Transcript Poster Theme- 36x48.ppt

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
In a mouse model of mechanobiology at a
healing bone-implant interface, we control axial
motion of the implant to provide control over
strain patterns in healing interfaces. We use
screws and pins (polymer and metal) to generate
different strain distributions in healing tissue.
Methods described here allow us to quantify the
experimental state of strain at an actual
interface. Poster #1693 presents data on implant
displacement and force during in vivo testing in
a series of mice. Poster #1026 describes initial
cellular and molecular findings.
MATERIALS AND METHODS
The jig is placed in the micro-CT in the orientation
shown in Fig. 2 and the scans are done before and
after implant displacement in the interface.
The
stage allows rotation of the tibia about its long axis in
small angular steps (~0.5º). Images have a pixel size
of 5.959 µm and are processed in Analyze software.
Images before and after displacement are then
analyzed via DISMAP (Kim et al. 2005, Nicolella et
al. 2001) to determine strain fields.
(a)
RESULTS
Example raw images from the Analyze program
before and after implant displacement are shown in
Fig. 3.
(a)
(b)
Fig. 3: Images before (a) and after (b) implant axial
displacement of approximately 150 µm via the jig in the
micro-CT described in Fig. 2.
(b)
Fig 1: Implant system in tibia with (a) and without
(b) protective cap which shields the implant from
unwanted motion in between micromotion periods.
Strain distributions (below), come from analyses of
selected regions in the sample shown in Fig. 3
(above). In this example case, the bone-implant
interface in the tibia healed for 7 days in the absence
of implant micromotion.
Following the prescribed healing period the
mouse is euthanized and tibia is dissected out.
The dissected tibia is mounted for micro-CT
scanning on a specially designed jig to allow
for implant displacement during scanning.
Fig. 2: Jig (A) used to hold the tibia
containing the implant system (C).
The screw (B) is used to displace
the implant approximately 150 µm
in the axial direction during microCT scanning.
(b)
1
2 2 2

(1   2  1 2 ) 2 ,
3
In our micromotion system, a central test
implant is displaced within a specially designed
bone plate held onto the anterior proximal
mouse tibia by two side screws (Fig. 1). The tip
of the test implant is 0.5 mm in diameter and
resides in a 0.8 mm diameter hole through one
cortex of the tibia, resulting in a bone-implantgap-interface (BIGI).
(a)
Ridges
Fig. 5: Effective strain levels,  eff
in mock interfaces; note strains on the left, right and
bottom of a pin implant (a) and a screw implant (b)
after implant motion of approximately 150 µm and
strain concentrations at the circumferential ridges
on the pin & threads of the screw.
The mock interface was created with a rubber
(Reprorubber®, Small Parts, Inc. Miami Lakes,
FL) to simulate the contents of a bone-implant
gap interface early after surgery. The areas of
high strain were concentrated around the
ridges of the pin and also along the sides of the
screw, where 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 88.69% respectively. The average
effective strains in the gap on the left and right
of the screw were 63.27% and 31.68%
respectively.
DISCUSSION
This work demonstrates the feasibility of
making strain measurements at interface around
implants in mouse tibiae using micro-CT and
DISMAP methods. The values of strain here
compare reasonably with previous finite
element simulations of similar situations. The
spatial resolution of strain is at a level that is
meaningful for our purposes in the mouse
model.
REFERENCES
Kim, DG, Brunski, JB, Nicolella, DP (2005) 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.
(a)
AFFILIATED INSTITUTES
(b)
1
2 2
1
dist  [(1  2 ) 2  (2  3 )2  (3  1) ]
3
Fig. 4: Distortional strain levels,
,
in the tissue on the left (a) and right (b) sides of the
interface after implant motion of approximately ~54
microns following 7 days ofhealing.
**Stanford University, Stanford, CA
***Mayo Clinic, Rochester, MN
****Southwest Research Institute, San Antonio, TX
SUPPORT
NIH R01 EB000504-02