Ultrasonography screening of DDH is recommended for all newborns [16, 17]. However, some guidelines recommend that only newborns with risk factors undergo such screening . If all newborns are screened by ultrasonography, issues, such as an increased use of braces  and detection of abnormalities that spontaneously improve , can arise. In addition, Biedermann et al. reported that 99.6% of Graf Type I infants at the age of 1 month were Graf Type I at the age of 3 months . Thus, 0.4% of those diagnosed at the age of 1 month may experience subsequent deterioration.
In the present study, ultrasonography screening was only performed for infants with risk factors at the age of 4 months. However, none of the hips diagnosed as Graf Type I at the age of 4 months showed worsening from Type I to ≥Type II at the age of 7 months. None of the patients experienced dislocation at the age of 1 year. The hip joints of infants at the age of 4 months thus appear to have already stabilized. Furthermore, even if there are limitations in abduction and flexion, the Graf classification will not worsen in 4-month-old infants with Graf Type I.
By contrast, when DDH treatment is initiated at the age of 4 months, it may be more difficult to treat compared with treatment started in the neonatal period; however, in all of our patients, treatment was feasible with a Pavlik harness, and there were no subsequent issues. Moreover, although there is a possibility of missing DDH when only infants with risk factors are examined, but our institution is the only institution treating DDH in Nagasaki City and patients with DDH who started walking have not been encountered in the past 5 years. Thus, it is believed that only infants with risk factors should undergo selective ultrasonography screening at the age of 4 months.
The “Guide to hip dislocation in infant health check-ups” published by the Japanese Society of Pediatric Orthopaedic Surgery was created by referring to the so-called Matsudo method. Shinohara performed X-rays in all patients during infancy. The results of this test were evaluated in detail to identify the risk factors of DDH. A method of selective X-rays for infants with appropriate risk factors was devised in 1974. This is called the Matsudo method because it was implemented in Matsudo City . When reporting on the results of this method in 2014, Shinada stated that out of 196,643 patients who were screened during a 41-year period, diagnosis was delayed for 10 patients only .
Yamamura et al. stated in 1975 that the mean acetabular index at the age of 1 year was 24.3° in males and 23.1° in females , which was not markedly different from our results. In addition, the report stated that the confidence limits of the acetabular angle at a risk of 5% was 30.8° in females and 30.5° in males at the age of 1 year . Our results showed that the acetabular index was 30° in 8.7% of the patients at the age of 1 year. Dornacher et al. stated that there were no significant correlations between the Graf classification and the radiological outcome at follow-up . A diagnosis of acetabular dysplasia was made at the age of 1 year for some patients who were diagnosed with Graf Type I at the age of 4 months, indicating that X-ray testing should be performed at least once.
This study had some limitations. We did not investigate data for all infants born in Nagasaki City, and the follow-up period was relatively short. Even if acetabular dysplasia is diagnosed at the age of 1 year, if the condition improves by the age of 4–5 years, it may not be necessary to perform X-ray testing at the age of 1 year.
Performing selective hip dislocation screening at the age of 4 months did not lead to any refractory cases or delayed diagnoses. At the age of 1 year, no patients had dislocations as per X-ray. However, some patients were diagnosed with acetabular dysplasia.