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Technical Briefs

The Effect of Screw Pullout Rate on Screw Purchase in Synthetic Cancellous Bone

[+] Author and Article Information
Rad Zdero1

Martin Orthopaedic Biomechanics Laboratory, St. Michael’s Hospital, Shuter Wing (Room 5-066), 30 Bond Street, Toronto, ON, M5B 1W8, Canadazderor@smh.toronto.on.ca

Emil H. Schemitsch

Martin Orthopaedic Biomechanics Laboratory, St. Michael’s Hospital, Shuter Wing (Room 5-066), 30 Bond Street, Toronto, ON, M5B-1W8, Canada; Department of Surgery, Faculty of Medicine, University of Toronto, Toronto, ON, Canada

1

Corresponding author.

J Biomech Eng 131(2), 024501 (Dec 10, 2008) (5 pages) doi:10.1115/1.3005344 History: Received May 07, 2008; Revised August 07, 2008; Published December 10, 2008

Clinically, orthopaedic fracture fixation constructs are mounted using screws inserted into cancellous bone, while biomechanical studies are increasingly using commercially available synthetic bones. The goal of this study was to examine the effect of screw pullout rate on cancellous bone screw purchase strength in synthetic cancellous bone. Sixty synthetic cancellous bone cubes (40×40×40mm3) each had one orthopaedic cancellous bone screw (major diameter=6.5mm) inserted to a depth of 30mm. Screws were extracted to obtain outcome measures of failure force, failure shear stress, failure energy, failure displacement, resistance force, and removal energy. The ten test groups (n=6 cubes per group) had screws extracted at pullout rates of 1mmmin, 2.5mmmin, 5mmmin, 7.5mmmin, 10mmmin, 20mmmin, 30mmmin, 40mmmin, 50mmmin, and 60mmmin. The aggregate average results for failure force, failure stress, failure energy, failure displacement, resistance force, and postfailure removal energy for combined pullout rates were, respectively, 984.8±63.9N, 3.5±0.2MPa, 298.3±41.7J, 0.53±0.08mm, 453.8±19.6N, and 5420.1±489.7J. Most statistical differences (40 of 47) involved either the 5mmmin or the 60mmmin rates being compared to other rates. Failure force, failure stress, and resistance force increased and were highly linearly correlated with pullout rate (R2=0.78, 0.76, and 0.74, respectively). Failure energy, failure displacement, and removal energy were relatively unchanged over the pullout range tested, yielding low correlation coefficients (R2<0.05). Failure force, failure stress, and resistance force were affected by bone screw pullout rate in synthetic cancellous bone, while failure energy, failure displacement, and removal energy remained unchanged. This is the first study to perform an extensive investigation of cancellous bone screw pullout rate in synthetic cancellous bone.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 2

Cancellous screw pullout force-displacement curve. Phase 1 represented the initial rise in force. Phase 2 showed drop in force postfailure. Phase 3 illustrated resistance to screw motion from friction and obstruction. Phase 4 was final full extraction of the screw. Failure indicated primary initial structural collapse of cancellous bone material around the screw threads. Samples of the lowest (1mm∕min) and highest (60mm∕min) pullout rates showed good overlap.

Grahic Jump Location
Figure 3

Cancellous material failure mode. During the screw pullout process, material was cleanly sheared off at the screw thread-bone interface. A layer of cancellous material covered the screw threads. No screws were broken, bent, or damaged during pullout tests.

Grahic Jump Location
Figure 1

Cancellous screw pullout test setup. The apparatus was comprised of a (1) universal swivel joint pin, (2) a support pin that passed through the eye of the universal swivel joint, (3) a screw extraction grip, (4) a cancellous bone screw, (5) side mounting plates, (6) an aluminum support plate, and (7) a synthetic cancellous test cube. The large white arrow shows the direction of screw extraction.

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