7.1 Introduction

Proper knowledge of normal sagittal balance and underlying concepts is paramount when evaluating and treating patients with spinal pathologies. While postural control and compensatory mechanisms can vary with growth and aging, understanding the differences between children and adults is also important when assessing sagittal balance. In this chapter, references values and key concepts of sagittal balance in normal children and adults are described.

7.2 Parameters of Sagittal Balance

Various parameters have been used to describe normal sagittal balance. Regional parameters typically describe the morphology and orientation of the various segments of the human body: cervical, thoracic and lumbosacral spine, pelvis, and lower extremities. Global parameters are used to describe the overall sagittal alignment for adjacent anatomical segments. When compared to global parameters, regional parameters are typically associated with larger variability among normal individuals. ¹

It is important to distinguish morphological from orientation parameters of sagittal balance. Morphological parameters such as pelvic incidence describe an anatomical feature that is unaffected—or negligibly affected—by changes in position of the individual. Conversely, orientation parameters are position dependent, and therefore require standardized positioning during radiographic acquisition to ensure adequate reproducibility.

Variations in parameters of sagittal balance can also be amplified by the radiographic and/or measurement techniques that are used. While angular parameters are only slightly influenced by the radiographic and measurement techniques, linear measurements are highly dependent on the radiographic technique and calibration. Angular, adimensional, or qualitative assessments with respect to reference points (Fig. 7‑1) are therefore often preferred in an attempt to reduce the errors associated with linear measurements.

7.3 Normal Sagittal Balance: Underlying Concepts

Despite large variations in parameters of sagittal balance, normal individual stand with a balanced posture for which the spine, pelvic, and lower extremities are aligned to minimize the energy expenditure and preserve horizontal gaze. There are basic concepts underlying this principle that need to be considered when evaluating sagittal balance. First and foremost, adjacent anatomical regions are interdependent, and their relationships result in a stable and balanced posture. ^{2 ,​ 3} Although normative values of sagittal balance are useful clinical guidelines for evaluating and treating patients with spinal disorders, ensuring and preserving the close relationships between adjacent anatomical regions is of paramount importance. In addition, even if parameters of sagittal balance differ between pediatric and adult subjects, the scheme of correlations between parameters remains similar in the normal pediatric and adult populations.

In the presence of pathology in the spine, pelvis, or lower extremities, compensatory mechanisms to maintain adequate sagittal balance can occur either locally at the level of the pathology and/or remotely in another anatomical region. While regional parameters of sagittal balance can vary widely in the normal population, on some occasions, the presence of an underlying pathology can be reflected only by abnormal relationships among these parameters. When compensatory mechanisms are exceeded to the point where sagittal imbalance occurs, regional parameters of sagittal balance may still remain within normal limits, but parameters of global balance will likely fall outside of normal limits. Accordingly, global parameters of sagittal balance are maintained in a narrow range within the normal population. Clinically, the assessment of global sagittal balance is an important aspect of the evaluation of patients with spinal pathology, of surgical planning, and to minimize complications such as adjacent segment disease, sagittal imbalance, and pseudarthrosis. The importance of global parameters of sagittal balance is also supported by studies ^{4 ,​ 5 ,​ 6} demonstrating its significant relationship with health-related quality of life in the presence of spinal deformity.

Considering the variability within the normal population, a longitudinal evaluation of sagittal balance is most useful to detect compensatory mechanisms and/or impending sagittal imbalance. In particular, progressive pelvic retroversion, hip and knee flexion, and/or forward displacement of the C7 plumb line should raise a suspicion for an underlying spinal disorder or sagittal imbalance.

