Of twenty-one patients with juvenile chronic arthritis (seventeen with
juvenile rheumatoid arthritis and four with juvenile ankylosing
spondylitis) who had total hip replacement before the age of thirty years,
thirteen patients (62 per cent) showed different degrees of ectopic
ossification: nine (53 per cent) of the seventeen with juvenile rheumatoid
arthritis and all four of those with juvenile ankylosing spondylitis. Human
leukocyte antigen B5 was found with an increased frequency in the patients
with ectopic ossification. Functional capacity was not impaired regardless
of the severity of the ectopic ossification. As in patients with
adult-onset disease, ectopic ossification after total hip replacement
occurred with a higher frequency in the patients with juvenile-onset
ankylosing spondylitis than in those with juvenile-onset rheumatoid
arthritis.