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Technical Briefs

A Nitinol Based Flexor Tendon Fixation Device: Gapping and Tensile Strength Measurements in Cadaver Flexor Tendon

[+] Author and Article Information
Shawn P. Reese

CoNextions Inc.,
Salt Lake City, UT 84104;
Department of Bioengineering,
The University of Utah,
Salt Lake City, UT 84112
e-mail: s.reese@conextionsmed.com

Erik N. Kubiak

CoNextions Inc.,
Salt Lake City, UT 84104;
Division of Orthopaedic Trauma,
Department of Orthopaedic Surgery,
The University of Utah,
Salt Lake City, UT 84112

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received May 8, 2013; final manuscript received September 30, 2013; accepted manuscript posted October 19, 2013; published online December 3, 2013. Assoc. Editor: Zong-Ming Li.

J Biomech Eng 136(1), 014501 (Dec 03, 2013) (5 pages) Paper No: BIO-13-1221; doi: 10.1115/1.4025779 History: Received May 08, 2013; Revised September 30, 2013; Accepted October 19, 2013

In this study, a new nitinol based fixation device was investigated for use in repairing severed digital flexor tendons. The device, composed of superelastic nitinol, is tubular in shape with inward facing tines for gripping tissue. Its cellular structure was designed such that it has a large effective Poisson's ratio, which facilitates a “finger trap” effect. This allows for reduced tendon compression during a resting state (to permit vascular perfusion) and increased compression during loading (to drive the tines into the tissue for gripping). To test the feasibility of using this device for flexor tendon repair, it was tested on cadaver flexor digitorum profundus tendons. The tendons were excised, cut in the region corresponding to a zone II laceration, and repaired using the device. The device was easy to install and did not prevent the tendon from bending. Constant strain rate tensile testing revealed a mean tensile strength of 57.6 ± 7.7 N, with a force of 53.2 ± 7.8 N at a 2 mm gap. This exceeds the suggested primary repair strength of 45 N, which has been proposed as the necessary strength for enabling early mobilization. Although considerable future studies will be needed to determine the suitability of the new repair device for clinical use, this study demonstrates the feasibility of utilizing a tubular, nitinol repair device for flexor tendon fixation.

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Figures

Grahic Jump Location
Fig. 3

Application of nitinol fixation device. (a) The tendon stumps were pulled into the opened device using loops of suture material (direction of pull indicated by yellow arrows) such that an overlap of approximately 3 mm was obtained. The side laces were tightened, swages were crimped tight, excess wire was trimmed, and suture loops pulled out. (b) Overhead image of the device after application to a severed tendon. (c) Overhead image of the repaired tendon in the tissue clamps prior to testing.

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Fig. 2

Nitinol fixation device. (a) 2D CAD drawing of the laser cut sheet component, with the center region and regions 1–3 delineated. (b) Side view schematic of the assembled device showing the top and bottom sheet components (heat set tines represented with dotted lines) connected on the sides with nitinol wire. Two separate wires were used for each side and were laced through the holes in the two opposing sheet parts. The ends of the wire lace were passed through a stainless steel swage (dark gray boxes at the ends). (c) Overhead view of a sheet component after the tines were heat set into a downward facing position. (d) Close up view of a tine. (e) Side view of a device ready for use.

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Fig. 1

Palmar view of the hand, with flexor tendons (red) overlaid on the bones. Zones I–V are labeled and delineated with a black line. The five pulleys (shown as blue rectangles) are shown on the index finger and labeled, but omitted from the other fingers for clarity.

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Fig. 4

Force–strain response of repaired tendons. The force–strain response was nearly linear for all samples tested (R2 values ranged from 0.98 to 1.00, best fit lines not shown). Curves were truncated at the failure point for each curve for the purpose of clarity.

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Fig. 5

Mean gapping and tensile strength. Force at 2 mm of gapping exceeded 45 N (represented by the dotted line), which is considered the necessary repair strength for early mobilization. No significant differences were present between the failure force, force at 2 mm of gapping or force at 1 mm of gapping. Error bars represent the standard deviation.

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Fig. 6

Typical failure behavior of a repaired tendon. Most samples failed near the 2 mm gapping point. After failure, load decreased in an erratic manner as strain was increased. The locations of 1 mm gap formation, 2 mm gap formation, and failure are overlaid on the plot.

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