Slipped Upper Femoral Epiphysis (SUFE)

SUFE (also called Slipped Capital Femoral Epiphysis, SCFE) is the commonest hip disorder of adolescence, in which the femoral head (capital epiphysis) slips posteriorly and inferiorly off the femoral neck through the growth plate. It is a Salter-Harris type I injury of the proximal femoral physis under chronic shear stress, classically seen in the obese pubertal boy presenting with a limp and knee pain. Early diagnosis is sight-saving for the hip because the dreaded complications — avascular necrosis (AVN) and chondrolysis — are largely preventable with prompt fixation.

Definition & key concept

• The slip is described by the displacement of the neck relative to the epiphysis (the head stays in the acetabulum, held by the ligamentum teres; the neck displaces antero-superiorly). Hence the femoral neck moves up and forward while the epiphysis appears to slip postero-inferiorly.
• It is fundamentally a disorder of the hypertrophic zone of the physis (the same weak zone involved in Salter-Harris fractures), where the growth plate is mechanically weakened and shear forces of the obese adolescent overwhelm it.
• Peak incidence: boys 10–16 years, girls 8–15 years (girls earlier because they mature earlier). Slips rarely occur after physeal closure.
• Left hip is more commonly affected, and it is bilateral in 20–40% of cases (higher in endocrine-associated disease), so the contralateral hip must always be watched.

High-yield: SUFE is the most common adolescent hip disorder and is a Salter-Harris type I injury through the hypertrophic zone of the proximal femoral physis. The epiphysis slips postero-inferiorly.

Etiology & pathophysiology

The cause is multifactorial — a combination of mechanical load and biochemical weakening of the physis.

Mechanical factors

• Obesity (most important; raises shear load across the physis).
• Physiological femoral retroversion in this age group, which increases shear.
• A relatively horizontal (more vertical/oblique) physeal orientation during the pubertal growth spurt.

Endocrine / metabolic factors — suspect these in an atypical SUFE: a child who is young (<10 yr), thin, very tall, or short for age, or who has bilateral slips.

• Hypothyroidism (classic association — always check thyroid function in atypical cases).
• Growth hormone therapy / GH deficiency (slips during treatment).
• Renal osteodystrophy / chronic kidney disease (renal rickets) — produces bilateral slips through metaphyseal bone.
• Hypogonadism, panhypopituitarism.
• Prior radiation to the pelvis.

High-yield: Atypical SUFE = young, thin, or short child, or bilateral slips → screen for hypothyroidism, renal osteodystrophy, GH disturbance and hypogonadism. These two endocrine associations (hypothyroidism, renal osteodystrophy) are the most repeated exam facts.

Pathologically, the periosteal sleeve is weak, the physis widens, and progressive shear leads to displacement. The blood supply to the epiphysis runs along the postero-superior neck via the medial femoral circumflex artery (lateral epiphyseal vessels / posterosuperior retinacular vessels) — this is why forceful reduction of an unstable slip tears these vessels and produces AVN.

Classification

Two classification systems are tested — the older temporal system and the prognostically vital Loder (stability) system.

Temporal classification (by symptom duration)

Type	Duration of symptoms	Notes
Pre-slip	Physis widened, no displacement	Limp/ache only; physis irregular
Acute	< 3 weeks	Sudden, often trauma-related
Chronic	> 3 weeks	Commonest; gradual limp & out-toeing
Acute-on-chronic	Chronic symptoms + sudden worsening	Acute displacement over established slip

Loder classification (stability) — the prognostic one

Feature	Stable	Unstable
Weight-bearing	Able to bear weight (± crutches)	Unable to bear weight, even with crutches
Mechanism	Usually chronic	Often acute
Risk of AVN	Low (~0–10%)	High (up to ~50%)
Prognosis	Good	Guarded

High-yield: The Loder (stability) classification best predicts AVN. Unstable = cannot bear weight even with crutches → AVN risk approaches 50%. This single fact is the most exam-relevant point in the whole topic.

Severity grading

• Southwick angle (head–shaft angle on frog-leg lateral, compared with the normal side): Mild < 30°, Moderate 30–50°, Severe > 50°.
• Wilson grading (% of epiphyseal displacement on the neck): Grade I < 1/3, Grade II 1/3–1/2, Grade III > 1/2.

Clinical features

The hallmark is a limping adolescent with hip, groin, thigh or KNEE pain — and the classic trap is isolated knee pain (referred via the obturator nerve), which leads to missed diagnoses when only the knee is examined.

