After knee replacement, some patients feel that the joint gets stiffer the more they practise, and that bending to a certain angle is like hitting a wall. Is this just slower recovery, or has something gone wrong inside the joint? Among the stiff cases after knee replacement, one cause that must be checked carefully is arthrofibrosis, in which excess scar tissue grows inside the joint and locks the range of movement tightly.
The key points first. Arthrofibrosis means that excess scar tissue inside the joint limits bending and straightening and causes pain. About 4–5% of knee replacements are affected, and women and people with obesity are at higher risk. The earlier the patient starts active ROM training, or, where necessary, has manipulation under anaesthesia (MUA) within 90 days, the easier it is to regain a good range of movement. Revision surgery is the last resort (Tibbo 2019; Postler 2025). The sections below go through how stiffness is defined, the risk factors, the mechanism, and the three-step treatment ladder.
Telling early tightness after surgery from pathological stiffness
The first step is not to mistake the normal tightness in the first few weeks after knee replacement for a disease. The medical literature has a fairly clear consensus on "acquired idiopathic stiffness". The range of movement must stay below 90°, there must be no progress for more than 12 weeks, and external factors such as infection or a poorly positioned implant must have been excluded, before it counts as pathological stiffness (Tibbo 2019; Buchalter 2024). In other words, a bending and straightening range that is not yet ideal early after surgery is the best period for working hard to catch up, and it is not a diagnosis of stiffness.
| Normal recovery period | Swelling and pain in the first few weeks after surgery, or a bending and straightening angle that has not reached target, are part of normal recovery as long as there is still progress every week. The focus then is to keep up frequent and sufficient joint range of movement (ROM) practice. |
| Pathological stiffness | If the range of movement stays below 90°, there has been no progress at all for more than 12 weeks, and infection and the position of the implant have been ruled out, the clinical criteria for arthrofibrosis are met (Tibbo 2019). |
| How to tell them apart | The assessment should look at whether the angle keeps improving, and not only whether it reaches 90°. If there is a little progress every week, recovery is just slower. If progress has stopped completely, the joint feels as though it is blocked by something hard, and it cannot be pushed further even with outside force, it should be checked for pathological stiffness (Buchalter 2024). |
This threshold is closely tied to the angle goals discussed in the parent article. Only by first knowing how much bending each daily activity needs can one tell whether the angle is "not yet enough" or the joint is "already stuck". See bending angle goals after knee replacement for details.
How common arthrofibrosis is, and the risk factors
Arthrofibrosis is not rare, but not every patient having knee replacement will meet it. A systematic review and meta-analysis of 35 studies covering 48,873 primary knee replacements (Tibbo 2019) found an average prevalence of acquired idiopathic stiffness of about 4% (ranging from 1–5% across studies), while a German guideline-level review put it at about 5% (Postler 2025). It should be noted that the risk is not spread evenly:
For people in the high-risk groups, the clinical message from these figures is direct. The attitude should not be to wait and see whether the joint stiffens. ROM practice should start early after surgery and be progressed regularly, to make the most of the window while the scar tissue has not yet set.
Where the scar comes from: the overreaction of myofibroblasts and TGF-β
To understand why earlier intervention is better, the way scar tissue forms must be understood. The core pathology of arthrofibrosis is the abnormal activation and growth of scar cells called myofibroblasts, which deposit a lot of type I collagen in a pro-inflammatory environment, so the joint capsule and surrounding soft tissues become tight and hard. The clearest signalling pathway driving the whole process is TGF-β (Ramos 2023; Thompson 2019).
This mechanism has two important clinical implications. First, the later scar tissue is treated, the more mature and harder to release it becomes, which is the physiological reason for a time-limited treatment window. Second, the clinical diagnosis relies mainly on history and physical examination, with other external causes excluded, and generally does not need biomarker tests or a routine biopsy (Ramos 2023). When a joint is stiff, instead of spending time worrying about special tests, it is better to step up range-of-movement training early, before the scar has set.
Step 1 of the ladder: start active ROM physiotherapy early (first-line, with a chance of avoiding surgery)
The first-line approach, and the step most likely to avoid another operation, is physiotherapy under professional supervision after surgery. It is both prevention and the core of early treatment, and its results depend on the timing of intervention, the intensity of training and individual adjustment of progress (Cheuy 2017). This conservative treatment is backed by solid data. A retrospective review of more than 11,000 patients with knee arthrofibrosis (Stinton 2022) confirmed that non-surgical high-intensity home mechanical stretching increased bending on average from 79.5° to 108.4° (a large gain of nearly 30°). Most patients reached their target within two months, and more than 90% of severely stiff cases avoided another operation as a result.
Practical points for home ROM and stretching (the exact grading and number of repetitions are prescribed by the therapist case by case):
- Train both bending and straightening—While working hard to catch up on bending, full straightening must be practised at the same time. If only knee bending is practised while the knee is left unable to straighten, walking is badly affected.
- Early enough and regular enough—The patient should use the golden first 12 weeks after surgery. Several practices a day are far more effective than an occasional hard stretch, because the joint's range of movement can then be kept up continuously before the scar sets.
