Would you still have numbness in the sternum area ten months after open heart surgery?

Possible : And perhaps forever! the little nerves may be interfered with by the surgical dissection, or surgical closure done perfectly! sometimes they gradually return and reinervate the area. It is not a danger sign.

Possibly: Scar tissue stays numb. Particularly if an artery in your chest (the left internal mammary artery) was harvested as part of your surgery, you can expect numbness for many months.