According to the “Wrestling Observer Newsletter” by way of 411mania.com, Ambrose’s injury initially took place at September’s “Tables Ladders and Chairs 2017” pay per view. During the match, Ambrose’s arm hit a table but the table didn’t break, causing the arm to swell up. Ambrose didn’t take any time off to heal the injury, causing it to get even worse.
This probably didn’t help things.

Seth Rollins and Dean Ambrose cause carnage with elbow drops through tables: WWE TLC 2017
Opinion: I can’t say I’m surprised. A Tables, Ladders and Chairs match is one of the most dangerous matches you’re going to see in wrestling and it’s no surprise that someone is going to get hurt. The fact that Ambrose seemingly refuses to take time off didn’t help things either. Hopefully he’s back sooner rather than later as this could cost him “Wrestlemania XXXIV”.
Are you surprised Ambrose got hurt? If he’s back, what do you want to see him do at “Wrestlemania XXXIV”? Let us know in the comments below.