Evaluation of pullout strength and failure mechanism of posterior instrumentation in normal and osteopenic thoracic vertebrae

Laboratory investigation

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Object

There is limited data on the pullout strength of spinal fixation devices in the thoracic spine among individuals with different bone quality. An in vitro biomechanical study on the thoracic spine was performed to compare the pullout strength and the mechanism of failure of 4 posterior fixation thoracic constructs in relation to bone mineral density (BMD).

Methods

A total of 80 vertebrae from 11 fresh-frozen thoracic spines (T2–12) were used. Based on the results from peripheral quantitative CT, specimens were divided into 2 groups (normal and osteopenic) according to their BMD. They were then randomly assigned to 1 of 4 different instrumentation systems (sublaminar wires, pedicle screws, lamina claw hooks, or pedicle screws with wires). The construct was completed with 2 titanium rods and 2 transverse connectors, creating a stable frame. The pullout force to failure perpendicular to the rods as well as the pattern of fixation failure was recorded.

Results

Mean pullout force in the osteopenic Group A (36 vertebrae) was 473.2 ± 179.2 N and in the normal BMD Group B (44 vertebrae) was 1414.5 ± 554.8 N. In Group A, no significant difference in pullout strength was encountered among the different implants (p = 0.96). In Group B, the hook system failed because of dislocation with significantly less force than the other 3 constructs (931.9 ± 345.1 N vs an average of 1538.6 ± 532.7 N; p = 0.02). In the osteopenic group, larger screws demonstrated greater resistance to pullout (p = 0.011). The most common failure mechanism in both groups was through pedicle base fracture.

Conclusions

Bone quality is an important factor that influences stability of posterior thoracic implants. Fixation strength in the osteopenic group was one-fourth of the value measured in vertebrae with good bone quality, irrespective of the instrumentation used. However, in normal bone quality vertebrae, the lamina hook claw system dislocated with significantly less force when compared with other spinal implants. Further studies are needed to investigate the impact of different transpedicular screw designs on the pullout strength in normal and osteopenic thoracic spines.

Abbreviation used in this paper: BMD = bone mineral density.

Article Information

Address correspondence to: Avinash G. Patwardhan, Ph.D., Department of Orthopaedic Surgery and Rehabilitation, Loyola University Medical Center, 2160 South First Avenue, Maywood, Illinois 60153. email: apatwar@lumc.edu.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Photographs of instrumented vertebrae with the 4 posterior thoracic systems used in the study: pedicle screws (A), sublaminar wires (B), laminar claw hooks (C), and pedicle screws with sublaminar wires (D).

  • View in gallery

    Radiograph showing the trajectory of the transpedicular screws. All screws were inserted with an anteromedial angulation of approximately 15º on each side, achieving an overall screw convergence (triangulation) of 30º.

  • View in gallery

    Photograph of the sublaminar wire instrumentation. Two loops of annealed wire passed around the lamina. The construct was completed with placement of 2 titanium rods measuring 10 mm in length and 5 mm in diameter. Two custom-made steel cross-connectors were used to provide further rigidity to the posterior instrumentation construct.

  • View in gallery

    The pullout test setup. The instrumented vertebra was secured in a special adjustable jig by means of a metal rod inserted into the spinal canal. The jig was adjusted so the rods were perpendicular to the tension force of the material testing machine. A displacementcontrolled test to failure was conducted, applying traction through a loop of cable.

  • View in gallery

    Graph of mean pullout strength values for the 4 instrumentation systems in vertebrae with BMD < 150 mg/ml (Group A) and BMD > 150 mg/ml (Group B).

  • View in gallery

    Examples of the pullout test to failure. A: Radiograph of a pedicle fracture at the junction with the vertebral body after placement of a lamina hook construct. B: Pedicle base fracture after fixation with sublaminar wires with complete separation of the posterior arch. C: Pullout of transpedicular screw.

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