• Obese adolescent, often with delayed sexual maturity (Fröhlich/adiposogenital habitus) OR tall and thin.
• Antalgic gait, out-toeing of the affected limb; in chronic cases, shortening.
• Pain may be vague over weeks (chronic) or sudden and severe (unstable).
• Drehmann sign (pathognomonic): when the hip is passively flexed, the thigh obligatorily rolls into external rotation and abduction (cannot flex in neutral). This reflects the antero-superior position of the neck.
• Loss of internal rotation, abduction and flexion; limb lies in external rotation.
• Trendelenburg gait/sign with chronicity.

Clinical approach (flow):

Limping obese adolescent with knee/hip pain → examine the HIP (not just the knee) → look for obligate external rotation on flexion (Drehmann) & loss of internal rotation → assess weight-bearing ability (stable vs unstable) → AP + frog-leg lateral pelvis X-ray → confirm Klein's line disruption / Trethowan sign → urgent orthopaedic referral.

High-yield: Always X-ray the hip in an adolescent presenting with knee pain. Missed SUFE because the knee was examined in isolation is a classic clinical/medicolegal pitfall and a favourite MCQ stem.

Diagnosis & investigation of choice

Plain radiograph — investigation of choice (first-line)

Obtain AP pelvis AND frog-leg (Lauenstein) lateral of both hips. The lateral view is most sensitive because the slip is postero-inferior; in a pre-slip or subtle slip, the AP may look near-normal.

Radiographic signs (high-yield):

Sign	Description
Klein's line	Line drawn along the superior border of the femoral neck on AP view should intersect the lateral epiphysis. In SUFE it fails to do so (epiphysis slips below it) = Trethowan sign.
Steel / metaphyseal blanch sign of Steel	Crescent-shaped dense area in the proximal metaphysis (double density from the slipped overlapping epiphysis) — early sign.
Capener sign	In a normal hip the posterior acetabular margin cuts across the medial neck metaphysis; lost in SUFE.
Widened, irregular physis	Especially in the pre-slip stage.
Loss of "S" curve (Shenton's line)	Disrupted hip architecture.

High-yield: Klein's line drawn on the superior femoral neck normally transects part of the epiphysis; failure to do so = Trethowan sign = SUFE. A modified Klein's line (asymmetry > 2 mm vs the normal side) improves sensitivity for subtle slips.

Other modalities

• MRI: most sensitive for the pre-slip stage (physeal oedema, marrow changes) and to assess early AVN; useful when X-ray is equivocal.
• Ultrasound: can detect effusion and physeal step-off.
• Bone scan / MRI post-fixation to assess epiphyseal perfusion (AVN).
• Blood tests: TFT, renal profile/PTH, calcium-phosphate — in atypical/bilateral cases to find an endocrine cause.

Management

SUFE is a surgical emergency-ish condition: once diagnosed, the child should be non-weight-bearing and admitted, because a stable slip can become unstable. The goal is to stabilise the slip in situ and prevent further displacement — not to reduce a chronic slip.

Drug of choice / first action

There is no "drug" of choice — the definitive treatment is surgical. The first action is strict non-weight-bearing (bed rest, no further ambulation) to prevent progression while planning urgent fixation. Analgesia is supportive only.

Surgical management (procedure of choice)

• In-situ fixation with a single cannulated cancellous screw is the gold standard for mild–moderate stable slips. A single central screw crossing the physis prevents further slip while minimising the risk of joint penetration.
• The screw is placed perpendicular to the physis, with the threads crossing into the epiphysis; avoid the postero-superior quadrant to protect the retinacular vessels and avoid joint penetration.
• Unstable slips: urgent fixation; gentle (incidental) reduction only — avoid forceful manipulation, which precipitates AVN. Some advocate the modified Dunn osteotomy (surgical hip dislocation, open reduction) for severe unstable slips in experienced hands, but it carries its own AVN risk.
• Severe / chronic slips with deformity: corrective osteotomy after physeal closure (e.g. proximal femoral / intertrochanteric Southwick osteotomy, or neck osteotomy) to address residual deformity and cam impingement.
• Prophylactic fixation of the contralateral hip: considered in patients at high risk — endocrinopathy, very young age (open triradiate cartilage), or unreliable follow-up — because of the high bilaterality.

High-yield: Treatment of choice for a stable mild/moderate slip = in-situ pinning with a single cannulated screw. Do NOT attempt closed reduction of a chronic slip — forceful reduction is the chief avoidable cause of AVN.

Why not reduce?

Anatomic reduction tensions/tears the posterosuperior retinacular (lateral epiphyseal) vessels from the medial femoral circumflex artery → epiphyseal ischaemia → AVN. Accepting a mild residual deformity and fixing in situ is safer; remodelling occurs in the skeletally immature.

