Laboratory Studies
A renal panel in patients with renal failure–associated slips may reveal elevated blood urea nitrogen (BUN) and creatinine levels.
An endocrine panel is appropriate if endocrinopathy is suspected. Hypothyroidism is a common endocrinopathy associated with slipped capital femoral epiphysis (SCFE). In cases where hypothyroidism is suspected, triiodothyronine, thyroxine, and thyrotropin levels must be obtained. Other endocrinopathies also may be associated with SCFE (see Pathophysiology). In these cases, the appropriate investigations must be performed.
Plain Radiography
In the preslip stage (see Classification), the only positive findings on radiography are a widening and irregularity of the physis with rarefaction of its juxtaepiphysial portion.
Early diagnosis is made by using a Lauenstein (frog-leg) lateral view (see the image below) or true lateral view of the hip because small slips may be missed on the anteroposterior (AP) view. [47] A frog-leg lateral view should not be attempted in persons with acute or unstable slips, because it may cause further displacement.
Frog-leg (Lauenstein) lateral view, showing a mild slip that can easily be missed on an anteroposterior view.
The Klein line, a line along the superior border of the femoral neck, intersects less of the femoral head than it does on the opposite side (Trethowan sign) on the AP radiograph (see the image below). [48] Green et al, in a study of the diagnosis of SCFE using the Klein-line AP radiograph or the frog-leg lateral radiograph, found that the classic Klein line failed to identify 60% of slips; a modification of the Klein line by the authors increased sensitivity to 79%. [48]
Klein line (line drawn along the superior border of the neck intersects less of the capital epiphysis than on the unaffected side).
Localized rarefaction of the inferior medial metaphysis of the neck may be evident. The so-called metaphyseal blanch of Steel is a crescent-shaped area of increased density overlying the metaphysis adjacent to the physis on the AP radiograph. [49] This is an early sign of a posterior slip without significant medial slip and indicates the need for a lateral radiograph to identify the slip.
In moderate-to-severe slips, the overlap of the head and the metaphysis is visible.
Remodeling in chronic slips is seen as callus on the posteroinferior portion of the neck and rounding-off of the anterosuperior bare area of the neck, which is seen as a rounded hump.
Bone Scanning
Bone scanning [50] can be used to identify patients at high risk for avascular necrosis (AVN). A cold bone scan (demonstrating an absence of vascularity) essentially is seen only in unstable cases. When a patient has such a bone scan finding, the risk that AVN will subsequently develop is 80-100%.
Bone scans help in the early detection of AVN (ie, decreased activity/uptake on the femoral side). These scans also help in the early detection of chondrolysis (ie, increased activity/uptake on both the acetabular and the femoral sides).
CT, US, and MRI
Computed tomography (CT) can be used to measure the amount of retroversion and the degree of slip. Three-dimensional imaging of the upper femur, which may be useful in the preoperative planning of osteotomies, is also possible. CT scans may incidentally reveal asymptomatic, mild contralateral slips.
On ultrasonography (US), the presence of an effusion indicates an unstable slip, whereas the presence of remodeling indicates a stable slip.
Magnetic resonance imaging (MRI) is used for early detection of AVN.
Classification
Traditional classification
The traditional classification of SCFE is as follows:
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Preslip - Discomfort (pain and restricted internal rotation) is mild; the diagnosis is based primarily on radiologic findings, with widening and irregularity of the physis and generalized osteopenia in the affected hemipelvis [51] ; a scoring system for staging a preslip on the contralateral side has been developed [46]
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Chronic slip (85% of cases) - Symptoms persist for more than 3 weeks [43]
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Acute on chronic - An acute-on-chronic SCFE is one associated with chronic symptoms initially and subsequent development of acute symptoms, as well as a sudden increase in the degree of slip, usually with minor trauma
Bilateral slipped capital femoral epiphysis. One side shows evidence of remodeling of the neck and an anterior bone bump that restricts flexion. The other side demonstrates an acute slip as seen by the absence of any evidence of remodeling.
This traditional classification system may not be accurate and is not useful for prognosis. [53, 54, 52, 55, 56]
Classifications based on stability
The focus of classification has shifted to determining the stability of the slip because stability significantly affects the prognosis. The classification into stable and unstable categories is important because it is predictive of the prognosis. The traditional classification of slips as acute or chronic is misleading in that it does not take into account the stability of the hip, which is highly predictive of the development of AVN.
