Trunk asymmetry (TA) is defined as a clinical expression of scoliosis1 and appears to correlate with the prediction of future scoliosis in adolescents; however, it is not a sensitive clinical sign of scoliosis.2–4 Changes in TA mainly develops during the pubertal growth spurt in girls and boys and shows a strong surface-spine relation with scoliosis.5–7

The aetiology of scoliosis is multi-factorial, and includes genetic, tissue, hormonal, and neurosensorial factors.8 Side dominance (SD), one of the behavioural markers of early neurodevelopment, can be defined as preference or hand-differences in task performance and represents an expression of the motor cortex asymmetry of the brain.9 A positive association has been reported between laterality of the curve in scoliotic adolescents and hand dominance,10 but further studies are needed to confirm this. One of the hypotheses involves the cerebral cortex functioning as an aetiopathogenic factor for the development of TA.11 In other studies, a correlation between SD and the scoliotic curve has a prevalence in right thoracic scoliosis and right-handed patients.12 The obvious predominance of right convex thoracic curve patterns in scoliosis has always suggested a parallel with hand preference and with other patterns of cerebral lateralisation.13 This has prompted hypotheses that associate the two, either with the dominant side causing scoliosis or the reverse, scoliosis determining SD, but curve convexity itself is not a major predictive sign.1,14

For some authors, the laterality evaluation should form an integral part of the scoliotic patient's assessment; the majority of these studies have only evaluated the graphic manual laterality,15 but the development of laterality is a complex process that involves a different part of the cortex.9,16 For this reason, it is preferable to use an evaluation of the hand, podal and visual laterality in subjects to verify a possible aetiopathogenesis hypothesis of side dominance.17 Therefore, the objective of this study is to evaluate the prevalence between TA and side dominance in hand, foot and visual laterality in adolescents.

The overall characteristics of the included population are reported in Table 1Table 1.

Of the 1029 subjects included, 79.9% were symmetric (ATR <4°), 18.0% were asymmetric with an ATR between 4° and 7°, and 2.1% were asymmetric with an ATR ≥7°. Overall, 89.5% of the sample showed right-side dominance, 6.7% reported left-side dominance and 3.8% were ambidextrous. In total, 96.1% of boys and 93.5% of girls were symmetric in the thoracic region; for the thoracolumbar and lumbar regions, the corresponding percentages were 94.6–89.4% and 96.1–89.2%, respectively. ATR mean values in the subjects with TA were 4.8±1.1, 5.3±1.6 and 5.1±1.5 in the thoracic, thoraco-lumbar and lumbar spine, respectively, as shown in Table 1. No significant differences were detected between subjects with or without a SD and TA with respect to gender.

The association of right-side dominance with TA was estimated to have a prevalence of 0.43%, 1.01% and 0.87% for right thoracic, right thoracolumbar and left lumbar curves, respectively. It was not possible to consider the right-side dominance as a predictor of TA and vice versa, due to the very low sensitivity, specificity, positive and negative predictive values and likelihood ratios (Table 2Table 2).

The association of left-side dominance with TA was estimated to have a prevalence of 2.72%, 2.54% and 0.65% for right thoracic, right thoracolumbar and left lumbar curves, respectively. It was possible to consider a prevalence of left-side dominance in TA in thoracic and thoraco-lumbar curves and vice versa due to the high sensitivity, specificity, positive and negative predictive values and likelihood ratios (Table 3).

The present study investigated the correlation between TA and SD with evaluations of the hand, foot and visual laterality in 1029 adolescents. The results showed a prevalence of left-side dominance in TA with the possibility of considering the left-side dominance as a predictor of TA in thoracic and thoraco-lumbar curves and vice versa.

We expect these results to be generalisable since these data refer to a general population in which no specific selection processes were applied. Some of the schools involved were in a large town, while others where in a nearby rural area. Obviously, the population refers to Italy, so it is possible that the results could be different in other countries, even if there are no real physiological reasons why this would happen. In terms of the prevalence of trunk asymmetries, our data corresponded to previous results.18

The literature showed that the normal spine has a slight rotation and curvature with the same laterality as TA, which may explain why the laterality of the curve pattern may differ from the pathogenesis of TA and scoliosis.20 The superficial extrinsic back muscles on the right side, which contribute to the movement of the upper limbs, should be stronger than those on the left side as they should help to restrict the tendency of flexion and rotation of the spine. Humans will most likely use the right upper limb as it seems to be lighter due to the stronger superficial extrinsic back muscles. This may be the reason for the right laterality of handedness.14,21 As a result of the trend of right flexion and rotation of the spine in most right-handed people, the right convex curve pattern is frequent. The same mechanism might be valid for left SD related to TA. Milenkovic et al.22 showed that left-handedness might be significantly related to scoliosis in females during early adolescence.22 In addition, genetical and environmental factors, such as lateralisation of cerebral motor cortex function11 and carrying a backpack in an asymmetrical manner,23 may affect the spine in the age near the growth spurt and influence the correlation between SD and TA.10

Consequently, the previous management of TA is very important to prevent its progression because it may cause painful spine degenerative processes24,25 and finally, cardiovascular26 and respiratory complications requiring operative treatment. It is important to consider the left SD as a possible predictor of TA in the screening process of adolescents, and to plan in-depth sessions for them. For left-sided adolescents, there are also many important specific preventive measures concerning board adjustment, writing technique, body posture while writing, and left-side dominance specific school supplies that can be used before puberty.27

This study has some limitations, such as the screening procedures used which precluded a real study of the bones; the low prevalence of scoliosis and asymmetries in our sample yielded only a few scoliosis subjects, even though it was reported to be representative of a general population. Also, poor vision was not considered an exclusion criterion.

This study showed a prevalence of left-side dominance in trunk asymmetry in thoracic and thoraco-lumbar curves and vice versa. This is a crucial point for the prompt treatment of trunk asymmetry and the prevention of further progression. Further studies are necessary to investigate the development of SD in TA in adolescents.

The author declares no conflicts of interest.