The effect of HCL concentration on the Solubility of Hydroxyapatite
The $Ksp$ expression for hydroxyapatite is: $\ce{ Ca5(PO4)3(OH)_{(s)} -&gt; 5Ca^{2+}_{(aq)} + 3PO4^{3-}_{(aq)} + OH-_{(aq)}}$. 

When $\ce{HCl_{(aq)}}$ is added, it reacts with the dissolved $\ce{OH-}$ to form water, thereby decreasing the $\ce{OH-}$ concentration. Le Chatlier's principle predicts that more hydroxyapatite will dissolve (shift to the right).