Developmental dysplasia of the hip is the most common musculoskeletal disorder in infancy, with a reported prevalence of 2% of all newborns. Although newborn screening programs based on clinical examination with Ortolani and Barlow tests were introduced in the 1950's and 1960's with early abduction splinting of the 2% testing positive, the prevalence of late cases warranting surgery has remained stable, around one per 1000. This has led to the introduction of ultrasound as an additional diagnostic tool, resulting in treatment rates of until 5-6%. This three fold increase in abduction splinting treatment is partly due to the initiation of treatment of infants in whom mild hip dysplasia but no hip instability has been identified. The benefit of early treatment of mild dysplasia in a hip that is neither dislocated nor dislocatable is unclear. Further, abduction splinting is not without risk, with avascular necrosis being reported in around 1%. The investigators conducted a masked, randomized, controlled trial to examine whether mildly dysplastic but stable or instable hips would benefit from early treatment, as compared to watchful waiting.

140 newborns with stable (not dislocatable or dislocated) but mildly dysplastic hips, born at the Maternity Hospital, Haukeland University Hospital, from 1997 onwards.

The newborns will be randomly assigned to one of two groups (number in sealed envelope, see flow chart). Because the newborns are recruited from a high risk group (60% with a positive family history and 35% breech), these risk factors are considered to be equally distributed in the two study groups. Stratification is therefore considered unnecessary.

For the controls, treatment will be started at age 1.5 months in cases of persistent dysplasia, i.e. a α-angle <50º, while treatment will be continued in the treatment group if the α-angle is ≤53º. At 3 months, treatment will be discontinued if the α-angle is ≥55º or started if α-angle is<55º. Treatment continued beyond 3 months will be discontinued when the AI is within two standard deviations according to the reference values from Tönnis and Brunken.

The study will require randomisation of 128 subjects into two equally sized groups to obtain 80% statistical power to detect a 3º difference of the α-angle. Less than 128 infants will be required to detect a similar difference in AI on radiography. To compensate for an expected rate of ineligibility and loss to follow-up of up to ten per cent, 140 patients will be enrolled.

PRACTICAL ISSUES The clinical hip-examinations will be performed at the maternity ward during day 1-3, by a physician with at least 2 years of pediatric experience. The hip joints will be classified as stable, unstable, dislocatable or dislocated. TM has the responsibility for the clinical re-examination prior to enrollement.

Ultrasound screening of newborns with increased risk for CDH is common practice at the maternity unit. Newborns eligible for the present study will be recruited from this high-risk group (about 13% of all newborns). To avoid inter-examination bias, all the ultrasound examinations will be performed by one examiner (KR), using a GE RT 3000, 5 MHz linear probe at KKB, and an ATL HDI 3000 machine with a 5 MHz linear probe or an Acuson 10XP, 5 MHz linear probe at the Section of Pediatric Radiology).

The ultrasound examinations will be performed according to a modified Graf procedure (Rosendahl), including both hip morphology and hip stability. Newborns with stable hips and a confirmed mild dysplasia on the second ultrasound examination will be re-examined clinically by TM, HR or TA prior to invitation to the study. After written informed consent has been given, the patient will attend the out patient clinic at BKB. A nurse will open the sealed envelope with a random number, and the newborn will enter the control or the treatment group. All data will be exported into SPSS by KR. RTL is the statistical adviser.

Inclusion Criteria:

• Birth weight > 2500 g
• Mild hip dysplasia on ultrasound day 1-2
• Written informed consent given

Exclusion Criteria:

• Birth weight less than 2500 g and/or severe congenital malformation(s)