Lateral Patellar Instability (Kneecap Dislocation)

Description

The kneecap (patella) rides in a groove at the end of the thighbone, like a train in a track. When the knee bends and straightens, the kneecap should glide up and down the center of that groove. Lateral patellar instability means the kneecap slips toward the outside of the knee, either partway (a subluxation) or all the way out (a dislocation). It is one of the most common knee injuries in children and teenagers, with a peak between ages 10 and 17, and it is more common in girls.

A first dislocation often happens during sport, with a twist on a planted foot, and the kneecap usually slides back on its own when the knee is straightened. The real question afterward is whether it will happen again, and that depends on the shape of the knee.

How it happens (the mechanism)

Think of the kneecap as being pulled in two directions. The thigh muscle (quadriceps) and its tendon pull it upward and slightly outward. On the inside of the knee, a short, strong ligament called the medial patellofemoral ligament (MPFL) tethers it toward the midline and is the main thing stopping it from sliding out in the first part of bending. The groove itself keeps it on track once the knee bends deeper.

When the kneecap dislocates, the MPFL tears, almost every time. That ligament does not heal back to full strength on its own. So after one dislocation, the knee is relying on the groove and the muscles alone, which is why about one in three teenagers will dislocate again, and more than that if the knee has the risk factors below.

The risk factors I measure

Some knees are shaped in ways that make the kneecap want to slip. I measure each of these on the exam, on X-rays, and on an MRI or CT, because together they decide how likely another dislocation is and which operation, if any, would fix the cause.

1. Torsion of the leg (pan genu torsion). The thighbone can be twisted inward and the shinbone twisted outward, so the knee points in while the foot points out. The kneecap then sits on a groove that is turned away from the direction the leg is moving, and every step nudges it outward. This is the "miserable malalignment" seen in some teenagers who were in-toeing children.
2. Increased TT-TG distance. The tibial tubercle is the bump below the kneecap where the patellar tendon attaches. If that bump sits far to the outside of the groove (a large tubercle-to-groove distance on the scan, over about 20 millimeters), the tendon pulls the kneecap outward with every contraction of the thigh.
3. Trochlear dysplasia. The groove is shallow, flat, or even domed instead of V-shaped. A train on a flat track derails easily. This is the single strongest risk factor for repeat dislocation.
4. Patella alta. The kneecap sits higher than normal, so it does not drop into the groove until the knee is bent further. In the first part of bending, when most dislocations happen, it is floating above the track.
5. Genu valgum (knock-knee). When the knees angle inward, the line of pull of the thigh runs to the outside of the kneecap.
6. Increased Q angle. The Q angle is the angle between the pull of the thigh muscle and the patellar tendon. A larger angle means a larger outward pull. It is related to knock-knee and to the width of the pelvis.
7. MPFL deficiency. The inside tether is torn from a previous dislocation, or was loose to begin with in someone with generally loose joints.

A knee with none of these can dislocate once with a hard enough twist and may never do it again. A knee with three or four of them can dislocate with ordinary activity, and will keep doing so until the cause is addressed.

Symptoms

• At the time of injury: a sudden giving-way, severe pain, often a visible lump on the outside of the knee until the kneecap slides back, and rapid swelling from bleeding into the joint.
• Afterward: tenderness along the inside of the kneecap where the MPFL tore, swelling for one to three weeks, and a fear of the knee giving way with twisting or stairs.
• With repeated episodes: slipping with less and less force, a grinding feeling under the kneecap, and a sense that the knee cannot be trusted.

Doctor examination and imaging

I check how far the kneecap can be pushed outward and whether that makes you apprehensive, where the tenderness is, and how the whole leg lines up from hip to foot: the rotation of the hips, the angle of the knees, and the position of the feet. X-rays show the height of the kneecap, the shape of the groove, and any chips of bone. An MRI after a first dislocation is routine, because about one in four dislocations knocks a piece of cartilage and bone off the kneecap or the thighbone, and a loose piece in the joint changes the plan. The MRI and sometimes a CT scan measure the groove, the TT-TG distance, and the torsion.

Treatment

After a first dislocation without a loose fragment. A brace for comfort and a few weeks of protected walking, then physical therapy focused on the inner thigh muscle, the hip muscles that control the knee's position, and landing mechanics. My home program for patellar instability is the starting point. Most teenagers are back to sport in six to twelve weeks. A kneecap brace with a lateral buttress helps during the return.

When surgery makes sense.

• A loose fragment of cartilage and bone in the joint needs an operation soon, to fix the piece back in place or remove it.
• A second dislocation, or a first one in a knee with several of the risk factors above, is a reason to rebuild the MPFL. A new ligament is made from a strip of hamstring tendon and anchored to the inside of the kneecap and the thighbone. In a growing child, the anchor on the thighbone is placed carefully below the growth plate so that growth is not disturbed. Return to sport is usually at five to six months, and the chance of another dislocation falls to under one in ten.
• A large TT-TG distance in a teenager whose growth plates have closed is corrected by moving the bump of bone on the shin inward a few millimeters and fixing it with screws (a tibial tubercle osteotomy), usually combined with the ligament reconstruction. This is not done while the growth plate under that bump is still open.
• Knock-knee in a child who is still growing can be corrected gradually with a small plate across the growth plate on the inside of the knee (guided growth), which lets the leg straighten over a year or so with no cutting of bone.
• Severe torsion is occasionally corrected by cutting and rotating the bone (a derotation osteotomy), and a very flat groove by reshaping it (a trochleoplasty). Both are uncommon and reserved for knees that have failed the simpler steps.

The principle is to match the operation to the cause. Rebuilding the ligament in a knee whose real problem is bone shape will fail; reshaping bone in a knee that only needs the ligament is more surgery than it needs.

Recovery and long-term outlook

Most teenagers with a single dislocation and a normally shaped knee do well with therapy alone. Those with repeated dislocations do well after a reconstruction that addresses their particular risk factors, and the large majority return to sport. The reason to take recurrent instability seriously is cartilage: every dislocation scrapes the underside of the kneecap, and repeated episodes lead to early wear under the kneecap in adulthood.

When to call the doctor

Call if the knee locks or will not fully straighten, which can mean a loose fragment, if the kneecap dislocates again, if the swelling is large or the knee is hot and red, or if the knee is still giving way after eight weeks of therapy.

Learn more

OrthoInfo (AAOS), Unstable Kneecap: orthoinfo.aaos.org (opens in a new tab)

My home program for patellar instability, and my guides to knock-knees and in-toeing.

This information is for educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. For specific concerns about your child, consult a board-certified, fellowship-trained pediatric orthopaedic surgeon.

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