# Slipped Capital Femoral Epiphysis

## Introduction

Slipped capital femoral epiphysis (SCFE) is the most common hip disorder in adolescents, characterized by displacement of the femoral epiphysis relative to the metaphysis through the physis. The epiphysis typically displaces posteriorly and inferiorly while the metaphysis displaces anteriorly and superiorly. Prompt diagnosis and appropriate surgical stabilization are essential to prevent the devastating complications of avascular necrosis and chondrolysis.

## Epidemiology

SCFE has an incidence of approximately 10 per 100,000 adolescents. Peak age is 12 to 15 years in boys and 10 to 13 years in girls, coinciding with the adolescent growth spurt. Males are affected approximately twice as often as females. Bilateral involvement occurs in 20 to 40% of cases, and the contralateral slip may be simultaneous or sequential. Obesity is the strongest modifiable risk factor, with most patients above the 90th percentile for BMI. Endocrine disorders including hypothyroidism, growth hormone deficiency, hypogonadism, and renal osteodystrophy are also associated. African American and Pacific Islander populations have a higher incidence.

## Pathophysiology

The physis is weakened by the combination of hormonal changes involving growth hormone, sex hormones, and thyroid hormone alongside increased mechanical load from obesity. The hypertrophic zone of the physis becomes widened and disorganized. Shear forces across the weakened physis cause the epiphysis to displace posteriorly and inferiorly. In endocrine-related forms, the physis is abnormally weak independent of mechanical factors.

## Classification

| Classification | Criteria | Details |
|---------------|----------|---------|
| Stable | Able to bear weight ± crutches | ~90% of cases; <5% AVN risk |
| Unstable | Unable to bear weight | ~10% of cases; 20-50% AVN risk |
| Mild (Southwick) | <30° slip angle | |
| Moderate | 30-50° slip angle | |
| Severe | >50° slip angle | |

### By Onset

The stability classification is the most clinically important. A stable SCFE means the patient is able to bear weight with or without crutches and comprises approximately 90% of cases. An unstable SCFE means the patient is unable to bear weight even with crutches, accounting for approximately 10% of cases but carrying a 20 to 50% risk of avascular necrosis.

### By Chronicity

Acute slips present with symptoms for less than 3 weeks and are typically unstable. Chronic slips have symptoms for more than 3 weeks and are typically stable with remodeling changes visible on imaging. Acute-on-chronic slips represent an acute exacerbation superimposed on chronic symptoms.

### By Severity (Southwick Angle)

Severity is measured as the difference in the head-shaft angle between the affected and unaffected sides on the frog-lateral radiograph. Mild slips have less than 30 degrees of displacement. Moderate slips range from 30 to 50 degrees. Severe slips exceed 50 degrees.

## Clinical Presentation

The typical presentation is groin, thigh, or knee pain with a limp in an overweight adolescent. Knee pain is the presenting complaint in up to 25% of cases, which is a common cause of delayed diagnosis, so the hip should always be examined. The Drehmann sign demonstrates obligatory external rotation of the hip during passive flexion. Physical examination reveals limited internal rotation, flexion, and abduction. An antalgic gait with the affected limb held in external rotation is characteristic. In unstable SCFE, the patient presents with severe pain and inability to bear weight, similar to a fracture presentation.

## Imaging

### Plain Radiographs

AP pelvis and frog-lateral views of both hips are mandatory. Klein's line, drawn along the superior femoral neck on the AP view, should intersect the lateral epiphysis, and failure to do so indicates a slip. Widening and irregularity of the physis may be visible on the AP view. The blanch sign of Steel shows a double density at the metaphysis due to the overlapping posteriorly displaced epiphysis. The frog-lateral view is more sensitive for detecting mild slips. The Southwick angle is measured on the frog-lateral for severity grading.

### Advanced Imaging

MRI is useful for pre-slip diagnosis, revealing physeal widening and bone marrow edema, and for evaluating epiphyseal perfusion in unstable SCFE. CT can assess the degree of slip and help with surgical planning for osteotomy.

