This study sought to determine the genetic contribution of Perthes' disease, using the world's largest twin-registry.

We extracted all twin pairs from the Danish Twin Registry (DTR) in which at least one individual had Perthes' Disease. The DTR captures every twin pair born alive in Denmark. Those with Perthes' disease were identified using health record linkage to the Danish Morbidity Record. Probandwise concordance was calculated to describe the likelihood that any given individual had LCPD if their co-twin was also diagnosed.

There were 81 twin pairs; 10 monozygotic (MZ), 51 dizygotic (DZ), and 20 unclassified (UZ). There was no association between birth weight and being the affected co-twin. Four pairs (two dizygotic and two unclassified) were concordant for LCPD, which is greater than would be expected assuming no familial aggregation. There were no concordant MZ twin pairs. The overall probandwise concordance was 0.09 (95% CI 0.01–0.18): 0.00 for the MZ, 0.08 (95% CI 0.00–0.18) for the DZ, and 0.18 (95% 0.00–0.40) for the UZ twin pairs.

This study found evidence of familial clustering in LCPD but did not demonstrate a genetic component. The absolute risk that a co-twin of an affected individual will develop LCPD is low, even in the case of MZ twin pairs.