Your dog is happily playing in the yard, jumps up to catch a ball, lands badly, and comes up not just lame but able to use a back leg normally.  The toes knuckle under.  Perhaps the dog even sort of tilts downward unable to rise up properly in the back end.  After checking the dog over, you find no indication of any injury.  There are many conditions that might fit, however, neuroligical knuckling and absence of a sore spot, suggests FCE

To understand FCE, you must also understand how the vertebral column works.  It consists of many small bones called “vertebrae” which are linked together by special joints called “intervertebral discs.”  The discs are similar to the joints that connect arm or leg bones together.  They allow flexibility between the vertebrae enabling the animal to arch or twist the back voluntarily.   The discs are unique because they are more like a cushion between the end plates of the vertebrae.  They are round thus the name “disc” and they are fibrous on the outside with a soft gelatinous material inside to absorb the forces exposed to the bones.  This soft gelatinous, jelly-like material is called the “nucleus pulposus” and it is this material that makes up the fibrocartilaginous embolus. 

The spinal cord is the cable of all nerve connections that transmit messages to and from the brain and also controls the reflexes of the body.  The spinal cord is fed by a network of arteries.  In FCE, the material from the nucleus pulposus enters the arterial system and is carried to the spinal cord where it becomes a blood vessel obstruction: an “embolism”.  It is this area of the spinal cord that dies.  The process is not painful but generally recovery is not very likely.  The neurological loss that has occurred within the 1^st 24 hours is likely to be permanent.

Most FCE dogs are between 3 and 6 years of age. There may initially be a yelp at the time of trauma but the injury itself is not painful usually.  There is a 50:50 chance that the lumbar area of the spinal cord will be affected which will affect the hind legs only.  The loss of function will not get worse as after the 1^st 24 hours, the maximum function loss has already occurred.  Maximum improvement has been seen by 2 weeks following injury or trauma.   Some dogs end up completely paralyzed.  Some are simply weak in the affected limbs and may or may not need assistance in getting around, depending on how severe the embolism was and where in the spinal cord it occurred.  

Acute neurological weakness after trauma could also be caused by Type 1 Disc Herniation or by spinal cord trauma.  In Type 1 Disc Herniation, a mineralized intervertebral disc “slips” upward pressing on the spinal cord.  The pressure may be relieved by medication or surgery may be needed.  The spot where the disc is pressing on the spinal cord is very painful.  X-rays or a myelogram will show any abnormalities. If there is an area of compression of the disc, it will be visible with a myelogram. 

In FCE, there is no compression.