Fig. 5.

Cylindrical hydroxyapatite needle oriented along vector e ${}_r$, and inclined by the Euler angles $\mathit{\vartheta }$ and $\mathit{\phi }$, with respect to the reference base frame defined through the unit vectors e ${}_1$, e ${}_2$, and e ${}_3$; the local base frame, defined by unit vectors e ${}_r$, e ${}_{\mathit{\vartheta }}$, and e ${}_{\mathit{\phi }}$, is attached to the cylindrical inclusion; vector $\mathbf{n}$, oriented perpendicular to e ${}_r$, is further defined by angle $\mathit{\psi }$