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Technical Brief

An Instrumented Pendulum System for Measuring Energy Absorption During Fracture Insult to Large Animal Joints in Vivo

[+] Author and Article Information
B. W. Diestelmeier

Boston Scientific,
St. Paul, MN 55112-5798

M. J. Rudert, T. E. Baer, D. C. Fredericks

Department of Orthopaedics and Rehabilitation,
University of Iowa,
Iowa City, IA 52242-1100

Y. Tochigi

Department of Orthopaedics,
Dokkyo Medical University Koshigaya Hospital,
Saitama 343-8555, Japan

T. D. Brown

Department of Orthopaedics and Rehabilitation,
University of Iowa,
Iowa City, IA 52242-1100
Department of Biomedical Engineering,
University of Iowa,
Iowa City, IA 52242-1527

Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received March 3, 2013; final manuscript received June 26, 2013; accepted manuscript posted July 30, 2013; published online April 23, 2014. Assoc. Editor: Richard Neptune.

J Biomech Eng 136(6), 064502 (Apr 23, 2014) (5 pages) Paper No: BIO-13-1110; doi: 10.1115/1.4025113 History: Received March 03, 2013; Revised June 26, 2013; Accepted July 30, 2013

For systematic laboratory studies of bone fractures in general and intra-articular fractures in particular, it is often necessary to control for injury severity. Quantitatively, a parameter of primary interest in that regard is the energy absorbed during the injury event. For this purpose, a novel technique has been developed to measure energy absorption in experimental impaction. The specific application is for fracture insult to porcine hock (tibiotalar) joints in vivo, for which illustrative intra-operative data are reported. The instrumentation allowed for the measurement of the delivered kinetic energy and of the energy passed through the specimen during impaction. The energy absorbed by the specimen was calculated as the difference between those two values. A foam specimen validation study was first performed to compare the energy absorption measurements from the pendulum instrumentation versus the work of indentation performed by an MTS machine. Following validation, the pendulum apparatus was used to measure the energy absorbed during intra-articular fractures created in 14 minipig hock joints in vivo. The foam validation study showed close correspondence between the pendulum-measured energy absorption and MTS-performed work of indentation. In the survival animal series, the energy delivered ranged from 31.5 to 48.3 Js (41.3 ± 4.0, mean ± s.d.) and the proportion of energy absorbed to energy delivered ranged from 44.2% to 64.7% (53.6% ±4.5%). The foam validation results support the reliability of the energy absorption measure provided by the instrumented pendulum system. Given that a very substantial proportion of delivered energy passed—unabsorbed—through the specimens, the energy absorption measure provided by this novel technique arguably provides better characterization of injury severity than is provided simply by energy delivery.

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Figures

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Fig. 6

Absorbed energy (J) versus delivered kinetic energy (J) for 14 survival-animal hock joint fractures produced by the pendulum device

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Fig. 5

Pendulum-impact absorbed energy (J) versus MTS-impact absorbed energy for 18 foam surrogate blocks

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Fig. 4

Absorbed energy (J) versus penetration depth (mm) for pendulum and MTS impacts of 18 identical foam surrogate blocks

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Fig. 3

Pendulum-created porcine cadaver distal tibia fractures produced during experiments to establish the appropriate system parameters for subsequent use in live-animal tests. The fractures exhibit the general clinical appearance of human fractures: the fracture lines are predominately medial-lateral and an anterior fragment is minimally displaced. (A = anterior, P = posterior, L = lateral, M = medial).

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Fig. 2

The pendulum system configured to measure energy absorbed in indentation of a rigid polyurethane foam block. At impact, the pendulum drives the block into the indenter, which is rigidly mounted on the low-friction sled. As the block is driven into the indenter, the sled moves to the right, resisted by an initially uncompressed spring. The energy absorbed by the block equals the difference between the kinetic energy of the mass and the energy through-passed to the spring. The spring constant (36.7 N/mm) had been determined by empirical measurement.

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Fig. 1

The instrumented pendulum system is shown in (a). (b) illustrates its placement relative to the operating room table, such that sterile field conditions are maintained. Energy delivered to the specimen to create a fracture is controlled by the release height of the pendulum mass. (c) schematically illustrates a purpose-designed tripod pin fixation system for creating distal tibial articular surface fractures. A subarticular surface stress–rising saw cut is employed to ensure reproducibility of fracture morphology. Energy that passes through the specimen is determined by displacement of the sled.

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