Shorter screw lengths in dynamic Dynesys fixation have less screw loosening - From clinical investigation to finite-element analysis

Sep 29, 2026

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Background: The dynamic Dynesys Stabilization System preserves lumbar mobility at instrumented levels. This study assessed how screw length influences screw loosening after dynamic Dynesys fixation and how screw tips displace during lumbar motion, combining retrospective clinical investigation with finite-element (FE) analysis. Methods: Fifty patients with degenerative spondylolisthesis treated in 2011 were analyzed. Horizontal and vertical screw-tip displacements at L4 in L4-L5 under multiple motions were computed with displacement-controlled FE models using different screw lengths. Results and Conclusion are reported in the paper.

LanguageEnglish

CategoryHealthcare

FormatPDF

Pages8

Reading Time20 min

AccessFree to download

Abstract

1. INTRODUCTION

Background and clinical need

Methods

Clinical retrospective study

Finite-element analysis setup

Results

Screw loosening rates

Displacement patterns by screw length

Conclusion

J Chin MedAssoc Original article Shorter screw lengths in dynamic Dynesys fixation have less screw loosening: From clinical investigation to finite-element analysis Fang-Chi Hsua, Chen-Sheng Chena, Yu-Cheng Yaob,c, Hsi-Hsien Linb,c, Shih-Tien Wangb,c, Ming-Chau Changb,c, Chien-Lin Liub,c, Po-Hsin Choub,c,* aDepartment of Physical Therapy and Assistive Technology, National Yang Ming Chiao Tung University, Taipei, Taiwan, ROC; bSchool of Medicine, National Yang Ming Chiao Tung University, Taipei, Taiwan, ROC; cDepartment of Orthopedics and Traumatology, Taipei Veterans General Hospital, Taipei, Taiwan, ROC Abstract Background: The dynamic Dynesys Stabilization System preserves lumbar mobility at instrumented levels. This study investigated the effect of screw length on screw loosening (SL) after dynamic Dynesys fixation and screw displacement during lumbar motion, using clinical investigation and finite-element (FE) analysis. Methods: Clinical data of 50 patients with degenerative spondylolisthesis treated with decompression and Dynesys fixation in 2011 were analyzed retrospectively. Horizontal sliding displacement and vertical displacement of screw tips at L4 were analyzed postoperatively using displacement-controlled FE analysis at the L4-L5 level with screw lengths 45 (long screw), 36 (median screw), and 27 (short screw), and 6.4 mm in diameter, under flexion, extension, lateral bending, and rotation. Results: In 13 patients (13/50, 26%), 40 screws (40/266, 15%) were loose at mean follow-up of 101.3 ± 4.4 months. Radiographic SL at 35, 40, 45, and 50 mm were 7.7%, 10.7%, 12. 1%, and 37.5%, respectively, regardless of the fixation level (p = 0.009) . FE analysis revealed that the long screw model with corresponding longer lever arm had maximal horizontal sliding displacement under all directions and maximal vertical displacement, except for lateral bending. Conclusion: Shorter screws in Dynesys fixation may help avoid dynamic SL. Clinically, 50 mm screws showed the greatest SL and median screw screws demonstrated the least displacement biomechanically. Keywords: Finite-element analysis; Fracture fixation; Internal; Lumbar vertebrae; Pedicle screw; Prosthesis failure 1. INTRODUCTION With the rapid increase in older adults in the global population, the number of aging patients needing spine surgery has increased as the average lifespan increased.1 Degenerative lumbar spondylolisthesis (LDS) with lateral recess and foraminal stenosis is a common pathologic condition in the aging spine,2 for which decompression along with spinal fusion is the gold standard for those who have failed conservative treatment.3 Fusion of rigid pedicle screws significantly reduces range of motion (ROM) atthe fused segments.4 The dynamic Dynesys Stabilization System (Zimmer Inc., Warsaw, IN) is one of the most frequently used * Address correspondence. Dr. Po-Hsin Chou, Department of Orthopedics and Traumatology, Taipei Veterans General Hospital, 18F, 201, Section 2, Shi-Pai Road, Taipei 112, Taiwan, [ROC. E-mail address: choupohsin@gmail.com](ROC. E-mail address: choupohsin@gmail.com) (P.-H. Chou). Author contributions: Dr. Fang-Chi Hsu and Dr. Chen-Sheng Chen contributed equally to the manuscript. Conflicts of interest: The authors declare that they have no conflicts of interest related to the subject matter or materials discussed in this article. Journal of Chinese Medical Association. (2023) 86: 330-337. Received April 29, 2022; accepted November 12, 2022. doi: 10. 1097/JCMA.0000000000000864. Copyright © 2022, the Chinese Medical Association. This is an open access article under the CC BY-NC-ND license ([http://creativecommons.org/licenses/](http://creativecommons.org/licenses/)[ ](http://creativecommons.org/licenses/)[by-nc-nd/4.0/](by-nc-nd/4.0/)) posterior motion-preserving implant methods,3 designed to preserve mobility at instrumented levels with satisfactory clinical outcomes.5,6 Screw loosening (SL) i

FAQs

What question does the study address about Dynesys fixation?

The study investigates whether shorter screw lengths reduce screw loosening after dynamic Dynesys fixation and how screw-tip displacement behaves during lumbar motion.

How were patients and screw-tip displacement evaluated?

Fifty patients treated with decompression and Dynesys fixation in 2011 were analyzed retrospectively for screw loosening. Displacement-controlled finite-element analysis at the L4-L5 level quantified horizontal and vertical screw-tip displacements under flexion, extension, lateral bending, and rotation.

What did the FE analysis suggest about long versus short screws?

The long-screw FE model showed the maximal horizontal sliding displacement under all directions and maximal vertical displacement, except during lateral bending.

Sep 29, 2026

Uploaded by Rizky