You Cannot
Assurance states that:
Even in the worst circumstances, you can perform basic tasks. Choose a skill you’re trained in. You can forgo rolling a skill check for that skill to instead receive a result of 10 + your proficiency bonus (do not apply any other bonuses, penalties, or modifiers).
While risky surgery states that:
Your surgery can bring a patient back from the brink of death, but might push them over the edge. When you Treat Wounds, you can deal 1d8 slashing damage to your patient just before applying the effects of Treat Wounds. If you do, you gain a +2 circumstance bonus to your Medicine check to Treat Wounds, and if you roll a success, you get a critical success instead.
When you use assurance you can't roll a success for risky surgery, because you forgo that roll. Instead, you receive a result (10 + 6, for a 2nd level Expert), which against a DC 15 means you receive a success that risky surgery can't improve.
As you've already noted, risky surgery's +2 circumstance bonus is also useless with assurance, so risky surgery deals 1d8 damage forgrants no benefit when combined with assurance.