Neoplasia

	Because ÒBonnieÓ is an older large breed dog, we cannot rule out the possibility 
of a tumor in the stifle region/distal femur.  Osteosarcoma, synovial cell sarcoma, 
chrondrosarcoma and metastatic neoplasia should all be considered.  ÒBonnieÓ has a 
history of being very active.  Her owner takes her on walks in the mornings and evenings 
and they jog a couple of times a week.  ÒBonnieÕsÓ active lifestyle causes a chronic low- 
grade bone and joint trauma that can lead to osteosarcoma and other primary neoplasias.  

	.  Neoplasia can have a rapid course of disease explaining the quick onset of 
ÒBonnieÕsÓ clinical signs.  ÒBonnieÓ has a partial weight-bearing lameness of the left hind 
leg, which could be due to the pain caused from neoplastic infiltration of the bone and 
periosteal elevation in the case of a bone tumor. Distension of the fibrous joint capsule in 
the case of a tumor involving joint components will also elicit pain.  The pain is also 
manifested by ÒBonnieÓ barely toe touching when she stands still.  The lameness is 
localized to the stifle region supporting a primary bone tumor.  Most tumors adjacent to 
the stifle joint will eventually involve both the bone and the joint. As the tumor grows, it 
will invade and destroy the surrounding bone, tendons, ligaments and capsule of the stifle 
joint causing incongruity, weakening of the supporting structures and inflammation.  The 
released inflammatory mediators will increase vascular permeability causing joint 
effusion and additional pain.  This entire process can lead to joint laxity and a weakening 
support of the left hind leg.  The neoplastic degeneration of the stifle ligaments may have 
predisposed ÒBonnieÓ to tear the cranial cruciate ligament during exercise explaining the 
positive cranial drawer sign.  Another possibility could be that the aggressive osteolytic 
nature of a bone tumor resulted in a pathologic fracture of the distal femur.  The fracture 
causes malalignment of the distal femur and instability of the limb.  This may also have 
predisposed ÒBonnieÓ to further damage the stifle joint and ligaments the next time she 
exercised.  ÒBonnieÓ is overweight which puts extra pressure on her bones and joints 
increasing her chances of injury to the compromised stifle region.