Normative values of spino-pelvic sagittal alignment, balance, age, and health-related quality of life in a cohort of healthy adult subjects

The human skeleton works like a “reverse pendulum” during standing with the chain of balance starting from the support polygon (both feet), then moving to the lower limb skeleton with the ankles, knees, hip joints, the pelvic vertebra; then the spinal segments; and finally, the cephalic vertebra working as a pendulum to achieve horizontal vision and balance. These elements together contribute to the characteristic erect posture of humans, where the “cone of economy” is perfectly balanced with minimal muscle action [1]. Deterioration of standing spinal balance decreases health-related quality of life (HRQOL) [2‐6]. Based on conventional X-ray, Schwab et al. reported that pelvic tilt (PT), pelvic incidence (PI), and lumbar lordosis (LL) combined with the sagittal vertical axis (SVA) predict disability, and proposed threshold values for severe disability [Oswestry Disability Index (ODI) >40] including: PT ≥22°, SVA ≥47 mm, and PI–LL ≥11° [7]. In case of decompensated standing balance, the trunk shows an increased SVA, the pelvis retroversed, the hip extended, and the knee flexed, suggesting the uneconomic alignment affects HRQOL [1, 8]. Therefore, a better understanding of alignment from the head to the feet will elucidate the “cone of economy” mechanism. Standardized data for whole skeletal alignment in the standing position have not been fully provided, however, likely due to the limitations of conventional X-ray measurements in which a fan-beam X-ray significantly magnifies the objects at the cassette margin [9].

A new scanning X-ray imaging system (EOS Imaging, Paris, France) was developed by multidisciplinary investigators to overcome the limitations of conventional X-ray measurement. Simultaneous anteroposterior and lateral X-rays of the whole body can be obtained using the three-dimensional bone external envelop technique, allowing for three-dimensional reconstruction at every level of the osteo-articular system, especially the spine, in the standing position. This X-ray system allows for more precise bone reconstruction in orthopedics, especially at the level of the spine, pelvis, and lower limbs, with limited X-ray exposure [10, 11]. In 2013, the authors initiated prospective clinical studies of standing spino-pelvic alignment and various pathologies using the X-ray imager.

In the present study, we hypothesized that the whole body standing sagittal alignment and HRQOL deteriorate with age even in the healthy population, but the compensatory mechanism plays a role to maintain the global alignment to preserve a horizontal gaze. The purposes of this study were (1) to elucidate the normative values of the whole body sagittal alignment of a healthy population in the standing position; and (2) to clarify the relationship among the alignment, age, and HRQOL.

Following approval by the institutional review board, 136 volunteers without history of treatment for spinal disease were enrolled. Informed consent was obtained from all patients. After X-ray images were obtained as described below, we excluded four cases with lumbarisation, two cases with sacralisation, two cases with 11 thoracic vertebrae, and two cases with scoliosis >20° Cobb angles, so that accurate radiographic measurements could be obtained. Exclusion of the transitional vertebrae is important because transitional vertebrae affect spinal and pelvic parameter measurements. Consequently, we analyzed a total of 126 cases [mean age 39.4 years (20–70 years); 30 men, 96 women] prospectively. The epidemiologic and morphologic characteristics of this cohort were obtained from the following data: age, sex, weight, and height. Body mass index (BMI) was calculated as weight in kilograms divided by square of the height in meters.

In the present study, we investigated sagittal whole body skeletal alignment and standing balance used a scanning X-ray imager with a biplanar upright scanning imaging modality to achieve reduced X-ray particle scatter, improved image quality, and significantly reduced radiation to the patient [10, 11, 14]. While artifacts in the images due to body sway during scanning may affect the measurements, these are minimized by the rapid scan rate (7.6 cm/s) and X-ray detection time (every 0.8333 ms). Furthermore, we strictly excluded all subjects with anomalous vertebrae such as transitional vertebrae and scoliosis with a Cobb angle >20°, which can affect measurement precision. Thus, the data in this study are considered to be precise. The present study yields a physiologic standard for several angular pelvic and spinal parameters that describe sagittal whole spinal alignment, including knee alignment, in a cohort of 126 healthy adult volunteers (Table 1). The standing sagittal alignment was similar to previous studies. The correlations among the radiographic parameters of sagittal alignment were also comparable with previous reports [17, 20‐34]. The correlation was stronger between the parameters of the adjacent structures, indicating the chain of balance advocated by Dubousset (Table 3) [1]. More than two decades ago, Duval–Beaupère’s group found PI, which is the most significant unique parameter of individual standing spinal alignment, i.e., a large PI is associated with a great SS and a pronounced LL, and a low PI is associated with a smaller SS and a subtle LL, leading to a basic concept of “équilibre économique” in standing [1, 17, 23, 29, 35, 36]. Itoi et al. [37] investigated the relationship between sagittal posture of the spine and the lower extremities in osteoporotic subjects, and found that thoracic kyphosis, a primary deformity, was compensated for by the lumbar spine, sacroiliac joint, hip joint, and knee joint. Another study suggested that upright posture is secondary to hip extension and LL, and an optimal and economic standing posture is obtained when these two parameters are balanced [8]. These reports suggest that lower extremities are also crucial factors for standing balance. When the biomechanics of standing balance are taken into account, contact force on the distal lumbar spine is the sum of the gravity force and the force acting on the posterior muscles to maintain an erect position. The more unbalanced the body (forward inclined), the greater the increase in gravity force, and the more the force acting on the muscles must compensate for increasing contact force [36]. In the present study, SVA, TPA, SFA, KneeFlex, and AnkleFlex, but not CAM–GL, increased with age (Fig. 5), suggesting that hip extension, knee flexion, and ankle flexion are also an important compensation mechanism for deteriorating standing alignment with degenerative changes or aging. A linear regression analysis between standard spinal parameters and (PI–LL), the key parameter of spino-pelvic mismatch, showed that PT more significantly correlated with PI–LL (R = 0.8565, p < 0.0001) than Kyph (R = 0.3626, p < 0.0001). On the other hand, KneeFlex and AnkleFlex were not significantly correlated with PI–LL (KneeFlex: R = 0.0696, p = 0.4386, AnkleFlex: R = 0.0871, p = 0.3323) (Table 3), suggesting that pelvic alignment, represented by PT, is the primary mechanism for compensation in cases of spino-pelvic mismatch and the alignment of lower extremities are the last resort to compensate for standing whole skeletal alignment when PT is maximally functioning. These findings are compatible with a recent report [38].

