Dislocated kneecap

The kneecap, or patella, runs up and down a groove at the end of the thigh bone rather like a slider along its rail. When it jumps out of that groove the leg loses its support instantly and the person drops on the spot. From the outside it looks as though the knee itself has gone, but the joint is intact: one of three bones has shifted. That distinction matters a great deal. A dislocated kneecap treated promptly ends well, whereas a dislocation of the knee joint proper is a rare injury measured in hours, because the blood supply to the whole lower leg is at stake. What follows is how to tell the two apart, what to do in the first hour, and why in some people the kneecap keeps coming out again and again.

How it usually happens

Contrary to what most people assume, a direct blow to the knee is not the commonest cause. Far more often the kneecap comes out during a sharp twist of the body over a planted, slightly bent leg: playing football, dancing, landing awkwardly, sometimes simply turning on a slippery floor. The thigh rolls inwards, the shin turns outwards, and the kneecap is effectively dragged sideways. It nearly always goes outwards, towards the outer side of the knee; inward displacement is very rare and usually follows a heavy direct blow.

What a person feels and sees at that moment:

• a pop, or a sensation that something has shifted inside the knee;
• the leg gives way and no weight can be kept on it;
• the knee locks slightly bent and will not straighten;
• the front of the knee looks oddly flat, while a bony lump can be felt or even seen at the side;
• severe pain that eases a little the moment the kneecap goes back.

There is a detail here that makes the injury easy to underestimate. A great many dislocations reduce themselves: the person straightens the leg by reflex and the kneecap slides back into its groove. By the time anyone examines it, all that is visible is a swollen, painful knee, and the story of it going out and back sounds trivial. In fact the ligament holding the kneecap on the inner side tears in almost every first dislocation, and how that knee is managed afterwards decides whether the whole thing repeats itself six months later.

It is also worth learning to recognise rapid swelling. If the knee balloons while you watch, within minutes or an hour or so, that is almost certainly blood inside the joint rather than ordinary bruising. Blood means something substantial has torn, or that a flake of cartilage with bone underneath has broken off. Such a knee should be seen by a doctor even if walking is possible.

When this is an emergency

Go straight to an emergency department if, after a knee injury:

• the knee or lower leg has visibly changed shape, or the leg lies at an unnatural angle;
• the foot is pale or bluish, cold or numb, tingling, or you cannot move the toes;
• the knee has swollen up within minutes;
• you can neither stand on the leg nor straighten it;
• bone is showing through a wound, there is a deep wound over the joint itself, or heavy bleeding.

A pale, cold or numb foot is not a leg that has gone to sleep. That is how compression or rupture of the popliteal artery announces itself, and it happens when the whole knee joint dislocates, not just the kneecap. The displacement may well reduce itself before help arrives, leaving a knee that looks almost normal from outside while the artery is already damaged. If blood flow is not restored within the first hours, the leg is lost. An injury with these features therefore goes to hospital at once rather than being watched until morning.

Do not drive yourself and do not walk the injured person anywhere. Ask someone to take you, and if that is not possible or the leg looks bad, call an ambulance (in Spain, Italy, Portugal, Poland and Ukraine the single number 112 works). Take a list of the medicines you are on, particularly any blood thinners.

What to do while you wait

The aim during that time is simple: do no further harm.

• Leave the leg in whatever position hurts least and do not force it straight.
• Put something soft under the knee and calf — rolled-up clothing, a towel, a cushion — so the leg rests supported rather than hanging.
• Apply something cold wrapped in cloth for fifteen or twenty minutes. Ice never goes straight onto skin.
• Do not put weight on the injured leg and do not try to walk it off.
• Do not push the kneecap back yourself, and do not let anyone who "knows how" do it. A fracture may be hiding behind the displacement, and a loose flake of cartilage may be sitting in the joint, where a rough manoeuvre would drive it between the bones.
• Do not eat or drink until you have been examined: if the kneecap has to be put back under sedation or a general anaesthetic, a full stomach delays it.

What the doctor will do

If the kneecap is still out, it is put back: the doctor slowly straightens the leg and eases the kneecap towards its groove. The manoeuvre takes seconds and the pain settles almost at once; if the muscles are in spasm, pain relief or light sedation is given first.

The knee is then always investigated, even if it went back on its own during the journey. X-rays are taken in several views: they show fractures and, more importantly, any osteochondral fragment that has broken off. Such a fragment stays inside the joint, jams the movement and calls for a decision of its own. If there is a lot of blood in the knee or the picture is unclear, an MRI scan is arranged, since it shows the torn stabilising ligament, the state of the cartilage and loose fragments that X-rays miss.

Three things can follow. Most often the knee is held in a brace or splint for a few weeks, weight-bearing is allowed as the pain settles, and the patient is referred to a physiotherapist. Less often — when a loose fragment is sitting in the joint — an arthroscopy is needed to remove it or pin it back. And surgery for people whose kneecap dislocates repeatedly is a separate story, covered below.

How recovery goes

Full recovery from a first dislocation usually takes six to eight weeks, sometimes longer. Walking, at least a little and with support, is something most people manage within a few days.

What helps:

• over-the-counter painkillers when pain gets in the way of moving and sleeping; which one suits you is best checked with a doctor or pharmacist, especially if you already take something;
• keeping the leg raised while resting, which brings the swelling down;
• the exercises the physiotherapist sets, done every day in small amounts rather than once a week to exhaustion; the main aims are to rebuild the quadriceps at the front of the thigh and to teach the buttock muscles to stop the knee collapsing inwards;
• ordinary everyday activity within the limits you are given — a knee that only rests recovers less well.

What to avoid: twisting the knee, walking without the brace if one has been prescribed, and going back to sport before the leg matches the healthy one for strength. That last point is decided by the doctor or physiotherapist on specific tests, not by the calendar. Returning to sport on how the knee feels is the commonest reason for a second dislocation.

Why the kneecap comes out again

For most people the first dislocation is also the last, but in a sizeable minority it repeats, and each time it takes less to bring it on: at first a football match, later just turning round in the kitchen. The reason is rarely bad luck; it is usually the architecture of that particular knee. The risk is raised by:

• a groove in the thigh bone that is too shallow, leaving the kneecap nothing to sit in;
• a high-riding kneecap that only engages the groove when the knee is well bent;
• generally lax ligaments, with loose joints all over the body;
• knock knees and an inward-rotated thigh;
• weak buttock and front-of-thigh muscles;
• a first dislocation in the teenage years, and the same history in close relatives.

Once the kneecap has come out even twice, the approach changes: a brace and exercises are no longer enough. An operation is discussed to reconstruct the torn ligament on the inner side of the kneecap and, in some cases, to correct the bony geometry itself. Imaging is done first to measure how shallow the groove is and how high the kneecap sits. The decision rests not on a count of episodes but on how much the knee is limiting the person's life.

There is also a side of this that gets less attention. Cartilage is damaged with every dislocation, and repeated episodes eventually mean persistent pain behind the kneecap and early arthritis. That is a reason not to put off investigation, even if the knee behaves itself between episodes.

Online consultation

A remote appointment is a convenient way to settle the questions that most often go unanswered: the kneecap went back on its own, so are any tests needed now; what swelling that has not settled in three weeks means; and when the brace can come off. The doctor will ask exactly how the leg gave way, what you felt and how the knee behaves now, help you work out whether an MRI is worth arranging, and point you towards the right specialist if dislocations keep happening. It is equally useful for planning a return to sport and for working out why an apparently healed knee still lets you down. The features on the emergency list are not assessed remotely — with those, go straight to hospital.

This material is for information only and does not replace medical advice.