Bone Damage Evolution Around Integrated Metal Screws Using X-Ray Tomography - in situ Pullout and Digital Volume Correlation

Abstract

Better understanding of the local deformation of the bone network around metallic implants subjected to loading is of importance to assess the mechanical resistance of the bone-implant interface and limit implant failure. In this study, four titanium screws were osseointegrated into rat tibiae for 4 weeks and screw pullout was conducted in situ under x-ray microtomography, recording macroscopic mechanical behavior and full tomographies at multiple load steps before failure. Images were analyzed using Digital Volume Correlation (DVC) to access internal displacement and deformation fields during loading. A repeatable failure pattern was observed, where a ∼300-500 μm-thick envelope of bone detached from the trabecular structure. Fracture initiated close to the screw tip and propagated along the implant surface, at a distance of around 500 μm. Thus, the fracture pattern appeared to be influenced by the microstructure of the bone formed closely around the threads, which confirmed that the model is relevant for evaluating the effect of pharmacological treatments affecting local bone formation. Moreover, cracks at the tibial plateau were identified by DVC analysis of the tomographic images acquired during loading. Moderate strains were first distributed in the trabecular bone, which localized into higher strains regions with subsequent loading, revealing crack-formation not evident in the tomographic images. The in situ loading methodology followed by DVC is shown to be a powerful tool to study internal deformation and fracture behavior of the newly formed bone close to an implant when subjected to loading. A better understanding of the interface failure may help improve the outcome of surgical implants.

Keywords: Digital Volume Correlation; X-ray tomography; bone; in situ loading; metallic screw.

FIGURE 1

Overview of the x-ray μCT set-up (A) and zoom on the custom-made loading device (B) to pullout the screw in situ, with the sample wrapped in gauze inside the polycarbonate chamber. A view of the sample inside the chamber is presented in the insert. The blue arrow indicates the loading direction.

FIGURE 2

Tomography scan cuts (top row) after each loading step for sample S2. Cutting direction of the scans is presented in the insert to the right. Three lower rows present the corresponding DVC results between loading steps, with vertical displacements (Z-displ), shear and volumetric strains. Bone detachment close to the threads can be observed in the last scan (step 4). High strains were already detected step 2–3 (before failure) where the crack will later develop (arrows).

FIGURE 3

Zoom in on two last steps of Sample S4, to illustrate the detachment of the bone close to the threads from the trabecular network. From left to right, tomographic slices with yellow dotted line highlighting the bone detaching, and DVC results for the same position.

FIGURE 4

The left column represents the same cutting direction as Figure 2. Right side shows 3 horizontal cuts of the threaded region: close to the screw tip (yellow line), the middle (red) and close to the end of the screw threads (green). Note the envelope of vertical displacement (∼4 voxels) around the screw (white arrow) inducing a specific strain pattern with a strain-free regions (1, red arrow), followed by high strains region (2, red arrow), and again a low strain region (3, red arrow).

FIGURE 5

(A) Cutting direction for the following results and loading curve of sample S1. (B) From top row to bottom row, tomographic scans cuts at the tibial plateau region, and corresponding DVC shear and volumetric strains for all loading steps. Crushing of bone close to the support (arrows 1) can be seen in the scans (see also Supplementary Data Sheet 1, Files 2, 3). Crack opening (2) is also seen in scans in relatively dense bone. However, the connection (3) between 1 and 2 was not clearly observed in the tomographies (see Supplementary Data Sheet 1, File 3). Note the strains spread in the bone structure at early loading stages, gathering where the cracks are to open, and finally relaxing in the bone while the cracks were opening (from step 4–5).