risk.
Retrospective review recommending geneticist referral to stratify which children benefit most from surveillance - supports surveillance while noting the isolated-overgrowth risk is lower than classic figures.

Clinical implication: Surveillance is warranted, but the risk is NOT uniform - syndromic overgrowth and isolated hemihyperplasia are different propositions, and this paper's own recommendation is referral to a clinical geneticist to stratify rather than blanket imaging of everyone. Where surveillance is undertaken the two tests run for different durations: three-monthly abdominal ultrasound to age 7 for Wilms tumour, and three-monthly serum alpha-fetoprotein to age 4 for hepatoblastoma.

Limitation: A RETROSPECTIVE TEN-YEAR REVIEW FROM ONE PAEDIATRIC HOSPITAL, AND THE SYNDROMIC ARM IS TEN CHILDREN - a single hepatoblastoma in 10 patients is reported as a '10 per cent incidence', which at that denominator carries a confidence interval running from roughly 0.3 to 45 per cent and should not be quoted as a rate at all. The isolated-hemihyperplasia figure is the usable one, and even 3 of 250 gives a confidence interval of about 0.3 to 3.5 per cent. Case ascertainment is by diagnostic coding at a referral orthopaedic hospital, so children whose hemihyperplasia was mild or who presented to a geneticist rather than an orthopaedic surgeon are missing, and follow-up duration is not stated - a tumour arising after the review window is simply not counted, which biases the incidence DOWN. Note also that the authors' own conclusion is not the blanket three-monthly protocol conventionally taught: they recommend referral to a clinical geneticist to identify the higher-risk subgroups most likely to benefit from imaging surveillance.

Verify on PubMed (PMID 22411341)

Evidence

Leg Length Discrepancy After Femoral Shaft Fractures in Children - Review After Skeletal Maturity

LoE 4

Stephens MM, Hsu LC, Leong JC • J Bone Joint Surg Br (1989)

Key Findings:

Thirty patients reviewed and radiographed AT SKELETAL MATURITY after an isolated closed femoral shaft fracture in childhood treated conservatively
Where the fracture occurred between ages 7 and 13, the limb overgrew by ABOUT 1 CM - and did so regardless of sex, upper-limb dominance, age, fracture site or fracture configuration
Excessive fracture overlap AT THE TIME OF INJURY increased overgrowth; overlap at union did not
Angulation remodelled in children injured under 10 years, but in older children it sometimes ADDED to shortening rather than correcting
The practical target: aim for about 1 cm of overlap at union in the 7-to-13-year-old, and correct angular deformity in children over 10 - this keeps the maximum discrepancy at maturity to about 1 cm

Clinical implication: Overgrowth after a childhood femoral shaft fracture is real, modest and predictable, which is why mild shortening is accepted at the time of treatment rather than corrected. It is a reason for reassurance and observation, not for intervention.

Limitation: THIRTY PATIENTS, RETROSPECTIVE, FROM ONE HONG KONG UNIT IN THE 1980s, AND ALL TREATED CONSERVATIVELY - which is precisely the treatment that has since been abandoned for most of this age group. Modern practice uses flexible intramedullary nails, external fixation or submuscular plating in the 7-to-13-year-old, so the central recommendation of this paper (aim for 1 cm overlap at union) describes a technique few surgeons now perform, even though the overgrowth phenomenon it documents is unchanged. Thirty patients is too few to detect the effect of the variables the authors report as making no difference - 'regardless of sex, dominance, age, site or configuration' is an underpowered null across five subgroups, not a demonstration that none of them matters. Measurement was radiographic without a stated technique or observer reliability, and the 7-to-13-year window excludes the under-7s in whom overgrowth is conventionally said to be greatest.

Verify on PubMed (PMID 2768309)

Narrowing It Down

Working in order. Each step either stops the workup or narrows it, which is why the sequence is worth keeping: the child whose true lengths are equal never needs the scanogram, and the child whose long limb is the outlier may need surveillance arranged before any orthopaedic plan is made.

