Obesity is a medical problem which is associated with various musculoskeletal disorders, especially the spine [13, 3]. Although the interaction between spinopelvic disease and obesity is not clear, the relationship between low back pain and BMI has been demonstrated by many studies [9–12, 28]. To our knowledge, the parameters of spinopelvic have clinical implications. For example, the risk of early distal discopathy increases in patients with low PI and a flat back [21]. Patients with chronic low back pain typically have a low SS, LL, and small PI [12]. That means biomechanics play an important role in the initiation and progression of several spine pathologies, and it is imperative to understand spinopelvic alignment in obese patients. However, no study has reported the significance of spinopelvic parameters for the diagnosis and treatment of human musculoskeletal disorders. Thus, the present study made an objective analysis of the correlation between spinopelvic parameters and the obesity parameters.
Our results showed that coronal spinopelvic parameters have no correlation with both BMI and WC, which means that obesity may not affect the coronal spinopelvic arrangement. In the male group, BMI and WC were not correlated with sagittal spinopelvic parameters yet. But in the female group, the sagittal spinopelvic parameter (PI) was correlated with both BMI and WC strongly. It seems that obese female tends to have higher PI values. PI, an anatomical parameter, is unique to each individual and independent of the spatial orientation of the pelvis [22]. It has a geometrical relationship with two positional parameters, PT and SS: PI = PT + SS [23]. A study concluded that PI is significantly correlated with SS and there is a positive linear relationship between SS and LL. This implied that, in females, with the PT increasing, the balance of the sagittal spinopelvic arrangement was maintained by an increasing LL. This may explain why obese females tend to have hyperlordosis and their pelvis is usually forward-inclined.
Whitcome et al. [24] pointed out that in bipeds, the upper body is stabilized by the positioning of the center of mass (COM) on the trunk above the hips. However, in women who are pregnant, the body shape changes as a result of the extra mass, which results in a shift of the body's COM towards ahead. In a typical pregnant human female with a naturally extended back, the COM was recovered through increased lumbar lordosis, a stable positional alignment with reduced hip torque but exacerbated spinal shearing load. This indicated that the increased abdominal circumference might affect the sagittal spinopelvic alignment. The abdominal circumference can reflect the form of the abdomen, and it was an appropriate index for evaluating the influence of obesity on the spinopelvic alignment under certain conditions. However, as an abdominal circumference can only be measured in a single plane at a time, this may not suitable for evaluating changes of COM in the sagittal and coronal planes.
Kvist et al. [25] proposed the use of TAD and SAD to estimate visceral obesity. Therefore, we thought it necessary to determine the correlation of SAD and TAD with the spinopelvic parameters. The results indicated that TAD was not correlated with any of the spinopelvic parameters in the two groups. Concerning SAD, it was weakly correlated with PA and LL in the male group, and with PA and PT in the female group. Further, the results of the correlation analysis showed that SAD was linearly correlated with PA and PT in both groups, but the correlation was not strong [Figure. 4]. In the female group, a positive correlation was found between SAD and PA. As an increase in PA and PT was often associated with pelvic incline and greater LL, this may have important implications [28]. Further, RR (the ratio of SAD to TAD) had a positive linear correlation with PA and PT (Figure. 5). These results indicated that for people with abdominal obesity, SAD may have a greater impact on the spinopelvic parameters than TAD.
From the findings of our research, we can deduce that the changes in the shape of the lumbar spine caused by obesity impacts females more than males. This may be attributed to the differences in the shape of the spine between males and females. The spine and pelvis of females were more dorsally inclined, either as a whole (T1–L5–SSI) or specifically about the thoracic and thoracolumbar vertebrae. Because the biomechanical demands of pregnancy exerted an early selection pressure on the evolution of lumbar lordosis in bipedal hominins [24, 26]. The explanation can also be applied to the obese individuals with increased pressure on the spine and pelvis, which results in shifting of the COM backward to maintain standing balance. This may change the position of the lumbar spine in the long term. Hyperlordosis results in degenerative spondylolisthesis and its main symptoms are lumbago, degenerative discopathy, and Baastrup disease. It consequently leads to a disturbance of the global sagittal balance of the spine [21, 27]. Altogether, these findings highlight the importance of maintaining an appropriate body weight by incorporating an appropriate amount of exercise to keep the spine and pelvis in a healthy condition.