developmental dysplasia of the hip comprises of disorders of hip development that present in different forms at different ages . the common etiology is excessive laxity of the hip capsule , which fails to maintain the femoral head within the acetabulum . in newborns , the syndrome consists of instability of the hip such that the femoral head can be displaced from the acetabulum . the hip may also rest in a dislocated position and be reducible . over time , the femoral head becomes fully dislocated and can not be reduced by changing the position of the hip . the syndrome may manifest later in childhood as a developmental dislocation of the hip ( ddh ) or in adolescence as a hip with poorly developed acetabular coverage ; the latter is termed acetabular dysplasia ( ad ) . in general , infants are screened for ddh in the first 4 months of life by clinical examination . if the infant demonstrates limited abduction of the affected hip , further physical examination , plain radiography , and an ultrasound are performed . infants diagnosed with ddh are usually treated with a pavlik harness at the time of diagnosis . in addition , traction , closed reduction , and open reduction procedures are also selected as treatment options depending on the patient 's age . from 8% to 60% of patients with a history of treatment for ddh osteoarthritis ( oa ) is an age - related degenerative disease that is common in both middle - aged and older women . primary oa of the hip is an extremely rare condition in japan ; most patients have secondary oa due to ad and ddh . japanese patients with oa of the hip have been reported to have higher rates of dysplasia than american patients ( 46% vs 4.5% ) , and significantly worse dysplasia compared with that in british populations ( mean center - edge angle of 37 vs 31 ) . patients with ad have been reported to exhibit an abnormal morphology of the pelvis and have a high rate of comorbid spinal congenital anomalies , such as spina bifida occulta . in a recent study , bone mineral density was reported to be higher in patients with ad than that in normal controls . these reports suggest there is a genetic link to the development of ad . in this study , to investigate the relationship between ad and ddh , we examined the percentage of ad patients with hip pain at skeletal maturity also having a history of ddh in infancy and the correlation between the severity of ad at skeletal maturity and a history of treatment for ddh . this retrospective diagnostic study received permission for publication from the institutional review board of our institute . for this study , we selected patients diagnosed with ad of the hip as well as pre - arthritis or early - stage osteoarthritis . patients with advanced or end - stage osteoarthritis were excluded from the study so that we could more accurately evaluate ad by excluding osteophyte formation . patients less than 16 years of age were not included to exclude premature hip joints and those older than 60 years were excluded because of the difficulty of confirming a history of treatment for ddh . in addition , patients with a history of hip osteotomy , hip dislocation into the gluteal muscles , secondary post - traumatic osteoarthritis , inflammatory rheumatic disease , osteonecrosis , or infectious diseases were excluded from the study . all patients in the study visited our hospital for consultation regarding hip joint pain between 2009 and 2012 . a total of 245 patients were radiographically examined and questioned for any history of ddh treatment in infancy . the study population comprised 226 women and 19 men with a mean age at examination of 40.7 years ( range 1759 years ) . study parameters evaluated were center - edge angle ( ce angle ) , acetabular head index ( ahi ) , acetabular angle , and acetabular roof angle ( figure 1 ) . ad was defined as a ce angle less than 20 , ahi less than 75% , acetabular angle more than 45 , or acetabular roof angle more than 15 on anteroposterior radiographs . when bilateral ad was observed in a patient , the joint with a higher degree of ad diagram showing the radiological parameters ( a , center - edge angle ; b , acetabular angle ; c , acetabular roof angle ; acetabular head index ( ahi )  =  a / b    100 ) . osteoarthritis of the hip was classified into the following four stages : pre - arthritis stage with no osteoarthritic change ; early stage with narrowing of the joint space associated with sclerosis of the subchondral bone ; advanced stage with partial disappearance of the joint space and evidence of cystic radiolucencies and osteophytes , and end stage with almost a total disappearance of the joint space and marked osteophyte formation . anteroposterior radiographs were taken using a source - to - film distance of 110  cm . the patient 's feet were internally rotated with the toes at 15    5 to ensure that the x - ray beam was centered on the superior aspect of the pubic symphysis . to analyze the relationship between the severity of ad at skeletal maturity and history of treatment for ddh , patients were divided into three groups ( mild , moderate , and severe ad ) , comprising equal number of patients ( table 1 ) . variables in mild , moderate , and severe acetabular dysplasia groups to test the reproducibility of the radiographic measurements , three authors ( ko , ky , and yn ) measured the ce angle , ahi , acetabular angle , and acetabular roof angle in five randomly selected hips . each hip was measured three times with an interval of 1 week between measurements , and the values were subsequently averaged . the data were analyzed for intra- and inter - observer variances , and the coefficient of variation was calculated to be less than 5% . differences between values in ddh and non - ddh groups were tested using the mann differences in the percentage of ddh patients between the mild and severe dysplasia groups were tested using fisher exact test . the cochran  armitage test was used to test for trends in the percentage of ddh patients according to the three groups . logistic regression analysis was used to evaluate the simultaneous effect of various factors on ddh . the significance level of the hypothesis test was chosen as p  <  0.05 . all statistical analyses were performed with ezr ( saitama medical center , jichi medical university ) . a total of 88 patients ( 36% ) had a past history of treatment for ddh ( ddh group ) and the remaining 157 patients ( 64% ) had no history of ddh ( non - ddh group ) . although the average age was lower and the acetabular angle was larger in ddh group , no significant differences of height , weight , ce angle , ahi , and acetabular roof angle