Effusion and motion
Swelling and range of motion after ACL surgery
Effusion and extension are the two things that decide the first three months, and both are measurable in under a minute. Here is how, and what to do with the answer.
Everyone remembers to ask about strength. Almost nobody asks about swelling and full extension, and those two are upstream of everything else. Fluid in the joint inhibits the quadriceps, so a swollen knee cannot be strengthened. A knee that does not fully straighten changes your gait, keeps the quadriceps inhibited, and is the hardest deficit in the entire recovery to reverse once it has set.
The stroke test, and why it beats a tape measure
The stroke test grades joint effusion by sweeping fluid up the medial side of the knee and watching what returns to the medial joint line. It is graded zero, trace, 1+, 2+ and 3+. It takes fifteen seconds, needs no equipment, and it is reliable: across 75 test pairs, two therapists reached a kappa of 0.75 with 73% exact agreement, and no pair differed by more than two grades.
Circumference measured with a tape captures swelling in the whole segment, including muscle and soft tissue, and it drifts with where you place the tape. For effusion inside the joint, which is the thing that inhibits your quadriceps, the stroke test is the better instrument.
| Grade | What it means | What to do |
|---|---|---|
| Zero | No fluid returns to the medial joint line | Progress as planned |
| Trace | A small wave returns slowly | Fine early, watch it after sessions |
| 1+ | A larger wave returns | Hold the current load, address swelling |
| 2+ | Fluid returns without stroking, on its own | Back off, this is not a session to push |
| 3+ | Fluid cannot be swept out of the medial compartment | Stop adding load, tell your clinician, especially if it appeared suddenly |
Swelling that appears late is information
Effusion at week two is surgery. Effusion that shows up at month six after a running session is your knee reporting that the load exceeded what the leg can absorb. The correct response is to look at the load and the strength, not to ice it and repeat the same session.
Extension is the priority, not flexion
Extension is compared against your other knee, not against zero on a goniometer, because many people are naturally a few degrees hyperextended. A knee that stops 5 degrees short of the other one is not a small cosmetic difference: it forces the quadriceps to work constantly in standing, changes the way you load in gait, and tends to keep the joint irritated.
The work is unexciting and it is daily: prone hangs, low-load long-duration stretching, heel props, extension with the quadriceps active, and patellar mobilization so the kneecap does not become the limiting structure. Every week that a deficit persists makes it harder to reverse, which is why this is a first-month priority and not a month-three project.
Flexion, in its place
- Around 90 degrees by week two is a normal expectation, enough to sit, cycle and start closed-chain work.
- Full flexion typically returns somewhere between week six and week twelve.
- A flexion deficit at week four is a normal thing that resolves. An extension deficit at week four is a problem that needs a plan this week.
- Chasing deep flexion aggressively while the knee is still swollen usually buys more swelling.
What to do about swelling that keeps coming back
Grade it, do not eyeball it
Stroke test after every hard session, same time of day. Written down, a pattern appears within two weeks.
Look at the load, not the ice
Compression, elevation and cryotherapy manage the symptom. The recurring cause is almost always volume or intensity ahead of current capacity.
Check strength
A quadriceps 30% behind cannot absorb landing forces, so the joint takes them. Recurrent effusion at month five and a stalled strength number are usually the same problem.
Escalate what does not fit the pattern
Sudden swelling, a hot knee, fever, or swelling with a mechanical block or new instability is a call to your surgeon's office, not a rehab adjustment.