- Intensity must be enough—High-intensity mechanical stretching can increase bending by nearly 30° on average in severe cases (Stinton 2022), but the intensity must be adjusted stage by stage by the physiotherapist, to avoid injury from going too fast.
- Record progress in numbers—At each practice, record the bending and straightening angles accurately, and judge progress with objective numbers and not just the subjective feeling that it seems a bit looser today.
Catching up on ROM at home each day also depends on strong thigh muscles to support knee extension and a normal gait. See quadriceps strength training for how to train them.
Step 2 of the ladder: manipulation under anaesthesia (MUA) and the 12-week / 90-day window
If conservative treatment does not regain a good range of movement, the next common medical option is manipulation under anaesthesia (MUA), which comes before going straight to revision surgery. Whether this step works depends largely on timing. The study by Mamarelis and colleagues (2015) showed that MUA done within 12 weeks of surgery gives clearly better results than delayed treatment.
Done within 12 weeks (early intervention)
- Average gain in bending about 36.5°
- Final range of movement about 119°
- Knee Society Score about 89
Done after 12 weeks (delayed intervention)
- Average gain in bending only about 17°
- Final range of movement only about 95°
- Knee Society Score about 84
The figures show that the gain in bending with early intervention is almost twice that with delayed intervention. Even after 12 weeks, MUA within about 26 weeks may still bring some benefit, but if it is delayed beyond that, open arthrolysis is usually needed instead (Mamarelis 2015). National cohort data also support this order of management. Patients who had MUA had clearly higher revision-free survival at 2 years and 10 years after surgery than the group that went straight to revision surgery, which shows that MUA is suitable as a first-line intervention (Sanghvi 2025; Postler 2025). Whether MUA is needed, and when, is of course a clinical decision for the orthopaedic doctor. What rehabilitation does is use the time window to win as much angle as possible, because on the day of the follow-up visit this set of objective degrees is what the surgeon uses for the assessment.
Step 3 of the ladder: arthroscopic release and revision surgery
Only when MUA also fails to solve the stiffness are arthroscopic lysis of adhesions (aLOA) or revision total knee replacement (rTKA) considered. A systematic review of 40 studies (Haffar 2022) compared these three options directly, and clearly showed that revision surgery has the worst outcomes and most often needs further treatment:
- Proportion of studies reaching a range of movement above 90°: aLOA about 71%, MUA about 70%
- Proportion needing further intervention: MUA about 17%, aLOA about 25%
- Relatively conservative and repeatable, and usually done earlier in the order
- Proportion of studies reaching a range of movement above 90° only about 30%
- Proportion needing further intervention as high as about 43%
- The last step, and not an option to switch to early
Revision surgery is still not without value. When conservative treatment and MUA have both failed and rTKA is clinically needed, a retrospective review of 42 cases (Rockov 2023, average follow-up 4.5 years) found that after surgery the final flexion increased on average by 18.4°, extension by 6.8° and total arc of movement by 25.2°, and the final range of movement was similar to that before the first knee replacement. Because this kind of surgery carries high physical cost, risk and rate of further treatment, the clinical ladder puts it as the last step.
Challenges of living in Hong Kong, and when to ask a physiotherapist for help
In Hong Kong, a stiff knee can quickly become a daily difficulty in a cramped home. Many buildings have stairs and homes are generally small. Once the bending range after surgery stays below 90°, basic actions such as going down stairs, sitting on a low sofa and squatting to pick things up become very hard. Follow-up appointments at public hospitals after knee replacement are also far apart, and in those weeks the joint range easily slips back because the patient is afraid of pain and dares not move. Yet this period is exactly when frequent, professionally supervised ROM training is most needed. The value of home physiotherapyis that the therapist comes to the patient's own living space, designs a daily ROM and stretching timetable for the individual, and makes use of the golden 12-week window after surgery to catch up on angle.
In clinical practice, when a visiting physiotherapist meets a patient 6 to 10 weeks after knee replacement whose bending is still stuck at 85–90°, the therapist treats this as a key period in which the angle can still be won. Besides arranging frequent daily ROM practice and stretching first, the therapist also reminds the patient to tell the operating surgeon promptly, so that if needed an MUA can be arranged within 90 days, before the stiffness sets completely. In daily guidance, the therapist also explains clearly the difference between normal "stretching pain" and abnormal "injury pain", and turns each angle measured into recorded data, so that small gains are clearly visible and the patient is less likely to lose range by stopping practice for fear of pain. The registered physiotherapists at Nova Health follow these principles and design and adjust the home ROM programme stage by stage for each patient according to the clinical criteria, risk factors and time window. If there are signs that the stiffness is setting, they advise the patient to return to the orthopaedic clinic early, so that the doctor can assess the timing of MUA. A patient who wishes to use public rehabilitation services can be referred by the treating doctor to the orthopaedic and physiotherapy departments in the Hospital Authority clusters, and enquiries can be made through the Hospital Authority hotline 2300 6555 (www.ha.org.hk). For public support on mobility and fall prevention in older adults after surgery, the Elderly Health Service of the Department of Health can be reached on 2121 8080. All of the rehabilitation arrangements above concern care after total knee replacement has been done, and do not cover medical decisions such as whether to have a joint replacement or how severe an X-ray grade is.