Yonsei Med J.  2014 Sep;55(5):1386-1394. 10.3349/ymj.2014.55.5.1386.

Biomechanical Analysis of Fusion Segment Rigidity Upon Stress at Both the Fusion and Adjacent Segments: A Comparison between Unilateral and Bilateral Pedicle Screw Fixation

• Kim HJ
• Kang KT
• Chang BS
• Lee CK
• Kim JW
• Yeom JS

• KMID: 1799507
• DOI: http://doi.org/10.3349/ymj.2014.55.5.1386

Abstract

PURPOSE
The purpose of this study was to investigate the effects of unilateral pedicle screw fixation on the fusion segment and the superior adjacent segment after one segment lumbar fusion using validated finite element models.
MATERIALS AND METHODS
Four L3-4 fusion models were simulated according to the extent of decompression and the method of pedicle screws fixation in L3-4 lumbar fusion. These models included hemi-laminectomy with bilateral pedicle screw fixation in the L3-4 segment (BF-HL model), total laminectomy with bilateral pedicle screw fixation (BF-TL model), hemi-laminectomy with unilateral pedicle screw fixation (UF-HL model), and total laminectomy with unilateral pedicle screw fixation (UF-TL model). In each scenario, intradiscal pressures, annulus stress, and range of motion at the L2-3 and L3-4 segments were analyzed under flexion, extension, lateral bending, and torsional moments.
RESULTS
Under four pure moments, the unilateral fixation leads to a reduction in increment of range of motion at the adjacent segment, but larger motions were noted at the fusion segment (L3-4) in the unilateral fixation (UF-HL and UF-TL) models when compared to bilateral fixation. The maximal von Mises stress showed similar patterns to range of motion at both superior adjacent L2-3 segments and fusion segment.
CONCLUSION
The current study suggests that unilateral pedicle screw fixation seems to be unable to afford sufficient biomechanical stability in case of bilateral total laminectomy. Conversely, in the case of hemi-laminectomy, unilateral fixation could be an alternative option, which also has potential benefit to reduce the stress of the adjacent segment.

Keyword

Unilateral pedicle screw fixation; lumbar fusion surgery; adjacent segment degeneration; finite element model

MeSH Terms

Biomechanical Phenomena
*Computer Simulation
Humans
Lumbar Vertebrae/surgery
Male
Middle Aged
*Models, Anatomic
*Pedicle Screws
*Range of Motion, Articular
Software
Spinal Fusion
Stress, Mechanical

Figure

• 

  Fig. 1 FE models in the current study. (A) The BF-HL model. (B) The BF-TL model. (C) The UF-HL model. (D) The UF-TL model. The BF-HL model, bilateral fixation and hemilaminectomy model; The BF-TL model, bilateral fixation and total laminectomy model; The UF-HL model, unilateral fixation and hemilaminectomy model; The UF-TL model, unilateral fixation and total laminectomy model.

• 

  Fig. 2 The comparison between the current intact model and previous studies for the validation. (A) Four pure moments (comparison with Renner, et al.28). (B) Four pure moments (range of motion at L2-3 was compared with that of Wilke, et al.30). (C) Intradiscal pressure.

• 

  Fig. 3 The percent change of ROM at the adjacent segment (L2-3) from intact model under 4 pure moments. The BF-HL model, bilateral fixation and hemilaminectomy model; The BF-TL model, bilateral fixation and total laminectomy model; The UF-HL model, unilateral fixation and hemilaminectomy model; The UF-TL model, unilateral fixation and total laminectomy model; ROM, range of motion.

• 

  Fig. 4 The percent change of remaining ROM (100-percent change) at the fusion segment from the intact model under 4 pure moments. The BF-HL model, bilateral fixation and hemilaminectomy model; The BF-TL model, bilateral fixation and total laminectomy model; The UF-HL model, unilateral fixation and hemilaminectomy model; The UF-TL model, unilateral fixation and total laminectomy model; ROM, range of motion.

• 

  Fig. 5 Comparison of percent increment of AF stress at the adjacent segment (L2-3) from intact model among four models under 4 pure moments. AF, annulus fibrosus; The BF-HL model, bilateral fixation and hemilaminectomy model; The BF-TL model, bilateral fixation and total laminectomy model; The UF-HL model, unilateral fixation and hemilaminectomy model; The UF-TL model, unilateral fixation and total laminectomy model.

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