After a knee replacement, many patients fix on a single number: has the knee bent to 90° yet? But "how many degrees of flexion is enough" is not one single dividing line. It depends on what the patient needs to do in daily life, and level walking and going up and down stairs already ask for quite different amounts of knee flexion. In addition, the direction patients more often overlook is whether the knee can straighten fully to 0°.
Put simply, flexion after a knee replacement has to be won back in stages. Level walking needs about 60–70°, sit-to-stand and stairs need 90–120°, and soaking in a bath needs about 135° (Rowe 2000). At the same time, full extension to 0° is essential for normal gait. For every 1° of extension that is lacking, the Oxford Knee Score falls by 1.0–1.6 points (Booij 2021). If flexion is under 90° and the joint is stiff, release under anaesthesia within 12 weeks of surgery gives the best results (Mamarelis 2015). The sections below go through the angle needed for each activity, the cost of lacking extension, and how to judge for yourself and seek help when flexion is limited.
The knee flexion angle needed for different daily activities
First, the most practical question: how many degrees must the knee bend to cope with daily life? Rowe and colleagues (2000) used a flexible electrogoniometer to measure, in real settings, the knee angles of 20 healthy older adults (average age 67) during walking, stairs, sit-to-stand and bathing, giving a set of objective reference values backed by measured data and not estimated.
These data show two points. First, whether an angle is "enough" depends on what the person actually needs to do, and not on a single standard. If the person only needs to walk on the flat, 60–70° is enough. But the stairs and bathtubs common in Hong Kong homes raise the demand to 90–120° or even 135°. Second, the 110° target often set clinically is not a casual estimate but corresponds to the practical upper limit of daily function. Movements that need more than this angle (such as a deep squat) are fairly rare in daily life, and for such high-flexion movements aids or alternatives can be used, with no need to push for the extreme angle.
Why flexion and full extension to 0° are equally important
Many patients put all the focus of rehabilitation on flexion and overlook extension. In fact, lacking extension directly damages walking ability. Booij and colleagues (2021) analysed the gait of patients after knee replacement and found that limited knee extension during walking was significantly related to a lower Oxford Knee Score: for every 1° of extension lacking during gait, the score fell by about 1.0–1.6 points.
More notable is that, even when static examination found no mechanical block to extension, patients with lower scores still showed limited extension during walking. This suggests the problem is not purely in the joint structure and also involves motor control and active muscle effort. In other words, extension training must never give way to flexion training. A knee that cannot straighten makes the quadriceps keep working tightly while walking, which slows walking speed and increases the load on the other leg, and in the long run it holds back overall function. For a deeper look at why the knee must reach full extension to 0° after a knee replacement, and how lacking extension affects gait and the knee step by step, see the more detailed article on that topic. Flexion and extension have to be worked on side by side, and this is also why quadriceps strength training is equally essential in rehabilitation after surgery.
| Flexion (bending) | The flexion angle decides whether the patient can manage stairs, sit down, stand up and bathe. Different activities need different angles, from 60–70° up to 135° (Rowe 2000). |
| Extension (straightening) | The extension angle decides whether the walking pattern looks natural, how fast the patient walks, and how heavy the load on the other leg is. The target for extension is 0°, and for every 1° short, the Oxford Knee Score falls by 1.0–1.6 points (Booij 2021). |
| A common misunderstanding | A common mistake is to practise only flexion and let the knee stay unable to straighten. In fact flexion and extension must be practised together, and extension range must never be given up to gain a few more degrees of bending. |
How fast flexion recovers after surgery, and what to monitor
A patient who feels anxious because flexion is still under 90° in the first few weeks after surgery can first relax a little. Among the rehabilitation measures, the speed of recovery of flexion is naturally the slowest. Bade and colleagues (2010) compared patients after knee replacement with healthy adults and found that by 6 months after surgery most functional measures had recovered to close to the level before surgery, but knee flexion range of motion (ROM) was the only one that had not caught up and was still limited as before surgery. This shows that recovery of flexion is slower by nature, and patients need a more active and persistent rehabilitation strategy.
However, "recovering more slowly" does not mean "it can be left alone". At present, rehabilitation guidance after knee replacement differs quite widely between healthcare organisations, is often left to individual therapists to judge, and lacks one unified standard for range of motion (ROM) (Mistry 2016). For this reason, when setting a flexion target, the actual functional need (for example the Rowe reference angles in the table above) should be the guide, and a single number should not be applied blindly. In addition, early joint swelling and wound pain also truly hinder flexion progress, and the mechanism is explained in how swelling and the wound limit flexion. A clear timeline of the stages of rehabilitation can be found in the recovery timeline for knee replacement.
What really needs attention is not whether the knee has reached 90°, but whether the angle keeps improving. It is quite normal for flexion to recover slowly, but progress that stops altogether is a clear warning sign. If for several weeks in a row the range of flexion or extension stays the same, the joint jams hard at a certain angle, or it cannot even reach the target angle with outside help, consider whether the joint has become stiff, and arrange a follow-up appointment promptly for further assessment.
