More than 54 million Americans have or are at high risk of developing a metabolic bone disease; disorders of bone strength that leave individuals with fragile bones and disabilities. The gold standard to evaluate these diseases is dual energy x-ray absorptiometry, but this only measures mineral content. These diseases, however, impact collagen and mineral integrity which impede the bone’s ability to store hormones, proteoglycans, and glycoproteins imperative to homeostasis. We have established a second harmonic generation (SHG) polarimetric assay that describes bone collagen organization. To further our analysis, we propose multimodal optical evaluation of bone quality with third harmonic generation (THG) to measure osteocyte dendritic processes. This method of analysis could be used to evaluate the disease state of bone and response to therapy.
Bone growth and strength is severely impacted by Hypophosphatasia (HPP). It is a genetic disease that affects the mineralization of the bone. We hypothesize that it impacts overall organization, density, and porosity of collagen fibers. Lower density of fibers and higher porosity cause less absorption and scattering of light, and therefore a different regime of transport mean free path. To find a cure for this disease, a metric for the evaluation of bone is required. Here we present an evaluation method based on our Phase Accumulation Ray Tracing (PART) method. This method uses second harmonic generation (SHG) in bone collagen fiber to model bone indices of refraction, which is used to calculate phase retardation on the propagation path of light in bone. The calculated phase is then expanded using Zernike polynomials up to 15th order, to make a quantitative analysis of tissue anomalies. Because the Zernike modes are a complete set of orthogonal polynomials, we can compare low and high order modes in HPP, compare them with healthy wild type mice, to identify the differences between their geometry and structure. Larger coefficients of low order modes show more uniform fiber density and less porosity, whereas the opposite is shown with larger coefficients of higher order modes. Our analyses show significant difference between Zernike modes in different types of bone evidenced by Principal Components Analysis (PCA).
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