For this sound I used a Neumann 81i shotgun and ran it into an Avalon M5 preamp and through my RME Fireface 800 to my Mac. I started with breaking celery sticks and realized I needed a little more umph so I went outside and found a few stay twigs on the ground. Using a combination of celery and sticks, the right amount of EQ and compression, I came up with the sound. Sounds like a bone breaking to me!
Here's the recording: Bone Break