Biomechanical advantage of the index-level pedicle screw in unstable thoracolumbar junction fractures

Presented at the 2010 Joint Spine Section Meeting 

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Unstable fractures at the thoracolumbar junction often require extended, posterior, segmental pedicular fixation. Some surgeons have reported good clinical outcomes with short-segment constructs if additional pedicle screws are inserted at the fractured level. The goal of this study was to quantify the biomechanical advantage of the index-level screw in a fracture model.


Six human cadaveric T10–L4 specimens were tested. A 3-column injury at L-1 was simulated, and 4 posterior constructs were tested as follows: one-above-one-below (short construct) with/without index-level screws, and two-above-two-below (long construct) with/without index-level screws. Pure moments were applied quasistatically while 3D motion was measured optoelectronically. The range of motion (ROM) and lax zone across T12–L2 were measured during flexion, extension, left and right lateral bending, and left and right axial rotation.


All constructs significantly reduced the ROM and lax zone in the fractured specimens. With or without index-level screws, the long-segment constructs provided better immobilization than the short-segment constructs during all loading modes. Adding an index-level screw to the short-segment construct significantly improved stability during flexion and lateral bending; there was no significant improvement in stability when an index-level screw was added to the long-segment construct. Overall, bilateral index-level screws decreased the ROM of the 1-level construct by 25% but decreased the ROM of the 2-level construct by only 3%.


In a fracture model, adding index-level pedicle screws to short-segment constructs improves stability, although stability remains less than that provided by long-segment constructs with or without index-level pedicle screws. Therefore, highly unstable fractures likely require extended, long-segment constructs for optimum stability.

Abbreviations used in this paper: LZ = lax zone; ROM = range of motion.

Article Information

Address correspondence to: Neil R. Crawford, Ph.D., c/o Neuroscience Publications, Barrow Neurological Institute, 350 West Thomas Road, Phoenix, Arizona 85013. email:

Please include this information when citing this paper: published online January 7, 2011; DOI: 10.3171/2010.10.SPINE10222.

© AANS, except where prohibited by US copyright law.



  • View in gallery

    Photographs of representative specimens in each instrumented condition studied. A: Posterior view of short-segment (T12–L2) fixation without index-level screws. B: Posterior view of short-segment (T12–L2) fixation with index-level screws. C: Posterior view of long-segment (T11–L3) fixation without index-level screws. D: Posterior view of long-segment (T11–L3) fixation with index-level screws.

  • View in gallery

    Anteroposterior (left) and lateral (right) radiographs of a sample specimen after destabilization showing entry points and trajectories of index-level screws.

  • View in gallery

    Graph showing the mean unidirectional T12–L2 angular ROM. Error bars show standard deviation.

  • View in gallery

    Graph showing the mean bidirectional T12–L2 angular LZ. Error bars show standard deviation.


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