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The authors have declared that no competing interests exist.

Conceived and designed the experiments: SAS PA MB CC CHR. Performed the experiments: SAS. Analyzed the data: PA SAS. Contributed reagents/materials/analysis tools: MB PA. Wrote the paper: PA MB AL CC CHR SAS.

Vertebral wedging is associated with spinal deformity progression in adolescent idiopathic scoliosis. Reporting frontal and sagittal wedging separately could be misleading since these are projected values of a single three-dimensional deformation of the vertebral body. The objectives of this study were to determine if three-dimensional vertebral body wedging is present in mild scoliosis and if there are a preferential vertebral level, position and plane of deformation with increasing scoliotic severity.

Twenty-seven adolescent idiopathic scoliotic girls with mild to moderate Cobb angles (10° to 50°) participated in this study. All subjects had at least one set of bi-planar radiographs taken with the EOS® X-ray imaging system prior to any treatment. Subjects were divided into two groups, separating the mild (under 20°) from the moderate (20° and over) spinal scoliotic deformities. Wedging was calculated in three different geometric planes with respect to the smallest edge of the vertebral body.

Factorial analyses of variance revealed a main effect for the scoliosis severity but no main effect of vertebral Levels (apex and each of the three vertebrae above and below it) (F = 1.78, p = 0.101). Main effects of vertebral Positions (apex and above or below it) (F = 4.20, p = 0.015) and wedging Planes (F = 34.36, p<0.001) were also noted. Post-hoc analysis demonstrated a greater wedging in the inferior group of vertebrae (3.6°) than the superior group (2.9°, p = 0.019) and a significantly greater wedging (p≤0.03) along the sagittal plane (4.3°).

Vertebral wedging was present in mild scoliosis and increased as the scoliosis progressed. The greater wedging of the inferior group of vertebrae could be important in estimating the most distal vertebral segment to be restrained by bracing or to be fused in surgery. Largest vertebral body wedging values obtained in the sagittal plane support the claim that scoliosis could be initiated through a hypokyphosis.

Scoliosis is a three-dimensional (3D) deformity of the spine with curvatures occurring in all three planes

There are however a few studies on 3D vertebral morphology. Masharawi et al.

Frontal plane wedging has been well documented for scoliotic patients with severe deformity. In Modi et al.

In this study we have combined the approaches described by Parent et al.

Twenty-seven adolescent idiopathic scoliotic girls having an average age of 13.1±1.7 years and diagnosed by an orthopedic surgeon according to the criteria given by Bunnel

The girls were then divided into two groups to separate the mild from the moderate spinal scoliotic deformities. The median Cobb angle was 20° and it was selected as the division point between these two groups. Such a distribution was used in previous studies

Group | n | Age (years) | Mean Cobb angle (degrees) | Cobb angle range (degrees) |

Mild scoliosis | 20 | 12.5±1.5 | 14.9±3.1 | 10–19 |

Moderate scoliosis | 20 | 13.6±1.7 | 28.2±8.1 | 20–50 |

Forty standardized bi-planar radiographs were taken with the EOS® X-ray imaging system based on the Nobel Prize work of Georges Charpak. This system allows for full-body weight-bearing standing orthogonal set of digital radiographs. These are taken simultaneously to ensure that the same spine configuration is projected onto the postero-anterior and sagittal radiographs simultaneously. The EOS® system has been developed for orthopedic applications especially for assessing spinal deformities

Each radiograph was manually digitized to obtain the planar coordinates of 8 bony landmarks on each of the 12 thoracic and 5 lumbar vertebrae. These points correspond to the vertebral body corners

Points represent the eight vertebral body corners as seen on the radiographs. The spatial angles were measured with respect to the smallest edge (SE) representing the frontal, sagittal and diagonal planes wedging. Ψ, φ and θ represent respectively the postero-frontal, sagittal and diagonal angles with 1 and 2 indicating the superior and inferior angles. Each superior and inferior angles of a plane were summated to characterize vertebral wedging.

