Abstract
Study design
A n 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 digi-tized 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 (A VA R) and thoracic kyphosis. A simulation of a segmental pedicle screw fixation, with rod derotation for scoliosis, was performed. The changes in the Cobb o rod derotations. o and 90 o, 60 o, 45 o, 30 o, 15 angle, kyphotic angle, A VA R and rib hump were compared after 0
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, o rod derotation, a decrease in the kyphosis, and increases in the rib hump and A VA R were observed. after a 30
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Table 1.
Kinematics | Cobb Angle | Kyphosis | AVAR∗ | Rib Hump (mm) |
---|---|---|---|---|
Distraction (mm) | ||||
0 | 42° | 29° | 24° | |
10 | 39° | 32° | 23° | |
20 | 33° | 34° | 20° | |
30 | 25° | 37° | 19° | |
40 | 17° | 41° | 19° | |
Translation (mm) | ||||
00 | 42° | 29° | 24° | |
10 | 39° | 28° | 26° | |
20 | 36° | 28° | 28° | |
30 | 33° | 28° | 29° | |
40 | 30° | 28° | 31° | |
50 | 27° | 27° | 32° | |
Derotation (degrees) | ||||
0 | 42° | 29° | 24° | 1.1 |
10 | 42° | 29° | 26° | 4.4 |
20 | 41° | 30° | 29° | 14.5 |
30 | 41° | 30° | 33° | 27.7 |
40 | 43° | 32° | 37° | 42.2 |
50 | 44° | 36° | 42° | 66.4 |