Fact List
Labrador- Active, large breed dogs.  Large breed dogs are predisposed to certain diseases.  Their physics causes increased stress on joints.

6years old-This helps rule out congenital defects as the cause of lameness.  Also this is a large breed dog so 6 is an age that can be concurrent with neoplasia as well as joint/ligament damage.

active dog/chases dogs and cats when allowed to do so, jogs with owner, plays fetch. Increases likelihood of traumatic injury.  

partial weight bearing lameness-This helps identify possible causes of lameness and eliminates severe traumatic fractures.

effusion and distension of left stifle joint capsule-This information localizes at least a partial cause of the lameness to the stifle and indicates inflammation.

pain on manipulation of stifle-This further localizes the lameness to the stifle.

overweight-Excess weight causes increased concussive forces on joints exacerbating any abnormalities of the bone or soft tissue as well as predisposing the animal to ligament rupture.

cranial drawer under sedation-This finding is indicative of cranial cruciate rupture.

started last Wednesday-Fairly acute onset helps rule out chronic progressive diseases.

Up to date on vaccinations and on heartworm prevention/vets noted that she was in "excellent health" on previous check ups-This would help to confirm a more acute onset of her current problems.