Sagittal alignment of the cervical spine: do we know enough for successful surgery?

Abstract

Over the past decade, there has been growing interest in the sagittal alignment of the cervical spine and its correlation to clinical outcomes. It is now known that cervical lordosis is not universally physiological and should not be pursued in all patients undergoing surgery. Rather, it is increasingly understood that it is how these angular parameters (lordosis or kyphosis) interact with translational parameters that is reflective of overall cervical spine and whole spine balance, which in turn impacts patient outcomes. This review synthesises currently available evidence relating to the sagittal alignment of the cervical spine. Radiographic assessment of the cervical spine including horizontal gaze is discussed and alignment in physiological and pathological states analysed. The interdependence of spinal segments is reviewed, with cervical alignment correction influencing the adjacent thoracic spine, and similarly lumbar lordosis (LL) and global balance correction changing cervical spine alignment. Cervical kyphosis with associated cervical sagittal imbalance is known to lead to poorer post-operative outcomes, and this dichotomy in outcomes has not been shown to improve even with alignment correction. Further work is required to uncover the extent to which cervical spine alignment correction should be attempted toward improved patient outcomes, in order to plan and deliver patient-specific surgical realignment targets.

Keywords: Alignment; cervical spine; corpectomy; discectomy; fusion; kyphosis; laminectomy; laminoplasty; lordosis; sagittal balance; surgery.

Figure 1

The five cervical sagittal alignment subtypes.

Figure 2

The various radiographic measures of horizontal gaze. (A) Chin brow vertical angle; (B) slope of line of sight—angle between a line drawn from inferior margin of the orbit to the superior border of the external auditory meatus and the horizontal; (C) McGregor’s slope—angle between a line drawn from the posterosuperior corner of hard palate to opisthion and the horizontal; (D) the ‘3-6-12’ rule: a = vertical distance from horizontal line drawn from middle of orbit to base of sella turcica. b = angle between line drawn from middle of orbit to internal occipital protuberance and horizontal.

Figure 3

Spontaneous increase in compensatory cervical lordosis following increase in lumbar lordosis and thoracic kyphosis, to maintain overall cervical sagittal balance (C2-C7 SVA 0.9 to 2.7 cm) and global spinal balance (SVA 2.4 to 3.6 cm). SVA, sagittal vertical axis.

Figure 4

Increase in T1-slope and thoracic kyphosis following increase in cervical lordosis.