J Korean Soc Spine Surg.  2003 Mar;10(1):14-24. 10.4184/jkss.2003.10.1.14.

The Explanation of Postoperative Change of Vertebral Rotation and Rib Hump Using 3 Dimensional Finite Element Scoliosis Model

• Ha JH
• Lee JH
• Ahn YJ
• Son CK
• Lee KH
• Choi HY
• Kim YE
• Lee CK

• KMID: 2209700
• DOI: http://doi.org/10.4184/jkss.2003.10.1.14

Abstract

STUDY DESIGN: An analytical study using a mathematical 3-D finite element model for thoracic scoliosis.
OBJECTIVE
To find the important kinematics and post-operative changes of the spine and rib cage, in the corrective surgery for scoliosis, using the rod derotation method. SUMMARY OF LITERATURE REVIEW: A conventional corrective surgery for scoliosis was performed, based on empirical knowledge, and an increase in the secondary postoperative change in the rib hump, and a shoulder level imbalance, were reported. However, no analytical data exists for the kinematics and optimal correction method.
MATERIALS AND METHODS
A mathematical finite element model of a normal spine, including the rib cage, sternum, both clavicles and pelvis, was developed. Using geometric mapping, with standing radiographs and CT images, a 3-D FEM of scoliosis was reconstructed, after translating and rotating the 3-D FEM of a normal spine, with the amounts analyzed from 12 built-in digitized coordinate axes for each vertebral image. With this model, three elements; distraction, translation and derotation, in operative kinematics, were investigated by analyzing the Cobb angle, apical vertebrae axial rotation (AVAR) and thoracic kyphosis. A simulation of a segmental pedicle screw fixation, with rod derotation for scoliosis, was performed. The changes in the Cobb angle, kyphotic angle, AVAR and rib hump were compared after 0 degrees, 15 degrees, 30 degrees, 45 degrees, 60 degrees and 90 degrees rod derotations.
RESULTS
In kinematics, the vertebral rod derotation of a major curve, without rod deformation, is less influential in the correction of scoliosis, simply causing an increase in the rib hump. During the simulation, the co-action of distraction and translation, during rod insertion, has a major impact on the decrease in the Cobb angle and in the maintenance of the kyphotic angle. However, after a 30 degrees rod derotation, a decrease in the kyphosis, and increases in the rib hump and AVAR were observed.
CONCLUSIONS
The distraction and translation factors were more important in operative kinematics than the rod derotation. With excessive rod derotation, the Cobb angle progressively decreased, but increases in the secondary change in the rib hump and rotation of the apical vertebrae were found.

Keyword

Scoliosis; Rib hump; FEM; Derotation; Kinematics

MeSH Terms

Biomechanical Phenomena
Clavicle
Kyphosis
Pelvis
Ribs*
Scoliosis*
Shoulder
Spine
Sternum
Translating

Figure

• 

  Fig. 1. Normal 3-Dimensional Spine Finite Element Model including rib cage, sternum, pelvis, clavicle, scapula, intervertebral disc and ligaments.

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  Fig. 2. A. Digitization of 12 Coordinates from roentgenogram of King-Moe type II scoliosis. B. Conversion from normal 3-D FEM spine model to scoliosis model by displacement of vertebral body center and rotation. C. Developed 3-Dimensional Scoliosis Finite Element Model similar to King-Moe type II scoliosis.

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  Fig. 3. Measurement of Apical Vertebrae Axial Rotation (AVAR) from 3-D FEM of scoliosis. AVAR was defined as the rotation of the apex vertebra about its local z-axis.

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  Fig. 4. Measurement of Rib Hump from 3-D FEM of scoliosis. The rib hump was defined as a distance between lines drawn vertically at most protruded rib surface and at opposite side of rib surface equidistant apart.

• 

  Fig. 5. A. Distraction of 3-D FEM of scoliosis. The upper end vertebra (T5) was distracted 40 mm to cranial direction. At each decadal point, 3-D FEM of scoliosis in coronal plane was displayed. B. The results of Cobb angle, angle of kyphosis, and AVAR during distraction. With distraction, Cobb angle and AVAR decreased, but the angle of kyphosis increased slightly.

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  Fig. 6. A. Translation of 3-D FEM of scoliosis. The apex vertebra (T8) was translated 50 mm medially toward spinal column axis. At each decadal point until 50 mm, 3-D FEM of scoliosis in coronal plane was displayed. B. The results of Cobb angle, angle of kyphosis, and AVAR during translation. There was no change of the angle of kyphosis, but it showed a linear decrease of Cobb angle, and a slight increase of AVAR during translation.

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  Fig. 7. A. Rotation of 3-D FEM of scoliosis. The assumptions in simulation of rod rotation are, there was no change of scoliosis curve during rod insertion and rod was deformed to fit shape of scoliosis. The inserted rod was derotated toward posteromedial direction, as it did in real operation with CD instrumentation of scoliosis. At each decadal point until 50。, 3-D FEM of scoliosis in coronal plane was displayed. B. The results of Cobb angle, angle of kyphosis, and AVAR during rod rotation. There were slight change of Cobb angle and kyphosis angle, and a linear increase of AVAR during rod rotation. C. With rod rotation, a linear increase of rib hump (about 70 mm at 50。 rotation) was observed.

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  Fig. 8. A. This picture shows the change of spinal column in coronal plane during the 1st step of operative simulation. The change was divided into quarter stage. B. The results of Cobb angle, angle of kyphosis, and AVAR during 1st step of simulation. Cobb angle decreased with rod insertion, but there was a little change of kyphosis and AVAR.

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  Fig. 9. A. The 2nd step of operative simulation. After rod insertion, the rod derotation toward posteromedial direction was done until 90。 . The spinal column changes in coronal plane were shown. B, C. The results of Cobb angle, angle of kyphosis, AVAR, and rib hump during 2nd step of rod derotation. Even if there was a great decrease of Cobb angle with derotation, but an associated increase of AVAR and rib hump size were found after 30o of rod derotation.

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