Knee instability: why a knee gives way and what can be done

Knee instability describes a knee that does not feel secure. People describe it as the knee buckling, collapsing or giving way, often when pivoting, stepping off a kerb, going downstairs or after a stumble. It may happen once after an injury or repeatedly in everyday activities.

The knee is held in position by muscles, cartilage and ligaments. When a ligament is torn or stretched, the way the joint moves can change. Milton Keynes University Hospital explains that after an ACL injury the motion of the knee can become abnormal so that it collapses on some movements, and that the ligament also contains nerves that tell the brain where the knee is, which stop working after injury and cause a feeling of instability.

Instability is therefore partly mechanical, because a structure is damaged, and partly about control and muscle strength. That is one reason rehabilitation can help even when a ligament remains torn.

The NHS knee pain page lists, among causes of knee pain after an injury, an unstable knee that gives way, cannot straighten and may have popped during injury, which may be a torn ligament, tendon or meniscus or cartilage damage. A kneecap that changes shape after a collision or sudden change of direction may be a dislocated kneecap. Knee symptoms with no clear injury can come from osteoarthritis and other joint conditions.

ACL injury is among the most frequent sports knee injuries. The Royal Orthopaedic Hospital says it accounts for around 40% of all sports injuries and lists football, netball, skiing and rugby as common settings. Common mechanisms include twisting when landing from a jump, sudden changes of direction, stopping suddenly and collisions. Milton Keynes University Hospital adds that the injury does not have to be particularly violent and can occur with a simple stumble.

Other ligaments, the menisci and the joint surface can be damaged at the same time, which affects how the knee feels and what treatment is advised.

The NHS advises getting advice from NHS 111 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;

• your knee locks, gives way or painfully clicks (painless clicking is normal);

• you have a very high temperature, or feel hot, cold or shivery, with redness or heat around the knee, which can be a sign of infection.

Milton Keynes University Hospital says an ACL injury can usually be diagnosed from the history of what happened at the time, the trouble since and an examination of the knee. X-rays, an MRI scan, examination under anaesthetic and arthroscopy (keyhole surgery) may be used where necessary. It can be difficult to tell early on whether a ligament is completely ruptured, and the hospital notes that whether it is partial or complete is less important than how the knee functions.

In many areas people can get help such as physiotherapy from NHS community musculoskeletal (MSK) services without a GP referral, according to the NHS knee pain page. A GP can also refer you and can arrange a scan or specialist opinion when needed.

Milton Keynes University Hospital describes three main options: physiotherapy, a brace and surgery. Specific advice and exercises are enough for nearly all people with a partial rupture and many with a complete rupture. A simple knee support may give confidence, but properly stabilising the knee needs a fairly large and complicated brace, which may suit people who cannot have surgery or are waiting for an operation. Some sports do not allow braces.

The Royal Orthopaedic Hospital says it is entirely reasonable to begin with a period of physiotherapy to strengthen the thigh muscles and improve balance and general fitness. Some people continue with work and activity reasonably free of symptoms, and others remain symptom-free after changing high-risk activities such as football or skiing. The downside is that the knee may continue to be unstable despite 3 or 4 months of rehabilitation and then need surgery.

Factors that influence the decision include age, lifestyle, how often you play sport, occupation, how unstable the knee is and whether other injuries such as meniscus tears are present.

ACL reconstruction rebuilds the torn ligament using tissue taken from the patient, such as hamstring tendons, patella tendon or quadriceps tendon, or from a donor or a synthetic graft. At the Royal Orthopaedic Hospital the vast majority of patients have a hamstring graft. The operation takes between 45 minutes and 1.5 hours and usually needs a maximum of an overnight stay, though it can sometimes be done as a day case.

Rehabilitation is long. Milton Keynes University Hospital says surgery usually needs one night in hospital, crutches for 2 to 3 weeks and no driving for generally six weeks, and that most people can return to some sport at about 10 to 12 months. The Royal Orthopaedic Hospital notes that the medical literature has not shown that ACL reconstruction changes future arthritis risk. As with any operation, risks such as infection and blood clots exist and are discussed with the surgeon.

Milton Keynes University Hospital says the future is difficult to predict. Some people have no trouble, some have a feeling of weakness when playing sport and some have repeated giving way during simple day-to-day activities. If the knee does not keep giving way, serious trouble is unlikely. A very unstable knee, or continuing sport at a high level, carries a greater risk of further ligament injury, torn cartilage and wear and tear arthritis later in life.

Practical steps include progressing exercise with a physiotherapist, avoiding the movements that trigger giving way until strength improves and seeking review if episodes continue.

On the NHS, assessment is through GP or MSK physiotherapy services, with orthopaedic referral for scans or surgery when rehabilitation does not restore stability. Private physiotherapy, scans and surgical opinions are available, but the treatment decision usually follows the same questions of function, activity level and rehabilitation commitment. It is sensible to ask what rehabilitation programme will follow any operation before agreeing to it.