How twin studies help to understand inflammatory joint disease.

Inflammatory joint disease (IJD) is a group of conditions that target the joints and periarticular structures. The contribution of genetic factors to these conditions is often less than 50%, suggesting a major role for environmental influences. Twin studies are the best means of assessing the role for genetic factors in IJD. Conclusive evidence has been provided by a few studies in vast samples of monozygotic and dizygotic twins, with only one twin in each pair having IJD. These studies have been most successful in ankylosing spondylitis and psoriatic arthritis. The other IJDs have proven more difficult to evaluate. This review demonstrates that genetic and environmental factors are inextricably linked and that ascribing IJDs to one or the other is misguided. Awareness of the limitations and possible sources of bias in twin studies is important when seeking to understand the development of these complex diseases.