Complications

Two complications dominate exams: AVN and chondrolysis.

Complication	Key points
Avascular necrosis (osteonecrosis)	Most feared. Linked to unstable slips and to forceful reduction; can also follow malpositioned/posterosuperior screw. Leads to femoral head collapse and early arthritis.
Chondrolysis	Acute cartilage loss with joint-space narrowing and stiffness; classically associated with pin/screw penetration into the joint (persistent penetrance). Presents with pain and restricted motion.
Femoroacetabular impingement (cam type)	Residual neck prominence (the "metaphyseal bump") abuts the acetabulum → labral damage → early secondary osteoarthritis.
Coxa vara / limb shortening / out-toeing	From residual deformity in severe slips.
Progression of slip	If fixation inadequate or delayed; or development of a contralateral slip.

High-yield: AVN ↔ unstable slip & forced reduction. Chondrolysis ↔ intra-articular screw penetration. Knowing which complication pairs with which cause is a classic single-best-answer discriminator.

Key differentials

A limping adolescent with hip/knee pain has a focused differential:

Condition	Distinguishing features
Perthes disease	Younger (4–8 yr), thin/short, no obesity link; AVN of femoral head is the primary event; AP shows sclerosis, fragmentation, "crescent sign".
Transient (toxic) synovitis	Post-viral, well child, mild effusion, settles in days; afebrile or low-grade; normal/near-normal X-ray.
Septic arthritis	Acutely unwell, fever, raised CRP/ESR/WCC; Kocher criteria; emergency aspiration.
Developmental dysplasia of hip (DDH)	Presents earlier; positive Ortolani/Barlow in infancy, leg-length discrepancy.
Fracture / trauma	History of significant injury; physeal or neck fracture on X-ray.
Juvenile idiopathic arthritis	Multiple joints, morning stiffness, systemic features.

Kocher criteria (to separate septic arthritis from transient synovitis): non–weight-bearing, fever > 38.5°C, ESR > 40 mm/hr, WBC > 12,000/µL. (Useful contrast item in MCQs.)

Recently asked / exam angle

• "Most common adolescent hip disorder" → SUFE.
• Klein's line / Trethowan sign asked as the radiographic sign on an AP pelvis image of an obese teen.
• Loder unstable slip = inability to bear weight and its ~50% AVN risk — repeatedly tested as the worst-prognosis subgroup.
• Treatment of choice = in-situ single-screw pinning; "what NOT to do" = forceful closed reduction (causes AVN).
• Endocrine associations (hypothyroidism, renal osteodystrophy) in bilateral/atypical slips.
• Direction of slip of the epiphysis → postero-inferior (neck goes antero-superior). Frequent MCQ trap.
• Knee pain referred to the hip — clinical reasoning stem ("adolescent with knee pain, normal knee X-ray, next investigation?" → AP & frog-leg lateral of the hip).
• Drehmann sign (obligate external rotation on hip flexion) appearing as the pathognomonic clinical sign.
• Chondrolysis ↔ screw joint penetration as a pair-matching distractor against AVN.
• Image-based: frog-leg lateral being more sensitive than AP for an early slip.

Rapid revision

1. SUFE = commonest adolescent hip disorder; Salter-Harris type I through the hypertrophic zone of the proximal femoral physis.
2. Typical patient: obese pubertal boy, 10–16 yr; left hip; bilateral in 20–40%.
3. Epiphysis slips postero-inferiorly; neck displaces antero-superiorly.
4. Atypical SUFE (young/thin/short/bilateral) → screen for hypothyroidism & renal osteodystrophy (also GH therapy, hypogonadism).
5. Classic presentation: limp + referred KNEE pain — always examine and X-ray the hip.
6. Drehmann sign = obligate external rotation/abduction when the hip is flexed (pathognomonic); internal rotation lost.
7. Investigation of choice: AP + frog-leg lateral pelvis; MRI best for the pre-slip stage.
8. Klein's line fails to intersect the epiphysis = Trethowan sign; also Steel's metaphyseal blanch sign.
9. Loder classification: unstable (cannot bear weight) → AVN risk up to ~50%; stable → low risk.
10. Treatment of choice for stable mild/moderate slip: in-situ pinning with a single cannulated screw.
11. Never force-reduce a chronic slip — tears the medial femoral circumflex retinacular vessels → AVN.
12. AVN ↔ unstable/forced reduction; chondrolysis ↔ intra-articular screw penetration; consider prophylactic contralateral pinning in endocrine/very-young patients.