The importance of curve severity, type and instrumentation strategy in the surgical correction of adolescent idiopathic scoliosis: an in silico clinical trial on 64 cases

Abstract

Adolescent idiopathic scoliosis is a three-dimensional deformity of the spine which is frequently corrected with the implantation of instrumentation with generally good or excellent clinical results; mechanical post-operative complications such as implant loosening and breakage are however relatively frequent. The rate of complications is associated with a lack of consensus about the surgical decision-making process; choices about the instrumentation length, the anchoring implants and the degree of correction are indeed mostly based on personal views and previous experience of the surgeon. In this work, we performed an in silico clinical trial on a large number of subjects in order to clarify which factors have the highest importance in determining the risk of complications by quantitatively analysing the mechanical stresses and loads in the instrumentation after the correction maneuvers. The results of the simulations highlighted the fundamental role of the curve severity, also in its three-dimensional aspect, and of the instrumentation strategy, whereas the length of the fixation had a lower importance.

Figure 1

Six representative finite element models of the thoracolumbar spine of scoliotic subjects. For each model, the coronal, sagittal and axial views are shown.

Figure 2

The seven screw patterns investigated: segmental (S), convex minimal (C_M), apical key vertebrae (A_P_K), alternate (A), convex alternate (C_A), periapical dropout (P_D), convex periapical dropout (C_P_D). UIV upper instrumented vertebra, APEX apex of the curve with the largest Cobb angle, LIV: lower instrumented vertebra.

Figure 3

Distributions of the post-operative major Cobb angle in the coronal plane (a), of the post-operative transverse rotation of the apical vertebra (b), of the difference between the target thoracic kyphosis (c) and lumbar lordosis (d) and the predicted values. In (c,d), negative values should be intended as undercorrections, and positive values as overcorrections.

Figure 4

Relative importance of the features in determining the maximal stress in the rods (a) and the maximal force at the screw–rod interface (b). Lenke Lenke type, Cobb Cobb angle in the coronal plane of the largest curve, sc. pat. screw pattern, in. len. instrumentation length, apex location of the apex of the largest curve, apex rot. rotation of the apex in the transverse plane, u. cur. upper end vertebra of the deformity, l. cur. lower end vertebra of the deformity, UIV upper instrumented vertebra, LIV lower instrumented vertebra, lord. change in lumbar lordosis with respect to the pre-operative condition, kyph. change in thoracic kyphosis.

Figure 5

Distribution plots showing the maximal rod stress (a) and maximal screw–rod force (c) with respect to the transverse rotation of the apex of the largest curve. Heatmaps and p values of the statistical comparisons between groups regarding the maximal rod stress (b) and the maximal screw–rod force (d).

Figure 6

Distribution plots showing the maximal rod stress (a) and maximal screw–rod force (c) with respect to the Cobb angle of the largest curve in the coronal plane. Heatmaps and p values of the statistical comparisons between groups regarding the maximal rod stress (b) and the maximal screw–rod force (d).

Figure 7

Distribution plots showing the maximal rod stress (a) and maximal screw–rod force (c) with respect to the screw pattern. Heatmaps and p values of the statistical comparisons between the different screw patterns regarding the maximal rod stress (b) and the maximal screw–rod force (d). S segmental, C_M convex minimal, A_K_V apical key vertebrae, A alternate, C_A convex alternate, P_D periapical dropout, C_P_D convex periapical dropout.

Figure 8

Distribution plots showing the maximal rod stress (a) and maximal screw–rod force (c) with respect to the Lenke type. Heatmaps and p values of the statistical comparisons between the different Lenke types regarding the maximal rod stress (b) and the maximal screw–rod force (d).

Figure 9

Frequency plot of the position of the maximal rod stress (a) and of the maximal screw–rod force (b) with respect to the apex of the largest curve. Negative values (− 1, − 2, etc.) indicate the vertebrae above the apex, i.e. proceeding in the cranial direction, whereas positive values (+ 1, + 2, etc.) indicate the vertebrae caudal to the apex.