The Loder classification categorizes SCFE as follows [57] :
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SCFE is considered stable when the child is able to walk with or without crutches
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SCFE is considered unstable when the child cannot walk with or without crutches
The Kallio classification is a radiographic categorization that depends on the presence or absence of a hip effusion on US, as follows [58, 59] :
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If the ultrasonograms demonstrate the absence of metaphyseal remodeling and the presence of an effusion, an acute event is likely to have occurred, and the SCFE is considered unstable
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If the ultrasonograms demonstrate metaphyseal remodeling and the absence of an effusion, an acute event has not occurred, and the SCFE is considered stable
Classifications based on severity
One approach to a severity-based classification of SCFE makes use of the epiphyseal-metaphyseal angle. On a true lateral radiograph, the capital femoral epiphysis and the femoral neck usually lie at right angles to each other (up to 87° is considered normal). The physeal-neck angle decreases in proportion to the amount of slip. Categories are as follows:
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Mild - The neck is displaced less than one third the diameter of the femoral head
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Moderate - One third to one half of the diameter of the femoral head is displaced
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Severe - Displacement is more than one half the diameter of the femoral head
Another approach uses the epiphyseal-shaft angle (angular measurement method), as follows:
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Mild - Angle < 30°
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Moderate - Angle of 30-60°
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Severe - Angle >60°
Depending on the epiphyseal-shaft angle (slip angle of Southwick), this classification is probably most important with regard to long-term prognosis (see the image below). [60, 48] Mild and moderate slips have an excellent long-term prognosis when treated with in-situ pinning, whereas severe slips are associated with a more rapid decline in hip function over time. [61, 62, 63]
Southwick head-shaft angle (angle between the metaphyseal surface of the physis and the shaft of the femur on a frog leg lateral view). Difference from the opposite side is used to grade the severity of the slip.
Histologic Findings
In SCFE, the zone of hypertrophy is thickened, and the normal columnar arrangement of chondrocytes is lost. Chondrocyte clustering and disarray are evident. [64, 65]
Anatomically, the slip occurs in the layer of cartilage adjacent to the zone of provisional calcification (unlike a type 1 Salter-Harris injury, which occurs between the proliferating zone and the hypertrophic zone). However, the plane of cleavage may be irregular and may pass through different zones of the physis.
Ultrastructural studies show defective collagen fibrils and defects in collagen banding in the zone of hypertrophy and changes in the proteoglycan and glycoprotein concentrations in the zone of proliferation, with increased glycoprotein staining in the territorial matrix and increased proteoglycan staining in the extraterritorial matrix. [66]
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Valgus slip (rare).
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Frog-leg (Lauenstein) lateral view, showing a mild slip that can easily be missed on an anteroposterior view.
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Klein line (line drawn along the superior border of the neck intersects less of the capital epiphysis than on the unaffected side).
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Southwick head-shaft angle (angle between the metaphyseal surface of the physis and the shaft of the femur on a frog leg lateral view). Difference from the opposite side is used to grade the severity of the slip.
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Bilateral slipped capital femoral epiphysis. One side shows evidence of remodeling of the neck and an anterior bone bump that restricts flexion. The other side demonstrates an acute slip as seen by the absence of any evidence of remodeling.
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Chronic slipped capital femoral epiphysis showing the extent of remodeling along the anterior neck (arrow).
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Pin placement (anteroposterior view). A: The entry point must be at or above the level of the lesser trochanter to avoid the risk of subtrochanteric fracture. B: The pin (screw) should be in the center of the epiphysis. C: At least 2.5 threads engaging the epiphysis should be used for a secure hold.
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Pin placement (lateral view). Arrow shows entry point in the anterolateral cortex.
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Osteotomies in slipped capital femoral epiphysis (SCFE). A: Subcapital osteotomy. B: Base of the neck compensatory osteotomy. C: Intertrochanteric compensatory osteotomy. Note that the amount of correction increases from subcapital to intertrochanteric osteotomies.
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Principle of the Dunn osteotomy. Reduction of the slip after shortening the neck to avoid stretch on the vessels and decrease the risk of avascular necrosis.
