Cervical sagittal imbalance is implicated in the development of various spinal disorders[1, 2] and associated with patient’s health-related quality of life [4, 5]. However, there were few studies focusing on the correlation between BMI and cervical sagittal alignment. After analysis, we derived clear connections between BMI and C2-C7SVA, TZC2-C7, CrT, CeT, CL, NT, TIA (Fig. 2), which indicated that surgeons should take BMI into consideration in reconstructive surgery of cervical sagittal alignment.
Forward Head Posture Increases As Bmi Rises
Several studies[1, 9, 11, 17] testified that C2-C7SVA is a crucial parameter in cervical sagittal balance, which was related to clinical symptoms. In study, C2-C7SVA were positive correlated with BMI, which reflected forward head posture increases as BMI rises. In line with our result, Oe et al[9] implied the correlation between C2-C7SVA and BMI in the result of his study. TZC2-C7 is a good addition for C2-C7SVA[18], and we also found TZC2-C7 increases as BMI rises.
CrT is an angle formed by the plumb line and the line connecting the center of T1 upper end plate with the tip of the dens, which is contacted to the flexion state of the cervical spine[19]. In study, we found CrT increases as BMI rises, which also reflected the status of forward head posture. Similar to CeT, CeT is also a common parameter to reflect cervical sagittal alignment, and CeT decreases as BMI rises[11, 19].
Combining C2-C7SVA, TZC2-C7, CrT, and CeT, we assessed forward head posture increases as BMI rises. This phenomenon may be caused by two reasons. On the one hand, we guessed that the pathological fat infiltration in paraspinal muscle lead to forward head posture. Accumulating evidences demonstrated that BMI was positive associated with fatty infiltration of paraspinal muscle[20–22], and previous studies showed that pathological muscle influences the cervical sagittal alignment[23] and quality of life[24]. On the other hand, the anterior shift of the center of gravity may be an explanation of the compensatory increasement of forward head posture. Accumulating evidences reported that obese individuals have significantly greater trunk mass and BMI is positively correlated with increased abdominal fatness[25, 26]. And increased abdominal fatness leads to anterior shift of the center of gravity. Previous articles showed that the anterior shift of the center of gravity is compensated with the posterior tilt in the pelvis and the thoracic region[27, 28], which explains the anterior tilt in cervical region.
In line with our observed phenomenon, Brink et al.[29] pointed out that overweight or obese students have more neck flexion than thinner students, when working on desktop computers in their school computer classroom. And the clinical studies certified that obese negatively effect on postural stability, not only in one leg standing but also moving from sit to stand[30, 31]; Excessive forward head posture and increased abdominal fatness were regarded as the potential factor of postural instability[32, 33].
Thoracic Inlet Lifts As Bmi Rises
As the important element of cervicothoracic junction, thoracic inlet is a circle, made up by T1 vertebral body, first ribs and the upper part of sternum. As previous studies[34–36] described, parameters of thoracic inlet, such as TIA and NT, were significant correlative with cervical sagittal balance.
TIA is formed by a line perpendicular to the superior endplate of T1 and a line connecting the center of the T1 upper end plate and the upper end of the sternum. Different from TS, there is no significant change of TIA in different positions[37, 38], which is an advantage to guide surgery when patients lay, not stand, on the operating table. And TIA was found to markedly increase with age by previous studies[11, 12, 39], which was consistent with our result. As for BMI, we found there was a correlation between BMI and TIA. Like TIA, NT is also positively related to BMI, which reflect the phenomenon thoracic inlet lifts as BMI rises, and Oe et al.[9] study also implied that NT was correlative with BMI in the result.
Combining the tender of TIA and NT, we assessed thoracic inlet lifts as BMI rises and the change came from sternum, not T1 vertebral body. We guessed that a rising position of manubrium related to the level of T1 leads to a larger TIA and NT. Shi et al. and Kent et al. certified that as BMI rises, the ribs became more perpendicular to the spine and rib cage depth increased, which lead to a rising position of manubrium related to the level of T1[40, 41].
Combining our results with those of previous studies, we assessed that forward head posture increases and thoracic inlet lifts, as BMI rises, in asymptomatic population (Fig. 3). corresponding to our result, Fabris et al[25] showed that postural changes in morbidly obese patients, and Koller et al[42] showed that the risk for revision of adult scoliosis surgery was increased, as BMI rises. Of course, the role of BMI cannot be further exaggerated. Because we found that only significant increase of BMI, such as obese, leads to cervical sagittal imbalance, and the cervical sagittal alignment of underweight people is normal.
This is the first preliminary analysis of the change of cervical spine alignment along with BMI in asymptomatic population. In clinic, we advise obese patients with neck pain to lose weight to maintain cervical sagittal balance and reduce neck pressure. In surgery, surgeons can properly evaluate cervical alignment of obese patients with cervical disorders, and map out more precise cervical realignment parameters in obese patient with cervical deformity in infusion operation.
This study has several limitations in fact. First, the number of volunteers can be more to support our conclusions and a larger scale study is our next proposal. Second, our volunteers are all Asians, for which a multi-ethnic study is need in our future. Third, besides fists-on-clavicles position, lying position and sitting position are the next aim.