Stiffness and pain after knee replacement each have a documented pattern and a documented list of causes.

Early stiffness is expected, and motion before surgery predicts motion after

Swelling, pain, and healing tissue limit bending in the first weeks. AAOS OrthoInfo notes that restoring full motion is uncommon, and that the motion a knee has before surgery predicts the motion it will have after ¹. Most patients can expect to almost fully straighten the knee and to bend it enough for stairs and for getting in and out of a car ¹.

A review of stiffness after knee replacement reached the same conclusion: reduced motion before surgery is the most important risk factor for stiffness after it. Diabetes, smoking, and previous knee surgery are also listed ².

The range-of-motion figures that get discussed

Motion is measured in degrees of bend, called flexion. The review cites 65° to walk on flat ground, 70° to rise from a chair, and 90° to go down stairs ². A replacement aims for 0° to 120°, though 5° to 95° is enough for most daily activities ².

The figure that matters most clinically is 90°. The commonly accepted threshold for considering manipulation is flexion below 90° at six weeks ². Reported rates of stiffness after knee replacement range from 1.3% to 5.3% across series ², and a 2015 review put it at roughly 5% to 7% ³.

Arthrofibrosis, and what manipulation under anaesthesia is

Arthrofibrosis is excess scar tissue inside and around the joint that binds the knee and limits bending. Manipulation under anaesthesia, or MUA, is a procedure in which the surgeon bends the knee while the patient is asleep to break up that scar tissue. It carries its own risks. Reported complications include wound breakdown, patellar tendon avulsion, bleeding into the joint, heterotopic bone, fracture above the knee, and pulmonary embolism ³. That is one reason the decision rests with the surgeon.

Timing is the debated question, with proposals ranging from two weeks to four months after surgery ³. One series found manipulation within 12 weeks gained a mean 36.5° of flexion and reached 119°, against 17° and 95° when performed later ³.

A series of 7,386 knee replacements, in which 1.8% had a manipulation, also found that early manipulation (within three months) gained more flexion, 33.1° versus 24.3°. Both groups, though, ended at a similar final flexion of about 105° ⁴. So the studies agree that earlier manipulation gains more, and disagree on how much the end result differs. The stiffness review also advises a delay of at least three weeks after surgery to protect healing tissue ².

Persistent pain has a documented list of causes

A systematic review of prospective cohorts found that 10% to 34% of patients report an unfavourable long-term pain outcome after knee replacement ⁵. In the higher-quality studies the figure was about 20% ⁵. The causes the literature lists fall into two groups.

• Inside the knee: infection, aseptic loosening of a component, instability, polyethylene wear, patellofemoral (kneecap) arthritis, and component malposition ⁶.
• Outside the knee: in one series of painful knee replacements, 16% had moderate to severe arthritis of the hip on the same side. Another 18% had degenerative lumbar spine disease, and in 20% no cause was found ⁶.

Instability deserves its own line. It accounted for 17.4% of single-stage revision knee replacements in the UK National Joint Registry. Patients typically describe the knee giving way going down stairs, difficulty rising from a chair, and repeated swelling. That swelling is reported in 70% to 85% of those with flexion instability ⁷.

Working out which cause applies takes an exam, X-rays, and often blood tests or a joint aspiration. That sequence is the surgeon's to order, from your exam, imaging, and history.

When to call your surgical team

• Fever higher than 100°F, chills, increasing redness, tenderness or swelling at the wound, drainage from the wound, or knee pain that increases both with activity and at rest. AAOS lists these as possible signs of infection ¹.
• Increasing calf pain, new swelling in the calf, ankle, or foot, or sudden shortness of breath or chest pain. These are listed as possible signs of a blood clot ¹.
• Bending that was improving and then goes backwards, or flexion still below 90° at six weeks. That is the threshold at which manipulation is commonly considered ².
• A knee that gives way on stairs or swells repeatedly ⁷.

For everything else, write down what the knee lets you do and what it does not, and bring that to the next visit. Your surgeon decides from your exam, imaging, and history whether the answer is more therapy, a manipulation, a work-up for pain, or time.