Screening for DDH appears to be performed during the neonatal period in Germany and Australia [4, 5]. In Japan, it was reported in the article by Tanabe et al. in 1972 . A neonatal examination using the click test was carried out. Despite the absence of abnormalities during the neonatal period, there were a number of paediatric patients in whom DDH was later diagnosed, indicating that it is of little significance to diagnose during the neonatal period. Instead, they recommended to perform screening at 3 months of age, when a diagnosis can easily be made. In Japan, the incidence of DDH was 1.1–3.5% prior to 1965; however, Yamamuro et al. reported that the incidence of DDH decreased to 0.2% or less after a national campaign was launched to provide guidance on holding newborn babies and putting on diapers in 1975 . Based on these changes, a policy to carry out preventive measures for hip dislocation, perform hip dislocation examinations at 3 months of age, and to perform treatment if there is DDH to all infants in Japan was established in the late 1970s. Subsequently, the conduct of hip dislocation examinations was also changed to 4 months of age since the timing of infant health examinations by paediatricians changed from 3 months to 4 months after birth.
Hattori et al. performed a nationwide Japanese survey via a multi-centre study. There were 1295 patients with DDH from 2011 to 2013 of whom 119 (15%) were diagnosed at the age of 1 year or older. An additional 36 patients (3%) were diagnosed at the age of 3 years or older . This report raised the issue of delayed DDH diagnosis in Japan. The Japanese Pediatric Orthopaedic Association released publication, the ‘Guide for hip dislocations in pediatric health examinations,’ on their webpage in 2014 . Secondary hip dislocation examinations based on this publication were started from 2015 in Nagasaki City, although secondary hip dislocation examinations had already been carried out at that point publication.
The Japanese Pediatric Orthopaedic Association’s publication was prepared based on techniques referred to as the Matsudo method. Shinohara performed x-rays on all patients during infancy. The results were evaluated in detail and risk factors for DDH were extracted. In 1974, Shinohara developed a method for selectively x-raying infants with risk factors. It is called the Matsudo method since it was carried out in Matsudo city . The results were reported in 2014 by Shinada. During the 41-year period, 196,643 infants were screened, and 10 were reported to have delayed diagnoses .
Limitations of abduction in flexion were the most frequent reason for referral to our department for secondary hip dislocation examinations (Table III). However, only 20 out of the 124 infants (16.1%) actually had limitations of abduction in flexion. Since it is the paediatrician who performs the primary screening for hip dislocation, the cause may be that accuracy decreases when compared to the screening performed by paediatric orthopaedic specialists. Additionally, since there is a time lag from the primary screening for hip dislocation to the secondary hip dislocation examination, improvements may have occurred during that time.
Asymmetry of femoral or inguinal skin creases was the second most frequent reason for referral to our department for secondary hip dislocation examination. Asymmetry was actually confirmed in only 20 out of 82 infants (24.4%). This rate may have been due to the fact that asymmetry was evaluated with hips extended and flexed and determined to be positive if observed in both or one as stated in the publication. We believe that femoral or inguinal skin crease asymmetry in a hip extended position has no pathological significance and therefore should not be identified as such. As Anderton et al.  have suggested, isolated asymmetrical skin creases are a not reliable clinical sign in the diagnosis of pathological DDH and we agree with this stance. Another common reason for being referred to our department was a combination of being a female infant and having asymmetry of the femoral or inguinal skin creases (Table IV). In the future, revisions to the Japanese Pediatric Orthopaedic Association’s publication may be necessary.
None of the infants diagnosed with Graf Type I at 4 months of age worsened from Graf Type I to Graf Type II or higher at 3 months. Thus, a second ultrasound is not necessary in infants with Graf Type I hip. Biedermann et al. stated that 99.6% of infants with Graf Type I hip at 1 month of age had Graf Type I at 3 months of age . This suggests that 0.4% of infants diagnosed at 1 month of age may worsen thereafter. The mean age at initial visit in our study was 4.6 months. We believe that the hips of infants at this time are already stable. Even if there are tentative limitations of abduction in flexion, the belief is that infants diagnosed with Graf Type I at 4 months of age will not worsen in terms of Graf classification. However, even with guidance provided on holding infants, limitations of abduction in flexion and femoral or inguinal skin crease asymmetry may remain 3 months later.
In a 1975 report, Yamamuro et al. stated that the mean acetabular index at the age of 1 year was 24.3° in males and 23.1° in females, which did not differ substantially from our results . Additionally, the confidence limit for the acetabular index at 5% risk was 30.8° in females and 30.5° in males at the age of 1 year . In our results, the acetabular index was 30° at the age of 1 year in 7.7% of the total. Dornacher et al. noted that there were no significant correlations between Graf classification and radiological outcomes at follow-up . In some infants diagnosed with Graf Type I hip dislocation at 4 months of age, acetabular dysplasia was diagnosed at the age of 1 year, and x-rays are considered to be required at least once.