Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Stanley is carrying his rear leg and refusing to bear weight on it today after previously favoring it.
Because there is no history of trauma a fracture is unlikely, though possible if he is very fine boned and jumped off a high piece of furniture.
However in a small breed dog with rear leg lameness the two most common reasons are a luxating patella (knee cap that pops in and out of place, or stays out of place) or a condition called Legg-Calve-Perthes disease which is a congenital problem with the growth and maturation of the hip joint.
With a patellar luxation the kneecap slides out place rather then staying in the patellar groove. When that occurs the leg cannot bend or flex as it should and the dog becomes lame.
This condition is usually congenital, meaning the puppy is born with the genetics to develop the condition as they grow. It doesn't mean that the pup is affected as soon as they are born however, and in most cases it is a condition that we see develop as the bones, ligaments and tendons that form the knee joint grow and develop or in an older pup that gets arthritic. Not all pups in a litter are necessarily affected.
If your fellow has this your veterinarian can examine him, take some radiographs and tell you whether surgery is needed now or he could benefit from joint supplements such as glucosamine/chondroitins and omega 3 fatty acids or a supplement called Duralactin as well as pain medication. I do think that Stanley is painful because he is carrying his leg.
If you'd like to read more about pateller luxations here is a link to an excellent article written by the American College of Veterinary Surgeons: http://www.acvs.org/AnimalOwners/HealthConditions/SmallAnimalTopics/MedialPatellarLuxations/
In mild cases some dogs learn to kick their rear leg and pop the knee cap back into place. Over time however because of the constant wear and rubbing of the knee cap moving in and out of place we see secondary arthritis forming. Whether he needs surgery or not now will depend upon how severely he is affected. In most cases most dogs do need surgery at some point.
Legg-Calve-Perthes disease happens when the blood supply to the femoral head is interrupted, resulting in the bone dying. When the blood supply is re-established the femoral head is rebuilt and remodeled, but the head often collapses and reforms in an abnormal fashion creating an irregular fit in the socket part of the joint, the acetabulum. This process of bone cells dying and collapsing followed by new bone growth and remodeling of the femoral head and neck, can lead to stiffness and pain.
This is an inherited disease so dogs that are diagnosed should not be used for breeding.
Some dogs will tolerate the arthritic pain and not limp but suddenly become lame if a piece of the arthritic joint breaks off and floats in the joint, causing acute pain.
Treatment will depend upon how severely he is affected by his disease.
It may be a simple as medications to control pain and lessen arthritis formation or he may need surgery to keep him comfortable.
I would start with medication to see if we can get him relatively comfortable now.
Long term for any joint pain I recommend using a combination of a glucosamine/chondroitin product (examples are Dasuquin or Cosequin) and an omega 3 fatty acid (like 3V Caps or Derm Caps). I recommend an omega 3 fatty acid dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give him 20mg of EPA per pound of body weight per day. For example a 15 pound dog could take 300mg of EPA per day.
Omega 3's and glucosamine/chondroitins work synergistically, better together than either one alone, and improve cartilage health and joint fluid quality and quantity as well as reducing inflammation. They can take several weeks to see full improvement but some dogs do very well with them alone. They are available over the counter.
Another option is a product called Duralactin. This is an anti-inflammatory product derived from milk proteins and it also has omega 3 fatty acids incorporated into it which can be very helpful. See this link for further information: http://www.duralactin.com/products_canine.htm
If those medications are not enough his veterinarian can prescribe drugs that are more potent. Veterinary drugs we can add include a nonsteroidal like Metacam, Deramaxx, Previcox or Rimadyl. If those aren't enough we can add another drug in the opiod family called Tramadol and/or another drug called Gabapentin.
If he has Legg-Calve-Perthes disease and his discomfort is not controlled with pain medications then another option is a femoral head and neck ostectomy (FHO). This surgery removes the painful femoral head and neck and secondary arthritic changes and allow a more comfortable false joint to form.
Another option is a total hip replacement, a complete replacement of the hip joint. These are rarely done in small breed dogs as this is major surgery and they seem to do just as well with the FHO surgery.
Finally because this worsened suddenly another possibility is a complete tear of his anterior cruciate ligament in his knee. This ligament crosses the knee and stabilizes the joint. When the ligament tears there is no stability to the knee joint and the dog suddenly becomes leg carrying lame. It can happen suddenly with the smallest slip, especially one that causes a twisting motion to the knee.
Ideally surgery would be done to give his knee normal stability because without surgery secondary arthritis formation will occur sooner and to a more severe degree then with surgery. It is also more likely that he will rupture the cruciate ligament in his other knee because he will be putting more stress and strain on the other leg.
Your veterinarian will likely examine him, paying close attention to the ability to manipulate the joint and check "drawer" or forward laxity of the knee. We will often sedate our patients if they are very nervous or painful to check the knee and take radiographs of the knee joint to look for a characteristic change in the location of the fat pad in the knee joint.
If surgery is absolutely not in your budget you can try strict rest. Given his small size that may be a reasonable plan depending on how painful he is and whether we are able to get him more comfortable with medications and rest. With very strict rest the knee will form scar tissue and gain some stability with time but it won't ever be normal and it will be arthritic. When I say strict rest I mean cage rest, no running, jumping, climbing stairs or playing for at least 6 to 8 weeks. He needs to go outside to eliminate on a leash so he is not overly active.
Surgery or not I also recommend keeping him on the thin side, or weight loss if he is overweight to decrease stress on his knees.
Long term for cruciate injuries I recommend using a combination of a glucosamine/chondroitin product (examples are Dasuquin or Cosequin) and an omega 3 fatty acid (like 3V Caps or Derm Caps). As with the other conditions we are trying to slow arthritis formation.
Best of luck with your pup, please let me know if you have any further questions.