How to build 2D Bones local matrices ?

Hi everyone,
I'm parsing an array similar to https://docs.unity3d.com/ScriptReference/U2D.SpriteDataAccessExtensions.GetBones.html
I've made several attemps to build local matrices for there using Martrix4*4::TRS but with no luck.
How should I take the bone length into consideration ?

If someone could help, that would be great.
Thanks