Firstly a bony landmark was identified on the postero-anterior radiograph. A horizontal line drawn from this point was projected on the lateral radiograph. The intersection between the horizontal line and the edge of the vertebral body ensured that the same bony landmark was identified in each pair of radiographs.

When a second compensatory curve was present, the largest between the two was selected for the analysis. The apical or apex vertebra and three vertebrae above and three below it were analyzed. Wedging was measured by using the 3D coordinates of the eight points reconstructed from the bi-planar radiographs. Since the vertebral body is not necessarily a perfect cylinder of constant height, its shortest vertical edge was identified first as shown in

Wedging was calculated with respect to the three other vertebral body edges as shown in

A factorial analysis of variance was performed to determine the effect of the scoliosis severity on seven vertebral Levels (apex and each of the three vertebrae above and below the apex). A second factorial analysis of variance was performed to determine the effect of the scoliosis severity on three Positions (3 superior vertebrae, apex vertebra and 3 inferior vertebrae) and three wedging Planes (postero-frontal, sagittal and diagonal). A p value equal or less than 0.05 was considered as statistically significant. A Bonferroni post-hoc analysis was performed if a statistically significant difference was found.

Data related to this study was not made publicly available as it would breach confidentiality related to human subject research and that all data must be destroyed after a given number of years by the principal investigator.

The extent of wedging at the apex and at each of three vertebrae above and below it for the mild and moderate scoliosis groups is shown in

Circles represent the average wedging and bars represent the 95% confidence interval. The mild scoliosis group is in blue and the moderate scoliosis group is in green.

The combined effect of three superior and three inferior vertebrae wedging compared to that at the apex vertebra for the two scoliotic groups are shown in

Circles represent the average wedging and bars represent the 95% confidence interval. The mild scoliosis group is in blue and the moderate scoliosis group is in green.

The final results were to determine if there was a preferential plane of vertebral wedging.

Circles represent the average wedging and bars represent the 95% confidence interval. The mild scoliosis group is in blue and the moderate scoliosis group is in green.

According to Wang et al.

Stokes and Aronsson

Vertebral wedging has been reported to be the largest at the apex vertebra and to become smaller as the inferior and superior vertebrae are more distant to the apex

Our method of calculating vertebral wedging differs from others because it involves bi-planar radiography and 3D calculations as enabled by the modern EOS® system. By using the projection of the endplates on a postero-anterior radiograph, the orientation of the lateral edges of the vertebra is excluded. With 3D measurements, the endplates and vertebral edges are not considered horizontal or vertical. In other words the vertebral body is not considered as a slanted but rather a deformed cube. In this method, wedging is calculated in a reference system specific to each vertebra. A future study involving the calculation of wedging in the plane of maximal deformity could bring additional information on the influence of each vertebra on the scoliotic deviation.

The progression of vertebral wedging could be attributed to an abnormal vertebral growth in the skeletally immature spine of scoliotic patients

Wedging in the sagittal plane was reported by Parent et al.

Vertebral axial rotation could affect the apparent wedging as seen from the posterior-anterior radiograph. In such a case the lateral plane wedging is in part projected on the frontal plane. According to Stokes and Aronsson

Though no definite conclusions can be drawn about the causal relationship between the vertebral wedging and spinal curve progression in adolescent idiopathic scoliosis, the present study brings new insight in the development of scoliosis from its onset. Few studies have reported quantitative measurements of vertebral wedging in girls with mild scoliosis. The progression of vertebral wedging from the early on-set observed in girls with a mild scoliosis could lead to better corrective spinal surgical strategies and more efficient corrections

The main findings of this 3D measurements study are that vertebral body wedging is present in girls with a mild scoliosis of less than 20° and that it progresses. Wedging was more pronounced in the three vertebral bodies immediately located below the apex. This could be an important consideration in estimating the most distal vertebral segment to be restrained by bracing or to be fused in surgery. Though postero-frontal plane wedging values increased with the progression of the spinal deformity, values obtained in the sagittal plane were the largest. This supports the claim of Dickson

The authors wish to express their gratitude to Mrs. Mèlodia Veclabéaime, Mr. Marcel Beaulieu and Mr. Benjamin Michaud for their technical assistance.