The scars on his chest art partly from canon and partly from fanon. Damian is shown in the initial Batman and Robin run while he's getting treated at talia's place with an open back, cut in a y-shape. Now, since we know that Talia genetically engineered Damian to be perfect, I suspected he might have similar scars on his chest. I researched the cuts on autopsy corpses and added them into the design to fit this idea. I hope this answers your question!