Hypothesis #1:  Cranial cruciate ligament injury due to trauma

A positive cranial drawer sign is diagnostic of cranial cruciate ligament injury.  Acute trauma that puts excess strain on the ligament can cause the cranial cruciate 
ligament to rupture partially or totally.  An example of trauma that could cause this 
is hyperextension of the tibia, resulting from stepping in a hole such that the tibia 
stays immobile and the rest of the body continues forward.  Another example of 
causative trauma would be a rotation of the tibia internally while the rest of the 
body and femur rotate externally.  

A partial or total rupture of the cranial cruciate ligament initially causes intra-
articular hemorrhage and cell injury.  This causes the synoviocites  to release 
inflammatory mediators such as cytokines and prostaglandins.  This is thought to be 
in response to phagocytosis of collagen and proteoglycan fragments by synoviocytes. 
The synovial lining layer also undergoes hypertrophy and hyperplasia. The synovial 
vasculature becomes more permeable in response to injury and/or inflammation, 
resulting in increased fluid production. The distension caused by this effusion and 
hemorrhage will eventually limit joint laxity and stop the hemorrhage.  The 
distension will also create pressure on the joint capsule, resulting in nociceptor 
stimulation and subsequent pain.  The sensitivity of the nociceptors is increased by 
inflammatory mediators such as prostaglandins. The ruptured portion  of the 
ligament will retract and fan out, resulting in a mop-end appearance.  

Over time, the condition can lead to secondary degenerative joint disease.  This is 
characterized by osteophyte development, which are commonly seen on the distal 
border of the patella, the trochlear groove, periarticular medial and lateral margins 
of the distal femur and the proximal tibia, and on the medial tibial plateau at the 
site of attachment of the cranial cruciate ligament.