7.4 Normal Pediatric Sagittal Balance

Sagittal curvatures of the spine—mainly lumbar lordosis—develop during infancy along with the development of the erect position. Thereafter during growth, the morphology and orientation of the spine, sacrum, and pelvis will change to acquire stable sagittal balance and bipedal gait, ^{7 ,​ 8 ,​ 9} and to accommodate for physiological and morphological changes. To this extent, the pelvis has a central role in the development of a stable sagittal balance as it is the link between the upper and lower body. The sagittal morphology of the pelvis is best measured from the pelvic incidence. Pelvic incidence increases after the acquisition of bipedalism and stabilizes during adulthood. ^{7 ,​ 8 ,​ 9 ,​ 10} The increase in pelvic incidence with age is small but constant throughout childhood and adolescence. ¹⁰ The increase in pelvic incidence mainly results in a proportional increase in pelvic tilt, while sacral slope remains relatively stable. Table 7‑1 presents values of pelvic parameters reported in the literature for different age groups in the normal pediatric population. ^{7 ,​ 11}

Thoracic kyphosis and lumbar lordosis also tend to increase with age (Table 7‑1), although the correlations are small. ^{2 ,​ 10} Accordingly, Cil et al ¹² observed a tendency for increasing thoracic kyphosis and lumbar lordosis between different age groups. Thoracic kyphosis was 45° ± 11°, 48° ± 11°, 46° ± 11°, and 53° ± 9° in 3–6 year, 7–9 year, 10–12 year, and 13–15 year age groups, respectively. Lumbar lordosis was 44° ± 11°, 52° ± 12°, 57° ± 10°, and 55° ± 10° in 3–6 year, 7–9 year, 10–12 year, and 13–15 year age groups, respectively. Similarly, Voutsinas and MacEwen ¹³ reported a slight increase in thoracic kyphosis (1.8° mean difference) and lumbar lordosis (4.4° mean difference) from 5 to 20 years old.

The C7 plumb line tends to move backward slightly during growth ^{11 ,​ 14} until adulthood where it stabilizes before moving forward when degenerative changes occur. Females also show a tendency for having a slightly more posterior C7 plumb line. ¹⁴ Cil et al ¹¹ noted progressive backward displacement of the C7 plumb line with respect to the posterosuperior corner of S1 vertebral body during growth, with mean values of 2.5 ± 4.3 cm, 0.7 ± 4.6 cm, -0.1 ± 4.1 cm, and -0.9 ± 4.4 cm, respectively, for 3–6 year, 7–9 year, 10–12 year, and 13–15 year age groups. However, angular and adimensional parameters of global sagittal balance ^{15 ,​ 16} are usually preferred (Fig. 7‑1, Fig. 7‑2) because they are less sensitive to variations in the radiographic technique, thereby facilitating comparisons with normative values and between independent studies. Table 7‑2 presents global parameters of sagittal spinal balance, while Table 7‑3 shows the typical distribution of global balance types (Fig. 7‑1) observed in the normal pediatric population. ¹⁶ Overall, asymptomatic children and adolescents tend to stand with a relatively stable global spinal balance, with a narrow range of values for spinosacral angle and spinal tilt. Therefore, a spinosacral angle between 115° and 149°, and a spinal tilt between 84° and 102° are typically expected in 95% of the normal pediatric population. In addition, the line from the center of C7 vertebral body to the center of the upper sacral endplate used to measure the spinal tilt should be very close to the vertical line. ¹¹ Analysis of the global balance type (Fig. 7‑1, Table 7‑3) reveals that 27% of juvenile and 11% of adolescent individuals normally stand with a C7 plumb line in front of the sacrum and hip axis (types 3 and 6). This observation suggests that anterior global sagittal balance is not necessarily associated with spinal pathology, particularly in children. For these young individuals, it is assumed that a smaller pelvic incidence will lead to smaller sacral slope and lumbar lordosis, which decreases the ability to position the C7 plumb line behind the sacrum and hip axis.

The most clinically relevant correlations between parameters of sagittal balance involve the lumbosacral–pelvic relationships that highly influence global balance, and that need to be preserved or restored when planning spine surgery. Fig. 7‑3 shows the chain of correlations that is typically observed in the normal pediatric population. ^{2 ,​ 14 ,​ 17}