Evidence checked as of October 2026. Written by The Wellness Team.
A knee that feels stiff is not always the same as a knee that is physically stuck. Pain and swelling can make it difficult to straighten the joint. Sometimes a mechanical problem inside the knee prevents normal movement. The distinction changes what should happen next.
If the knee has suddenly locked, especially after an injury, seek prompt medical assessment rather than trying to force it straight or booking an injection first. The priority is to understand what is limiting movement.
Why can I not straighten my knee fully?
Several problems can limit movement. Swelling and pain can make you hold the knee bent. A meniscal tear may cause catching or locking. Other injuries and joint conditions can also affect movement. A description such as cartilage damage does not, on its own, identify which structure is responsible.
The menisci are pieces of cartilage that help cushion the knee. The American Academy of Orthopaedic Surgeons describes pain, stiffness, swelling, catching or locking and loss of full movement among the symptoms of a meniscal tear.
Symptoms overlap. A clinician needs the history and examination, and sometimes imaging, to decide whether the problem is a true mechanical block, pain-limited movement or another condition.
What is the difference between locking and clicking?
A click or crunch during movement does not necessarily mean that the knee is locked. True locking describes a knee that gets stuck and cannot move through its usual range. Pain-related hesitation can feel similar, which is why the words alone do not make the diagnosis.
Describe what actually happens: whether the knee stops at the same angle, whether you can walk, whether it followed a twist and whether there is swelling. Tell the clinician if the knee releases and locks again.
Do not repeatedly twist or force the joint to test it. That may worsen pain and does not establish which treatment you need.
When should you seek urgent help?
A newly locked knee, inability to bear weight after an injury or a significant loss of movement needs prompt clinical assessment. A visibly deformed knee, a cold or numb foot, severe injury or rapidly increasing pain needs emergency care.
A hot, red, swollen knee with fever or feeling unwell also needs urgent assessment. Infection must not be mistaken for a routine arthritis flare. Do not wait for a planned PRP appointment if those symptoms appear.
The urgency depends on the whole picture. If you are unsure, explain that the knee will not straighten and whether there was an injury, rather than simply asking for an injection.
Does a locked knee always mean surgery?
No. Not every meniscal tear or episode of painful stiffness requires an operation. Treatment depends on the type of problem, whether there is a displaced tear, the examination, symptoms and other changes in the joint.
AAOS explains that many meniscal tears do not need immediate surgery, particularly when symptoms do not persist and there is no locking or swelling. That is not a reason to ignore a knee that is actually locked. A mechanical block can need a different and more urgent pathway.
The useful question is not whether surgery or an injection is better in general. It is which problem has been identified and what treatment addresses it.
What happens at an assessment?
A clinician asks about the injury, pain location, swelling, previous episodes and any instability. They assess movement and the structures around the knee. An X-ray can help identify bone and arthritis changes but does not show a meniscal tear itself. MRI may be useful when the clinical question concerns soft tissue.
Bring existing imaging reports and images if available. An old scan may not explain a new loss of movement, and a scan finding may not be the source of every symptom. The assessment connects the two rather than treating a report in isolation.
Contact The London PRP Clinic about a non-urgent knee assessment. Tell the team if you cannot straighten the knee, so they can advise on the appropriate route. Urgent symptoms should go to urgent care instead.
Where does PRP fit?
PRP uses a preparation from your own blood. Evidence and suitability vary by condition. It may be discussed for selected knee problems after assessment, but it is not a way to unlock a mechanically blocked knee or repair every torn structure.
AAOS describes the evidence for PRP as condition-dependent. A discussion about PRP for knee osteoarthritis cannot automatically be applied to a displaced meniscal tear, an acute fracture or an infected joint.
If PRP is suitable for a separate, established problem, ask what symptom it is intended to improve, how the response will be measured and what alternatives remain. No clinician should promise that an injection guarantees normal movement or avoids surgery.
What can you do while waiting for advice?
Avoid the activity that triggered the problem and do not force the knee straight. If you have already been given an injury plan, follow it. Seek advice about pain relief that fits your medicines and medical history.
A graded rehabilitation programme may help many knee conditions, but exercises should fit the diagnosis and stage of recovery. The AAOS knee-conditioning guide advises working with a doctor or physiotherapist and not ignoring pain during exercise.
General strengthening advice is not a substitute for assessment of a new locked knee. Return to sport should follow a plan, not a test of whether you can push through.
The value of getting the diagnosis right
A clear assessment can save time spent on treatments that do not address the cause. It should leave you knowing why movement is limited, whether further imaging or specialist care is needed and what the next review should establish.
For an appropriate non-emergency concern, ask The London PRP Clinic about assessment and treatment options. The team can confirm the appointment and costs before you book. The first decision is the right pathway, not the biggest treatment package.
Evidence and further reading
This information cannot determine the cause of a knee that will not straighten. Sudden locking or urgent symptoms need direct medical assessment.