My little pug Maggie was a rear amp, her mast cell tumor was in her knee.  Her surgeon amputated high up on femur. His explanation was that it was a less complicated surgery than dis-articulating the hip joint.  He padded the remaining femur bone with muscle.  Looking at her you could not tell there was any femur left, you could feel it though.  She usually sat on her amp side, and often laid on it too.  She often used a sit maneuver to change directions. All of that to say that she never had any problems with the way the surgery was done in her almost 4 years as a tri-pug.

It is maybe a preference by the surgeon?- assuming the tumor is at the knee or lower.

You could always post this question in the 'Ask A Vet' forum and get a perspective from our resident vet Dr. Pam (posts as tazziedog).

Karen and the pugapalooza