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Related Experiment Videos

The "Z-effect" phenomenon defined: a laboratory study.

Eric J Strauss1, Frederick J Kummer, Kenneth J Koval

  • 1Department of Orthopaedic Surgery, NYU-Hospital for Joint Diseases, 301 East 17th Street, New York, New York 10003, USA.

Journal of Orthopaedic Research : Official Publication of the Orthopaedic Research Society
|June 27, 2007
PubMed
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The Z-effect phenomenon, a complication in hip fracture fixation, occurs when screws migrate. This study reproduced the inferior screw migration in lab models with higher femoral head strength than the neck.

Area of Science:

  • Orthopedic surgery
  • Biomechanical engineering
  • Materials science

Background:

  • The Z-effect phenomenon is a known complication associated with two-lag screw intramedullary nail designs for intertrochanteric hip fractures.
  • This phenomenon involves the migration of the inferior lag screw laterally and the superior lag screw medially under physiological loading.

Purpose of the Study:

  • To experimentally reproduce the Z-effect phenomenon in a controlled laboratory setting.
  • To investigate the relationship between femoral head and neck compressive strengths and lag screw migration.

Main Methods:

  • Sixteen simulated femoral head and neck constructs with varying polyurethane foam densities were created.
  • Constructs were instrumented with a two-screw cephalomedullary intramedullary nail and subjected to cyclic loading (250 N) for up to 10,000 cycles.

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  • Lag screw displacement was measured after each loading increment.

    • Main Results:

    • The inferior lag screw migration component of the Z-effect was reproduced in constructs where the femoral head's compressive strength exceeded the femoral neck's.
    • The most significant inferior lag screw displacement occurred in specimens with the greatest head-neck compressive strength difference.
    • A specific construct (0.91 MPa neck, 8.8 MPa head) showed over 1 cm of inferior lag screw lateral migration after 10,000 cycles.

    Conclusions:

    • Femoral head compressive strength higher than the femoral neck can lead to inferior lag screw migration, mimicking the Z-effect.
    • This finding may help simulate fracture patterns prone to varus collapse, particularly those with medial cortex comminution.
    • The study provides insights into the biomechanics underlying lag screw migration in hip fracture fixation devices.