Answer: Where the bone broke.There are three primary types of bone: woven bone, cortical bone, and cancellous bone. Woven bone is found during embryonic development, during fracture healing (callus formation), and in some pathological states such as hyperparathyroidism and Paget disease. It is composed of randomly arranged collagen bundles and irregularly shaped vascular spaces lined with osteoblasts. Woven bone is normally remodeled and replaced with cortical or cancellous bone.
Answer: the tibia is commonly referred to as the shin bone- if fractured at the distal end, it would involve the ankle- immobilization w/ a splint (casting) is the usual treatment. A tibial fracture at the proximal end -near the knee is more complex and may require surgery. All fractures of bones require immobilization of the affected portion to heal initially.
Answer: Yes, if you are not under immediate threat, then you are in the legal wrong. If someone is making a verbal threat, you would be expected to seek aid, and possibly seek a restraining order. You need to speak to law enforcement are an attorney to find out more of what your rights are.
Added: As satisfying as it may seem (or as ievitable as it may become) you are not allowed to use more offensive action against them than sufficient to make them stop the attack. If this is less-than-lethal, then that is what is necessary.