Clinical Algorithm

Structured Approach to a Child with Suspected Limb Length Discrepancy

1
Step 1 — Is it true or apparent?
Do the block test standing, then measure true (ASIS to medial malleolus) against apparent (umbilicus to medial malleolus) lengths supine with the pelvis squared.
Equal true lengths with unequal apparent lengths means pelvic obliquity from a hip contracture or spinal deformity — treat the contracture or the curve, not the bone. Only a TRUE discrepancy proceeds to scanography.
2
Step 2 — Which segment, and does the foot contribute?
Perform the Galeazzi test to localise femur versus tibia, then compare shoe sizes and calcaneal height.
Femoral-only shortening points to PFFD, DDH, Perthes or a distal femoral physeal bar. Tibial-only shortening points to fibular or tibial hemimelia, proximal tibial arrest, or Blount disease. Multi-segment shortening with a small foot suggests a neuromuscular or global congenital cause.
3
Step 3 — Congenital or acquired?
Date the onset from the history: present since birth, or following a documented fracture, infection, tumour treatment or arthritis.
A congenital discrepancy projects from a single measurement by the multiplier method. An acquired one demands physeal imaging and serial measurement.
4
Step 4 — Is the short side abnormal, or is the long side abnormal?
Measure each limb against the child's own height percentile rather than only against each other, and decide which limb is the outlier.
If the LONG limb is the outlier, think hemihypertrophy, vascular malformation, JIA hyperaemia or post-fracture overgrowth — and in hemihypertrophy this is the point at which tumour surveillance is triggered, before any orthopaedic plan. If the SHORT limb is the outlier, think physeal arrest, congenital deficiency or neuromuscular disuse.
5
Step 5 — Is there associated deformity or joint disease?
Look specifically for angular deformity that worsens with growth, for a stiff, painful or unstable hip, for anterolateral tibial bowing, and for a ball-and-socket ankle.
Progressive angulation indicates an asymmetric (peripheral) physeal bar. A stiff hip suggests Perthes, post-septic sequelae or slipped epiphysis. Anterolateral bowing with a short leg raises NF1 and congenital pseudarthrosis. A ball-and-socket ankle is strongly associated with longstanding fibular deficiency or tarsal coalition.
6
Step 6 — Are there systemic or syndromic features?
Examine the skin, face, abdomen and neurology, not just the legs.
Cafe-au-lait macules and axillary freckling (NF1), macroglossia and omphalocele (Beckwith-Wiedemann), port-wine stain and varicosities (Klippel-Trenaunay), chronic knee swelling and uveitis (JIA), or spasticity and equinus (cerebral palsy). Each redirects the workup away from isolated orthopaedics, and two of them are urgent.
7
Step 7 — What will the discrepancy be at maturity?
Obtain a bone age (Greulich and Pyle, left hand) and at least two scanograms 6-12 months apart, then project: multiplier method for a congenital discrepancy from a single visit, Moseley straight-line graph or Green-Anderson charts for acquired arrest.
Only the PROJECTED MATURE discrepancy dictates treatment, never the current one: observation and a shoe raise below about 2 cm, contralateral epiphysiodesis for roughly 2-5 cm, lengthening or amputation with prosthetic fitting above about 5-6 cm.

Four paired-limb AP radiographs labelled A to D, each limb numbered 1 and 2, showing the bowing lessening as the bones grow. — Serial films of both lower legs on four occasions (A to D), the two limbs in each panel numbered 1 and 2: both are bowed in A, limb 2 stays the more curved in B and C, and by D both are comparatively straight with the bones larger and their ends broader. Behaviour like this is what a projection is built on, and it takes more than one film to see it.Credit: Kamath S et al., Journal of Orthopaedic Surgery and Research 2025 (CC BY 4.0).

Posteroanterior radiograph of a child's left hand and wrist, shown plain in panel a and with the assessed physes labelled in panel b. — Every projection needs a bone age: the left hand and wrist, plain in (a) and in (b) with lettered leaders onto each physis that is graded — the phalangeal and metacarpal growth plates of the thumb, middle and little fingers and the distal radius and ulna. The film is matched against atlas standards to convert today's measurement into a projected discrepancy at maturity.Credit: Tewattanarat N et al., BMC Medical Imaging 2026 (CC BY 4.0).

