The paediatric hip: SUFE, Perthes and DDH

Written by the X-Ray Atlas editorial team from the sources listed below · Medical review pending · Updated 24 October 2026 · Level: Intermediate

A child with a limp or hip pain is a common emergency presentation, and the likely diagnosis depends heavily on age. Developmental dysplasia of the hip (DDH) affects infants, Perthes disease typically affects young school-age children, and a slipped upper femoral epiphysis (SUFE; US: slipped capital femoral epiphysis, SCFE) affects adolescents. Septic arthritis can occur at any age and must never be missed. Each condition has a small set of radiographic lines and signs that make it visible. This page teaches them, along with the views that make them easiest to see.

1. Think by age

Age (approximate)	Conditions to consider first	First imaging
Birth to about 4–6 months	DDH	Ultrasound: the femoral head is still cartilage and invisible on X-ray.
Older infant and toddler	DDH (late presentation), septic arthritis, toddler's fracture, non-accidental injury	X-ray, plus ultrasound for an effusion.
About 4–10 years	Transient synovitis, Perthes disease, septic arthritis	X-ray of the pelvis; ultrasound if an effusion is suspected.
About 10–16 years	SUFE, apophyseal avulsion injuries, septic arthritis, tumours	AP pelvis and lateral views of both hips.

Ages overlap and are only a guide. A child of any age with fever and a refusal to bear weight needs septic arthritis excluded.

2. Views and how to look

• AP pelvis showing both hips allows side-to-side comparison, which is the single most useful trick in the paediatric hip.
• Frog-leg lateral (hips flexed and abducted) shows the femoral head–neck junction and is more sensitive than the AP for SUFE. Some centres prefer a cross-table lateral if the child cannot move the hip or the slip appears acute and unstable; follow local protocol.
• Use an MSK system (see describing a fracture): alignment, bone density and texture, cartilage (joint space and physis), soft tissues.
• Compare: epiphyseal height, density and shape; physeal width; the medial joint space (teardrop distance); and the lines described below.

3. Slipped upper femoral epiphysis (SUFE)

In SUFE the femoral head (epiphysis) slips off the neck through the growth plate: it is a Salter-Harris type I injury (see Salter-Harris fractures). The epiphysis moves posteriorly and medially (and inferiorly) relative to the neck. It typically affects children aged about 10–16, more often boys, and is associated with obesity. Endocrine causes such as hypothyroidism should be considered, especially when the child is outside the usual age range.

It is bilateral in about 20–40% of children; reported figures vary with how long patients are followed and whether imaging-only slips are counted. Always look at the other hip.

Signs

• Klein's line: on the AP, draw a line along the superior border of the femoral neck. It should cut through the lateral part of the epiphysis. If it misses the epiphysis, or cuts less of it than on the other side, a slip is likely (the Trethowan sign).
• A widened, irregular or blurred physis, an early sign.
• Reduced epiphyseal height on the AP, because the slipped head is seen at a different angle.
• The metaphyseal blanch sign of Steel: a crescent of increased density over the metaphysis where the displaced epiphysis overlaps it.
• On the frog-leg lateral, the epiphysis is seen sitting posterior to the neck, like a scoop of ice cream slipping off a cone. Mild slips may be visible only on this view.

SUFE needs same-day orthopaedic referral. The child is usually kept non-weight-bearing while awaiting surgery, which is commonly fixation in situ. An unstable slip (one where the child cannot bear weight, even with crutches) carries a higher risk of avascular necrosis of the femoral head.

4. Perthes disease (Legg-Calvé-Perthes)

Perthes disease is an idiopathic avascular necrosis of the femoral epiphysis, typically at about 4–10 years and more often in boys. It may be bilateral in about 10–15%, usually at different times. The child often presents with a painless or mildly painful limp. The bone dies, fragments and is then gradually replaced, so the X-ray appearance depends on the stage.

Stage	X-ray findings
Early	The film may be normal. Then a small, dense (sclerotic) epiphysis and a widened medial joint space.
Subchondral fracture	A thin lucent crescent sign just under the articular surface, best seen on the frog-leg lateral.
Fragmentation	The epiphysis breaks into dense and lucent fragments and flattens.
Re-ossification and healing	New bone fills in. The head may remain large and flat (coxa magna), with a short, broad neck.

Classification systems (for example Catterall and the Herring lateral pillar classification) grade how much of the head is involved and help predict outcome; they are specialist tools. MRI can show early disease when the X-ray is normal.

