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Locked Internal Fixator: Sensitivity of Screw/Plate Stability to the Correct Insertion Angle of the Screw

Kaab, Max J., Frenk, Andre, Schmeling, Andreas, Schaser, Klaus-Dieter, Schuetz, Michael, & Haas, Norbert (2004) Locked Internal Fixator: Sensitivity of Screw/Plate Stability to the Correct Insertion Angle of the Screw. Journal of Orthopaedic Trauma, 18(8), pp. 483-487.

Abstract

Objective: Internal fixators with angular stability have been developed to provide high stability without compression of the plate on to the bone. Angular and axial stability of a plate-screw construct can be achieved using a conically threaded screw head undersurface and a corresponding conically threaded plate hole. Furthermore, the insertion angle of the screw must correspond precisely to the axis of the screw hole. This is not always achieved in clinical practice and may result in screw loosening. The objective of this study was to examine the relationship between the stability of the locked screw-plate on the insertion angle of the screw.

Methods: Locking screws were inserted in an isolated (Point Contact Fixator, PC-Fix) or combined (Locking Compression Plate, LCP 4.5) locking hole with the use of an aiming device. The optimal insertion angle for these plates is perpendicular to the plate surface. The screws were inserted with an axis deviation of 0[degrees] (optimal condition), 5[degrees], and 10[degrees] respective to the optimal angle (variance +/- 1[degrees]). The samples were tested under shear or axial (push out) loading conditions until failure occurred. An Instron materials testing machine was used.

Results: Locking screws inserted in the isolated locking hole (PC-Fix) showed a significant decrease of failure load if inserted at 5[degrees] and 10[degrees] angle. Using an optimal insertion angle (0[degrees]), failure load was 1480 +/- 390 N, with 5[degrees] axis deviation 780 +/- 160 N, P = 0.0001, and with 10[degrees] axis deviation 550 +/- 110 N, P = 0.0001. Screws inserted in the combined locking hole (LCP) also showed a significant decrease of push-out force of 77% (4960 +/- 1000 N versus 1120 +/- 400 N) with 10[degrees] axis deviation. Compared to optimal insertion angle (0[degrees]), bending load to failure did decrease up to 69% (1240 +/- 210 N vs. 390 +/- 100 N) with 10[degrees] axis deviation.

Conclusion: A locking head screw exhibits high stability with a moderate axis deviation in the angle of insertion of up to 5[degrees]. However, there is a significant decrease in stability with increasing axis deviation (>5[degrees]). An aiming device is recommended to provide optimal fixation with angular stability.

Impact and interest:

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36 citations in Web of Science®

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ID Code: 10222
Item Type: Journal Article
Additional Information: Self-archiving of the author-version is not yet supported by this publisher. For more information, please refer to the journal’s website (see hypertext link) or contact the author.
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ISSN: 1531-2291
Divisions: Past > QUT Faculties & Divisions > Faculty of Built Environment and Engineering
Copyright Owner: Copyright 2004 Lippincott Williams & Wilkins
Deposited On: 17 Oct 2007
Last Modified: 29 Feb 2012 23:09

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