Diagram of the six Fishman middle-phalanx maturation stages used in skeletal age assessment. — Reading maturity without an atlas: the Fishman middle-phalanx maturation stages, drawn as MPM1 to MPM6 — the epiphysis first a separate piece with a clear gap beneath it, then broadening, then capping the corners of the shaft, and across the last three stages the gap closing to a narrow irregular seam and finally to a thin pale line across an almost continuous outline.Credit: Inchingolo AM et al., Oral Radiology 2026 (CC BY 4.0).

Evidence

Multiplier Method for Predicting Limb-Length Discrepancy

LoE 3

Paley D, Bhave A, Herzenberg JE, Bowen JR • J Bone Joint Surg Am (2000)

Key Findings:

The premise, and the reason the method works only where it works: in CONGENITAL and developmental discrepancy the short limb grows at a rate PROPORTIONAL to the normal limb, so the discrepancy stays a constant fraction of limb length
The multiplier is mature length divided by length at each age; it proved EQUIVALENT ACROSS ALL PERCENTILE GROUPS, varying only with age and sex
For a congenital discrepancy the mature value is simply current discrepancy multiplied by the age- and sex-specific multiplier - obtainable from a SINGLE measurement, with no graphs
For progressive developmental (non-congenital) discrepancy the formula differs: current discrepancy plus growth inhibition times growth remaining - two different formulae, and using the wrong one is the error
Multipliers derived from radiographic, clinical and anthropological databases were similar despite differences in race, ethnicity and generation; predictions correlated well with the Moseley method and with actual outcomes in lengthening and epiphysiodesis groups

Clinical implication: Use the multiplier for a congenital discrepancy, where one clinic measurement suffices; use Moseley or Green-Anderson for an acquired arrest, where the discrepancy does not scale with growth. The method also gives the timing of epiphysiodesis.

Limitation: A METHOD DERIVED FROM EXISTING GROWTH DATABASES AND THEN VALIDATED BY ITS OWN AUTHORS ON TWO OF THEIR OWN PATIENT GROUPS - there is no independent prospective validation here, and the verification cohorts were children with congenital shortening managed at the developing centre, so the population that fits the method best is the population it was tested on. The claim that multipliers are independent of race, ethnicity, generation and socioeconomic class rests on comparing historical databases rather than on prospectively measuring diverse contemporary children, and the source databases are largely mid-twentieth-century North American. The critical practical caveat is built into the method rather than being a flaw in it: the constant-percentage assumption FAILS in acquired arrest, in progressive syndromic overgrowth and after physeal injury, and the paper supplies a different formula for those which requires an estimate of growth inhibition - a quantity that is itself uncertain and needs serial measurement. No confidence interval is given for an individual prediction, so the output looks more precise than it is.

Verify on PubMed (PMID 11057472)

MCQ Practice Points

Key Point

Q: A child has equal true limb lengths (ASIS to medial malleolus) but a 3 cm apparent difference. What is the mechanism? A: Pelvic obliquity, most often from a fixed hip adduction contracture on the apparently short side (or abduction contracture on the opposite side), or from a structural spinal curve. The treatment is directed at the contracture or the spine, not at the bone length.

Key Point

Q: Which method best predicts mature discrepancy from a single clinic visit in a congenital case? A: The multiplier method. Congenital discrepancy remains a constant percentage of the growing limb, so current discrepancy multiplied by an age- and sex-specific factor gives the predicted mature value. Acquired arrests are better handled with the Moseley straight-line graph or Green-Anderson charts because the percentage is not constant.

Key Point

Q: A 5-year-old had a mid-shaft femoral fracture at age 3 and now has a 1 cm LONGER injured limb. Management? A: Reassurance and observation. Hyperaemic overgrowth after paediatric femoral shaft fracture is well recognised, is usually modest, and plateaus by about 2 years post-injury. It is one reason for accepting mild shortening at the time of fracture treatment in young children.

Key Point

Q: Which physeal bar is potentially amenable to resection? A: A bar involving less than roughly half the physeal area, in a child with at least 2 years or 2 cm of growth remaining, with correctable or minimal associated deformity. Larger bars, near-mature children and post-infective bars have poor results, and completion of the arrest with subsequent length equalisation is preferred.

