Locked knee: why a knee will not straighten and what to do

People use the word locked in two ways. Sometimes it means a knee that feels stiff or will not move after sitting, which is common with arthritis or swelling. True locking is different: the knee suddenly sticks and a mechanical block stops you straightening it fully. The Hull University Teaching Hospitals leaflet on soft tissue injury of the knee describes locking as an inability to straighten the knee, and lists it alongside pain, swelling, bruising and giving way as symptoms of an acute knee injury.

The knee is a complex joint that relies on ligaments, the meniscus (the cartilage that acts as a shock absorber) and the surrounding muscles. Any of these can be injured. Locking is most often linked to the meniscus, because torn cartilage can wedge between the thigh bone and shin bone.

Sherwood Forest Hospitals describes the menisci as c-shaped cartilage structures that sit between the thigh bone and shin bone. It says that if the tear is larger, or part of the meniscus becomes loose, the knee may lock in a bent position, and that you may need to move your leg with your hands to straighten it.

The NHS lists a torn ligament, tendon or meniscus and cartilage damage among the causes of a knee that is unstable, gives way when you try to stand or cannot be straightened, sometimes with a popping sound at the time of injury. The NHS meniscus page says a tear usually happens when you twist your knee while playing sport, but it can also happen from more minor injuries such as twisting when standing up.

Age matters. The Sherwood Forest leaflet says there are acute tears due to a specific injury and degenerative tears that are not due to a specific injury, which are common in people aged over 40, and that half of people over 50 will have a degenerative tear, yet most show no symptoms at all. That is why a scan finding does not always explain pain or locking.

The NHS knee pain page says to get advice from 111 now if your knee is very painful, you cannot move your knee or put any weight on it, your knee is badly swollen or has changed shape, or your knee locks, gives way or painfully clicks. It adds that a very high temperature, or feeling hot, cold or shivery, with redness or heat around the knee can be a sign of infection.

The NHS meniscus page says to get an urgent GP appointment or help from NHS 111 if you have very bad knee pain after a fall or injury, are unable to walk or put weight on your knee, the skin around the knee is swollen and hot, you have knee pain and a high temperature, or you cannot bend or straighten your leg. It says to go to A&E if your knee has moved out of place, changed shape or is pointing at an odd angle, or if you have tingling or no feeling in your knee, leg or toes. The NHS asks people not to drive themselves to A&E.

Sherwood Forest Hospitals says meniscal tears can often be diagnosed by a healthcare professional based on your description of symptoms combined with simple tests. An X-ray may be requested, which will not show the meniscus itself but helps check for bone issues such as osteoarthritis. An MRI scan can confirm the diagnosis, but is usually only required if you are experiencing severe pain or symptoms such as locking and giving way, or it is felt that you may need to see a surgeon.

The NHS says a doctor may refer you to hospital for a scan or specialist treatment if needed. In many areas you may be able to get help such as physiotherapy from NHS community musculoskeletal (MSK) services without needing a GP referral. If the knee is stuck and you cannot straighten it, seek an urgent assessment, as the NHS advice above describes.

For the first days after an injury the NHS advises stopping exercise and resting the knee when you can, using an ice pack wrapped in a towel for up to 20 minutes every 2 to 3 hours, wrapping a bandage or wearing a knee support (removed in bed), keeping the leg raised and taking paracetamol or ibuprofen gel or tablets. It advises against heat packs, hot baths, alcohol and massage in the early stage, and not completely stopping movement: after a few days of rest, try gentle movements.

The treatment of a tear depends on its size and type, the symptoms and the general condition of the knee. Sherwood Forest says many people benefit from physiotherapy and that people treated without surgery tend to do just as well as those who have surgery. Surgery may be required if you have a large tear with severe symptoms, including locking and giving way, though it is not always successful, especially for degenerative tears or when arthritis is already present. The NHS says arthroscopy surgery may be needed to repair or remove the damaged cartilage if the damage is severe.

The Sherwood Forest leaflet notes that small tears on the outer edge of the meniscus may heal over time because of a better blood supply, taking two to three months, whereas larger tears or those closer to the centre do not tend to heal.

The Hull leaflet suggests that with basic advice a soft tissue knee injury should take around six weeks to heal, but that recovery varies with the severity, the tissue involved and any other medical problems. After the first 72 hours it advises regular exercises to regain function, flexibility and strength, which also reduces the risk of the injury recurring. It also notes that smoking may delay the healing process.

On the NHS the usual route is GP, NHS 111 or an urgent treatment centre for the acute problem, then physiotherapy or an orthopaedic clinic. Private orthopaedic and physiotherapy clinics offer faster access to MRI and surgery, but the assessment and the treatment options described here are the same.