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					                               A Micro-CT Based System for Determining
                              Strain Fields at a Bone-Implant Interface in the
                                                 Mouse Tibia
                                *Currey, J.A., **Leucht, P., ***Vercnocke, A., ***Hansen, D.,
                                      ***Ritman, E.L., ****Nicolella, D., *Brunski, J.B.
                         *Dept. of Biomedical Engineering, Rensselaer Polytechnic Institute, Troy, NY



          INTRODUCTION                              The jig is placed in the micro-CT in the
                                                    orientation shown in Fig. 2 and the scans are done
In a mouse model of mechanobiology at a             before and after implant displacement in the
healing bone-implant interface, we control          interface. The stage allows rotation of the tibia
axial motion of the implant to provide control      about its long axis in small angular steps (~0.5º).
over strain patterns in healing interfaces. We      Images have a pixel size of 5.959 µm and are
use screws and pins (polymer and metal) to          processed in Analyze software. Images before
generate different strain distributions in          and after displacement are then analyzed via
healing tissue. Methods described here allow        DISMAP (Kim et al. 2005, Nicolella et al. 2001)
                                                                                                                                                                Ridge
us to quantify the experimental state of strain     to determine strain fields.                                                                                 s
at an actual interface. Poster #1693 presents                                                                                                             (a)                           (b)
data on implant displacement and force                                                                                                                                            2 2 2
                                                                                                                                                                                                      1

during in vivo testing in a series of mice.                                        RESULTS                                                 Fig. 5: Effective strain levels  eff  (1   2  1 2 ) ,
                                                                                                                                                                                                      2
                                                                                                                                                                                   3
Poster #1026 describes initial cellular and                                                                                                in mock interfaces; note strains on the left, right
molecular findings.                                 Example raw images from the Analyze program                                            and bottom of a pin implant (a) and a screw
                                                    before and after implant displacement are shown                                        implant (b) after implant motion of
                                                    in Fig. 3.                                                                             approximately 150 µm and strain concentrations
                                                                                                                                           at the circumferential ridges on the pin & threads
MATERIALS AND METHODS                                                                                                                      of the screw.
In our micromotion system, a central test
                                                                                                                                           The mock interface was created with a
implant is displaced within a specially
                                                                                                                                           rubber (Reprorubber®, Small Parts, Inc.
designed bone plate held onto the anterior
                                                                                                                                           Miami Lakes, FL) to simulate the contents
proximal mouse tibia by two side screws
                                                                                                                                           of a bone-implant gap interface early after
(Fig. 1). The tip of the test implant is 0.5 mm
                                                                                                                                           surgery. The areas of high strain were
in diameter and resides in a 0.8 mm diameter
                                                                                                                                           concentrated around the ridges of the pin
hole through one cortex of the tibia, resulting
                                                                                                                                           and also along the sides of the screw, where
in a bone-implant-gap-interface (BIGI).
                                                                                                                                           there was a periodicity that approximately
                                                                                                                                           matched the threads of the screws. The
                                                                                                                                           average effective strains in the gap on the
                                                                                                                                           left and right of the pin were 53.97% and
                                                                            (a)                                  (b)                       88.69% respectively. The average effective
                                                                                                                                           strains in the gap on the left and right of the
                                                    Fig. 3: Images before (a) and after (b) implant axial                                  screw      were    63.27%       and    31.68%
                                                    displacement of approximately 150 µm via the jig in                                    respectively.
                                                    the micro-CT described in Fig. 2.
                                                                                                                                                           DISCUSSION
                                                     Strain distributions (below), come from analyses                                      This work demonstrates the feasibility of
                                                     of selected regions in the sample shown in Fig. 3                                     making strain measurements at interface
          (a)                       (b)              (above). In this example case, the bone-implant                                       around implants in mouse tibiae using micro-
                                                     interface in the tibia healed for 7 days in the                                       CT and DISMAP methods. The values of
Fig 1: Implant system in tibia with (a) and
without (b) protective cap which shields the
                                                     absence of implant micromotion.                                                       strain here compare reasonably with previous
implant from unwanted motion in between                                                                                                    finite element simulations of similar
micromotion periods.                                                                                                                       situations. The spatial resolution of strain is
                                                                                                                                           at a level that is meaningful for our purposes
Following the prescribed healing period the                                                                                                in the mouse model.
mouse is euthanized and tibia is dissected
out. The dissected tibia is mounted for                                                                                                                   REFERENCES
micro-CT scanning on a specially designed
jig to allow for implant displacement during                                                                                               Kim, DG, Brunski, JB, Nicolella, DP (2005)
scanning.                                                                                                                                  J. Eng. In Med. Part H 219(2): 119-128.
                                                                                                                                           Nicolella, DP, Nicholls, AE, Lankford, J,
                                                                                                                                           Davy, D.T. (2001) J. . Biomechanics. 34(1):
                                                                                                                                           135-139.
                Fig. 2: Jig (A) used to hold the
                tibia containing the implant
                                                                           (a)                               (b)                            AFFILIATED INSTITUTES
                system (C). The screw (B) is
                used to displace the implant                                                 1
                                                                                                                                      1   **Stanford University, Stanford, CA
                                                   Fig. 4:Distortional strain levels, dist  [(1  2 )2  (2  3 )2  (3  1) ]
                                                                                                                                    2 2

                approximately 150 µm in the                                                  3                                            ***Mayo Clinic, Rochester, MN
                                                   in the tissue on the left (a) and right (b) sides of the                               ****Southwest Research Institute, San
                axial direction during micro-CT
                                                   interface after implant motion of approximately ~54
                scanning.                                                                                                                 Antonio, TX
                                                   microns following 7 days of healing.
                                                                                           
                                                                                                                                                                SUPPORT
                                                                                                                                           NIH R01 EB000504-02
                                                                                             NIH R01 EB000504-02

				
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posted:1/19/2013
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