Relationship between bodyweight and spinopelvic alignment in Chinese adult people: A pilot study

Background The purpose of this study is to evaluate the correlation between spinal alignment and the obesity parameters in healthy Chinese adult volunteers. Methods This pilot study included 100 Chinese healthy adult volunteers, 36 males and 64 females. The obesity parameters measured were BMI, waist circumference (WC), sagittal abdominal diameter (SAD), transverse abdominal diameter (TAD) and RR (the ratio of SAD to TAD). The sagittal spinopelvic parameters included pelvic incidence (PI), pelvic tilt (PT), pelvic angulation (PA), sacral slope (SS), sacral inclination (SI), lumbar lordosis (LL) and the disc angle of L5/S1. The coronal spinopelvic parameters included the Cobb angel of the major curve of the spine, lumbar scoliosis (LS) and pelvic obliquity (PO). Results The mean BMI of the males and females was 28.7 ± 3.7 kg/m2 and 26.8 ± 2.3 kg/m2 respectively; mean WC, 88.39 ± 9 cm and 82.6 ± 2.7 cm respectively. In the females, strong correlation was found between BMI and PI, WC and PT, and WC and PI. In the males, no strong correlation was found between the obesity and spinopelvic alignment parameters. RR showed a positive linear correlation with PA and PT in both groups. None of the coronal spinopelvic parameters showed a correlation with the obesity parameters in either group. Conclusions BMI and WC had a strong influence on some spinopelvic parameters, but only in females. In individuals with abdominal obesity, the sagittal spinopelvic alignment is likely to change, however there is little effect on the coronal spinopelvic.

Obesity is considered to be a strong risk factor for cardiovascular disease, cancer, stroke, and endocrine disorders. In addition, increase in body weight may increase the demand on the musculoskeletal system, particularly the lumbar spine [1,7]. This is supported by studies, which have shown that low back pain is correlated with obesity [8][9][10][11]27].
In the standing position, the human body is made up of a set of mutually articulating sections: The head is balanced on and connected to the trunk by the cervical spine; the trunk articulates with the pelvis, which in turn articulates with the lower limbs at the hip joints to maintain a stable posture and to expend the minimum amount of energy [13][14][15]. In the upright standing posture, the spinal load is primarily influenced by changes in body weight, especially the load on the lumbar vertebraes.
Excessive mechanical load on the trunk, applied intermittently or repetitively over time, are however believed to play a causative role in injures and degeneration [7]. Multiple studies have reported the normative values for parameters associated with spinopelvic alignment in populations of varying ages, races and pathologic conditions [16][17][18][19]. However, to our knowledge, the association of BMI, WC, SAD and TAD with spinopelvic parameters in the sagittal and coronal planes has never been studied before. Therefore, the purpose of this study is to evaluate the correlation between obesity parameters and spinopelvic alignment in healthy Chinese adult volunteers.

Materials And Methods Participants
This is a pilot study of 100 healthy Chinese adult volunteers (36 males and 64 females) who were recruited based on the following inclusion and exclusion criteria. We included individuals of the Han race who were aged between 18 and 45 years. Another requirement was for the participants to have an occupation that involved standing most of the time, such as sales jobs. The final criterion was the availability of a standing lateral radiograph of the spine and pelvis in which the spine from T1 to S1 and both femoral heads were visible.
We excluded individuals with sitting-type jobs, such as bus drivers. Those with a history of spine or pelvic surgery were also excluded. Further, individuals with a history of back pain, degenerative spine disease, or idiopathic scoliosis and those with hip or knee joint deformities were also excluded.
All subjects provided their written informed consent for the study after the experimental protocol was explained to them. Further, this study was approved by the Institutional Review Board of China-Japan Friendship Hospital.
Measurement of obesity parameters BMI was computed by dividing the weight in kilograms by the square of the height in meters (kg/m 2 ) [3]. Weight was measured using a platform scale to the nearest 0.1kg. Height was measured using a tape to the nearest 0.1cm. According to the classification of BMI by the World Health Organization, BMI in the range of 18.5-25.0 kg/m 2 is normal, 25.0-29.9 kg/m 2 is overweight grade 1; 30.0-39.9 kg/m 2 , overweight grade 2; and 40.0 kg/m 2 overweight grade 3.
WC was measured with a tape to the nearest 0.1 cm, with the patient wearing light garments. It was measured at the midpoint between the last rib and at the highest point of the iliac crest at the end of normal inspiration. Central obesity (indicated by increased waist circumference) was defined as WC 85cm for men and 80cm for women.
The anterior-posterior diameter, which is a measure of the abdominal extension along the sagittal plane in the standing position, is also called SAD. The transverse diameter, which is a measure of the abdominal extension along the coronal diameter in the standing position, is also called the TAD. SAD is determined using a radiograph, with the diameter extending from skin to skin through the center of the abdomen in the anterior-posterior direction. TAD is defined as the largest diameter from skin to skin through the center of the abdomen in both lateral directions in other image. These measurements were determined to the nearest 0.1 cm (Figure 1). Relative ratio (RR), which is defined as the ratio of SAD to TAD, was also used to study the relationship between the spinopelvic parameters and abdominal obesity.

