Morphological patterns of the rib cage and lung in the healthy and adolescent idiopathic scoliosis

Abstract

The morphology of the rib cage affects both the biomechanics of the upper body's musculoskeletal structure and the respiratory mechanics. This becomes particularly important when evaluating skeletal deformities, as in adolescent idiopathic scoliosis (AIS). The aim of this study was to identify morphological characteristics of the rib cage in relation to the lung in patients with non-deformed and scoliotic spines. Computed tomography data of 40 patients without any visible spinal abnormalities (healthy group) and 21 patients with AIS were obtained retrospectively. All bony structures as well as the right and left lung were reconstructed using image segmentation. Morphological parameters were calculated based on the distances between characteristic morphological landmarks. These parameters included the rib position, length, and area, the rib cage depth and width, and the rib inclination angle on either side, as well as the spinal height and length. Furthermore, we determined the left and right lung volumes, and the area of contact between the rib cage and lung. Differences between healthy and scoliotic spines were statistically analysed using the t-test for unpaired data. The rib cage of the AIS group was significantly deformed in the dorso-ventral and medio-lateral directions. The anatomical proximity of the lung to the ribs was nearly symmetrical in the healthy group. By contrast, within the AIS group, the lung covered a significantly greater area on the left side of the rib cage at large thoracic deformities. Within the levels T1-T6, no significant difference in the rib length, depth to width relationship, or area was observed between the healthy and AIS groups. Inferior to the lung (T7-T12), these parameters exhibited greater variability. The ratio between the width of the rib cage at T6 and the thoracic spinal height (T1-T12) was significantly increased within the thoracic AIS group (1.1 ± 0.08) compared with the healthy group (1.0 ± 0.05). No statistical differences were found between the lung volumes among all the groups. While the rib cage was frequently strongly deformed in the AIS group, the lung and its surrounding ribs appeared to be normally developed. The observed rib hump in AIS appeared to be formed particularly by a more ventral position of the ribs on the concave side. Furthermore, the rib cage width to spinal height ratio suggested that the spinal height of the thoracic AIS-spine is reduced. This indicates that the spine would gain its growth-related height after correcting the spinal deformity. These are the important aspects to consider in the aetiology research and orthopaedic treatment of AIS.

Keywords: AIS; biomechanics; chest; lung; morphology; rib cage; scoliosis; thorax.

FIGURE 1

Illustration of the global coordinate system defined by S1–T1 and the middle of the suprasternal notch as well as the position of the landmarks (extreme left). Exemplary patient cases of the three different spinal groups (right) [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 2

Morphological parameters calculated from the landmark locations. Landmarks were placed on the posterior position (postRib), medial position (medRib), and anterior position (antRib) of the respective rib, as well as the posterior point on the superior endplate of the vertebral body (supVert). Derived morphological parameters were rib length, area enclosed by the rib, depth and width of each rib level, inclination angle of the ribs. The inclination angle is the angle of the vector, pointing from the vertebral body to the anterior part of the rib and to the transverse plane of the global coordinate system [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 3

Example that illustrates how the anatomical proximity of the lung to the ribs was determined. (a) Patient case with the segmented bone and lung, as well as the position of the landmarks (yellow dots) on each rib in the posterior (post), middle (mid), and anterior (ant) sections. (b) Calculated nearest distance in mm from each landmark to the lung. (c) Simplified illustration, whether contact between lung and rib is present. All calculated distances below 20 mm are considered as full contact (1) and above 20 mm as no contact (0) [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 4

Morphological parameters of the healthy (black lines & dots) and scoliotic spines (coloured lines & dots): Lateral position of the vertebral bodies (VB) (a); dorso‐ventral position of the posterior rib section on the left (b) and right (c) sides; medio‐lateral position of the middle section of the left (d) and right (e) ribs; dorso‐ventral position of the anterior rib section on the left (f) and right (g) sides; “x” indicates a p‐value < 0.01 when comparing the AIS and healthy groups [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 5

(a) Relationship between the left and right lung volumes for each type of spinal curvature (mean and standard deviation). (b) Average region of the rib cage that is in anatomical proximity of the lung for the non‐deformed (healthy) spine group: anterior (ant), middle (mid), and posterior (post) regions of each rib. 0.0 indicates no contact (white) and 1.0 full contact within all subjects (blue) of the lung and rib in the specific region [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 6

Morphological parameters of the healthy (black lines & dots) and scoliotic spines (coloured lines & dots): (a) Relationship between the left and the right rib lengths. (b) Relationship between the depth and the width of each rib level. (c) Inclination angle of the left and (d) right ribs relative to the inclination angle of T1. (e) Relationship between the left and right areas that are enclosed by the rib. (f) Anterior (ant), middle (mid), and posterior (post) rib regions which are attached to the lung; 0.0 indicates no contact (white) and 1.0 full contact within all subjects (blue) of the lung and rib in the specific region. “x” indicates a p‐value <.01 when comparing the AIS and healthy groups [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 7

Relationship between the rib cage width, measured between the left and right medial points of the 6th rib, and the spinal length (a) and height (b). The spinal length was measured along the spinal curvature from T1 to T12 and the spinal height as the direct line between T1 and T12 [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE A1

Example CT images with the 3D‐reconstruction of two thoracolumbar AIS cases, which display an offset of the lung (blue) in the posterior region of the rib cage on the convex side of the spinal curvature [Colour figure can be viewed at wileyonlinelibrary.com]