Dynamic lumbar pedicle screw-rod stabilization: in vitro biomechanical comparison with standard rigid pedicle screw-rod stabilization

Laboratory investigation

Hakan Bozkuş Department of Neurosurgery, VKV Amerikan Hastanesi, Istanbul, Turkey; and

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Mehmet Şenoğlu Barrow Neurological Institute, Spinal Biomechanics Research Laboratory, Phoenix, Arizona

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Seungwon Baek Barrow Neurological Institute, Spinal Biomechanics Research Laboratory, Phoenix, Arizona

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Anna G. U. Sawa Barrow Neurological Institute, Spinal Biomechanics Research Laboratory, Phoenix, Arizona

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Ali Fahir Özer Department of Neurosurgery, VKV Amerikan Hastanesi, Istanbul, Turkey; and

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Volker K. H. Sonntag Barrow Neurological Institute, Spinal Biomechanics Research Laboratory, Phoenix, Arizona

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Neil R. Crawford Barrow Neurological Institute, Spinal Biomechanics Research Laboratory, Phoenix, Arizona

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Object

It is unclear how the biomechanics of dynamic posterior lumbar stabilization systems and traditional rigid pedicle screw-rod systems differ. This study examined the biomechanical response of a hinged-dynamic pedicle screw compared with a standard rigid screw used in a 1-level pedicle screw-rod construct.

Methods

Unembalmed human cadaveric L3–S1 segments were tested intact, after L4–5 discectomy, after rigid pedicle screw-rod fixation, and after dynamic pedicle screw-rod fixation. Specimens were loaded using pure moments to induce flexion, extension, lateral bending, and axial rotation while recording motion optoelectronically. Specimens were then loaded in physiological flexion-extension while applying 400 N of compression. Moment and force across instrumentation were recorded from pairs of strain gauges mounted on the interconnecting rods.

Results

The hinged-dynamic screws allowed an average of 160% greater range of motion during flexion, extension, lateral bending, and axial rotation than standard rigid screws (p < 0.03) but 30% less motion than normal. When using standard screws, bending moments and axial loads on the rods were greater than the bending moments and axial loads on the rods when using dynamic screws during most loading modes (p < 0.05). The axis of rotation shifted significantly posteriorly more than 10 mm from its normal position with both devices.

Conclusions

In a 1-level pedicle screw-rod construct, hinged-dynamic screws allowed a quantity of motion that was substantially closer to normal motion than that allowed by rigid pedicle screws. Both systems altered kinematics similarly. Less load was borne by the hinged screw construct, indicating that the hinged-dynamic screws allow less stress shielding than standard rigid screws.

Abbreviations used in this paper:

IAR = instantaneous axis of rotation; LZ = lax zone; ROM = range of motion; SZ = stiff zone.
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