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An Anesthesia, Surgery, and Harvest Method for the Evaluation of Transpedicular Screws Using an In Vivo Porcine Lumbar Spine Model

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Threaded screw head inserts improve locking plate biomechanical properties.

Joseph Bellapianta¹, Kyle Dow, Nicholas A Pallotta

¹Division of Orthopaedic Surgery, Albany Medical College, Albany, NY, USA.

Journal of Orthopaedic Trauma

|January 20, 2011

Summary

Filling the empty screw hole in one third tubular locking plates significantly improves their fatigue life and stiffness. This simple modification enhances implant mechanical properties, particularly for less rigid plates, without added cost or morbidity.

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Published on: April 11, 2012

Area of Science:

Orthopedic biomechanics
Biomaterials science
Surgical implant technology

Background:

Locking plates are crucial orthopedic implants for fracture fixation.
The mechanical integrity of locking plates, particularly their fatigue life and stiffness, is critical for successful bone healing.
Unfilled screw holes in locking plates may represent stress concentration points, potentially compromising performance.

Purpose of the Study:

To investigate whether filling the empty screw hole of one third tubular locking plates with a screw head enhances their stiffness and fatigue life.
To test the hypothesis that improved mechanical properties will be observed under physiologically relevant loads.

Main Methods:

Finite element analysis (FEA) and experimental testing were used to evaluate two manufacturers' Synthes (n) and Stryker (r) five-hole one third tubular plates.
Plates were tested in both open (O) and filled (F) center hole configurations under four-point bending.
Mechanical cycling was performed to determine stiffness, cycles to failure, and mode of failure.

Main Results:

FEA showed reduced maximum stress at the center hole periphery by over 2x when filled.
Experimental testing revealed a fourfold increase in fatigue life for Synthes plates (45,450 cycles F vs. 10,305 cycles O).
Both plate types demonstrated improved bending stiffness when the center hole was filled, with statistical significance for Stryker plates (P < 0.011).

Conclusions:

Filling the empty screw holes of locking plates with screw heads significantly enhances fatigue life and stiffness.
This simple, low-cost modification improves implant mechanical performance, especially in less rigid, lower-profile plates.
The findings suggest a broadly applicable method to improve the durability and effectiveness of various locking plate designs.