## Treatment

### Stable SCFE

In situ fixation with a single cannulated screw is the standard of care. The screw is placed percutaneously through the anterolateral femoral neck into the center of the epiphysis and should be positioned perpendicular to the physis in the center-center position on both AP and lateral views. Reduction of a stable SCFE should not be attempted, as manipulation increases the risk of AVN. Screw threads must cross the physis and engage the epiphysis to provide stabilization. Prophylactic fixation of the contralateral hip is controversial but should be considered in patients with endocrine disorders, younger age, or very high risk.

### Unstable SCFE

Unstable SCFE is considered a surgical urgency, with surgery recommended within 24 hours. Gentle reduction, if performed, is followed by single screw fixation. The role of reduction is debated, with some surgeons advocating gentle positioning reduction on the fracture table while avoiding forceful manipulation. Capsulotomy or aspiration to decompress the intracapsular hematoma may improve epiphyseal perfusion and reduce AVN risk, though this is emerging evidence not universally adopted. The modified Dunn procedure, which involves surgical hip dislocation with open reduction and fixation, is performed at specialized centers for severe unstable slips and allows direct visualization and protection of the retinacular blood supply.

### Severe Chronic Slips with Significant Deformity

In situ fixation remains the initial treatment even for severe slips. Proximal femoral osteotomy such as an Imhaeuser or base-of-neck osteotomy may be performed after physeal closure to correct residual deformity. Femoroacetabular impingement from the metaphyseal bump (cam lesion) may require osteochondroplasty after healing.

## Complications

### Avascular Necrosis (AVN)

AVN is the most devastating complication, with an incidence of less than 5% for stable SCFE and 20 to 50% for unstable SCFE. It results from disruption of the lateral epiphyseal (retinacular) blood supply. Risk is increased by forced reduction, multiple pin attempts, and screw malposition. No proven treatment exists once AVN is established, and arthroplasty may ultimately be required.

### Chondrolysis

Chondrolysis is acute cartilage destruction leading to rapid joint space narrowing, stiffness, and pain. It is associated with unrecognized screw penetration of the joint surface and prolonged immobilization. Incidence has decreased with modern imaging-guided techniques.

### Femoroacetabular Impingement

The metaphyseal prominence created by the slip causes cam-type impingement with flexion, leading to labral tears and early osteoarthritis if untreated. Osteochondroplasty may be indicated after physeal closure.

### Slip Progression

Continued slippage may occur if the physis is not adequately stabilized. It is essential to ensure the screw crosses the physis with threads engaging the epiphysis.

## Clinical Pearls

Any overweight adolescent presenting with hip, thigh, or knee pain should be evaluated for SCFE, as knee pain is the presenting complaint in 25% of cases and is a common cause of delayed diagnosis. Stable SCFE is treated with in situ fixation using a single percutaneous screw, and reduction of a stable slip should not be attempted because it increases the risk of AVN. Unstable SCFE carries a 20 to 50% risk of AVN regardless of treatment, but urgent stabilization within 24 hours and consideration of capsular decompression may improve outcomes. Radiographs of both hips should always be obtained, as bilateral involvement occurs in 20 to 40% of cases and prophylactic fixation should be considered in high-risk patients.

## References
1. Loder RT, Richards BS, Shapiro PS, et al. Acute slipped capital femoral epiphysis: the importance of physeal stability. *J Bone Joint Surg Am*. 1993;75(8):1134-1140.
2. Aronsson DD, Loder RT, Breur GJ, Weinstein SL. Slipped capital femoral epiphysis: current concepts. *J Am Acad Orthop Surg*. 2006;14(12):666-679.
3. Ziebarth K, Zilkens C, Spencer S, et al. Capital realignment for moderate and severe SCFE using a modified Dunn procedure. *Clin Orthop Relat Res*. 2009;467(3):704-716.
4. Sankar WN, Vanderhave KL, Herrera-Soto JA, et al. The modified Dunn procedure for unstable slipped capital femoral epiphysis. *J Bone Joint Surg Am*. 2013;95(7):585-591.