Sagittal balance correction of idiopathic scoliosis using the in situ contouring technique

Abstract

Purpose: Idiopathic scoliosis can lead to sagittal imbalance. The relationship between thoracic hyper- and hypo-kyphotic segments, vertebral rotation and coronal curve was determined. The effect of segmental sagittal correction by in situ contouring was analyzed.

Methods: Pre- and post-operative radiographs of 54 scoliosis patients (Lenke 1 and 3) were analyzed at 8 years follow-up. Cobb angles and vertebral rotation were determined. Sagittal measurements were: kyphosis T4-T12, T4-T8 and T9-T12, lordosis L1-S1, T12-L2 and L3-S1, pelvic incidence, pelvic tilt, sacral slope, T1 and T9 tilt.

Results: Thoracic and lumbar curves were significantly reduced (p = 0.0001). Spino-pelvic parameters, T1 and T9 tilt were not modified. The global T4-T12 kyphosis decreased by 2.1° on average (p = 0.066). Segmental analysis evidenced a significant decrease of T4-T8 hyperkyphosis by 6.6° (p = 0.0001) and an increase of segmental hypokyphosis T9-T12 by 5.0° (p = 0.0001). Maximal vertebral rotation was located at T7, T8 or T9 and correlated (r = 0.422) with the cranial level of the hypokyphotic zone (p = 0.003). This vertebra or its adjacent levels corresponded to the coronal apex in 79.6 % of thoracic curves.

Conclusions: Lenke 1 and 3 curves can show normal global kyphosis, divided in cranial hyperkyphosis and caudal hypokyphosis. The cranial end of hypokyphosis corresponds to maximal rotation. These vertebrae have most migrated anteriorly and laterally. The sagittal apex between segmental hypo- and hyper-kyphosis corresponds to the coronal thoracic apex. A segmental sagittal imbalance correction is achieved by in situ contouring. The concept of segmental imbalance is useful when determining the levels on which surgical detorsion may be focused.

Fig. 1

Right thoracic curve in the coronal plane with cranial hyperkyphosis (white arrow) and caudal hypokyphosis (black arrow) in the sagittal plane

Fig. 2

Correction pathway (continuous curved line) using in situ contouring at lumbar levels, pushing vertebrae successively forward (increasing lordosis) and medial (correction of coronal curve) while derotating on convex sided screws with a lever arm

Fig. 3

Correction pathway (continuous curved line) using in situ contouring at thoracic levels, pulling vertebrae successively backward (increasing kyphosis) and medial (correction of coronal curve) using pedicle hooks on the concave side of the curve

Fig. 4

Relationship between maximally rotated thoracic and lumbar vertebrae, apical vertebrae and sagittal hyphokyphotic zone. The cranial extremity of this zone (black arrow) corresponds to the turning point of the thoracic curve and separates caudal hypokyphosis and cranial hyperkyphosis

Fig. 5

Coronal curve correction and thoracic sagittal imbalance with cranial hyperkyphosis and caudal hypokyphosis before (white arrow) and after (black arrow) in situ contouring

Fig. 6

Comparison of two types of thoracic scoliosis with different sagittal plane deformities: global hypokyphosis (flat back) on the left and globally normal kyphosis (cranial hyperkyphosis and caudal hypokyphosis) on the right