Inside each of your dog's movable joints, from the hips and knees to the elbows and wrists, sits a small amount of thick, clear liquid called synovial fluid. It lets the cartilage-covered ends of the bones glide past each other with little friction, and it helps keep that cartilage healthy, since cartilage has no blood supply of its own.
When a joint wears down, the fluid changes with it, which is why vets sometimes sample it to find out what's going on. There's no way to measure or "top up" joint fluid at home, but gentle, regular movement and a lean body weight support the joints that rely on it.
We make a daily joint chew, Everhound Hip & Joint, and a common question is whether a supplement can add joint fluid. We answer that honestly further down, after the anatomy.
Where synovial fluid sits in a dog's joint
Hips, knees (the stifle), elbows, and wrists (the carpus) are all synovial joints. In each one, as the veterinary education wiki WikiVet describes it, a sheet of tissue called the synovial membrane encloses the joint cavity, a fibrous capsule and ligaments reinforce it, and smooth articular cartilage covers the ends of the bones.
| Part of the joint | What it does |
|---|---|
| Joint capsule | A tough fibrous outer layer that encloses and steadies the joint |
| Synovial membrane | The inner lining of the joint, which produces synovial fluid |
| Synovial fluid | Lubricates the joint and nourishes the cartilage |
| Articular cartilage | A smooth covering on the bone ends, with no blood vessels or nerves |
| Subchondral bone | The layer of bone just beneath the cartilage |
That detail about cartilage matters. With no blood vessels of its own, cartilage gets its nutrients by diffusion from the synovial fluid and nearby vessels, WikiVet notes. The canine rehabilitation clinic Paws4Paws describes synovial fluid as a viscous, egg-white-like lubricant that also nourishes the cartilage and keeps it hydrated.
What synovial fluid does for your dog's joints
- Lubrication. The fluid cuts friction inside the joint, so the bone ends glide instead of grinding. Mar Vista Animal Medical Center describes it as providing both lubrication and nutrition to the joint.
- Cartilage upkeep. VCA Animal Hospitals notes that joint fluid helps maintain cartilage health.
- Cushioning. Healthy fluid and cartilage together act as the joint's shock absorber, which is the capacity that shrinks when joints wear.
Two molecules do much of the lubricating. WikiVet notes that synovial fluid is usually very thick because of its high hyaluronic acid content, and Paws4Paws adds that the joint lining produces both hyaluronic acid and lubricin. A 2020 study in Scientific Reports describes lubricin as an important boundary lubricant that also helps protect cartilage.
Movement keeps the system working. Paws4Paws explains that moving a joint acts like a pump for synovial fluid, and that when a stiff, sore joint moves less, the pumping action weakens. It's one reason vets favor regular, gentle exercise for dogs with joint changes.
What happens to joint fluid as joints wear
In osteoarthritis, the fluid and the cartilage change together. Cornell's Riney Canine Health Center describes a process in which joint fluid decreases and cartilage thins, reducing the joint's ability to absorb shock, with bony changes developing around the joint as the cartilage keeps breaking down. The Merck Veterinary Manual adds that the fluid itself changes: it lists extra fluid in the joint (effusion), a thickened synovial membrane, and less viscous synovial fluid among the hallmarks of osteoarthritis.
It's common. VCA reports that osteoarthritis affects at least 20% of dogs over one year old and 80% of dogs over eight. VCA also stresses that simple aging is not a cause: body shape, body condition, joint development, activity, past injuries, surgery, and nutrition all play a part.
Injuries change the fluid, too. In the 2020 Scientific Reports study, joint fluid from 30 dogs with a ruptured cranial cruciate ligament had nearly 16 times more lubricin than fluid from 9 healthy dogs, and lubricin levels were linked to signs of osteoarthritis on X-rays. In other words, the fluid's makeup shifts after a joint is damaged.
Cornell lists the signs owners tend to notice: lameness or a change in gait, difficulty getting up from lying down, muscle loss, and reluctance to walk, run, or exercise. If you're seeing these, book a vet visit. Our article on arthritis in dogs covers what happens next.
When vets test synovial fluid
Your vet may suggest a joint fluid analysis, sometimes called a joint tap, if a joint is painful or swollen with extra fluid, or if a joint infection is possible. Here's how VCA describes it:
- Your dog may need sedation or anesthesia. The skin over the joint is shaved and scrubbed, and a fine, sterile needle draws a small amount of fluid from the joint space.
- The lab measures the fluid's protein and counts and identifies the cells in it.
- Normal joint fluid is thick, clear, and contains very few cells.
- More protein or more cells suggests inflammation. Many neutrophils, a type of white blood cell, point toward a joint infection or immune-mediated joint disease, while more mononuclear cells fit long-standing inflammation, as in degenerative joint disease.
A hot or swollen joint, a sudden limp, crying out, or refusing to put weight on a leg needs a veterinarian now, not a wait-and-see approach.
Can you increase synovial fluid in dogs?
Not in any way you can measure at home, and no chew, powder, or liquid can promise a set amount of new fluid. What you can do is support the joints that depend on it, with your vet setting the plan:
- Keep your dog moving, gently. Cornell suggests low-impact activity such as leash walks and swimming. Our guide to low-impact exercise for dogs has ideas you can start this week.
- Keep their weight lean. Cornell notes that obesity adds force to the joints and contributes significantly to osteoarthritis.
- Follow the vet's medical plan. For osteoarthritis, Cornell notes that vets commonly prescribe NSAIDs, and that dogs on them long term need routine blood work. Joint injections are veterinary procedures, too.
- Ask about supplements. Cornell lists glucosamine, chondroitin sulfate, and omega-3 fatty acids among the supplements used for joint support.
Where a daily joint chew fits
Everhound Hip & Joint doesn't contain hyaluronic acid, and we don't claim it adds synovial fluid. It's a daily chicken-flavor soft chew with glucosamine HCl 200 mg, chondroitin sulfate 65 mg, MSM 100 mg, and five more named actives per chew, and it supports joints, cartilage, and mobility.
The research on each active is on our science page, and it looks at the ingredients, not at this finished chew. For the cartilage side of the story, read glucosamine for dogs. If your vet gives the go-ahead, the serving goes by weight, from one to five chews a day with food, as laid out in our guide to giving joint chews with food.
FAQs
What color is normal synovial fluid in dogs?
VCA describes normal joint fluid as thick and clear, with very few cells. Changes in color, thickness, or cell count are lab findings for your vet to interpret after a sterile sample, not something to judge at home.
Is hyaluronic acid the same thing as synovial fluid?
No. Hyaluronic acid is one component of synovial fluid, the one that gives it its viscosity. The fluid also contains lubricin and other molecules. Supplements and injections that contain hyaluronic acid are separate products to discuss with your vet.
Can a dog run out of synovial fluid?
Not in the sense of a tank running empty. What changes in osteoarthritis is the amount and quality of the fluid: Cornell notes that joint fluid decreases as cartilage thins, while Merck lists less viscous fluid, and sometimes extra fluid in the joint, among the hallmarks. Your vet can assess those changes.
Why is my dog's joint swollen?
Swelling can mean extra fluid, inflammation, an injury, or an infection, and only a vet can tell which. VCA lists joint pain, or swelling from excess fluid, among the reasons vets analyze joint fluid. If the joint is also hot, or your dog won't use the leg, call your vet today.