Most people end up with a new knee that bends to somewhere around 120 degrees, and that goes fully STRAIGHT or within a few degrees of it. You need about 90 degrees of bend to sit in a chair and manage stairs comfortably, so that is the first line everyone aims for.
Some stiffness in the first weeks is normal. What matters is the DIRECTION. If your bend creeps up a little every week, you are on track, even if it feels slow.
If the bend stalls below about 90 degrees in the first two to three months, your surgeon may talk about a manipulation under anesthesia. Timing matters with this, so raise a stalled knee early. Your surgeon sets your targets. Use theirs.
How much bend do you actually need?
- Walking on the flat: about 60 degrees.
- Stairs: roughly 85 to 90 degrees, one step at a time at first.
- Getting up from a chair: a little more than 90 degrees. A higher chair or a cushion helps while you get there.
- Cycling: a full turn of the pedals needs more bend than stairs. Start with the seat high and rock back and forth until the pedal goes over.
- Picking something up from the floor: closer to 115 to 120 degrees.
- Kneeling, deep squats and sitting cross-legged: more than many new knees reach. Losing some of these is common and does not mean the operation failed.
Straightening matters just as much.
A knee stuck well short of straight makes every step harder work. See knee replacement recovery week by week for how to get it straight early.
What’s normal in the first three months
- Stiff first thing in the morning and after sitting. It loosens after a few minutes of moving. This can last for months.
- A tight, full feeling at the end of the bend. Swelling takes up space inside the joint and physically limits bend. As swelling goes down, bend usually comes up.
- Gains of a few degrees a week. Slow and steady is the normal pattern, not big jumps.
- Losing a few degrees after a busy day. You will often get them back within a day or two once swelling settles.
- Comparing yourself with someone else. A knee that only bent to 100 degrees before surgery rarely ends up at 130. Your own starting point sets a lot of your finish line.
The number matters less than the direction. A knee that gains a little every week is doing its job.
Why some knees get stiff
A genuinely stiff knee is usually defined as one that cannot bend past 90 degrees, or stays 15 degrees or more short of straight. It affects about 1 to 7 in every 100 people (figures vary with how stiffness is defined).
- How far it bent before surgery. This is the strongest predictor. Of people who could bend less than 90 degrees before surgery, about 7 in 10 reach 90 afterward. With better bend beforehand, it’s more than 9 in 10. See knee arthritis for keeping the knee moving before surgery.
- Too much scar tissue. Some people’s bodies lay down extra scar tissue inside the joint (arthrofibrosis), which binds tissues that should glide.
- Pain. It is hard to bend a knee that hurts. Pain that isn’t well controlled makes it hard to keep up the exercises that win bend back.
- Implant position or infection. Less common. This is why your surgeon may order X-rays, blood tests or a scan if the knee isn’t progressing.
What helps
- Time your pain relief. Take it as prescribed so it is working when you do your exercises, not wearing off halfway through.
- Little and often. Five to ten minutes of bending several times a day beats one long, painful session.
- Heel slides. Lying on your back, slide your heel toward your bottom, hold the stretch, then slide back. A towel under the heel helps it glide.
- The chair stretch. Sit near the front of a chair, slide your foot back under it, then use your other foot to push it a little further. Hold for about 30 seconds and repeat.
- Firm stretch, not sharp pain. A good stretch eases within an hour or so. If the knee is hot and more swollen the next day, you pushed too hard. Ease back for a day, don’t stop.
- Keep the swelling down. Elevate the leg with the foot above the knee and the knee above the hip, and ice after exercise.
- Get it measured. Ask your physio to measure your bend at each visit so you can see the trend, not just how it feels today.
- Bending machines (CPM): trials found they add very little bend. They don’t replace your own exercises.
When manipulation under anesthesia comes up
If your bend has stalled despite good rehab and pain control, your surgeon may suggest a manipulation under anesthesia. It is their call, based on your knee and how you are progressing.
- What it is: you are put to sleep and the surgeon bends the knee firmly to break up the scar tissue. It takes a few minutes, and most people go home the same day.
- When: usually within the first three months, while the scar tissue is still soft enough to stretch. After that, results are less reliable. See what AAOS OrthoInfo says about manipulation.
- What you gain: most people get back a useful amount of bend and are glad they had it done. Some stiffness can remain.
- Afterward: the knee is sore and swollen for several days. Physio starts again quickly and matters more than ever, because the scar tissue will try to form again.
- Risks: the most common is the stiffness coming back. A fracture is very rare.
- Later options: if a knee is still stiff after manipulation or months down the line, surgeons may check for infection or implant position, and sometimes suggest keyhole surgery to release scar tissue. A redo replacement is rarely needed.
Call your surgeon today if
- The knee becomes hotter, redder, more swollen and more painful, especially with a fever or chills.
- The wound is red, spreading, weeping or draining
- You suddenly lose bend after a fall, a twist or a clunk
- Your calf is painful, tender or newly swollen: possible clot in the leg. If you are suddenly short of breath or have chest pain, call emergency services.
Mention it at your next review if
- Your bend has not improved for a couple of weeks and is still below about 90 degrees
- The knee still won’t get within about 5 degrees of straight at six weeks
- Pain is stopping you doing your exercises
- The knee is getting stiffer or more painful months after surgery, when it had been improving
Stiff early is normal. Stuck early is worth a phone call.