Measurement of spinopelvic alignment parameters
Participants keep an upright relaxed position, looking forward, with their knees straight, arms gently clasped in front of their trunk and a distance of 25cm between their feet. All the subjects underwent frontal and lateral radiography of the entire spine and hip joints. Lateral radiographs of the lumbar spine were obtained on a vertical film (30×90 cm) while maintaining a constant distance between the subject and the radiographic source.
The sagittal and coronal spinopelvic parameters were measured on the radiographs. The sagittal spinopelvic parameters included pelvic incidence (PI), pelvic tilt (PT), pelvic angulation (PA), sacral slope (SS), sacral inclination (SI), lumbar lordosis (LL) and the disc angle of L5/S1. The coronal spinopelvic parameters included the Cobb angel of the major curve of the spine, lumbar scoliosis (LS) and pelvic obliquity (PO). These parameters are defined and described clearly in the legend to the figures, as it is easier to understand them from the depictions in the figures ( Figures. 2 and 3).
All the measurements were made by two experienced individuals who using software program that was designed to assess the sagittal and coronal alignment of the spine and pelvis. In order to determine the inter-observer and intra-test reliability, two observers measured the same radiograph two times. The angular parameters were expressed in degrees.

Statistical analysis
All data analyses were performed using SPSS version 15.0 (SPSS Inc. Chicago, IL). The mean values, and standard deviation (SD) were obtained for all the measurements. Pearson's correlation analysis was used to determine correlations between the obesity and spinopelvic alignment parameters. A correlation coefficient (CC) of<0.5 was considered to indicate relatively weak correlation, while a CC of >0.5 was considered to indicate relatively strong correlation. Levene's test for equality of variances was used to verify the gender-based differences between the measurements. All the results are expressed as mean ± standard deviation (SD).

Results
The participants were grouped according to gender (male, n = 36; female, n = 64 Levene's test for equality of variances showed the presence of gender-based difference in the variables. Therefore, Pearson's correlation analysis was conducted separately in the male and female groups. According to the results of Pearson's correlation analysis, in the male group, BMI, WC and TAD were not correlated with the obesity parameters, while SAD was weakly correlated with LL (CC = 0.459) and PA (CC = 0.331) ( Table 1). In female group, PI was strongly correlated with BMI (CC = 0.521). Furthermore, WC was correlated with SI (CC = 0.435), PT (CC = 0.551), PA (CC = 0.436), and PI (CC = 0.539) respectively. No spinopelvic parameter was correlated with TAD. However, SAD was correlated with both PT (CC = 0.450) and PA (CC = 0.393) weakly ( Table 2).
As shown in Table 1 Table 2, RR was correlated with PT (CC = 0.391) and PA (CC = 0.349). Contrary to the sagittal spinopelvic parameters, the coronal spinopelvic parameters did not show a strong correlation with the spinopelvic alignment parameters.

