Slipped Capital Femoral Epiphysis (SCFE)

A slipped capital femoral epiphysis is a Salter-Harris type I (occasionally II) separation through the proximal femoral growth plate. The slip happens through the hypertrophic zone of the physis — its weakest layer — under a combination of mechanical overload (obesity is by far the commonest risk factor) and hormonal change (the adolescent growth spurt, and endocrine disorders that weaken the physis). The epiphysis is held in the acetabulum, so it is really the neck and shaft that displace: the metaphysis moves anteriorly and rotates externally relative to the head, which is why the classic examination finding is obligate external rotation when the hip is flexed.

Two facts make SCFE high-stakes. First, it hides: the pain is often felt in the thigh or knee, not the hip, and a mild slip can look almost normal on an AP film — so it is a classic missed diagnosis. Second, its feared complication, avascular necrosis (AVN) of the femoral head, is driven largely by the initial vascular insult at the moment the slip becomes unstable and by any attempt to forcefully reduce it — not by how skilfully it is later pinned. Get the diagnosis early, classify stability correctly, and fix in situ, and most children do very well.

Where the slip happens. The growth plate is layered — from the epiphysis downward into the reserve (resting) zone, the proliferative zone, the hypertrophic zone, and the zone of provisional calcification — before it meets the metaphyseal primary spongiosa. The hypertrophic zone is the mechanically weakest layer, and in SCFE it is abnormally widened and disorganised under combined hormonal and mechanical influence. The adolescent physis is especially vulnerable because, around the growth spurt, it becomes more obliquely/vertically oriented (raising the shear across it), the reinforcing perichondrial ring thins, and sex-hormone / growth-hormone shifts plus excess body weight further reduce its resistance to shear — which is exactly why obesity and endocrinopathy are such strong risk factors. The slip therefore propagates as a cleavage through the hypertrophic zone and zone of provisional calcification: the epiphysis stays seated in the acetabulum while the metaphysis and shaft displace anteriorly and rotate externally. Recognising this cleavage plane explains both why the slip occurs where it does and why a single screw across the physis arrests it.

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Why avascular necrosis is the dominant fear. The femoral head is fed by the retinacular vessels (terminal branches of the medial femoral circumflex artery, MFCA), which run along the posterosuperior femoral neck and are held in a periosteal sleeve that stays attached to the epiphysis. When the slip occurs, those vessels are stretched and, in an unstable slip, may already be torn or thrombosed — which is why unstable slips carry such a high AVN rate regardless of treatment.

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Three classifications are used, but they are not equal: stability (Loder) predicts the outcome, while Southwick grades the deformity and the temporal labels describe the history.

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Southwick angleTemporal

Southwick (severity) is the lateral epiphyseal–shaft (head–shaft) angle, measured ideally on the frog-leg lateral of both hips. How to measure it:

1. Mark the anterior and posterior tips of the capital epiphysis at the physis and join them — the epiphyseal base line.
2. Draw a line perpendicular to that base line.
3. Draw the femoral shaft axis.
4. The angle between the perpendicular and the shaft axis is the head–shaft angle.
5. Subtract the contralateral (normal) hip's head–shaft angle — the difference is the Southwick slip angle. If both hips are slipped, use the normal reference of ~12°.

Mild

Southwick slip angle

Less than 30°

Clinical

Minimal deformity

Moderate

Southwick slip angle

30–50°

Clinical

Some deformity

Severe

Southwick slip angle

Greater than 50°

Clinical

Significant deformity, cam-FAI risk

Severity	Southwick slip angle	Clinical
Mild	Less than 30°	Minimal deformity
Moderate	30–50°	Some deformity
Severe	Greater than 50°	Significant deformity, cam-FAI risk

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Severe slips drive cam-type femoroacetabular impingement and early osteoarthritis, and may need later deformity correction.

Temporal labels describe the history rather than the prognosis: acute (symptoms under 3 weeks), chronic (over 3 weeks), or acute-on-chronic (a chronic slip with an acute exacerbation). Less useful than Loder stability for predicting outcome.

The hallmark sign is obligate external rotation (the Drehmann sign): as you flex the hip it is forced into external rotation, because the anteriorly-displaced metaphysis drives against the anterior acetabulum. It is, in effect, the slip made visible on examination.

The pathway is the same in principle for every slip — confirm stability, fix in situ, then decide about the other hip — but the urgency differs sharply with stability.

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The numbers worth carrying: AVN is under 1% in stable slips but around 47%unstable in unstable slips — and rises steeply with any reduction attempt. AVN, when it occurs, is the principal driver of a poor long-term hip; cam-FAI from residual deformity is the slower, later cause of osteoarthritis even in mild slips.

Chondrolysis deserves emphasis as a distinct complication. It is acute loss of articular cartilage, defined radiographically as joint-space narrowing to under ~3 mm (or under 50% of the contralateral hip), and it presents with progressive pain and a globally stiff, restricted hip. The classic and avoidable cause is persistent intra-articular screw penetration — so live fluoroscopy through a full arc (the "approach-withdrawal" technique) to exclude penetration is mandatory. Crucially, however, chondrolysis can also occur idiopathically — without any pin penetration — and is associated with severe slips and prolonged spica immobilisation. Management is to confirm there is no retained intra-articular hardware, then NSAIDs, analgesia and physiotherapy to maintain motion; it is often partly self-limiting but can leave permanent stiffness.

Everything below condenses SCFE for revision and viva practice — the high-yield points, the memory hooks, worked vivas, rapid-fire MCQ points, and a one-screen cheat sheet.