Developmental Dysplasia of the Hip
- Developmental dysplasia of the hip (DDH) is a spectrum: a hip socket that's too shallow, a femoral head that slips around in it, or one that's frankly dislocated.
- The single most important fact about imaging it is timing. Before the hip bones turn to bone, X-rays are nearly useless and ultrasound is the star. After they ossify, the roles flip.
- Ultrasound is the tool of choice in the early months because the cartilage you care about is invisible on X-ray but shows up beautifully on sound.
- Treated early, most hips do great. Found late, it gets a lot harder — which is exactly why screening exists.
Imagine a golf ball sitting in a tee. If the tee is a nice deep cup, the ball stays put no matter how you wiggle it. If the tee is a flat, lazy little dish, the ball rolls right off. The hip is supposed to be the deep cup. In developmental dysplasia of the hip — DDH — the cup came in too shallow, and the ball (the femoral head) is free to slide toward the edge, perch halfway out, or pop out entirely.
It's called developmental because it isn't always present and obvious at birth — some hips drift out of position over the first weeks and months. So this isn't a one-and-done diagnosis; it's a moving target, which is half of why it's a favorite exam topic.
Why the baby's age changes everything
Here's the twist that trips everyone up. A newborn's hip is mostly cartilage. The roof of the socket and the femoral head haven't turned to bone yet. And cartilage is essentially invisible on an X-ray — it lets the beam pass straight through, the same way water does (a quick refresher on why different tissues look different lives in the four radiographic densities).
So if you X-ray a young infant's hip, you're photographing a mostly empty room and trying to guess where the furniture is. Not helpful.
Sound, on the other hand, doesn't care about bone. Ultrasound sees cartilage just fine — it bounces sound off soft tissue and builds a picture from the echoes (the how-it-works version is in ultrasound physics). That's why, in the early months, ultrasound is the imaging tool of choice for the hip.
The rough rule of thumb: ultrasound is the workhorse in the early infant months, before the femoral head ossifies. Once that bony nucleus shows up — generally somewhere in the second half of the first year — the radiograph takes over. The exact crossover is a range, not a birthday, so don't memorize a magic number.
What the ultrasound is actually measuring
The classic hip ultrasound is done with the baby's hip held in a standard position, and the radiologist looks at two things: how deep and well-formed the bony socket is, and how much of the femoral head is actually tucked under the roof rather than hanging out over the edge.
A deep, sharp-angled socket cradling a well-covered head is reassuring. A shallow, rounded socket with the head sliding laterally is the worry. Some practices add a gentle stress maneuver — a controlled push to see if the head is stable or whether it slips — because a hip that looks okay sitting still but dislocates when you nudge it is still an unstable hip.
When the X-ray takes over
Once the femoral head has ossified, the radiograph becomes useful, and now you're reading geometry. The pelvis becomes a little map: you draw reference lines off the bony landmarks and check that the femoral head sits in the right quadrant, that the socket roof slopes at a reasonable angle, and that the two hips are symmetric. A head that's drifted up and out, with a steeper-than-normal socket roof, is the abnormal hip.
Don't get fooled
A normal newborn hip can feel a little loose and still be completely fine — mild laxity in the first days is common and often settles on its own. The flip side is the real trap: a dislocated hip that has been out for a while can stop clunking and start to feel deceptively "stable" because it has settled into a false position. Quiet is not the same as normal.
There are well-known risk factors worth flagging — being a first-born, breech positioning late in pregnancy, a family history, and being female all push the odds up. None of these make the diagnosis, but they're the babies people watch most closely.
Why we bother chasing it early
| When it's caught | Typical approach | General outlook |
|---|---|---|
| Early infancy | Bracing to hold the hip seated while the socket deepens | Usually very good |
| Later (walking age and beyond) | More involved interventions, sometimes surgical | Harder road |
The whole reason for screening newborns is buried in that table. A shallow socket isn't a fixed flaw — held in the right position early, it tends to deepen and mold around the head the way it was always meant to. Miss the window, and you're trying to remodel a joint that has already set in the wrong shape.
DDH imaging is a tale of two tools split by age: ultrasound for the cartilage-rich early months, the radiograph once the bone shows up. Match the tool to the hip's age and the rest follows.
One last note in keeping with caring for small patients: the move toward ultrasound first isn't only about seeing cartilage — it's also radiation-free, which is always the goal in kids (more on that mindset in radiation dose in children).