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Influence of Dynesys System Screw Profile on Adjacent Segment and Screw

Liu, Chien-Lin MD*; Zhong, Zheng-Cheng PhD; Shih, Shih-Liang MD; Hung, Chinghua PhD; Lee, Yong-Eng MD§; Chen, Chen-Sheng PhD

Journal of Spinal Disorders & Techniques: August 2010 - Volume 23 - Issue 6 - pp 410-417
doi: 10.1097/BSD.0b013e3181b63d89
Original Articles

Study Design: Displacement-controlled finite element analysis was used to evaluate the mechanical behavior of the lumbar spine after insertion of the Dynesys dynamic stabilization system.

Objective: This study aimed to investigate whether different depths of screw placement of Dynesys would affect load sharing of screw, range of motion (ROM), annulus stress, and facet contact force.

Summary of Background Data: In clinical follow-up, a high rate of screw complications and adjacent segment disease were found after using Dynesys. The pedicle screw in the Dynesys system is not so easy to implant into the standard position and causes the screw to protrude more prominently from the pedicle. Little is known about how the biomechanical effects are influenced by the Dynesys screw profile.

Methods: The Dynesys was implanted in a 3-dimensional, nonlinear, finite element model of the L1 to L5 lumbar spine. Different depths of screw position were modified in this model by 5 and 10 mm out of the pedicle. The model was loaded to 150 N preload and controlled the same ROMs by 20, 15, 8, and 20 degrees in flexion, extension, torsion, and lateral bending, respectively. Resultant ROM, annulus stress, and facet contact force were analyzed at the surgical and adjacent level.

Results: Under flexion, extension, and lateral bending, the Dynesys provided sufficient stability at the surgical level, but increased the ROM at the adjacent level. Under flexion and lateral bending, the Dynesys alleviated annulus stress at the surgical level, but increased annulus stress at the adjacent level. Under extension, the Dynesys decreased facet loading at the surgical level but increased facet loading at the adjacent level.

Conclusions: This study found that the Dynesys system was able to restore spinal stability and alleviate loading on disc and facet at the surgical level, but greater ROM, annulus stress, and facet loading were found at the adjacent level. In addition, profile of the screw placement caused only a minor influence on the ROM, annulus stress, and facet loading, but the screw stress was noticeably increased.

*Faculty of Medicine, National Yang-Ming University, Taipei, Taiwan

Department of Mechanical Engineering, National Chiao Tung University, Hsinchu, Taiwan

Department of Orthopaedic Surgery, Taipei-City Hospital, Taipei, Taiwan

§Department of Orthopaedic Surgery, Shin Kong Wu Ho-Su Memorial Hospital, Taipei, Taiwan

Department of Physical Therapy and Assistive Technology, National Yang-Ming University, Taipei, Taiwan

Reprints: Chen-Sheng Chen, PhD, Department of Physical Therapy and Assistive Technology, National Yang-Ming University, 155, Section 2, Li-Nung Street, Taipei, Taiwan 112 (e-mail: cschen@ym.edu.tw).

Received for publication November 27, 2008

accepted July 3, 2009

© 2010 Lippincott Williams & Wilkins, Inc.