A Micro-CT Based System for Determining
Strain Fields at a Bone-Implant Interface in the
*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)
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
during in vivo testing in a series of mice. RESULTS Fig. 5: Effective strain levels eff (1 2 1 2 ) ,
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.
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):
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) ]
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.
NIH R01 EB000504-02
NIH R01 EB000504-02