were observed between the ddh and non - ddh groups ( table 2 ) . younger age was significantly associated with ddh after adjustment for ce angle , ahi , acetabular angle , and acetabular roof angle [ odds ratio ( or )  =  1.09 , 95% confidence interval ( ci )  =  1.061.13 , p  <  0.0001 ] . variables in total , ddh , and non - ddh group there were significant differences in percentages of ddh and non - ddh patients between the mild and severe dysplasia groups classified by acetabular angle ( p  =  0.048 , figure 2c ) and acetabular roof angle ( p  =  0.013 , figure 2d ) ; however , no differences were observed on the basis of ce ( p  =  0.12 ) ( figure 2a ) and ahi ( p  =  0.28 ) ( figure 2b ) . there were significantly increasing trends in the percentage of ddh patients according to severity of ad classified on the basis of ce ( p  =  0.039 ) ( figure 2a ) , acetabular angle ( p  =  0.017 ) ( figure 2c ) , and acetabular roof angle ( p  =  0.0066 ) ( figure 2d ) . however , no trend was observed among the three groups classified on the basis of ahi ( p  =  0.12 ) ( figure 2b ) . graph showing the number of ddh ( black square ) and non - ddh ( white square ) patients in three groups ( mild , moderate , and severe dysplasia ) classified by ce angle ( figure 2a ) , ahi ( figure 2b ) , acetabular angle ( figure 2c ) , and acetabular roof angle ( figure 2d ) . previous reports have described that 8 to 60% patients treated for ddh in infancy have residual ad at skeletal maturity . in this study , we investigated the relationship between ddh in infancy and the later development of ad . we observed that 64% patients with ad at skeletal maturity have no history of treatment for ddh . although there was an increasing trend in the percentage of ddh patients associated with severity of ad at skeletal maturity , more than half of the patients with severe ad at skeletal maturity had no history of treatment for ddh . our study had several limitations with regard to obtaining the treatment history of ddh patients . first , with regard to questioning patients and their parents on the type of treatment received for ddh during infancy , although we were able to obtain treatment history from all patients in the ddh group , we were not able to confirm certain details , such as the side of the dislocated hip or the exact treatment method employed . these patients could describe the treatment history , but were unable to provide detailed aspects of the treatment methods used . second , there is a possibility that the non - ddh group in the present study may have included patients with untreated ddh that spontaneously improved . in this study , we observed several ad patients at skeletal maturity with no history of treatment for ddh . patients with ad have been reported to exhibit an abnormal morphology of the pelvis , a high rate of coexisting spinal congenital anomalies , such as spina bifida occulta , and high bone mineral densities of the lumbar spine , ultradistal radius , and calcaneus . as previously mentioned , primary oa of the hip is an extremely rare condition in japan . japanese patients with oa have been reported to have higher rates of dysplasia than american patients ( 46% vs 4.5% ) and significantly worse dysplasia than british populations ( mean ce angle 37 vs 31 ) . these reports and our data suggest that one - third of patients with ad in japan have residual ad affected by treatment of ddh , and the remaining two - third patients came from a genetic background characteristic of japanese individuals from that of caucasians . some previous studies described a natural course of osteoarthritis of the hip in patients with ad . hasegawa et al evaluated 86 hips in 59 patients with pre- or early stage osteoarthritis ( average age 29.9 years ) and 31 hips ( 66% ) with more progressive disease ( average age 7.8 years ) . small values for ce angle and ahi and large values for acetabular roof angle were observed in the hips of patients with pre - osteoarthritis , which progressed to an advanced stage . however , no differences in these radiographic parameters were observed in the hips of patients in early stages of osteoarthritis . hisatome et al reported that 7 of 61 ( 11% ) hips ( average age 38.2 years ) with pre- or early stage osteoarthritis progressed to advanced stages within an average of 10.1 years . no differences in radiographic parameters were observed between the hips with maintained and progressive disease . neither of these previous studies analyzed existing treatment for ddh in infancy . in this study , the average age of patients initially examined for hip pain was 40.7 years , and lower ages were observed in the ddh group ( 36.2 years ) compared with the non - ddh group ( 43.3 years ) . in addition , a significant difference was still observed after adjustment for radiographic parameters of ad . complications of dislocated hip joints in infancy ( elongated ligamentum teres , everted labrum , and a stretched hip capsule ) may accelerate development of pain in patients with ddh at skeletal maturity , prompting them to seek care for the pain earlier . there were significant differences between the mild and severe dysplasia groups , and an increasing trend in the percentage of patients with ddh associated with severity of ad classified using acetabular angle and acetabular roof angle ; however , no differences were observed in cases of severity of ad classified on the basis of ce angle and ahi . ce angle and ahi evaluations represent the position of the femoral head to the acetabulum . in contrast , acetabular angle and acetabular roof angle evaluation represents the whole structure and slope of the weight - bearing area of the acetabulum ( figure 1 ) . the labrum of a non - dislocated hip is a thin fibrocartilaginous rim around the periphery of the acetabular cartilage . this vital cartilaginous acetabular analogue is essential for normal growth and development of the acetabulum . at the fibrocartilage  hyaline junction of the labrum and acetabulum , there may be eversional or inversional hypertrophic changes ( neolimbus ) in the dislocated hip . neolimbus , which was formed during dislocation of the hip , may affect acetabulum growth even after reduction . in conclusion , we observed that 64% patients with ad at skeletal maturity have no history of treatment for ddh . ad patients with a history of treatment for ddh have hip joint pain at a younger age and severe dysplasia , particularly when evaluated by acetabular angle and acetabular roof angle . our data suggests that ad in patients without a history of ddh has different characteristics from ad in patients with a history of ddh .