Key Point

Q: Which limb is abnormal in a child with a chronically swollen knee from oligoarticular JIA? A: Early, the affected limb is LONGER because chronic synovial hyperaemia accelerates the adjacent physis, with epiphyseal overgrowth and a squared patella on radiographs. Late, premature physeal closure can reverse this into shortening. Concurrent ophthalmology referral for asymptomatic uveitis is essential.

Exam Viva Scenarios

Practise clinical reasoning and management decisions out loud

Viva scenarioStandard

Progressive shortening with valgus in a 9-year-old

Clinical prompt

“You are shown a standing full-length AP radiograph of a 9-year-old boy. The right leg is 2.5 cm shorter and there is a 12 degree valgus deformity at the right knee. Two years earlier he sustained a proximal tibial metaphyseal fracture treated in a cast.”

Viva scenarioAdvanced

An infant with a short thigh

Clinical prompt

“You are shown an AP pelvis and femora of a 14-month-old with a markedly short right thigh. The right proximal femur is short and bulbous, the acetabulum is shallow, and there appears to be no continuity between the femoral head and shaft.”

Viva scenarioStandard

An asymmetric toddler

Clinical prompt

“You are shown clinical photographs and a scanogram of a 3-year-old girl whose left leg is 1.5 cm longer than the right. The left arm and left side of the face also appear larger. She is otherwise well.”

Exam day cheat sheet

Limb Length Discrepancy — Rapid Recall

Confirm the sign

Block test standing = functional discrepancy including the foot
True length = ASIS to ipsilateral medial malleolus
Apparent length = umbilicus to medial malleolus (includes pelvic obliquity)
Galeazzi localises femur versus tibia
Imaging = standing full-length AP with ruler, patellae forward, pelvis levelled

Must-not-miss causes

Post-traumatic physeal bar — progressive shortening with angulation
Post-septic physeal destruction — history of neonatal sepsis, stiff hip
Hemihypertrophy — the LONG side is abnormal; screen for Wilms and hepatoblastoma
Neoplasia or post-irradiation arrest — night pain, oncology history

Short side abnormal

Congenital: PFFD, congenital short femur, fibular or tibial hemimelia
Acquired arrest: fracture, infection, irradiation, Blount disease
Hip disease: DDH, Perthes, SUFE sequelae
Neuromuscular disuse: hemiplegic CP — short, thin, osteopenic, small foot

Long side abnormal

Hemihypertrophy / Beckwith-Wiedemann
Vascular malformation: Klippel-Trenaunay (low flow), Parkes-Weber (high flow)
JIA hyperaemia around the knee
Post-fracture femoral overgrowth (plateaus by about 2 years)

Prediction and thresholds

Bone age plus 2 or more serial scanograms before any decision
Multiplier method for congenital; Moseley or Green-Anderson for acquired
Less than 2 cm projected: observe, shoe raise if symptomatic
Roughly 2-5 cm: contralateral epiphysiodesis, timed by bone age
Greater than 5-6 cm: lengthening (external fixator or intramedullary) or amputation with prosthesis

Answer-completing extras

Always state which segment is short and whether the foot contributes
Always mention remaining growth, not just today's number
Always screen hemihypertrophy for abdominal tumours
Minimise cumulative radiation — EOS where available

Cause

Typical age / setting

Discriminating feature

What confirms it

Physeal bar after fracture (Salter-Harris III-V, distal femur, proximal tibia)

Any age, months to years after a documented injury

Progressive shortening WITH angular deformity converging toward the bar; Park-Harris growth arrest line is oblique and converges on the bar rather than parallel to the physis

MRI physeal mapping (fat-suppressed 3D) or CT to size and locate the bar as a percentage of physeal area

Post-septic physeal destruction (neonatal osteomyelitis / septic arthritis of hip or knee)

Sequel of neonatal or infant sepsis, presenting at ages 2-10

Severe shortening far out of proportion to the history, often with femoral head loss and a stiff painless hip; frequently a history of NICU line sepsis

Radiographs showing absent or deformed capital femoral epiphysis; MRI for residual physis

Hemihypertrophy (isolated or Beckwith-Wiedemann)

