Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear that Caddie is lame on her left front leg. I understand your frustration because several radiographs of multiple joints have not been diagnostic.
That does mean that a fracture or bone abnormalities aren't likely to blame, but that can leave a whole list of possible soft causes.
The quickest way to fix something is to know what the primary problem is and address that. I don't believe we have that information yet with your girl.
It is certainly possible that she stretched or strained the ligaments in one of the joints in her leg jumping down off furniture, much like a sprained ankle in you and I. But I would expect that you would see some sort of swelling in that joint, especially if this has been going on as long as it has. Uncomplicated soft tissue injuries usually improve with 48 hours of strict rest and if you have been forcing her to rest and used anti-inflammatories she should be better. More severe injuries, with joint swelling can take longer, sometimes several weeks of strict rest and in those cases I do recommend using prescription anti-inflammatories for pain control and to reduce swelling. That sounds like it has been tried and it didn't work.
I would check her leg carefully, comparing it to her right front leg to see if you notice any difference in joint size or swelling.
If they appear to be similar, with no signs of joint swelling then I would be concerned about intervertebral disc disease in her neck affecting the nerve roots that innervate her front left leg.
Intervertebral discs are the spongy cushions between the individual vertebrae in their back and neck. These spongy discs can move or rupture and place pressure upon the spinal cord or nerve roots leaving the spinal cord which can lead to pain, and in severe cases paralysis.
Radiographs can sometimes be diagnostic but often early on in the disease process, because the discs are soft tissue not bone, everything will look normal. An MRI is the best way of diagnosing disc disease.
If the dog is painful but has no evidence of paralysis we can try strict rest, anti-inflammatories and pain medications for several weeks to allow healing.
If there is evidence or weakness or paralysis, or rest does not help control the pain and allow healing, then surgery by a board certified veterinary neurologist, as soon as possible, is indicated.
Ideally if your pup is still lame she needs to see her veterinarian for a recheck and probably get referred to an orthopedic specialist or neurologist veterinarian.
In the meantime your veterinarian can prescribe a steroid or nonsteroidal anti-inflammatory to relieve pressure on her spinal cord and nerve roots, as well as something for pain too, such as Tramadol. And if she is having painful muscle spasms then a muscle relaxant such as methocarbamol as well.
She should continue to be closely confined. No stairs, running or jumping. If you have a crate for her I highly recommend using it. The less she moves around the more comfortable she will be and the faster she will heal.
She should go out on a leash to relieve herself. Do not use a collar for her, a harness which more evenly distributes forces if she pulls on her leash is better.
You will need to confine her for several weeks, even as she starts to feel better or she may reinjure herself.
Keeping her on the thin side is recommended to decrease stress on her neck and back if this is disc disease, but is no guarantee that she won't have another episode. Once a dog has one bad disc the likelihood of another is very high.
If you are interested in reading more here is a link to an excellent article about intervertebral disc disease, its causes and therapy: http://www.petwave.com/Dogs/Dog-Health-Center/Bone-Joint-Muscle-Disorders/Intervertebral-Disk-Disease/Symptoms.aspx
There are other less common causes of back or neck pain such as infections, tumors of the vertebrae or the spinal cord itself or fibrocartilagenous emboli that block blood supply to the nerve roots, but far and away disc disease is the most common cause of neck or back pain and nerve root inflammation in dogs.
Given that she has blisters on her affected foot the other possible concern is a puncture wound with a foreign body lodged in her paw making it painful to bear weight. That's why I would have your veterinarian take a look before referring her. If the foreign material isn't metal it may not show up on a radiograph. Sometimes we only find them by surgically exploring the affected (blistered) area.
Please let me know if you have any further questions.