It is important to find developmental dysplasia of the hip early to treat it successfully. In the first few months of life, babies often can wear a type of harness to fix the problem. But babies older than 6 months often need surgery, and those who are older than 1 year almost always need surgery.
Treatment depends on how old your child is when DDH is found. All types of treatment have one goal: putting the hip joint back in place and keeping it there. This kind of treatment is called reduction, and there are several methods.

Reduction for Hip Dysplasia

Treatment from 0 to 6 months of age

Proper fit of Pavlik harness. Courtesy of 'Fundamentals of Pediatric Orthopedics,' © 2003 Lippincott Williams & Wilkins

The youngest babies may wear a Pavlik harness. This is a soft harness that flexes and pulls the baby’s legs away from their middle while allowing the baby to move their legs. The harness keeps the ball (femoral head) at the top of the thighbone positioned deeply in the socket.
This is the simplest form of reduction treatment and works 90% of the time. It allows the baby to exercise their legs while redirecting the thighbone into the hip socket.

Pavlik Harness-worn for 3 weeks 23 hours per day

Hip Abduction Orthosis (also called hewson brace, very similiar to the Rhino Cruiser)-worn for 8 weeks 23 hours per day pre closed reduction. At nightime and naps for 6 months prior to Salter Osteotomy

Treatment from 6 to 18 months of age

If the Pavlik harness is unsuccessful or if a child is older than 6 months, they may need a body cast or a brace to hold the thighbone in the hip socket.
Doctors either manipulate your child’s joint with a cast or harness (closed reduction) or operate to put the thighbone in the socket (open reduction).
Open reduction is necessary for about 10% to 20% of children with DDH who are 6 to 18 months of age. During the procedure, the doctor opens the hip and puts the ball directly into the hip socket.
In some children, the hip (psoas) tendon also is tight and must be released. Doctors do this by making a small cut through the skin to release the tendon.

Spica Cast after Closed Reduction applies 6/22/10, changed July 2010 and removed 8/23/2010

Treatment for children older than 18 months

Older children usually need surgery to redirect or reshape the bones of the hip, the pelvis or the thighbone so their hips will stay in proper alignment.
After surgery, your child will wear a special body cast, called a spica cast, on their hips and legs for about three months. The cast will be changed once, midway through the time period.
After the doctor removes the cast, your child will wear a plastic-and-metal brace to hold the hips in place until X-rays show that the hip socket is normal.
Spica Cast after Salter Osteotomy 3/22/2011

Information from Seattle Children's hospital website