Bilateral, symmetrical changes at the same stage should make you think of something else, such as a skeletal dysplasia or hypothyroidism, rather than Perthes disease.

5. Developmental dysplasia of the hip (DDH)

DDH covers a spectrum from a shallow acetabulum to a hip that is fully dislocated. In the UK, newborns are examined clinically, and babies with an abnormal examination or risk factors (such as breech presentation or a family history) are referred for hip ultrasound. Ultrasound is used until about 4–6 months, while the femoral head is mostly cartilage. After the head starts to ossify, an AP pelvis is used.

Lines on the AP pelvis

• Hilgenreiner's line: a horizontal line through both triradiate cartilages (the Y-shaped growth cartilage of the acetabulum).
• Perkin's line: a vertical line from the lateral edge of the acetabular roof, perpendicular to Hilgenreiner's line.
• These lines divide each hip into four quadrants. The ossified femoral head (or the medial beak of the metaphysis if the head is not yet ossified) should lie in the inferomedial quadrant. A dislocated head moves upwards and outwards.
• Shenton's line: a smooth curve from the inferior border of the superior pubic ramus to the medial femoral neck. A break suggests subluxation or dislocation (also used in adult hip fractures).
• Acetabular index: the angle between Hilgenreiner's line and a line along the acetabular roof. It is up to about 30° in newborns and decreases with age; normal values are age-dependent and differ between sources, so use age-specific references.
• The ossified femoral head on the affected side is often smaller or appears later.

6. The irritable hip: transient synovitis or sepsis?

Transient synovitis is the most common cause of an acutely painful hip in young children and settles on its own. Septic arthritis is far less common but destroys the joint within days if untreated. They cannot be separated on X-ray:

• The X-ray is usually normal in both, or shows subtle widening of the medial joint space from an effusion.
• Ultrasound confirms an effusion but cannot tell pus from sterile fluid.
• The decision is clinical and laboratory-based. The Kocher criteria (non-weight-bearing, fever above 38.5 °C, ESR above 40 mm/h, white cell count above 12 × 10⁹/L) raise the probability of sepsis as more are present. Local pathways may also use CRP.
• Suspected septic arthritis needs urgent orthopaedic referral and joint aspiration. Imaging should not delay it. See osteomyelitis and septic arthritis.

7. Pitfalls and mimics

• Relying on the AP for SUFE: mild slips can be invisible on the AP. Always obtain a lateral view of both hips.
• Assuming a bilateral slip is normal because both sides "match". Klein's line should still cut the epiphysis on each side, and the physes should be crisp.
• A normal X-ray does not exclude early Perthes disease, septic arthritis, osteomyelitis or a stress fracture. MRI or ultrasound may be needed.
• Pelvic apophyseal avulsions in adolescent athletes (for example at the anterior inferior iliac spine or ischial tuberosity) cause hip pain. Check the apophyses.
• Rotation of the pelvis distorts the acetabular index and the lines. Check that the obturator foramina are symmetrical.
• Tumours and infection can present with a limp. A permeative or destructive lesion needs urgent referral (see bone tumours).

Red flags: act now

What to focus on, by role

Role	Priorities
Medical student	The age table, Klein's line, the stages of Perthes disease and the DDH quadrants. Know that knee pain can come from the hip.
Nurse / ENP	Temperature and weight-bearing status in every limping child. Escalate fever with a painful joint. Keep suspected SUFE non-weight-bearing.
Junior doctor	Request both hips with a lateral view in adolescents, use the clinical criteria for sepsis, and refer SUFE the same day.
Radiographer	Symmetrical AP pelvis without rotation, and the appropriate lateral for the suspected slip according to protocol. Gonad shielding only as current guidance advises.
Advanced / FRCR	Subtle slips, Herring and Catterall grading, age-specific acetabular indices and the dysplasias that mimic bilateral Perthes disease.

Practise on paediatric X-rays →

References

1. Radiopaedia. Slipped capital femoral epiphysis. radiopaedia.org
2. Radiology Masterclass. Hip X-ray gallery. radiologymasterclass.co.uk
3. Kocher MS, Zurakowski D, Kasser JR. Differentiating between septic arthritis and transient synovitis of the hip in children: an evidence-based clinical prediction algorithm. J Bone Joint Surg Am 1999;81(12):1662–1670.
4. Don't Forget the Bubbles. Paediatric emergency medicine education. dontforgetthebubbles.com
5. British Orthopaedic Association. BOAST guidance. boa.ac.uk
6. Royal College of Radiologists. iRefer: making the best use of clinical radiology. irefer.org.uk

For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.