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Q: I have osteoarthritis in both knees. The left knee is especially painful and stiff. When should a person consider getting knee injections?

A: People can often manage the pain and improve mobility with lifestyle changes, such as weight loss, physical therapy, and exercises focusing on joint movement and strengthening leg muscles. Taking occasional over-the-counter or prescription pain relievers or anti-inflammatory medicines also can help. These can be taken in pill form or topically (applied to the skin).

But if these strategies don’t offer sufficient relief, or if symptoms impede your ability to move freely, an injection can provide immediate pain relief, reduce inflammation, and increase mobility.

But not everyone improves, and even if they are helpful, injections are a temporary treatment that won’t cure your knee osteoarthritis or change the course of the condition.

If you must rely on repeated injections for pain management and help with mobility, then it’s probably time to consider a knee replacement.

There are two main types of injections for knee osteoarthritis: corticosteroids and hyaluronic acid.

Corticosteroid injections (also known as steroid shots) contain a manufactured drug that resembles cortisol, a hormone the adrenal glands produce. They reduce inflammation, pain, and swelling in and around the knee joint. If the knee is swollen, your doctor may first remove excess fluid with a needle.

Joints are surrounded by synovial fluid, which helps cushion and lubricate the moving parts of the knee. However, when cartilage is damaged from osteoarthritis, the body responds by overproducing synovial fluid, which leads to swelling. Removing fluid relieves pressure and swelling around the knee joint and makes the steroid injection more effective.

Once any excess fluid is removed, the corticosteroid (usually mixed with a numbing agent) is injected into the knee joint. Relief from the numbing agent kicks in right away, while the corticosteroid takes about 24 hours to start working on your symptoms.

The corticosteroid injection’s effect can last from several weeks to months. Repeat injections can be given every three to four months. But over time, they may work less well and for shorter periods.

A hyaluronic acid injection is usually recommended if corticosteroid injections don’t work or the person needs to avoid injected corticosteroids.

Hyaluronic acid is a natural component of the synovial fluid in the joints. As osteoarthritis advances, hyaluronic acid decreases, which makes synovial fluid thinner and less effective. This can increase pain and stiffness, as there’s less lubrication to help the knee joint move freely. The synthetic hyaluronic acid acts like oil on a rusty joint.

If the hyaluronic acid injection provides significant benefit, a follow-up injection may be given after six months.

Q: I develop left leg pain when I try to walk faster. If I stop, the pain quickly improves. I can start walking again, but at a very slow pace. What’s the usual cause?

A: The three most common reasons are peripheral artery disease, sciatica and lumbar spinal stenosis.

Peripheral artery disease (PAD) occurs when the arteries that carry blood to the leg muscles narrow, most often because of a buildup of fatty plaque.

The first PAD symptom typically is pain in one or both legs when walking that goes away when you slow down or stop. When you walk, your leg muscles need more blood flow to deliver oxygen to the muscles. Narrowed leg arteries won’t allow the blood flow to increase, so the muscles hurt from a lack of oxygen.

Your doctor will feel the pulses in both legs and feet to identify any reduction in blood flow. The next assessment is often a comparison of the blood pressures and blood flow in your legs versus your arms to calculate an ankle-brachial index.

Treatment starts with adopting a heart-healthy diet, not smoking, and continuing to walk. Over time, you should find that walking becomes easier and you have fewer rest periods. Eventually, you want to build up to walking for at least 30 minutes at a time, five times a week.

Sciatica is the term for pain that happens because of pinching or irritation anywhere along the route of the sciatic nerve, which runs from the spine through the buttock and down the leg. While increased pain with walking may be one person’s predominant symptom, usually there are other symptoms, such as low back pain and difficulty sitting.

Your doctor can often diagnose sciatica just based on your history and a physical exam. If the symptoms are severe or unusual, your doctor might order an imaging test, most often an MRI.

Many people fear that activity will cause or worsen sciatica pain, but low-impact activity and exercise can help strengthen the areas along the nerve and prevent future attacks or at least lower their intensity and frequency.

In lumbar spinal stenosis, the space inside the lowest part of the spinal canal narrows, usually from degeneration of discs, ligaments or any of the joints between the interlocking vertebrae that form the spine. This can put pressure on the nerve roots as they exit the spinal cord.

Pain that occurs only when walking may be the only symptom of lumbar spinal stenosis. Many people discover that immediately squatting or sitting helps lessen the pain. Other symptoms include pain in the lower back, groin, buttocks or upper thigh.

Again, it’s best to stay physically active with a walking program or riding a stationary bike.

Howard LeWine, M.D., is an internist at Brigham and Women’s Hospital in Boston and assistant professor at Harvard Medical School. For additional consumer health information, see health.harvard.edu.

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