The Ruptured Cruciate Ligament

This is by far the most common orthopedic condition in animals and is due to degenerative reasons not traumatic reasons as with people, i.e. in dogs the ligament rupture happens slowly with a gradual joint fluid effusion.

The cruciate ligament has its own nerve supply so as it becomes inflamed it causes the bouts of pain that we see in most cases. Eventually when chasing a ball, on a run or during some mild exercise event, it ruptures fully. A genuine traumatic rupture can happen when a dog falls off a utility or motorbike at speed, or similar large forces. 

These articles from a practice in the USA deal with the issue clearly, explains the forces involved and explains the differences and reasons for using the two common repair techniques
 
http://vetsurgerycentral.com/tplo.htm
 
http://vetsurgerycentral.com/cruciatelrt.htm