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

MATERIALS AND METHODS

RESULTS

DISCUSSION

AFFILIATED INSTITUTES

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.

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-implant-gap-interface (BIGI).

**Stanford University, Stanford, CA***Mayo Clinic, Rochester, MN****Southwest Research Institute, San Antonio, TX

Fig 1: Implant system in tibia with (a) and without (b) protective cap which shields the implant from unwanted motion in between micromotion periods.

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.

Example raw images from the Analyze program before and after implant displacement are shown in Fig. 3.

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.

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.

SUPPORTNIH R01 EB000504-02

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 micro-CT scanning.

(a) (b)

(a) (b)

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.

(a) (b)

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.

REFERENCESKim, 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.

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.

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Fig. 5: Effective strain levels, 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.

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(a) (b)

Ridges