How can directional and fluctuating asymmetry help in the prognosis of scoliosis along the course of the condition?

Abstract

Fluctuating asymmetry (FA) is a phenomenon present in adolescent idiopathic scoliosis (AIS; Dangerfield et al., 1997;Goldberg et al., 1995Goldberg et al., , 1997b, which includes all random deviations between the left and the right sides of the human torso. It has been described in the literature as an expression of developmental instability and a consequence of environmental and/or genetic factors (Klingenberg, 2003a(Klingenberg, , 2003bValen, 1962). Hypothetically, the left and the right sides of the torso share the same genome, and as far as development is considered to be regulated by genetics, a perfectly symmetrical phenotype should be expected in the frontal plane. In the practice, the FA comprises the random deviations from the mean asymmetry of the shape in the sample subjected to study, a type of asymmetry that has been termed 'directional asymmetry (DA)' (Klingenberg, 2003a(Klingenberg, , 2003b. Nevertheless, the exposition to non-genetic factors like nutrition, gravity or lifestyle could modify the way an organism grows by perturbating its developmental system (Klingenberg, 2003a(Klingenberg, , 2003b. These environmental factors may cause the asymmetries in the human torso

Investigating Human Torso Asymmetries: An Observational Longitudinal Study of Fluctuating and Directional Asymmetry in the Scoliotic Torso

“…Then, the two-dimensional individual directional asymmetry score, which reflects the left-right curve direction in the X-ray using all landmark data, is essentially a two-dimensional variable, as the Cobb angle is. Due to the previous reason, the high correlation found in González-Ruiz et al [9] was not a surprise and made us postulate the same hypothesis in our three-dimensional research. However, the complexity of the three-dimensional shape data made the difference for two main reasons:…”

“…In fact, to our knowledge, this is the first study to assess individual directional and fluctuating asymmetry scores in the 3D shape of patients with scoliosis. It seems that the presence of fluctuating asymmetry even in mild scoliosis proves that fact, as well as the fact that the magnitude of directional asymmetry is proportional to the severity of the condition, as we have seen in our previous work based on the study of the two-dimensional shape with GMM [9]. In summary, directional and fluctuating asymmetry appear to be intrinsic to human developmental biology, even in control subjects.…”

“…In our previous two-dimensional study [9], the individual directional asymmetry score, extracted from a two-dimensional GMM analysis of X-rays of patients with AIS, showed a high predictive power of the Cobb angle (effect size = 0.58). This finding should also hold regarding the three-dimensional aspect of shape and is postulated as our second hypothesis.…”

“…Then, the covariance matrix of all individual's shapes was calculated and, as a result, the symmetric and asymmetric components of the shape were extracted and used to estimate the individual directional asymmetry score following the steps described in González-Ruiz et al [9]. A Procrustes ANOVA including the whole sample (AIS and controls) was conducted to test for the individual fluctuating asymmetry score and the measurement error.…”

“…Thus, higher levels of fluctuating asymmetry have been evidenced in other clinical conditions different from scoliosis in the form of positive correlation between asymmetries and symptoms [8]. Furthermore, in a recent study about fluctuating and directional asymmetries using two-dimensional GMM [9], the presence of both types of asymmetry has been revealed in a small group of patients with adolescent idiopathic scoliosis (AIS). However, the absence of a control group, the limited sample size and the two-dimensional approach to a three-dimensional condition such as idiopathic scoliosis led to three major limitations.…”