Developmental Dysplasia of Hip
- Developmental dysplasia of the hip (DDH) is a spectrum: the ball (femoral head) and the socket (acetabulum) don't fit snugly, ranging from a slightly loose, shallow socket to a fully dislocated hip.
- Under about 4-6 months, the femoral head is still cartilage, so ultrasound is the test — X-rays have almost nothing to show yet.
- Once that head ossifies (roughly 4-6 months and beyond), you switch to the frontal pelvic radiograph and start drawing lines.
- On ultrasound the two big numbers are the alpha angle (bony socket depth — bigger is better) and the beta angle (cartilage roof coverage).
- It's a clinical-plus-imaging diagnosis: physical exam (Barlow/Ortolani) plus risk factors decide who even gets scanned.
Picture a golf ball sitting on a tee. If the tee is a nice deep cup, the ball stays put no matter how you wave the club around. If the tee is a shallow, worn-down nub, the ball wobbles, slides to the edge, and eventually rolls off entirely. That, in one sentence, is developmental dysplasia of the hip — the femoral head is the golf ball, the acetabulum is the tee, and DDH is every flavor of "this tee is not doing its job."
The word developmental is doing real work there. This isn't one fixed deformity you're born with and stuck with; it's a moving target that can improve or worsen as the baby grows, which is exactly why we catch it early and re-check.
Why babies and not grown-ups
Here's the catch that trips everyone up at first: in a newborn, the femoral head isn't bone yet. It's a ball of cartilage, and cartilage is basically invisible on an X-ray — it lets the beam sail right through like a window. So if you order a radiograph on a two-week-old with a clunky hip, you're staring at a pelvis with a conspicuous absence of the very thing you wanted to see.
That's why the workhorse before roughly 4-6 months of age is ultrasound. Sound waves don't care that the head is cartilage — they bounce off it happily, so you get to actually watch the hip sit (or not sit) in its socket, and even stress it gently in real time to see if it slips.
The timing rule of thumb: ultrasound early, radiograph later. The switch happens once the femoral head ossifies enough to cast a shadow on film — generally around 4-6 months. Before that, an X-ray mostly wastes everyone's afternoon.
The ultrasound: angles, not vibes
A hip ultrasound for DDH is delightfully geometric. You line up a standard coronal view and draw three lines: one down the flat iliac bone (your baseline), one along the bony roof of the socket, and one along the cartilage roof. The angles between them are the whole exam.
| Angle | What it measures | Direction of "good" |
|---|---|---|
| Alpha (α) | Depth of the bony socket | Larger = deeper, more stable cup |
| Beta (β) | Coverage by the cartilage roof | Smaller is generally better |
A nice deep bony socket gives a high alpha angle — the deep golf tee. A shallow, immature socket gives a low alpha angle, and the cartilage ends up doing more of the holding-in work than it should. You also eyeball how much of the round femoral head actually sits inside the socket versus bulging out the edge.
The radiograph: lines and quadrants
Once the head ossifies, the frontal pelvis X-ray takes over, and now you're drawing a little grid. The classic construction uses Hilgenreiner's line (horizontal, across the lowest points of the iliac bones) and Perkin's line (vertical, down the outer edge of the bony socket). Together they carve the hip into four quadrants, and a normal ossifying femoral head should live in the lower-inner quadrant. A dysplastic or dislocated head drifts up and out.
You also trace Shenton's line — a smooth curve that should sweep continuously from the underside of the femoral neck onto the upper edge of the obturator foramen. In a healthy hip it's one graceful arc, like the line of a well-drawn eyebrow. In a dislocated hip that arc breaks and steps up. A broken Shenton's line is one of those findings that, once you see it, you can't unsee.
A normal newborn hip can look a little loose, and a baby's hips mature fast in the first weeks. Scanning too early can over-call mild physiologic laxity as disease. That's why screening ultrasound is often timed a few weeks out rather than on day one — and why borderline findings get re-imaged rather than rushed to treatment.
Who gets looked at, and why it matters
DDH loves a few setups: breech positioning, a family history, firstborn girls, and the left hip in particular (the left hip is the more commonly affected side). Tight swaddling that pins the legs straight out doesn't help either — hips like to be flexed and a little splayed, frog-style.
The reason we chase this so hard in infancy is leverage. Caught early, a hip can often be coaxed back into a good socket with a simple harness that holds it flexed and abducted — and a deep socket, once achieved, tends to keep developing normally. Caught late, you're looking at a shallow socket that has already "set," a child who limps, and a real risk of early arthritis down the road. Same disease, wildly different stakes, and the difference is mostly time.
This is also distinct from a true bony fracture line through a growth plate, which lives in a different chapter entirely — see Salter-Harris fractures for that crowd. DDH isn't a break; it's a socket that never grew up properly.
So the one thing to carry out the door: match the test to the bone. Cartilage hip, use ultrasound and measure the alpha angle. Ossified hip, use the radiograph and check that Shenton's line stays smooth. Get the timing right and DDH goes from a future hip replacement to a few months in a harness.