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Research Papers

High Speed Fracture Fixation: Assessing Resulting Fixation Stability and Fastener Withdrawal Strength

[+] Author and Article Information
Matthew Philip Prygoski

Department of Aerospace and
Mechanical Engineering,
University of Notre Dame,
150 Multidisciplinary Research Building,
Notre Dame, IN 46637
e-mail: matthew.prygoski@zimmer.com

Samuel Sanchez Caballero

Institute for Automotive
Manufacturing and Design,
Polytechnic University of Valencia,
Campus of Alcoy,
Alcoy 03801, Spain
e-mail: sasanca@dimm.upv.es

Steven R. Schmid

Department of Aerospace and
Mechanical Engineering,
University of Notre Dame,
150 Multidisciplinary Research Building,
Notre Dame, IN 46637
e-mail: schmid.2@nd.edu

Antony J. Lozier

Zimmer, Inc.,
Warsaw, IN 46581
e-mail: antony.lozier@zimmer.com

Miguel Angel Selles

Department of Mechanical and
Materials Engineering,
Polytechnic University of Valencia,
Campus of Alcoy,
Alcoy 03801, Spain
e-mail: maselles@dimm.upv.es

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received December 19, 2012; final manuscript received May 23, 2013; accepted manuscript posted May 28, 2013; published online July 10, 2013. Assoc. Editor: Sean S. Kohles.

J Biomech Eng 135(9), 091008 (Jul 10, 2013) (10 pages) Paper No: BIO-12-1629; doi: 10.1115/1.4024641 History: Received December 19, 2012; Accepted May 18, 2013; Revised May 23, 2013

A new method of bone fracture fixation has been developed in which fixation darts (small diameter nails/pins) are driven across a fracture site at high velocity with a pneumatically powered gun. When fixation darts are inserted oblique to one another, kinematic constraints prevent fragment motion and allow bone healing to progress. The primary aim of this study is to determine if fixation darts can provide reasonable fixation stability compared to bone screws, which were used as a benchmark since they represent a simple, yet well-established, surgical technique. The first objective was to evaluate macro-level stability using different numbers of darts inserted parallel and oblique to each other; experimental comparisons were undertaken in a bone analog model. Experimental results showed fixation darts could not be substituted for screws on a one-to-one basis, but that a plurality of fixation darts provided comparable fixation to two bone screws while allowing for faster insertion and damaging less bone. A second objective was to evaluate micro-level stability; a finite element model was created in order to provide a detailed look at the stress state surrounding the fixation darts and the evolution of the fracture gap. Even with relatively weak fixation dart configurations, the fracture gap was maintained below physiological thresholds for bone healing. Most failures of the fixed fractures were attributed to fixation dart pullout from the cancellous structure. The final objective of the study was to characterize this mode of failure with separate fixation dart and screw pullout tests conducted in Sawbones® cancellous foam and fresh porcine cancellous bone. The results showed that the cancellous foam was an acceptable substitute for real bone and provided a conservative estimate of the fixation darts' performance relative to bone screws. A final comparison between experimental and numerically predicted pullout strengths provided confirmation that the model and experiments were consistent.

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Figures

Grahic Jump Location
Fig. 1

Fixation darts driven at varying orientations across a fracture site prevent translation and rotation of a bone fragment through imposed kinematic constraints. The darts may pass through cortical and/or cancellous bone [8,12].

Grahic Jump Location
Fig. 2

The AO Foundation recommended treatment for medial condylar fracture with relatively small fragments. Two to four bone screws are inserted parallel to one another and parallel to the plane of the tibial plateau. Larger and/or more comminuted fractures are usually treated with intramedullary nails or lateral fixed angle devices [24].

Grahic Jump Location
Fig. 3

A visual comparison between the bone screws and steel darts used for fixation

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Fig. 4

The eleven configurations of fasteners tested in the study. The dot location marks the entry position and the dot shape signifies the fastener type (screws or steel darts/nails). The color signifies the orientation of the dart (perpendicular to the bone surface, perpendicular to the fracture plane, or parallel, as shown in Fig. 2). Configuration names summarize the important information; for example “3NF” has three steel nails (steel darts) inserted perpendicular to the fracture plane.

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Fig. 5

A Sawbones® femur is loaded in the custom fixtures. The lower fixture features a tibial insert that conforms to the condyles. The fixtures were based on those used by O'Connor-Read et al. [20].

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Fig. 12

The maximum fracture gap (magnitude) is plotted versus the global crosshead displacement for both the 2- and 3-nail (steel dart) configurations. The average experimental failure displacement is overlaid. Recall that the model does not simulate dart sliding so the results must be interpreted in the context of the experimental results.

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Fig. 11

The maximum frictional stresses in the fracture plane and at the nail (steel dart)-bone interface are plotted versus the global crosshead displacement for both the 2- and 3-dart configurations. The average experimental failure displacement is overlaid. Recall that the model does not simulate dart sliding so the results must be interpreted in the context of the experimental results.

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Fig. 10

(top) The frictional stress (MPa) on the surface of the steel darts is plotted for the 2-dart case. The upper dart has more regions of higher stress. (bottom) The fracture gap (mm) is measured for the 3-dart case. The largest gap (in magnitude) was located near the notch in the condyles. The dislocation patterns of the fragments agree with the fractured experimental samples.

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Fig. 9

The maximum displacement sustained during testing for a sample size of n = 4. No error bars indicate that all samples reached the machine limit. Columns 2-3 correspond to screws while columns 4-12 correspond to finish nails (steel darts). The orientation and number of fasteners is designated in the figure.

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Fig. 8

The maximum load sustained during testing for a sample size of n = 4. Columns 2-3 correspond to screws while columns 4-12 correspond to finish nails (steel darts). The orientation and number of fasteners is designated in the figure.

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Fig. 7

A typical load-displacement curve for the compression testing of Sawbones® femurs. Representative curves are shown for 3 parallel screws and 3 parallel nails (steel darts).

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Fig. 6

Steel darts were inserted into various locations in porcine femurs. “Femur 1” is shown as an example. (a) Darts inserted into the lateral condyle (1,2), supracondylar region (3,4), shaft (7-10), and greater trochanter region (11); (b) darts in the medial condyle (12,13); and (c) a supracondylar cut was made and four darts were fired directly into the cancellous structure.

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Fig. 13

A histogram showing the normalized pullout loads for 35 steel darts in isolated porcine cancellous bone harvested from six different femurs

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Fig. 14

Normalized pullout forces (low, average, and high values) are reported for steel darts shot into the lateral condylar cancellous bone of all six porcine femurs. Bones 5 and 6 were clearly the strongest.

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