Frontal and sagittal imbalance in patients with adolescent idiopathic deformity

Abstract

The human spinal column underwent many significant changes over the 4.5 million years of our ancestral bipedalism. The main change, however, came with acquiring multiple curvatures in the sagittal plane. This alteration seems to have exposed a weakness in our body's keystone and made us susceptible to thus far unbeknown problems of the spine because it has been noted that idiopathic scoliosis has not been observed in other primates. Adolescent idiopathic scoliosis (AIS) is a three-dimensional deformity of the spine causing an imbalance of the trunk as it increases in magnitude. A scoliotic curve comprises three components, the coronal, sagittal, and axial so that each curve can affect the global balance of the body differently. Patients with significant scoliotic deformities often find themselves at a biomechanical disadvantage when it comes to energy expenditure and keeping an upright stance. The pioneers of scoliosis research recognized the need for describing and quantifying deformity to better understand it, so they first translated clinical measurements to radiographs and built from there. The development of concepts like defining a curve by its end vertebrae and measuring its magnitude, assessing global spinal balance, describing the stable zone, and pinpointing the stable vertebra all followed suit. The importance of sagittal balance and restoring sagittal parameters during treatment was emphasized. In a quest to bring order to chaos, some tried to classify various scoliotic curve types. These classifications helped steer treatment decisions but were found lacking in many aspects. So far, a widely accepted three-dimensional classification of scoliosis still does not exist. This review aims to provide the reader with an overview of the development of balance and imbalance concepts in scoliosis.

Keywords: Adolescent; kyphosis; lordosis; scoliosis; spine.

Conflict of interest statement

Conflicts of Interest: The authors have no conflicts of interest to declare.

2020 Annals of Translational Medicine. All rights reserved.

Figures

Figure 1

Coronal balance. The C7PL is dropped from the center of the C7 vertebra in the same way it is done clinically and drawn parallel to the vertical edge of the image. The CSVL is drawn upward from the middle of S1, parallel to the vertical edge of the radiograph. In a healthy and balanced spine, these lines coincide. Offset between the C7PL and CSVL marks an imbalance in the coronal plane, easily quantified in millimeters by the formula B - A = ±X. Image courtesy of Scoliosis Research Society, from (28). C7PL, plumb line dropped from C7 vertebra; CSVL, center sacral vertical line.

Figure 2

Coronal trunk shift. Thoracic trunk shift is measured on upright PA (or AP) radiographs. First, the thoracic apical vertebra needs to be identified, and its center marked. A horizontal line is drawn through the center, and the edges of the apical ribs are marked. The midpoint between the two edge points is marked, and a perpendicular line is dropped as reference. This line is referred to as the vertical trunk reference line, and the trunk shift is measured as the linear distance in millimeters between this line and the CSVL. Shift to the right is marked as a positive and to the left of the CSVL a negative value. Image courtesy of Scoliosis Research Society, from (28). PA, posteroanterior; CSVL, center sacral vertical line.

Figure 3

Thoracic sagittal alignment. Thoracic kyphosis is measured from the upper-endplate of T2 to the lower endplate of T12 using the Cobb method. The upper thoracic spine is the most difficult to image, and it is a common occurrence not to have a clear shot of T2 on a radiograph, due to superposition of overlapping structures. Proximal thoracic kyphosis is measured from the upper-endplate of T2 to the lower endplate of T5. Mid/lower thoracic kyphosis is measured from the upper-endplate of T5 to the lower endplate of T12. Image courtesy of Scoliosis Research Society, from (28).

Figure 4

Thoracolumbar/lumbar sagittal alignment. Thoracolumbar sagittal alignment is measured from the upper-end plate of T10 to the lower endplate of L2 using the Cobb method. Lumbar sagittal alignment is measured from the upper-end plate of T12 to the S1 endplate. The S1 endplate can sometimes be difficult to identify by a straight line, so an alternative is to draw a line perpendicular to the posterior sacral cortex and draw a perpendicular to it at the level of S1. Image courtesy of Scoliosis Research Society, from (28).

Figure 5

Sagittal balance. The center of C7 vertebra is marked, and a plumb line has dropped perpendicular to the vertical edge of the radiograph. The posterosuperior corner of S1 is also marked, and a line perpendicular to the vertical edge of the radiograph is constructed. The horizontal difference measured in millimeters between these two lines gives the sagittal balance. No difference between the lines equals sagittal balance. Migration of the line to the front is marked as a positive value, while the movement to the back is negative. Image courtesy of Scoliosis Research Society, from (28).

Figure 6

Spinopelvic parameters. Sagittal pelvic parameters assessed from the standing lateral radiograph (in AIS, while in non-ambulating children with EOS the spinopelvic parameters can also be measured on sitting radiographs). Landmarks needed for measurement are the hip axis (located in the middle of the line connecting the centers of the femoral heads) and the middle of the sacral plate. Pelvic incidence is always equal to the sum of pelvic tilt and sacral slope. Image provided courtesy of Spine LWW, from (65). AIS, adolescent idiopathic scoliosis.