Functional evaluation and MRI of the shoulder rotator cuff : heritability estimates in monozygotic and dizygotic twin pairs.

Introduction The aim of our study is to establish if (a) the shoulder functionality, (b) specific anatomic characteristics predisposing to tendons lesions, (c) rotator cuff tear are genetically determined parameters. Materials and methods In the Italian Twin Registry, we identified 50 twin pairs of average age similar to that of patients with rotator cuff tear. We examined 31 twin pairs, 16 MZ (10 males; 6 females) mean aged 63y (range 53–72y) and 15 DZ (4 males; 8 females; 3 opposite sex) mean aged 63y (range 60–66y), without disease of the shoulder. All subjects underwent to: functional and subjective evaluation of the right shoulder (Constant Score; Simple Shoulder Test) and MRI. On the MRI images, we measured: acromio-humeral distance, angle of glenoid retroversion, area of the supraspinatus muscle; and we valued: degree of acromio-clavicular arthropathy, rotator cuff condition and Goutallier’s stage. Results Correlations of the three morphometric parameters were greater in MZ compared to DZ. The correlation of the acromiohumeral distance has been the highest (0.90 MZ; 0.24 DZ). Similar results have been obtained for the CS (MZ = 0.7; DZ = 0.5) and SST (MZ = 0.8; DZ = 0.6). From these correlations derivedheritability estimates of 32 and 34 % for the CS and the SST, respectively. Higher values of heritability included: glenoid retroversion (31 %) and acromio-humeral distance (91 %). The correlation of the variable (degeneration/tear of the infraspinatus tendon) resulted greater in MZ (0.91) compared to DZ (0.44). Excluding age and sex, a short acromio-humeral distance was significantly associated to a higher risk of degeneration/tear of the supraspinatus tendon (OR = 1.77, 95 % CI: 1.13–2.77, p = 0.01) and of the infraspinatus tendon (OR = 1.73, 95 % CI: 0.93–3.23, p = 0.08). Discussion Our data suggest that genetic factors play a preponderant role in determining the interindividual variability of the considered morphometric parameters. Conclusions This study is the first that uses the ‘‘twin design’’. Our results suggest that the variability for the acromion-humeral distance and the degeneration/tear of the posterior-superior rotator cuff in each twin pair depends more from genetic factors than from environmental.