INTRODUCTION
Hip osteoarthritis is an important cause of musculoskeletal disability. Epidemiological studies suggest that hip osteoarthritis has a multifactorial etiology and have identified several contributory risk factors [1], including increasing age, obesity, trauma, and occupational physical activity. In addition, clinical studies on hip osteoarthritis suggest that structural abnormalities of the hip that are present at delivery or develop in childhood may result in accelerated or premature joint degeneration [2]. Developmental dysplasia of the hip (DDH), formerly termed congenital dislocation of the hip (CDH), refers to a spectrum of anatomical abnormalities of the hip joint arising from a deviation in normal hip development during embryonic, fetal, and infantile growth periods [3]. Although, this problem normally resolves spontaneously within the first few months of life in most affected infants, persistent DDH can result in acetabular dysplasia followed by hip osteoarthritis. More than 70% of the cases of hip osteoarthritis in Japan are associated with acetabular dysplasia [4].
Clinical and epidemiological studies on DDH have been conducted. A greater number of subjects with DDH are born during winter than during warmer periods [5,6]. However, *Address correspondence to this author at the Department of Orthopedic Surgery, Faculty of Medicine, Saga University, Nabeshima 5-1-1, Saga 849-8501, Japan; Tel: +81-952-34-2343; Fax: +81-952-34-2059; E-mail: epc9719@yahoo.co.jp the incidence of hip osteoarthritis among patients born in different seasons has not been documented. Hip osteoarthritis and seasonal factors must also be related if there is a relationship between DDH and seasonal factors. The purpose of this study was to clarify the relationship between hip osteoarthritis and the month of birth in Japanese patients, using data from a hospital-based case control study. The study also addressed whether the season of birth affected the risk of hip osteoarthritis.

### Language ###
English

### Assessment ###
We could ask about the specific rate of hip osteoarthritis cases linked to acetabular dysplasia in Japan, which is clearly stated in the text.

### Query ###
Currently in Japan, what percentage of hip osteoarthritis can be traced back to acetabular dysplasia?
Currently in Japan, what percentage of hip osteoarthritis can be traced back to acetabular dysplasia?