Diagnosis

Traumatic Cranial Cruciate Ligament Rupture

	The cranial cruciate ligament is one of a pair of crossed 
ligaments which stretch between the femur and the tibia in the stifle 
joint.  The function of this ligament is to restrict cranial movement 
and internal rotation of the tibia in relation to the femur.  In an 
acute traumatic rupture, there are abnormal stresses placed on the 
joint which cause catastrophic loading of the ligament.  This often 
happens during athletic activity when the animal takes a misstep or 
steps in a hole and hyperextends the stifle joint.  Other possible 
etiologies involve forcible rotation of the tibia internally with the 
stifle joint flexed 20-50 degrees or rotation of the animal's body 
(femur) externally, which cause twisting and rupture of the cranial 
cruciate ligament.  In this case, Bonnie was probably chasing a 
squirrel in the backyard and either stepped in a hole or took a misstep 
and ruptured her cranial cruciate ligament.
	The diagnosis of a cranial cruciate ligament rupture is based on 
a test called the cranial drawer sign.  A cranial drawer sign (cranial 
movement of the tibia in relation to the femur) of greater than 3-5 mm 
is indicative of cranial cruciate ligament rupture.   A cranial drawer 
sign of less than 2 mm is normal.  Bonnie's cranial drawer sign in the 
left stifle was 4-5 mm, which is pathognomonic for a CCL rupture.
	The only significant changes seen on Bonnie's radiographs are 
stifle joint effusion and swelling.  There is no evidence of previous 
degenerative joint disease or other abnormalities.  If degenerative 
changes had been seen, that would indicate that the joint and possibly 
the ligament were already compromised, and the current cause of 
lameness may be something other than the ruptured ligament.
	When the cruciate ligament is ruptured, pain, intra-articular 
hemorrhage, and joint effusion may result.  The hemorrhage will cease 
with the subsequent joint distention, and joint instability will also 
be limited by the distention.  The ruptured ligament retracts and 
becomes frayed at the end.  The instability and abnormal joint function 
that result from the ligament rupture can cause meniscal injury, joint 
effusion, and pericapsular fibrosis.  Eventually, these changes in the 
joint result in osteophyte development, chronic instability, and 
degenerative joint disease. 
	The results of the joint tap are consistent with traumatic 
rupture of a ligament.  The mucin clot is good, and no etiologic agents 
are seen, which decrease the likelihood that the cause of the problem 
is infectious or degenerative.  The reddish-clear fluid indicates a 
small amount of hemorrhage mixed with normal synovial fluid.  In an 
acute rupture, a little bit of hemorrhage in an otherwise normal joint 
is expected.  The cell count is elevated, and consists mainly of 
neutrophils and monocytes.  These cells are the body's way of cleaning 
up the debris and red blood cells that are present within the joint.  
This, too, is a normal finding for a joint that has suffered trauma.
	 In conclusion, the facts of Bonnie's case support a diagnosis 
of a traumatic cranial cruciate ligament rupture.