onebear:it turns out he broke his tibia bone in his back leg.
Here is a description of that injury and its treatment: http://www.petplace.com/dogs/tibial-tuberosity-avulsion-fractures-in-dogs/page1.aspx
"Tibial Tuberosity Avulsion Fractures in Dogs
An avulsion fracture occurs when a bone breaks and a fragment of the bone is separated by the pull of an attaching muscle, tendon or ligament. The tibial tuberosity is the prominent bump on the front and top of the tibia, the shin-bone, below the knee joint. This tuberosity attaches the patella (knee-cap) via a strong thick tendon of the quadriceps muscle group. A fracture of the tibial tuberosity often results in an avulsion fracture, by virtue of the pull of the quadripceps muscles.
This fracture tends to occur in younger dogs, before this area of the tibia has fully grown and fused to the rest of the bone. The fracture occurs in puppies in association with trauma, often falling from a height and landing with the knee in flexion, tearing the bone fragment from its normal position."
Hope the surgery goes well!!