Porcine model of early onset scoliosis based on animal growth created with posterior mini-invasive spinal offset tethering: a preliminary report

Abstract

Several models of scoliosis were developed in the past 10 years. In most of them, deformations are induced in old animals and required long time observation period and a chest wall ligation ± resection. The purpose of the study was to create a scoliosis model with a size similar to an early onset scoliosis and an important growth potential without chest wall injuring. An original offset implant was fixed posteriorly and connected with a cable in seven (6 + 1 control) one-month-old Landrace pigs. The mean initial spinal length (T1-S1) was 25 cm and the mean weight was 9 kg. After 2 months observation, spinal deformities were assessed with a three dimension stereographic analysis. In four animals, the cable was sectioned and the deformities followed-up for next 2 months. No post-operative complication was observed. Mean weight growth was 10 kg/month and mean spine lengthening (T1-S1) was 7 cm/month. In 2 months, we obtained structural scoliotic curves with vertebral and disk wedging which were maximal at the apex of the curve. Mean frontal and sagittal Cobb angles was 45°. Chest wall associated deformities were similar to those observed in scoliotic deformities and were correlated to spinal deformities (p = 0.03). The cable section resulted in a partial curve regression influenced by disk elasticity and could probably be influenced by gravity loads (Decrease of the Cobb angle of 30% in the sagittal plane and 45% in the frontal plane). According to the results, the model creates a structural scoliosis and chest wall deformity that is similar to an early onset scoliosis. The spinal deformities were obtained quickly, and were consistent between animals in term of amount and characteristic.

Fig. 1

Representation of the curve lateralisation in accordance to the offset amount for an identical applied load (300 N) in several constructs. Resistance of the disk and of the vertebral body were simulated with finite element method

Fig. 2

Definitive offset design used with the pedicle screws, the cable with the olive fixation

Fig. 3

a Frontal view of a CT-scan with three dimensional reconstruction of the spinal and thoracic deformity. b Sagittal reconstruction of the thoracic deformity and thoracic parameters calculation. Posterior hemithoracic symmetry ratio: a line is drawn across the posterior aspect of the thorax on the basis of the location of the anterior tip of the rib heads articulating with the spine. The distance from the spine at each rib head to the inner border of the hemithorax is measured, and a ratio is derived by dividing the larger value by the smaller value. In a normal thorax, the ratio is one, but as a rib hump develops, the ratio increases, reflecting the onset of early windswept thorax. Thoracic ratio: it is represented by the angle between a line bisecting the thorax and the sagittal plane of the vertebra

Fig. 4

Mean value with standard deviation (black vertical segment) of the vertebral wedging. Vertical axis: Vertebral wedging in degrees

Fig. 5

Lateral tilt (dark grey line) and rotation of individual vertebrae (light grey line) of each specimens. The tethered segments are in between the vertical dotted lines