Did you remember to drop the feed dogs? Quilt too tight will also cause the need to bind & break. I have also had my darning foot wiggle loose and the needle would hit it, causing it to break. Normally the thread will rub on the foot and break before the needle hits it but sometimes if you are going too fast, the needle will hit the foot all of a sudden. My Elna 7200--very similar to your Janome--has a hole through the darning foot now because of that. Also, I never quilted with anything less than a size 14 needle and normally used an 18 to get through the seams without breaking a needle.