The kneecap (patella) is a disc- or cup-shaped bone located in front of the knee joint. It is embedded in the tendon of the powerful muscle on the front of the thigh (quadriceps femoris). The quadriceps femoris is one of the strongest muscles in the human body and is responsible for extending the knee joint. With every extension movement, the patella glides over the knee joint, using special articular surfaces designed for this purpose. These surfaces form what is known as a sliding bearing for the kneecap.
The patella in a healthy knee © Axel Kock | AdobeStock
Patellar dislocation is one of the most common injuries to the knee joint. As mentioned above, the kneecap (patella) normally sits centrally in front of the knee joint within its sliding bearing. If an unfortunate twisting of the knee joint or a collision occurs, the kneecap can slip out of its sliding bearing.
If the kneecap pops out completely, this is referred to as patellar dislocation. In this case, the patient is unable to walk and experiences severe pain. Since patellar dislocation generally requires severe trauma, a bone fracture (medically known as a patellar fracture) often occurs in addition to the dislocation. Furthermore, this very often results in bloody joint effusion, which can lead to pronounced swelling of the entire knee joint.
In cases of minor accidents involving only slight force, patellar subluxation is sometimes observed. In this instance, the patella pops out of its socket but then returns to its original position on its own, solely through muscle movement and the pull of the tendons. However, this requires that the patella be firmly embedded in the surrounding muscles and that there be minimal damage to that structure.
In cases of patellar subluxation, additional injuries to the joint capsule and ligaments are rare, as the mechanism of injury is less severe. In cases of patellar dislocation, however, accompanying injuries are often observed due to the strong force involved. In the worst-case scenario, this can result in severe cartilage damage, particularly behind the kneecap. This occurs, for example, when the kneecap is improperly reduced or when it is forced out of its gliding track with great force.
Immediately after the injury, severe bruising occurs in the knee joint, caused by damage to
.
This is accompanied by severe pain, which can make bending the leg or even walking nearly impossible. Therefore, in the case of a patellar dislocation, it is essential to seek professional treatment and physical therapy as soon as possible.
There are several factors that can increase the risk of patellar dislocation. External force, such as a collision, is the most common cause of the injury.
However, anatomical characteristics can also increase the risk. In cases of misalignment of the leg axes—known as bowlegs—the kneecap is pushed very far toward the outer edge of the gliding surface. As a result, less force or a smaller degree of twisting may be required to cause this injury.
Flattened patellae also pose a risk. Patients suffering from patellar dysplasia are also at risk. This is a congenital malformation of the patella.
Muscle strength also plays a role. If certain muscles are not strong enough, the patella can be pulled outward and thereby dislocated.
As previously described in detail, the capsular-ligamentous apparatus of the knee joint can be significantly damaged during patellar dislocations. This increases the risk of experiencing another patellar dislocation. However, particularly in cases of recurrent patellar dislocations—meaning the kneecap repeatedly pops out of place—cartilage and ligament injuries can occur.
Such recurrent patellar dislocations can eventually occur due to the pre-existing damage to the joint’s ligament and capsular structures, even after exposure to only minimal force. In some cases, they even occur spontaneously, while running or simply bending the knee joint.
Patellar dislocation causes very severe pain. The severity of the pain also depends on how many structures are affected by this injury. If
- cartilage
- tendons
- ligaments, or
- the joint capsule
are damaged, the pain is significantly more severe than when all structures remain intact. However, even without this “collateral damage” (a medical term referring to injury to surrounding structures), it is still a very painful condition.
Patellar dislocation is accompanied by severe pain in the knee © Aleksej | AdobeStock
If the kneecap dislocates, the affected person is often unable to fully straighten the leg. The leg is “locked” due to the displaced kneecap. Unfortunately—especially for sensitive patients—patellar dislocation is also visible. The kneecap pops out to the side, so that the knee simply no longer looks the way it should. It’s not a pretty sight, but it’s an unmistakable sign that the patella has slipped out of its track. Depending on which other structures are also damaged, it can also lead to a feeling of instability in the knee joint.
First, the kneecap must be expertly repositioned by an orthopedic surgeon. This procedure is very painful. Some patients require anesthesia or strong pain medication during the procedure. Afterward, the leg must be immobilized with a so-called Mecron splint. This keeps the leg straight and relieves pressure on the joint.
Now comes the crucial question: Should patellar dislocation be followed by surgery or conservative treatment?
If it was “only” a patellar dislocation in which no other structures were damaged, conservative therapy can be pursued. The first step here is to reduce the swelling in the knee. This is followed by mobilization and, with the help of targeted physical therapy, strengthening the muscles. The muscles on the front of the thigh are particularly important here, as they stabilize the kneecap in its position and help center it.
If recurrent dislocations occur or if other structures are damaged, knee surgery is usually required. During this procedure, the damaged structures are first repaired before, among other things, the so-called “lateral release” can be performed. This shifts the kneecap back toward the inner side.
Physical therapy is essential both for conservative treatment involving reduction and temporary immobilization and following surgery. It plays a key role in determining when athletes, for example, can resume their sport. This is particularly important in cases of anatomical conditions such as bowlegs, which can increase the risk of patellar dislocation. While there is never a 100% guarantee, the risk of another dislocation can be minimized through targeted exercise and muscle training.
Physical therapy is necessary after a patellar dislocation © contrastwerkstatt | AdobeStock
However, if the kneecap does become dislocated, a visit to a specialist is essential. The specialist will determine the next steps and discuss with the patient whether surgery is necessary. Patellar dislocation must always be followed by comprehensive therapy, as follow-up treatment is a crucial step toward complete recovery.
What is patellar dislocation?
In patellar dislocation, the kneecap pops out of its groove on the thighbone. This dislocation of the kneecap is also known as patellar dislocation. The dislocation often occurs laterally in the knee joint.
What are the symptoms of patellar dislocation?
Typical symptoms and complaints include severe pain, swelling, and instability of the kneecap. Many patients report that the kneecap suddenly pops out of place and experience pain in the front of the knee. In addition, cartilage damage or injuries to the medial patellofemoral ligament may occur.
How is the diagnosis made?
The diagnosis is made clinically through an examination of the knee, as well as via X-rays and other diagnostic tests. The orthopedic surgeon checks the patella’s alignment, the gliding mechanism, and any possible anatomically related changes, such as patella alta. Diagnosis and treatment depend on the severity of the condition and the stability of the knee joint.
When is surgery necessary?
Surgical treatment is often recommended for recurrent dislocations, bone injuries, or severe instability of the patella. The goal of surgery is to stabilize the patella and its supporting structures. Procedures such as MPFL reconstruction, arthroscopy, or trochleoplasty may be used.
Can patellar dislocation be treated conservatively?
Yes, conservative treatment is possible, especially after a first-time traumatic patellar dislocation. Physical therapy, muscle strengthening, and exercises for the quadriceps and vastus medialis muscles are helpful in this regard. The goal is to stabilize the knee joint and prevent further dislocations.
