Biomechanical analysis of four- Versus six-screw constructs for short-segment pedicle screw and rod instrumentation of unstable thoracolumbar fractures

Robert P. Norton, Edward L. Milne, David N. Kaimrajh, Frank J Eismont, Loren L. Latta, Seth K. Williams

Research output: Contribution to journalArticle

30 Citations (Scopus)

Abstract

Background context Conventionally, short-segment fusion involves instrumentation of one healthy vertebra above and below the injured vertebra, skipping the injured level. This short-segment construct places less surgical burden on the patient compared with long-segment constructs, but is less stable biomechanically, and thus has resulted in clinical failures. The addition of two screws placed in the fractured vertebral body represents an attempt to improve the construct stiffness without sacrificing the benefits of short-segment fusion. Purpose To determine the biomechanical differences between four- and six-screw short-segment constructs for the operative management of an unstable L1 fracture. Study design Biomechanical study of instrumentation in vertebral body cadaveric models simulating an L1 axial load injury pattern. Methods Thirteen intact spinal segments from T12 to L2 were prepared from fresh-frozen cadaver spines. An axial load fracture of at least 50% vertebral body height was produced at L1 and then instrumented with pedicle screws. Specimens were evaluated in terms of construct stiffness, motion, and rod strain. Two conditions were tested: a four-screw construct with no screws at the L1 fractured body (4S) and a six-screw construct with screws at all levels (6S). The two groups were compared statistically by paired Student t test. Results The mean stiffness in flexion-extension was increased 31% (p<.03) with the addition of the two pedicle screws in L1. Relative motion in terms of vertical and axial rotations was not significantly different between the two groups. The L1-L2 rod strain was significantly increased in the six-screw construct compared with the four-screw construct (p<.001). Conclusions In a cadaveric L1 axial load fracture model, a six-screw construct with screws in the fractured level is more rigid than a four-screw construct that skips the injured vertebral body.

Original languageEnglish
Pages (from-to)1734-1739
Number of pages6
JournalSpine Journal
Volume14
Issue number8
DOIs
StatePublished - Aug 1 2014

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Spine
Body Height
Cadaver
Students
Wounds and Injuries
Pedicle Screws

Keywords

  • Axial load fracture
  • Pedicle screw instrumentation
  • Short-segment fusion
  • Spine fracture biomechanics
  • Spine fracture fixation
  • Thoracolumbar instrumentation

ASJC Scopus subject areas

  • Clinical Neurology
  • Surgery

Cite this

Biomechanical analysis of four- Versus six-screw constructs for short-segment pedicle screw and rod instrumentation of unstable thoracolumbar fractures. / Norton, Robert P.; Milne, Edward L.; Kaimrajh, David N.; Eismont, Frank J; Latta, Loren L.; Williams, Seth K.

In: Spine Journal, Vol. 14, No. 8, 01.08.2014, p. 1734-1739.

Research output: Contribution to journalArticle

Norton, Robert P. ; Milne, Edward L. ; Kaimrajh, David N. ; Eismont, Frank J ; Latta, Loren L. ; Williams, Seth K. / Biomechanical analysis of four- Versus six-screw constructs for short-segment pedicle screw and rod instrumentation of unstable thoracolumbar fractures. In: Spine Journal. 2014 ; Vol. 14, No. 8. pp. 1734-1739.
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title = "Biomechanical analysis of four- Versus six-screw constructs for short-segment pedicle screw and rod instrumentation of unstable thoracolumbar fractures",
abstract = "Background context Conventionally, short-segment fusion involves instrumentation of one healthy vertebra above and below the injured vertebra, skipping the injured level. This short-segment construct places less surgical burden on the patient compared with long-segment constructs, but is less stable biomechanically, and thus has resulted in clinical failures. The addition of two screws placed in the fractured vertebral body represents an attempt to improve the construct stiffness without sacrificing the benefits of short-segment fusion. Purpose To determine the biomechanical differences between four- and six-screw short-segment constructs for the operative management of an unstable L1 fracture. Study design Biomechanical study of instrumentation in vertebral body cadaveric models simulating an L1 axial load injury pattern. Methods Thirteen intact spinal segments from T12 to L2 were prepared from fresh-frozen cadaver spines. An axial load fracture of at least 50{\%} vertebral body height was produced at L1 and then instrumented with pedicle screws. Specimens were evaluated in terms of construct stiffness, motion, and rod strain. Two conditions were tested: a four-screw construct with no screws at the L1 fractured body (4S) and a six-screw construct with screws at all levels (6S). The two groups were compared statistically by paired Student t test. Results The mean stiffness in flexion-extension was increased 31{\%} (p<.03) with the addition of the two pedicle screws in L1. Relative motion in terms of vertical and axial rotations was not significantly different between the two groups. The L1-L2 rod strain was significantly increased in the six-screw construct compared with the four-screw construct (p<.001). Conclusions In a cadaveric L1 axial load fracture model, a six-screw construct with screws in the fractured level is more rigid than a four-screw construct that skips the injured vertebral body.",
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T1 - Biomechanical analysis of four- Versus six-screw constructs for short-segment pedicle screw and rod instrumentation of unstable thoracolumbar fractures

