Evaluating differences in Young's Modulus of regenerated and uninjured mouse digit bone through microCT density-based calculation and nanoindentation testing

Abstract

The mouse digit tip amputation model is an excellent model of bone regeneration, but its size and shape present an obstacle for biomechanical testing. As a result, assessing the structural quality of the regenerated bone in this model has focused on mineral density and bone architecture analysis. Here we describe an image-processing based method for assessment of mechanical properties in the regenerated digit by using micro-computed tomography mineral density data to calculate spatially discrete Young's modulus values throughout the entire distal third phalange. Further, we validate this method through comparison to nanoindentation-measured values for Young's modulus. Application to a set of regenerated and unamputated digits shows that regenerated bone has a lower Young's modulus compared to the uninjured digit, with a similar trend for experimental hardness values. Importantly, this method heightens the utility of the digit regeneration model, allows for more impactful treatment evaluation using the model, and introduces an analysis platform that can be used for other bones that do not conform to a standard long-bone model.

Keywords: Bone; Digit regeneration; Micro-computed tomography; Nanoindentation; Young’s modulus.

Fig. 1.

Image processing and spatial analysis on bone data from μCT image stacks offers a path to predicting mechanical properties and regenerated bone quality.

Fig. 2.

Comparison of calculated Young’s modulus values from all four μCT density-modulus relationships (Eq. (1), Eq. (2), Eq. (3), Eq. (4), respectively) with those measured by nanoindentation experiment (NI), for mean values of A) whole UA digits (N = 3); B) UA proximal regions (N = 3); C) UA distal regions (N = 3); D) whole D42 digits (N = 3); E) D42 proximal regions (N = 3); and F) D42 distal regions (N = 3). Error bars indicate standard deviation. Eq. (4) shows best overall agreement with experimentally measured values.

Fig. 3.

Plotting of numerically calculated A) Unamputated and B) Regenerated Young’s modulus values versus proximal to distal distance. Local polynomial regression curve indicated in green, indicating the trend in modulus value from proximal to distal. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4.

Young’s Modulus (E, units of GPa) values in 3D with intensity reflecting magnitude as shown, calculated from mineral density data measured through μCT imaging, with Eq. (4). Whole digit rendering is displays of A) unamputated and B) regenerated representative digits, and planar vertical slices along the proximal to distal axis are used to create cross-sections of the same C) unamputated and D) regenerated digits.