The Explanation of Postoperative Change of Vertebral Rotation and Rib Hump Using 3 Dimensional Finite Element Scoliosis Model Jeong-Hyun Ha, Jae-Hyup Lee, Young-Jun Ahn, Chang-Kyu Son*, Kwang-Hee Lee**, Hyung-Yun Choi**, Young-Eun Kim*, Choon-Ki Lee Department of Orthopaedic Surgery, College of Medicine, Seoul National University Department of Mechanical Engineering, Dankook University* Department of Mechanical and System Design Engineering, Hongik University** 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 o, 15 o, 30 o, 45 o, 60 o and 90 o 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, dur- Address reprint requests to Choon-Ki Lee, MD Department of Orthopedic Surgery, Seoul National University Hospital #28, Yongun-dong, Jongro-gu, Seoul 110-744, Korea Tel: 82-2-760-2336, Fax: 82-2-764-2718, E-mail: choonki@plazasnuackr - 14 -
ing 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 o 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 Key Word: Scoliosis, Rib hump, FEM, Derotation, Kinematics 3, 3, 3,,,,, 3-3 (torsion), 2,11),,,,,,, 3 Table 1 Comparison of Operative Kinematics for Scoliosis Kinematics Cobb Angle Kyphosis AVAR* Rib Hump (mm) Distraction (mm) 00 42 o 29 o 24 o 10 39 o 32 o 23 o 20 33 o 34 o 20 o 30 25 o 37 o 19 o 40 17 o 41 o 19 o Translation (mm) 00 42 o 29 o 24 o 10 39 o 28 o 26 o 20 36 o 28 o 28 o 30 33 o 28 o 29 o 40 30 o 28 o 31 o 50 27 o 27 o 32 o Derotation (degrees) 00 42 o 29 o 24 o 011 10 o 42 o 29 o 26 o 044 20 o 41 o 30 o 29 o 145 30 o 41 o 30 o 33 o 277 40 o 43 o 32 o 37 o 422 50 o 44 o 36 o 42 o 664 * AVAR : Apical Vertebra Axial Rotation - 15 -
, 76867 (shell elements)782 (beam elements) (kinematic joint element) 6,, - S, in-vitro (rib cage) Fig 1 Normal 3-Dimensional Spine Finite Element Model including rib cage, sternum, pelvis, clavicle, scapula, intervertebral disc and ligaments, (Fig 1), Fig 2 A Digitization of 12 Coordinates from roentgenogram of King-Moe type II scoliosis Fig 2 B Conversion from normal 3-D FEM spine model to scoliosis model by displacement of vertebral body center and rotation Fig 2 C Developed 3-Dimensional Scoliosis Finite Element Model similar to King-Moe type II scoliosis - 16 -
3 3 12King-Moe type (Fig 2-C) CT II,, -, 12 (Fig 2-A),,,,,, (operative kinematics) (distraction),, (translation), (rod derotation), Cobb angle,, (apical vertebra axial rotation:avar) 6) (Fig 3), (offset), x y (Fig 2-B), (derotation) (Fig 4) 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 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 Fig 5 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 - 17 -
2, Cobb angle,,, 1 Cobb Angle,, 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 Fig 6 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 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 postero-medial 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 Fig 7 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 Fig 7C With rod rotation, a linear increase of rib hump (about 70 mm at 50 rotation) was observed - 18 -
(Fig 5-A,B) 50 mm, (center sacral 40 mm (cranial direction) l i n e ) 10 mm, Cobb angle,,, Cobb angle,, (Fig 6-A,B) 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 Fig 8 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 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 Fig 9 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 - 19 -
Table 2 Simulated Results of the derotation maneuver for Scoliosis Simulation Steps Cobb Angle Kyphosis AVAR Rib Hump(mm) 1 st Displacement (distraction 26mm, medial translation 42mm, posterior offset 15mm) 0% 42 29 24 25% 39 29 26 50% 34 29 26 75% 30 30 27 100% 24 30 27 2 nd Rod Derotation 0 24 30 27 86 15 23 30 27 108 30 22 30 30 199 45 21 28 31 273 60 18 27 33 331 90 11 22 30 241 * AVAR : Apical Vertebra Axial Rotation,,, 3,, z, 1, (Fig 7-A) 2 50, 10 1 3 26 mm, 42 mm, 15 mm,, 100%, 25%, 50%, 75% Cobb angle,, C o b b Cobb angle angle,,, (Fig 8-A,B) 2 0 o, 15 o,, 30 o, 45 o, 60 o, 90 o (Fig 9-A),, Cobb angle 2 1, (Table 1) 1 (Fig 7-B,C), 2 3 3, 30, Cobb angle,,, (Table 2)(Fig 9-B,C),, 3, King-Moe type II,,, - 20 -
King-Moe type 2 17,18), ( d e r o t a t i o n ),, 3, flexible beam 12 2 15),,,,, 2, 14),, 5) Andriacchi 1 ) 1 976 3 Milwaukee brace,, King-Moe type II ( t r a c t i o n ) (lateral force) 2, Aubin 4),,, (coupled mechanism), (coupled motion) Gianac 7 ), Stoke 15) 90 50, S t o k e, Cobb angle Laible 15) Harrington,, Gardner-MorseS t o k e 6 ), C D CD 6,,, Poulin 13 ) 4 CD CD, (spinal derotation),, (vertebral derotation) 8,,,,,,,,, 1960H a r r i n g t o n ( d i s t r a c t i o n ) 2 9),, Pollock 12) CD hook 30, CD, Lenke 10) 11, Gardner-MorseS t o k e 6 ) 8, 16, 19 ) FEM 8, FEM,,,,, - 21 -
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: 3 :, 2 3 :,,,,, 6,, 12,,, Cobb,, 0 o, 15 o, 30 o, 45 o, 60 o, 90 o,, : Cobb :,,, :,,,, - 24 -