Causes of limited flexion and stiffness (arthrofibrosis), and how to judge it yourself
When knee flexion is not simply "slow to progress" but "stuck", the usual culprit is arthrofibrosis. This means excessive scar tissue forming inside the joint and limiting the range of movement, and it is the main cause of blocked range of motion (ROM) after knee replacement (Haffar 2022). The situation differs in nature from ordinary slow recovery: in slow recovery there is still small progress step by step, whereas in arthrofibrosis there is no progress at all, and there may even be a step back. Patients can make a first self-assessment from a few simple warning signs: rehabilitation progress has stopped completely, the joint jams hard at a certain angle, and the target angle is not reached even with passive stretching.
For the detailed causes, risk factors and clinical staging of joint stiffness, and the treatment options at each stage (including observation, intensive physiotherapy and surgical release), see the article on how to recognise and manage a stiff joint after knee replacement.
When to consider manipulation under anaesthesia (MUA): the key window of 12 weeks after surgery
If joint stiffness is confirmed and the range of movement cannot be regained after conservative treatment, the usual next step is manipulation under anaesthesia (MUA) or arthroscopic release, and not going straight to revision surgery. When release treatment is considered, the timing of intervention directly governs the final result. If MUA is done within 12 weeks of surgery, the gain in flexion is clearly better than with delayed treatment (Mamarelis 2015).
- Average flexion gain about 36.5°
- Final range of motion about 119°
- Knee Society Score about 89
- Average flexion gain only about 17°
- Final range of motion about 95°
- Knee Society Score about 84
In other words, the gain in flexion from early intervention is about twice that of delayed intervention. When flexion is limited, the key is therefore not to keep stretching alone with no end, but to get a professional assessment early and make the treatment decision within the best time window. The details of the MUA procedure, how much range can be released under anaesthesia, how physiotherapy follows straight after to consolidate the result, and how the results compare with revision surgery are covered in a separate article on when MUA is needed for a knee that bends under 90° after replacement. As for the continuous passive motion (CPM) machine, which is often wrongly thought to help "increase flexion" during rehabilitation, the research evidence is covered in an article on the truth about CPM machines and is not repeated here.
Daily self-checks at home, and when to ask the therapist for help
The principles above come down to a practical daily self-check at home, with just three points at the core: measure the two angles accurately, compare with the needs of daily function, and tell whether progress is slow or stuck.
| Measure two numbers | After each exercise session, note down both the flexion and the extension angle, and do not look at flexion alone. Aim for 0° of extension, and compare flexion with the daily function needs below. |
| Compare with functional needs | Know how far the present angle is from the need and which direction to catch up in. For example, stairs need 90–120° and the target for extension is 0° (Rowe 2000; Booij 2021), instead of asking only whether 90 degrees has been reached. |
| Tell slow from stuck | If the angle improves a little each week, it is just the normal pattern of slower recovery. But if progress stops for a long time, the joint jams hard, and it cannot be pulled into position even with outside help, it may be joint stiffness and needs assessment promptly. |
| Mind the 12-week window | If joint stiffness is suspected, remember that the best period for release under anaesthesia is within 12 weeks of surgery (Mamarelis 2015). If it is delayed, the improvement is halved. |
In Hong Kong, the situation in which limited knee flexion causes most trouble in life is living in a tenement building or older public housing estate with no lift. Residents must walk up several flights of stairs to get in and out every day. Stairs need about 90–120° of flexion (Rowe 2000), so older people who have not yet reached the target can easily be stuck at home, and this is the functional complaint most often dealt with in home rehabilitation assessments. Another common difficulty is traditional sit-in bathtubs and bathrooms with high thresholds. Stepping into a bathtub and squatting to wash need close to 135° of flexion, and for patients who are unlikely to reach this in the short term, rather than pushing the angle, it is better first to add aids such as a shower stool and non-slip mats as an alternative.
From clinical practice, the cases that home physiotherapy meets most often are older people who are very worried because flexion is still under 90° 3–4 weeks after surgery. When Nova Health's registered physiotherapists visit, they first measure both flexion and extension accurately and combine them with the needs of daily function, so that the patient clearly understands "how far off the knee is now and which direction to catch up in", instead of answering vaguely whether 90 degrees has been reached. The therapist also tells whether the patient is simply recovering more slowly or the joint has become stuck (the arthrofibrosis warning signs). In the latter case the therapist advises going back to the orthopaedic clinic promptly, to assess the timing of MUA. Another common pattern is an older person who practises only flexion and lets the knee stay slightly bent and unable to straighten. For this, the therapist stresses that extension to 0° is equally key for normal walking, and adds targeted exercises such as prone hanging of the leg and passive extension in sitting to the home programme, so that flexion and extension go side by side. For public rehabilitation resources, the patient can ask the treating doctor for a referral to the orthopaedic and physiotherapy departments in each Hospital Authority cluster, and enquiries can be made on the Hospital Authority hotline 2300 6555 (www.ha.org.hk). For community rehabilitation and assistive device support, contact the Hong Kong Society for Rehabilitation on 2549 7744 (www.rehabsociety.org.hk). The clinical arrangements and exercise advice above are meant for people who have completed a total knee replacement, and details of the related services can be found on the page about physiotherapy after total knee replacement.