Table 3

Estimates of familial risk for anterior cruciate ligament rupture in monozygotic and dizygotic men and women.

MenWomenOpposite sex
DZ (n=8062)MZ (n=6418)DZ (n=9347)MZ (n=8113)DZ (n=12 267)
Pairs concordant for ACL rupture (n (%))2 (0.9)18 (8.4)4 (1.6)19 (7.6)7 (1.7)
Marginal risk (ie, population risk)*
(95% CI)
0.02 (0.01 to 0.02)0.02 (0.01 to 0.02)0.01 (0.01 to 0.02)0.02 (0.01 to 0.02)0.04 (0.04 to 0.06)
Casewise concordance†
(95% CI)
0.02 (0.00 to 0.07)0.14 (0.09 to 0.21)0.04 (0.01 to 0.09)0.13 (0.09 to 0.20)0.07 (0.04 to 0.13)
Familial risk (ie, relative recurrence risk ratio)‡ (95% CI)1.19 (0.00 to 2.81)8.89 (5.28 to 12.44)2.38 (0.14 to 4.62)8.71 (5.25 to 12.17)1.42 (0.68 to 2.17)
Tetrachoric correlation, r§
(95% CI)
0.03 (−0.20 to 0.25)0.48 (0.35 to 0.58)0.15 (−0.04 to 0.33)0.46 (0.34 to 0.56)0.09 (−0.06 to 0.23)
  • *The marginal risk is the population risk—that is, the risk of being diagnosed with ACL rupture without taking familial dependency into account.

  • †Casewise concordance is the risk of a twin being diagnosed with ACL rupture given that the co-twin has already been diagnosed with the injury.

  • ‡The familial risk is the excess risk of ACL rupture for a twin given that the co-twin has had the injury.

  • §The tetrachoric correlation is the intra-pair correlation between two theorised normally distributed continuous latent variables, from two observed binary variables (ACL rupture in twin 1 and ACL rupture in twin 2) and is used for calculating the heritability.

  • ACL, anterior cruciate ligament; DZ, dizygotic; MZ, monozygotic.