Recognised in infancy or early childhood

The LONG side is abnormal — asymmetry involves soft tissue, hemiface, tongue or trunk, not just the limb; asymmetry is proportional and non-progressive in ratio

Clinical asymmetry across body segments plus tumour surveillance ultrasound; genetic testing for 11p15 imprinting defect

Neoplasia adjacent to the physis (osteosarcoma, Ewing, or post-irradiation / post-chemotherapy arrest)

Ages 8-18 with pain, or a survivor of childhood cancer

Night pain, a metaphyseal lesion with periosteal reaction, or shortening dating precisely from radiotherapy or endoprosthesis

MRI of the whole bone; oncological history and radiotherapy field records

Idiopathic / constitutional discrepancy

Well child, any age, discrepancy typically less than 2 cm

Diagnosis of exclusion: no deformity, no dysmorphism, symmetric segments, ratio of shortening constant on serial measurement

Serial scanograms plus bone age showing a stable percentage difference; no imaging abnormality

Developmental dysplasia of the hip (dislocated / high-riding)

Infant to walking age; unilateral dislocation

Apparent rather than true shortening, with limited abduction, asymmetric thigh creases and a positive Galeazzi at the KNEE level

Ultrasound before 4-6 months; AP pelvis thereafter with broken Shenton line and proximal femoral migration

Perthes disease and other avascular necrosis of the capital epiphysis

Boys aged 4-8, painful limp

Modest shortening (usually less than 2 cm) with coxa magna, coxa breva and loss of hip internal rotation and abduction

AP and frog-lateral pelvis showing fragmentation and subchondral fracture; MRI if early

Cerebral palsy / hemiplegia and other neuromuscular disease

Spastic hemiplegia, walking age onwards

The affected limb is short, THIN and osteopenic overall, with equinus and reduced foot size — disuse affects the whole limb, not one physis

Neurological examination; radiographs show slender diaphyses and generalised osteopenia on the affected side

Fibular hemimelia (postaxial longitudinal deficiency)

Recognised at birth or in infancy

Short tibia with anteromedial bowing and a skin dimple, absent or short fibula, equinovalgus ball-and-socket ankle and missing lateral rays of the foot

AP and lateral tibia and foot radiographs counting rays and assessing the ankle mortise

Proximal focal femoral deficiency / congenital short femur

Birth

The whole shortening is femoral, the thigh is short bulbous and held in flexion, abduction and external rotation, and the knee often has cruciate deficiency

AP pelvis and femur assessing acetabulum and proximal femoral segment; Aitken classification; MRI in infants for cartilaginous continuity

Hyperaemic overgrowth after a diaphyseal femoral fracture

Ages 2-10, in the 1-2 years after a healed femoral shaft fracture

The previously FRACTURED side is longer, typically by up to about 1-2 cm, and the overgrowth plateaus by about 2 years post-injury

Serial scanograms showing overgrowth that stabilises; no physeal abnormality

Vascular malformation or arteriovenous fistula (Klippel-Trenaunay, Parkes-Weber)

Childhood, with a port-wine stain or varicosities

Overgrowth of the limb with a cutaneous vascular stain and lateral venous anomaly; warmth and bruit in high-flow lesions

Duplex ultrasound and contrast MR angiography of the limb

Juvenile idiopathic arthritis (mono/oligoarticular knee)

Girls aged 2-8 with a chronically swollen knee

The AFFECTED side is LONGER early (chronic synovitis hyperaemia accelerates the physis), then shorter if the physis closes prematurely; look for accompanying uveitis

Knee radiographs showing epiphyseal overgrowth and squared patella; MRI synovitis; ANA and slit lamp

Skeletal dysplasia or bone lesion (Ollier disease, multiple hereditary exostoses, fibrous dysplasia, neurofibromatosis type 1)

Childhood, often with multiple lesions

Discrepancy accompanied by a matrix-containing or expansile lesion abutting the physis, or anterolateral tibial bowing in NF1; often bilateral but asymmetric

Skeletal survey; characteristic lucent columns (Ollier), sessile exostoses, or ground-glass matrix

