19.1 Introduction

Sagittal balance is an essential component of everyday life. It is the result of complex interactions between the pelvis and the spine but also with the lower extremities. Any alteration between one of these structures can lead to abnormal sagittal balance and have a profound effect on the patient’s ability to stand and/or walk for prolonged periods of time. When sagittal balance is preserved, the human body is in a state of minimal energy expenditure.

Adolescent idiopathic scoliosis (AIS) is a three-dimensional deformity of the spine not only affecting the coronal and sagittal plane but also the axial plane. It is this spatial deformity of the spine that creates unbalance in the three planes. Locally, both the intervertebral disk and the vertebral body’s shape are affected, contributing to the regional and global deformity. When addressing these deformities surgically, the surgeon must have a good understanding of the impact of the corrective maneuvers and the extent of the instrumentation on the uninstrumented spine. Although surgeons historically focused on the correction in the coronal plane, there has been a renewed interest in the sagittal plane and the impact of different corrective strategies in AIS. From past experience with the Harrington instrumentation, it has become clear that special attention must be given to restore an optimal sagittal balance postoperatively to prevent long-term disability and reoperations.

19.2 Normal Spinopelvic Balance in Children and Adolescents

There are excellent references for sagittal spinopelvic balance in children and adolescents both for normal patients and for patients with AIS. ^{1 ,​ 2 ,​ 3 ,​ 4} Pelvic incidence (PI) is a pelvic morphometric parameter that increases during adolescent growth and stabilizes in adulthood. ⁵ PI regulates the relationship between sacral slope (SS) and pelvic tilt (PT), whereby PI = SS + PT. ⁶ PI is a determinant of lumbar lordosis (LL) and therefore is usually closely related to the lumbar sagittal curve. PI has been shown to be slightly higher in both AIS ⁷ and adults with idiopathic scoliosis ⁴ despite the finding that scoliosis deformity is often associated with a hypokyphosis or lordosis of the thoracic segment for thoracic structural curves. The type of curve has not been associated with a change in PI or sagittal balance ⁷ but the thoracic kyphosis is highly associated with the type of curve. The LL is not influenced by the type of scoliotic deformity but is associated with the pelvic morphology. ⁷

Global sagittal balance has also been described in children and adolescents. Different techniques have been proposed to assess global sagittal balance and C7 is generally used as a reference point to represent where the spine is in space relatively to the pelvis. These global measurements typically will encompass a larger number of parameters such as sacropelvic parameters, thoracic kyphosis and LL. One such parameter is based on the distance between the C7 plumb line and the anterosuperior corner of S1. This measurement was found to evolve between adolescents and adults with a progressive displacement of C7 over the pelvis denoting a form of modification in the sagittal curvatures with growth. ^{8 ,​ 9}

Linear parameters are, however, subject to measurement error if using uncalibrated radiographs. Descriptive angular parameters have been introduced to overcome these limitations. The spinosacral angle is an angle subtended by the upper sacral endplate and the line from the center of C7 vertebral body to the center of the upper sacral endplate, as described by Roussouly et al. ¹⁰ The spinal tilt (ST) angle is subtended by the horizontal line and the line from the center of the C7 vertebral body to the center of the upper sacral endplate. A value greater than 90° indicates that the center of the C7 vertebral body is behind the center of the upper sacral endplate, while for values less than 90°, the center of the C7 vertebral body is in front of the center of the upper sacral endplate. Spinopelvic tilt (SPT) is an angle subtended by the horizontal line and the line from the center of the C7 vertebral body to the hip axis. A value greater than 90° indicates that the center of the C7 vertebral body is behind the hip axis, while for values less than 90°, the center of the C7 vertebral body is in front of the center of the hip axis.

Finally, a classification has been proposed that does not use an angular or linear measurement but is based on the relative position of the C7 plumb line to both the sacrum and vertical axis. ¹¹ This provides a simple and intuitive understanding of the position of the spine relative to the sacrum and hips. Six types are proposed, three where the center of the upper endplate of the sacrum lies in front of the hip axis and three where it lies behind the hip axis. Types 3 and 6 represent the situation where C7 is in front of both the center of the upper endplate of the sacrum and the hip axis. ¹¹ This classification could be used to identify patients at higher risk of developing degenerative changes later in their adult life. Fig. 19‑1 shows the six types as described by Mac-Thiong et al. ¹¹ The classification determines global balance without relying on angular parameters. An evaluation of 646 children and adolescents has provided normative data for these types. A C7 plumb line in front of the hip axis and in front of the sacrum (types 3 and 6) is not necessarily associated with spinal pathology with 22% of asymptomatic patients having this configuration. The large variation in asymptomatic patients may reflect the complex interaction in patients’ shape, muscle tone, and spine morphology to obtain an upright stable posture.

19.3 How the Spine Reacts to Surgical Correction in Adolescent Idiopathic Scoliosis

Adolescents and young adults are particularly resilient to postural imbalance and will be able to accommodate for imperfect sagittal realignment through different mechanisms. Such mechanisms include pelvic retroversion and, as further imbalance occurs through degenerative changes, further pelvic retroversion, knee flexion and forward trunk displacement (Fig. 19‑2). Fortunately, this process usually happens over a long period of time and is more common in adult spinal deformities than in AIS. In adult spinal deformities, the focus is to restore sagittal balance to prevent proximal junctional issues and to minimize sagittal and coronal imbalance. The focus in recent years has been much more on restoring the sagittal alignment than on pure coronal correction, thereby creating a state of least energy expenditure. The long-term goal of AIS surgery is to prevent progression of the scoliotic deformity. This should also include the prevention of distal and proximal junctional issues as well as sagittal balance issues.

When surgical correction of a specific deformity is performed, there are typically two distinct mechanisms involved to produce the resultant sagittal alignment. First, there is an imposed correction that is highly dependent on the surgical maneuver and the instrumentation that will impose a fixed sagittal profile in the instrumented segment. The second mechanism is the reactive changes observed in the uninstrumented segment of the spine both above and below the instrumentation. A delicate balance between imposing too much correction in the instrumented segment and the extent of the fusion can limit the spontaneous adjustment usually observed following spinal arthrodesis.