Purpose: About 1% of the children are born with neonatal hip instability (NHI). By combining data from the Medical Birth Registry of Norway (MBRN) with that of the Norwegian Arthroplasty Register (NAR), the influence of NHI on the risk for total hip arthroplasty (THA) before 37 years of age are studied.

Materials and Methods: Since 1967 medical data, included stability of the hips, on all new-borns in Norway (2 092 536 babies) have been compiled. Since 1979 all THA performed in Norway are reported to the NAR. Until 2004 85,120 primary THAs were registered, of these 492 were performed on patients under 37 years of age. These two national registries were linked by using the unique person identification number assigned to each inhabitant of Norway.

Results: Of those 20 668 born with NHI (1%), 9 had received a THA before 37 years of age (43/100 000). Since only 18 of 100 000 new-borns without NHI had had THA, new-borns with NHI had 2,5 times increased risk for having a THA before they become 37 years.

Of the 492 THA in patients younger than 37 years in the NAR, 101 THA (20.5%) were, according to the surgeon, operated because of developmental dysplasia of hip (DDH). Since 13 of these were bilateral THA, the number of patients were 88. Only 9 of these 88 DDH-patients were, however, reported to have NHI. This is surprisingly few, since their dysplasia should be anticipated to be rather severe. Does this indicate that the hip-screening for new-borns in Norway should be changed?

Conclusions: New-borns with NHI has 2.5 times increased risk for THA before the age of 37 years compared to those with stable hips at birth. The absolute risk is, however, low, only 43/100 000. Of those 88 who received THA because of DDH before 37 years, 79 had, however, reported normal hips at birth.