ODI is a principal condition-specific outcome measure used in the management of low back disorders. Fairbank et al. stated that the mean ODI score in the normal population is 10.2. Thus, the mean ODI score of less than 10 reported by our patients (women: 5.5, men: 4.2, Table 2) can be regarded as normal [12, 39]. The score in the present study had a positively correlation with age. A recent report indicated that aging affects general health status measured by another HRQOL outcome measure (the SRS-22) with deteriorating standing alignment [40]. Therefore, the relationship between HRQOL and age is considered universal, and may be due to degenerative spinal changes. In the present study, PI–LL, a representative parameter of spino-pelvic mismatch [21], was positively correlated with age, suggesting that spino-pelvic harmony deteriorates with age. Furthermore, ODI score had also a tendency of positive correlation with PI–LL. This finding suggests that ODI is affected not only by degenerative processes, but also by spinopelvic malalignment.

Each patient’s spine status and shape is unique, even if general rules apply to most. In spinal reconstruction surgery, the final alignment of the spine can be planned before surgery. The surgical goal is to achieve optimal global sagittal alignment by restoring an optimal LL for patients with sagittal malalignment deformities [1, 19, 21, 28]. For this purpose, several formulae predicting an ideal LL have been reported. Duval–Beaupère’s group reported that a key point in standing sagittal alignment of the spine is a harmony of LL and PI and deduced the following formula: Ideal LL = −9.13847 + 0.19225 × T1–T12 Kyphosis + 1.54225 × SS − 0.26799 × PI + 1.39705 × T9 Tilt. (R ² = 0.9441, p < 0.0001) [19]. In the present study, we deduced the following equation with a least square analysis, Ideal LL = −11.30537 + 0.14094 × T1–T12 Kyphosis + 1.63650 × SS − 0.35750 × PI + 1.40833 × T9 Tilt (R ² = 0.935494, p < 0.0001). Although the subjects in the two reports differed in terms of age, sex, race, or X-ray measurement, the formulae are very similar. Xu et al. investigated standing spinal alignment in a Chinese population, and reported that PI and age are independent factors contributing to the difference of LL and deduced the predictive formula: LL = 28.6 + 0.508 × PI − 0.088 × Age [22]. Although the intercept and coefficient values of PI are similar, the coefficient of age was higher in our study. The reason for the difference between the two formulae is considered to be due to: (1) precision of X-ray measurement (conventional X-ray vs scanning X-ray imaging), (2) age distribution (our study included older subjects), and (3) race. Disk degeneration is more prominent in older people; thus more attention must be paid to age as a contributing factor to the formula for ideal LL. A recent study reported that ideal spino-pelvic alignment values that correspond to patient-reported outcomes increased with age, with older patients having substantially greater baseline deformity [41]. Further study is necessary to elucidate the effect of aging on the spino-pelvic alignment and HRQOL.

Human beings must maintain balance when standing still. Several clinical studies using a force plate have been performed to investigate the balance mechanism [42‐47]. El Fegoun et al. [43] reported a highly significant negative correlation between the gravity line and plumbline in the sagittal plane based on simultaneous assessment of full-length freestanding spine radiographs and the floor projection of the center of pressure (gravity line), suggesting that the value of the plumbline as a marker of true postural balance must be questioned. We investigated balance using a force plate measurement during X-ray scanning and found that a representative balance parameter, ENV, had a tendency of positive correlation with PI–LL, suggesting that spino-pelvic mismatch affects not only the static sagittal alignment but also dynamic alignment, balance. There are many subjects to clarify the profound mechanism of the “cone of economy” [1] of the chain of balance of the standing whole body.

In conclusion,