Knee Stiffness: Why Your Knee Won’t Straighten or Bend

When a patient tells me their knee is stiff, it usually means the knee has lost some ability to straighten, to bend, or both. In arthritis, the biggest reason is usually swelling inside the knee, along with bone-on-bone wear that physically gets in the way.

What knee stiffness usually means

Most people with a stiff knee have lost motion in one or both directions:

• Loss of straightening. The knee won’t go fully straight. This is called a flexion contracture, and most people with knee arthritis develop some degree of it. A knee that won’t straighten makes walking harder.
• Loss of bending. The knee won’t bend as far as it used to, which makes stairs and getting up from a chair harder.

Patients usually notice it as stiffness in the morning, tightness by the evening, or trouble going up and down stairs.

Why arthritic knees get stiff

In my patients, stiffness usually comes from one or both of two things:

• Swelling. Arthritis makes the knee produce extra fluid, called an effusion. Sometimes there’s enough to form a Baker’s cyst behind the knee. A knee full of fluid can’t bend or straighten all the way. I think swelling is probably what causes stiffness the most.
• Bone-on-bone wear. When the knee is bone-on-bone, the worn surfaces and bone spurs can block the knee mechanically, so it physically can’t move through its full range.

It’s often hard to say which one is doing it, and in many knees it’s both.

Other causes of a stiff knee

Many things can make a knee stiff besides arthritis:

• Meniscus tears
• Ligament injuries
• Muscle injuries
• Knee instability
• Scar tissue inside the knee, called arthrofibrosis, usually after an injury or surgery

On exam, I can usually tell if the knee’s alignment has changed and whether it has lost bending or straightening. When it has, that’s usually arthritis, but we always confirm it with X-rays.

The pillow under your knee

A lot of people with a stiff, sore knee sleep with a pillow under it. It feels better, because it holds the knee bent at about 15 to 30 degrees.

The problem is doing it night after night. Sleeping with the knee bent on a pillow can add to the stiffness and make it harder to get the knee fully straight. I explain more about this in knee pain at night.

Keep it moving

The most important thing you can do for a stiff knee is keep it moving: stretching, physical therapy, and staying active.

This matters even if surgery is in your future. Studies show that how well your knee moves before surgery, and how strong it is, help determine how well it moves after. Patients who work with therapy keep more motion, and patients with more motion and better strength do better. That’s why I want patients working with therapy before surgery, not just after.

Treatment, step by step

More on each option: physical therapy and nonsurgical care, knee braces, cortisone, gel, and PRP.

When to come in

If your knee is losing motion, or the stiffness is changing how you live, come in. We’ll examine the knee, get X-rays, and find out what’s making it stiff.

Surgery is based on quality of life. When a patient has severe, bone-on-bone arthritis, it has been going on for a while, and it’s seriously affecting their quality of life, that’s when we talk about a knee replacement. See the signs you need a knee replacement.