The Change of Sagittal Alignment of the Lumbar Spine after Dynesys Stabilization and Proposal of a Refinement.
OBJECTIVE
Dynesys® is one of the pedicle-based dynamic lumbar stabilization systems and good clinical outcome has been reported. However, the cylindrical spacer between the heads of the screws undergoes deformation during assembly of the system. The pre-strain probably change the angle of instrumented spine with time and oblique-shaped spacer may reduce the pre-strain. We analyzed patients with single-level stabilization with Dynesys® and simulated oblique-shaped spacer with finite element (FE) model analysis.


METHODS
Consecutive 14 patients, who underwent surgery for single-level lumbar spinal stenosis and were followed-up more than 24 months (M : F=6 : 8; age, 58.7±8.0 years), were analyzed. Lumbar lordosis and segmental angle at the index level were compared between preoperation and postoperative month 24. The von Mises stresses on the obliquely-cut spacer (5°, 10°, 15°, 20°, 25°, and 30°) were calculated under the compressive force of 400 N and 10 Nm of moment with validated FE model of the L4-5 spinal motion segment with segmental angle of 16°.


RESULTS
Lumbar lordosis was not changed, while segmental angle was changed significantly from -8.1±7.2° to -5.9±6.7° (p<0.01) at postoperative month 24. The maximum von Mises stresses were markedly decreased with increased angle of the spacer up to 20°. The stress on the spacer was uneven with cylindrical spacer but it became even with the 15° oblique spacer.


CONCLUSIONS
The decreased segmental lordosis may be partially related to the pre-strain of Dynesys. Further clinical and biomechanical studies are required for relevant use of the system.
Department of Mechanical Engineering, Kyung Hee University, Yongin, Korea.
Won Man
WM
Park
Department of Neurosurgery, Seoul National University Hospital, Seoul, Korea. ; Department of Neurosurgery, Seoul National University College of Medicine, Seoul, Korea. ; Neuroscience Research Institute, Seoul National University Medical Research Center, Seoul, Korea. ; Clinical Research Institute, Seoul National University Hospital, Seoul, Korea.
Chi Heon
CH
Kim
Department of Mechanical Engineering, Kyung Hee University, Yongin, Korea.
Yoon Hyuk
YH
Kim
Department of Neurosurgery, Seoul National University Hospital, Seoul, Korea. ; Department of Neurosurgery, Seoul National University College of Medicine, Seoul, Korea. ; Neuroscience Research Institute, Seoul National University Medical Research Center, Seoul, Korea. ; Clinical Research Institute, Seoul National University Hospital, Seoul, Korea. ; Department of Brain and Cognitive Sciences, Seoul National University College of Natural Sciences, Seoul, Korea.
Chun Kee
CK
Chung
Department of Neurosurgery, Seoul National University Hospital, Seoul, Korea. ; Department of Neurosurgery, Seoul National University Bundang Hospital, Seongnam, Korea.
Tae-Ahn
TA
Jahng
Degenerative disease; Dynamic; Finite element analysis; Lumbar; Spine; Stabilization