Hip and pelvic radiographic lines sharpen diagnosis across childhood and adolescence
Pelvic radiographs are not read by instinct alone. In the infant, the adolescent, and the mature hip, diagnosis depends on a set of reference lines, arcs, and angles that convert anatomy into measurable relationships. In developmental dysplasia of the hip, they help localize a femoral head that sits too far lateral or too high. In slipped capital femoral epiphysis, they expose subtle displacement between epiphysis, neck, and acetabulum. In the mature hip, they frame acetabular coverage and pelvic alignment.
No single sign carries the whole diagnosis.
Each line gains meaning only in context, because skeletal maturity changes the geometry of the pelvis and the way pathology projects on the film. A measurement that is foundational in the paediatric pelvis can be irrelevant in the adolescent hip, and a sign that is useful on an AP view may disappear if the slip is only visible on a lateral projection. The practical challenge at the bedside is not remembering names. It is matching the correct line to the correct age and clinical question.
Developing hip anatomy depends on a basic radiographic framework
Assessment of the developing hip begins with Hilgenreiner line, a horizontal reference drawn through the triradiate cartilages on the AP paediatric pelvis. It provides the baseline from which other measurements are built. With that line in place, Perkins line is drawn vertically through the lateral acetabular margin, perpendicular to Hilgenreiner line. Together they divide the developing hip into quadrants and help localize the proximal femur in developmental dysplasia of the hip.
The logic is simple but clinically powerful. If the femoral head lies outside the expected inferomedial quadrant, alignment has shifted and further evaluation becomes necessary. Acetabular development is then assessed with the Tönnis angle, which measures the acetabular roof angle between Hilgenreiner line and the roof itself. A larger angle indicates a steeper, less developed acetabulum and supports acetabular dysplasia.
The measurable advantage of this framework is reproducibility. A clinician looking at the same pelvis can separate orientation from coverage and coverage from migration, rather than relying on impression alone. In a child with suspected instability or limited abduction, that distinction determines whether the radiograph is merely describing anatomy or signaling true displacement.
Migration percentage turns displacement into a number
Reimers migration percentage pushes the analysis one step further by quantifying how much of the visible femoral head lies lateral to Perkins line. Calculated as a/b × 100, it converts displacement into a percentage that is easier to track over time than a qualitative estimate. The method is especially important in hip surveillance, particularly in children with cerebral palsy, where gradual lateral migration can be monitored serially.
A percentage is not just a number on a report. It is a way to follow whether the hip is staying contained or drifting out of the acetabulum. Serial measurements allow orthopaedic teams to compare one study with the next using the same geometric language, which matters when the child is growing and the pelvis is changing shape at the same time.
The Andrén–von Rosen view adds another projectional test. With the femora abducted at least 45° and internally rotated, the extended femoral shaft axis should point toward the acetabular margin in a reduced hip. In a dislocated hip, that axis points toward the anterior superior iliac spine. The value of the view lies in geometry rather than repetition: it looks at the hip from a different angle and can support the diagnosis when the standard AP pelvis is equivocal.
Walking-age hips and adolescent slips need a different set of signs
Once the child reaches walking age or adolescence, the radiographic questions change. Shenton line becomes useful as a smooth cervico-obturator arc that reflects the relationship between pelvis and proximal femur. When it is disrupted, displacement should be considered, although the sign remains non-specific. Its reliability is lower in neonates, where physiologic positioning can distort the arc, but it remains useful in walking-age children and in adolescent and adult hip pathology.
In slipped capital femoral epiphysis, AP and lateral views reveal different pieces of the same mechanical problem. Klein line extends the superior cortical margin of the femoral neck proximally; in a normal growing hip it intersects part of the lateral capital femoral epiphysis. With early slip, it intersects less of the epiphysis than expected. With greater displacement, it may miss the epiphysis entirely. The related Trethowan sign describes this abnormal neck–epiphyseal relationship.
The finding can be subtle. A child with hip, thigh, or knee pain may have a radiograph that looks nearly normal on first glance, especially if only the AP view is considered. The anatomy can still be slipping behind the radiograph’s apparent calm.
Supplementary signs can expose a slip before the classic line fails
The Capener triangle captures the normal dense triangular overlap of the inferomedial femoral neck or metaphysis with the posterior acetabular wall on the AP radiograph. In SCFE, posterior displacement reduces that overlap, so the triangle shrinks or disappears. It is a supplementary sign, not a stand-alone diagnosis, but it can add confidence when the slip is early or partial.
The metaphyseal blanch sign, also called the Steel sign, appears as a crescent-shaped area of increased density overlying the proximal femoral metaphysis adjacent to the physis. It is a projectional overlap caused by posterior displacement of the capital epiphysis behind the femoral-neck metaphysis. This sign may appear before Klein line becomes obviously abnormal, which is why a suspicious AP film should trigger a lateral radiograph rather than reassurance.
A single view can miss the problem. SCFE is a mechanical lesion, and mechanics often declare themselves only when two projections are compared.
On the frog-leg lateral, the S-sign evaluates contour continuity of the posteroinferior cortex of the femoral head and neck. Loss of that smooth contour supports the diagnosis of slip. On the AP pelvis, the abnormality may be subtle; on the lateral, the deformity can become easier to see because the displaced epiphysis no longer preserves the expected curve.
Mature hip measurements address coverage and pelvic alignment
In the mature hip, the key questions shift again. The Wiberg angle and Sharp angle assess acetabular coverage and orientation in a skeletally mature pelvis. Köhler line and Köhler teardrop are used as reference contours on the adult pelvic radiograph and contribute to assessment of pelvic alignment and acetabular anatomy.
These mature-hip measurements sit downstream of the pediatric framework. They do not diagnose the same conditions as Hilgenreiner or Perkins lines, but they extend the same radiographic habit: define a reference, compare anatomy against it, and decide whether coverage or alignment has departed from normal.
That continuity across age groups is what makes the system durable. The pelvis changes with growth, but radiographic logic remains the same: anchor the image to an anatomical landmark, then ask whether the femoral head is contained, centered, and aligned.
For clinicians reading a hip film, the consequence is immediate. Choose the wrong sign for the wrong developmental stage, and the image becomes misleading. Choose the right one, and a subtle dysplastic hip or early slip can be recognized before the deformity is fixed in bone.
Related reference names and historical attribution remain part of the vocabulary
Many of these measurements carry eponymous histories: Hilgenreiner, Perkins, Tönnis, Reimers, von Rosen, Shenton, Klein, Capener, and Steel. Some reflect the original observer; others preserve an adapted clinical usage that became standard over time. The names matter less than the anatomy they encode, but they remain embedded in day-to-day radiology and orthopaedic language.
What matters most is disciplined interpretation. Pelvic radiographs reward a methodical eye, especially when the image is being used to assess developmental dysplasia, SCFE, or adult acetabular coverage. The signs are complementary, not interchangeable, and each one becomes meaningful only when paired with the patient’s age and the clinical suspicion that brought the film to the screen.