Discussion
Obesity is a serious medical problem that is associated with various musculoskeletal disorders, especially the spine [12,2]. 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 [8][9][10][11]27]. Little is known about the importance of spinopelvic parameters in the diagnosis and treatment of human musculoskeletal disorders, even though these parameters are known to have clinical implications. For example, the risk of early distal discopathy increases in patients with low PI and a flat back [20]. In addition, patients with chronic low back pain typically have a low SS, LL, and small PI [11]. 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. Therefore, the present study made an objective analysis of the correlation between spinopelvic parameters and the obesity parameters.
Our analyses 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 male group, BMI and WC were not correlated with sagittal spinopelvic parameters. But in female group, sagittal spinopelvic parameter (PI) was correlated with both BMI and WC strongly. It seems that obese female tend to have higher PI values. PI, an anatomical parameter, is unique to each individual and independent of the spatial orientation of the pelvis [21]. It has a geometrical relationship with two positional parameters, PT and SS: PI = PT+SS [22]. A study concluded that PI is significantly correlated with SS, and that there is a positive linear relationship between SS and LL. This implied that in females, when PT increased, the balance of the sagittal spinopelvic arrangement was maintained by an increase in LL. This may explain why obese females tend to have hyperlordosis and why their pelvis is usually forward-inclined.
Whitcome et al. [23] pointed out that in bipeds, the upper body is stabilized by 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 [23]. In a typical pregnant human female with a naturally extended back, the COM is recovered by means of increased lumbar lordosis, a stable positional alignment with reduced hip torque but exacerbated spinal shearing load. This indicates that increased abdominal circumference might affect the sagittal spinopelvic alignment. Abdominal circumference can reflect the form of the abdomen, and it is an appropriate index for evaluating the influence of obesity on the spinopelvic alignment under certain conditions. However, as the abdominal circumference can only be measured in a single plane at a time, it may not suitable for evaluating changes in the COM in the sagittal and coronal planes.
Kvist et al. [24] proposed the use of forward TAD and SAD to estimate visceral obesity. We therefore 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. With regard to 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 increase in PA and PT is often associated with pelvic incline and greater LL, this finding may have important implications [27].  These findings indicate that people with abdominal obesity, SAD may have greater impact on the spinopelvic parameters than TAD.
From the findings of our research, we can deduce that 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 are more dorsally inclined, either as a whole (T1-L5-SSI) or specifically with regard to the thoracic and thoracolumbar vertebrae. This is because in bipedal hominids, selective pressure is exerted on the development of lumbar lordosis early on, which is traceable to the biomechanical demands of pregnancy [23,25]. This explanation can be extended to the case of obese individuals, in whom there is increased pressure on the spine and pelvis, which results in shifting of the COM backwards in order 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 results in disturbance of the global sagittal balance of the spine [20,26]. All together, these findings highlight the importance of maintaining an appropriate amount of weight by incorporating an appropriate amount of exercise to keep the spine and pelvis in a healthy condition.

Conclusion
This is the first analysis to investigate the association between sagittal and coronal spinopelvic parameters and abdominal obesity. The values of the spinopelvic parameters were found to be high in obese individuals, which means that BMI and WC may influence on the spinopelvic parameters.

Declarations
Ethics approval and consent to participate This study was approved by our Institutional Review Board.

Consent for publication
Not applicable.

Availability of data and materials
All data were provided in the table 1 and 2.    as the angle between the superior endplate of S1 and the horizontal axis.(5) Sacral inclination (SI) is defined as the angle subtended by a tangent to the posterior border of S1 and the vertical line. (6) The Cobb angle of L1-S1 in the sagittal plane is also known as lumbar lordosis(LL), and is measured using the Cobb method, between the upper endplate of L1 and the upper endplate of S1 in the sagittal plane. (7) The disc angle of L5/S1 is defined as the angle between the subordinate endplate of L5 and the superior endplate of S1. The relationship between sagittal abdominal diameter (SAD), pelvic tilt (PT) and pelvic angulation (PA) in the male and female group.The graph shows that SAD is linearly correlated with PA and PT in both male and female groups The relationship between RR (the ratio of sagittal abdominal diameter to transverse abdominal diameter) and pelvic tilt (PT) and pelvic angulation (PA) in male and female group. The graph shows that RR is linearly correlated with PA and PT in both male and female groups.