Group 11 - Hypothesis 1 - Trauma

	Several points in Bonnie's history and physical exam support the hypothesis of acute trauma causing rupture of the cranial cruciate ligament.  Firstly, she is overweight.  This would provide more force to a rotation of the tibia or hyperextension of the joint.  Also, there is a higher incidence of cruciate rupture in underconditioned dogs that exercise sporadically.  This could be due to weakness of the musculature supporting the joint.  The owner commented that the dog is very active.  She likes to play fetch in the backyard and chase other dogs and cats on occasion.  Also, the owner takes the dog to the woods many weekends where the dog may be allowed to run and play.  The cranial cruciate ligament could have been injured during any of these activities. 

	The cranial cruciate ligament is attached to a fossa on the caudal aspect of the medial side of the lateral femoral condyle.  It courses medially and distally across the intercondylar fossa and attaches to the cranial intercondyloid area of the tibia.  The cruciate ligaments function as constraints on the stifle joint.  Specifically, the cranial cruciate acts to prevent cranial displacement of the tibia from the femur, limit extensive internal rotation of the tibia on the femur, and to prevent hyperextension of the joint.  The mechanisms of injury to the cranial cruciate ligament can be related directly to its normal functions.  
	
The most common traumatic injury to the cranial cruciate ligament occurs due to sudden rotation of the stifle with the joint in 20 - 50 degree flexion.   The cruciate ligaments begin to twist on each other and themselves to limit the rotation of the tibia on the femur.   With excessive internal rotation of the tibia the cranial cruciate ligament becomes wound very tightly and is subject to injury from the lateral femoral condyle as it rotates against it.  This may cause the ligament to rupture in its mid portion or to avulse a portion of its bony attachment.  The latter is more common in younger animals.  A dog can cause rotational injury to the cranial cruciate if it is running and quickly changes direction while the paw is planted.  Another mechanism of injury to the cranial cruciate ligament is hyperextension.  The cranial cruciate ligament is the primary check ligament against hyperextension of the stifle joint.  Therefore, if the joint is hyperextended the cranial cruciate is the first structure subject to injury. Hyperextension of the stifle can occur when a dog is running and abruptly fixes the hind limb but the body continues to move forward, an example being stepping in a hole while running.

	Rupture of the cranial cruciate due to acute trauma will cause joint instability.  Rupture typically presents with acute hind-leg lameness.  Immediately after the trauma there will be pain with intra-articular hemorrhage and effusion.  Joint distention will ultimately stop the hemorrhage.  The ruptured ligament will retract and fan out.  The joint will be unstable without the support of the cranial cruciate.  Joint effusion may be noted for several days after the injury but subsides in chronic cases.  The presence of joint effusion and acute onset of lameness in this case indicates trauma that has occurred recently.  If the rupture is not treated the lameness from the initial trauma usually improves in 3-6 weeks, but will recur with exercise.  Instability causes abnormal forces in the joint, eventually leading to degenerative changes of the bony joint.  This will cause pronounced lameness due to destruction of cartilage and subchondral bone.  Since the owner stated that there has been no previous lameness, this is probably not a chronic case.	

	Trauma may also cause partial tearing of the cranial cruciate ligament rather than a complete rupture.  Partial tears present with mild lameness and joint effusion.   Cranial drawer is typically either absent or slight.  A partial tear will not necessarily change the prognosis or treatment since it has been shown that most will eventually progress to complete rupture.  
	
	Cranial cruciate rupture is typically diagnosed by demonstrating cranial displacement of the tibia with respect to the femur.  This is best done when the joint is in slight flexion, which is the "functional position" of the limb.  A positive cranial drawer sign, subluxation of the tibia cranial to the femur, indicates injury to the ligament.  This test is first done without sedation.  In large breed dogs muscle guarding can make eliciting the cranial drawer sign difficult.  In these instances the test should then be attempted under sedation.  In this case, a positive cranial drawer of 4-5mm was elicited in the left stifle under sedation.   There was also a positive cranial drawer of less than 2mm in the right stifle.  This is not necessarily pathologic.  There is individual variation in joint laxity, but less than 2mm movement is considered normal for the stifle joint.  Rupture of the cranial cruciate ligament is best diagnosed with the cranial drawer test, but another method to test insufficiency is the tibial compression test.  The metatarsus is grasped with one hand and the palm is placed over the cranial aspect of the distal femur and patella, extending the forefinger of the tibial tuberosity.  When the hock is flexed, the gastrocnemius muscle will tighten and compress the tibia and femur.  With cranial cruciate insufficiency, the tibial tuberosity will be felt sliding cranially.    

	Cranial cruciate ligament rupture is supported by history and physical exam.  A positive cranial drawer sign of 4-5mm in the left stifle indicates insufficiency of the ligament.  The history of physical activity and obesity support the potential etiology of trauma.