Biomechanics of thoracic short versus long fixation after 3-column injury

Laboratory investigation

Restricted access

Object

Posterior screw-rod fixation for thoracic spine trauma usually involves fusion across long segments. Biomechanical data on screw-based short-segment fixation for thoracic fusion are lacking. The authors compared the effects of spanning short and long segments in the thoracic spine.

Methods

Seven human spine segments (5 segments from T-2 to T-8; 2 segments from T-3 to T-9) were prepared. Pure-moment loading of 6 Nm was applied to induce flexion, extension, lateral bending, and axial rotation while 3D motion was measured optoelectronically. Normal specimens were tested, and then a wedge fracture was created on the middle vertebra after cutting the posterior ligaments. Five conditions of instrumentation were tested, as follows: Step A, 4-level fixation plus cross-link; Step B, 2-level fixation; Step C, 2-level fixation plus cross-link; Step D, 2-level fixation plus screws at fracture site (index); and Step E, 2-level fixation plus index screws plus cross-link.

Results

Long-segment fixation restricted 2-level range of motion (ROM) during extension and lateral bending significantly better than the most rigid short-segment construct. Adding index screws in short-segment constructs significantly reduced ROM during flexion, lateral bending, and axial rotation (p < 0.03). A cross-link reduced axial rotation ROM (p = 0.001), not affecting other loading directions (p > 0.4).

Conclusions

Thoracic short-segment fixation provides significantly less stability than long-segment fixation for the injury studied. Adding a cross-link to short fixation improved stability only during axial rotation. Adding a screw at the fracture site improved short-segment stability by an average of 25%.

Abbreviations used in this paper: BMD = bone mineral density; LZ = lax zone; ROM = range of motion; SZ = stiff zone; VB = vertebral body.

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: neuropub@chw.edu.

Please include this information when citing this paper: published online December 24, 2010; DOI: 10.3171/2010.10.SPINE09785.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Lateral fluoroscopic image (left) and anterolateral photograph (right) showing modeled wedge fracture.

  • View in gallery

    Posterior photographs in a representative specimen under the conditions of rigid fixation studied. A: Step A; two above–two below pedicle screws/rods with a central cross-link. B: Step B; one above–one below pedicle screws/rods without cross-link. C: Step C; one above–one below pedicle screws/rods with cross-link. D: Step D; one above–one below pedicle screws/rods without cross-link and with index-level pedicle screws. E: Step E; one above–one below pedicle screws/rods with cross-link and with index-level pedicle screws.

  • View in gallery

    Bar graph showing the mean unidirectional angular ROM across the destabilized levels (T4–6 or T5–7). Error bars show SDs.

  • View in gallery

    Bar graph showing the mean bidirectional angular LZ across the destabilized levels (T4–6 or T5–7). Error bars show SDs.

  • View in gallery

    Bar graph showing the mean unidirectional angular SZ across the destabilized levels (T4–6 or T5–7). Error bars show SDs.

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