Sagittal plane deformity: an overview of interpretation and management

Abstract

The impact of sagittal plane alignment on the treatment of spinal disorders is of critical importance. A failure to recognise malalignment in this plane can have significant consequences for the patient not only in terms of pain and deformity, but also social interaction due to deficient forward gaze. A good understanding of the principles of sagittal balance is vital to achieve optimum outcomes when treating spinal disorders. Even when addressing problems in the coronal plane, an awareness of sagittal balance is necessary to avoid future complications. The normal spine has lordotic curves in the cephalad and caudal regions with a kyphotic curve in between. Overall, there is a positive correlation between thoracic kyphosis and lumbar lordosis. There are variations on the degree of normal curvature but nevertheless this shape allows equal distribution of forces across the spinal column. It is the disruption of this equilibrium by pathological processes or, as in most cases, ageing that results in deformity. This leads to adaptive changes in the pelvis and lower limbs. The effects of limb alignment on spinal posture are well documented. We now also know that changes in pelvic posture also affect spinal alignment. Sagittal malalignment presents as an exaggeration or deficiency of normal lordosis or kyphosis. Most cases seen in clinical practise are due to kyphotic deformity secondary to inflammatory, degenerative or post-traumatic disorders. They may also be secondary to infection or tumours. There is usually pain and functional disability along with concerns about self-image and social interaction due to inability to maintain a horizontal gaze. The resultant pelvic and lower limb posture is an attempt to restore normal alignment. Addressing this complex problem requires detailed expertise and awareness of the potential pitfalls surrounding its treatment.

Fig. 1

a Chin-brow vertical angle, b Kyphosis tilt angle, c C7 plumb line (sagittal vertical axis), d Horizontal distance (HD) of displaced sagittal vertical axis from reference point on sacral end plate

Fig. 2

The 3 main pelvic parameters. When measuring pelvic incidence, the reference point is the centre of the femoral head (if the femoral heads superimpose) or the mid point of the line connecting the femoral heads (if they do not superimpose)

Fig. 3

a Spino-pelvic angle and b Spino-sacral angle

Fig. 4

Variations of spinal alignment. Types I–IV. Note the thoracolumbar split (as a measure of length ratio of the thoracic and lumbar curves) in each type: a Type I 80:20, b type II 60:40 c type III 50:50, d 20:80

Fig. 5

Pathological behaviour pattern of pelvis in presence of kyphosis shows a normal posture, b abnormal posture, and c compensatory posture. Pelvic tilt increases in order to maintain an upright posture

Fig. 6

Sagittal plane imbalance. Decompensated posture on the left versus compensated posture on the right. Note change in lower limb profile with extension of hip joints and flexion of knees

Fig. 7

Demonstration of relationship between pelvic incidence and pelvic tilt in compensatory adjustment for kyphosis. Smaller pelvic incidence indicates less compensatory ability

Fig. 8

Polysegmental wedge osteotomies showing resection of the lamina and facets. This leaves segmental gaps posteriorly at each level which are closed with compression to shorten the posterior column and correct deformity

Fig. 9

Pedicle subtraction osteotomy. a Lateral view of vertebra with wedge removed, b posterior vertebral wall with posterior arch and pedicles removed, c posterior view of vertebra after wedge excision. Re-alignment is effected by apposition of the remaining upper and lower portions of the vertebra by compression

Fig. 10

Change in lordotic profile of lumbar spine with shift in apex when osteotomy performed at different levels (L3 vs. L4). Note the shorter lumbar curve when the osteotomy is performed at L4

Fig. 11

Restoration of sagittal alignment is measured by the distance between the plumb lines (A) when the osteotomy is performed at B which is further away and distal to the apex of the deformity therefore requiring a smaller angle of correction C

Fig. 12

Restoration of sagittal alignment with osteotomy (X) closer to apex of deformity. Angle of correction C¹ required to give same amount of improvement in sagittal correction (A) is greater. In both cases, the plumb line (sagittal vertical axis, SVA¹) shifts posteriorly (SVA) with restoration of alignment

Fig. 13

Posterior vertebral column resection diagram to demonstrate margins of resection: 1 Posterior arch, 2 Adjacent lamina, 3 Transverse processes/ribs, 4 Discs, 5 Vertebral body. The anterior longitudinal ligament is left intact to act as a restraint to overcorrection

Fig. 14

Smith Petersen osteotomy: Posterior osteotomies (black lines) with deformity correction through disc space and anterior longitudinal ligament. The correction hinges on the middle and posterior columns. There is a risk of great vessel rupture with this method