AU - Norton, Robert P.

AU - Milne, Edward L.

AU - Kaimrajh, David N.

AU - Eismont, Frank J

AU - Latta, Loren L.

AU - Williams, Seth K.

PY - 2014/8/1

Y1 - 2014/8/1

N2 - Background context Conventionally, short-segment fusion involves instrumentation of one healthy vertebra above and below the injured vertebra, skipping the injured level. This short-segment construct places less surgical burden on the patient compared with long-segment constructs, but is less stable biomechanically, and thus has resulted in clinical failures. The addition of two screws placed in the fractured vertebral body represents an attempt to improve the construct stiffness without sacrificing the benefits of short-segment fusion. Purpose To determine the biomechanical differences between four- and six-screw short-segment constructs for the operative management of an unstable L1 fracture. Study design Biomechanical study of instrumentation in vertebral body cadaveric models simulating an L1 axial load injury pattern. Methods Thirteen intact spinal segments from T12 to L2 were prepared from fresh-frozen cadaver spines. An axial load fracture of at least 50% vertebral body height was produced at L1 and then instrumented with pedicle screws. Specimens were evaluated in terms of construct stiffness, motion, and rod strain. Two conditions were tested: a four-screw construct with no screws at the L1 fractured body (4S) and a six-screw construct with screws at all levels (6S). The two groups were compared statistically by paired Student t test. Results The mean stiffness in flexion-extension was increased 31% (p<.03) with the addition of the two pedicle screws in L1. Relative motion in terms of vertical and axial rotations was not significantly different between the two groups. The L1-L2 rod strain was significantly increased in the six-screw construct compared with the four-screw construct (p<.001). Conclusions In a cadaveric L1 axial load fracture model, a six-screw construct with screws in the fractured level is more rigid than a four-screw construct that skips the injured vertebral body.

AB - Background context Conventionally, short-segment fusion involves instrumentation of one healthy vertebra above and below the injured vertebra, skipping the injured level. This short-segment construct places less surgical burden on the patient compared with long-segment constructs, but is less stable biomechanically, and thus has resulted in clinical failures. The addition of two screws placed in the fractured vertebral body represents an attempt to improve the construct stiffness without sacrificing the benefits of short-segment fusion. Purpose To determine the biomechanical differences between four- and six-screw short-segment constructs for the operative management of an unstable L1 fracture. Study design Biomechanical study of instrumentation in vertebral body cadaveric models simulating an L1 axial load injury pattern. Methods Thirteen intact spinal segments from T12 to L2 were prepared from fresh-frozen cadaver spines. An axial load fracture of at least 50% vertebral body height was produced at L1 and then instrumented with pedicle screws. Specimens were evaluated in terms of construct stiffness, motion, and rod strain. Two conditions were tested: a four-screw construct with no screws at the L1 fractured body (4S) and a six-screw construct with screws at all levels (6S). The two groups were compared statistically by paired Student t test. Results The mean stiffness in flexion-extension was increased 31% (p<.03) with the addition of the two pedicle screws in L1. Relative motion in terms of vertical and axial rotations was not significantly different between the two groups. The L1-L2 rod strain was significantly increased in the six-screw construct compared with the four-screw construct (p<.001). Conclusions In a cadaveric L1 axial load fracture model, a six-screw construct with screws in the fractured level is more rigid than a four-screw construct that skips the injured vertebral body.

KW - Axial load fracture

KW - Pedicle screw instrumentation

KW - Short-segment fusion

KW - Spine fracture biomechanics

KW - Spine fracture fixation

KW - Thoracolumbar instrumentation

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