Clinical Algorithm

Structured Approach to a Child with Suspected Limb Length Discrepancy

1
Step 1 — Is it true or apparent?
Do the block test standing, then measure true (ASIS to medial malleolus) against apparent (umbilicus to medial malleolus) lengths supine with the pelvis squared.
Equal true lengths with unequal apparent lengths means pelvic obliquity from a hip contracture or spinal deformity — treat the contracture or the curve, not the bone. Only a TRUE discrepancy proceeds to scanography.
2
Step 2 — Which segment, and does the foot contribute?
Perform the Galeazzi test to localise femur versus tibia, then compare shoe sizes and calcaneal height.
Femoral-only shortening points to PFFD, DDH, Perthes or a distal femoral physeal bar. Tibial-only shortening points to fibular or tibial hemimelia, proximal tibial arrest, or Blount disease. Multi-segment shortening with a small foot suggests a neuromuscular or global congenital cause.
3
Step 3 — Congenital or acquired?
Date the onset from the history: present since birth, or following a documented fracture, infection, tumour treatment or arthritis.
A congenital discrepancy projects from a single measurement by the multiplier method. An acquired one demands physeal imaging and serial measurement.
4
Step 4 — Is the short side abnormal, or is the long side abnormal?
Measure each limb against the child's own height percentile rather than only against each other, and decide which limb is the outlier.
If the LONG limb is the outlier, think hemihypertrophy, vascular malformation, JIA hyperaemia or post-fracture overgrowth — and in hemihypertrophy this is the point at which tumour surveillance is triggered, before any orthopaedic plan. If the SHORT limb is the outlier, think physeal arrest, congenital deficiency or neuromuscular disuse.
5
Step 5 — Is there associated deformity or joint disease?
Look specifically for angular deformity that worsens with growth, for a stiff, painful or unstable hip, for anterolateral tibial bowing, and for a ball-and-socket ankle.
Progressive angulation indicates an asymmetric (peripheral) physeal bar. A stiff hip suggests Perthes, post-septic sequelae or slipped epiphysis. Anterolateral bowing with a short leg raises NF1 and congenital pseudarthrosis. A ball-and-socket ankle is strongly associated with longstanding fibular deficiency or tarsal coalition.
6
Step 6 — Are there systemic or syndromic features?
Examine the skin, face, abdomen and neurology, not just the legs.
Cafe-au-lait macules and axillary freckling (NF1), macroglossia and omphalocele (Beckwith-Wiedemann), port-wine stain and varicosities (Klippel-Trenaunay), chronic knee swelling and uveitis (JIA), or spasticity and equinus (cerebral palsy). Each redirects the workup away from isolated orthopaedics, and two of them are urgent.
7
Step 7 — What will the discrepancy be at maturity?
Obtain a bone age (Greulich and Pyle, left hand) and at least two scanograms 6-12 months apart, then project: multiplier method for a congenital discrepancy from a single visit, Moseley straight-line graph or Green-Anderson charts for acquired arrest.
Only the PROJECTED MATURE discrepancy dictates treatment, never the current one: observation and a shoe raise below about 2 cm, contralateral epiphysiodesis for roughly 2-5 cm, lengthening or amputation with prosthetic fitting above about 5-6 cm.

Evidence

Multiplier Method for Predicting Limb-Length Discrepancy

LoE 3

Paley D, Bhave A, Herzenberg JE, Bowen JR • J Bone Joint Surg Am (2000)

Key Findings:

The premise, and the reason the method works only where it works: in CONGENITAL and developmental discrepancy the short limb grows at a rate PROPORTIONAL to the normal limb, so the discrepancy stays a constant fraction of limb length
The multiplier is mature length divided by length at each age; it proved EQUIVALENT ACROSS ALL PERCENTILE GROUPS, varying only with age and sex
For a congenital discrepancy the mature value is simply current discrepancy multiplied by the age- and sex-specific multiplier - obtainable from a SINGLE measurement, with no graphs
For progressive developmental (non-congenital) discrepancy the formula differs: current discrepancy plus growth inhibition times growth remaining - two different formulae, and using the wrong one is the error
Multipliers derived from radiographic, clinical and anthropological databases were similar despite differences in race, ethnicity and generation; predictions correlated well with the Moseley method and with actual outcomes in lengthening and epiphysiodesis groups

Verify on PubMed (PMID 11057472)