The kneecap (patella) is the largest sesamoid bone in the body. It has two joint surfaces covered with thick articular cartilage because of the heavy loads it bears. The kneecap articulates with the thigh bone (femur), and more specifically with the femoral trochlea – a kind of “groove” – forming the patellofemoral joint of the knee. As long as the kneecap stays within the groove as the knee bends and straightens, a person can walk, run, sit and stand. When the kneecap “slips” out of the groove, it causes pain and problems with how the joint works.
Cause
The kneecap connects the muscles at the front of the thigh to the shin bone (tibia). As the knee bends and straightens, the kneecap is pulled up or down. As mentioned above, the femur has a notch or groove called the femoral trochlea. The kneecap articulates and moves within this groove. If the groove is shallow or uneven, however, the kneecap can slip out, resulting in a dislocation. Even a direct blow can cause the kneecap to dislocate.
Symptoms
- The knee locks and can no longer support the body's weight
- Pain at the front of the knee that increases with activity
- Pain when sitting
- Stiffness
- Crepitus (“grinding”) during movement
- Swelling
Diagnosis
During the physical examination, the orthopaedic surgeon asks the patient to walk and to straighten and bend the knee. This is followed by a careful examination of the area around the kneecap and measurements of its alignment.
X-rays: These are useful for assessing how the kneecap and the femur fit together and for ruling out other injuries.
Treatment
If the kneecap is dislocated, the first step is to reduce it (that is, put it back in place). Sometimes it goes back by itself. At other times the orthopaedic surgeon has to apply gentle pressure to push the kneecap back into place.
A kneecap dislocation can damage the joint surfaces of the kneecap and the femur, leading to further pain and osteoarthritis. Arthroscopic surgery can correct this.
If the kneecap has partially dislocated, it can be treated conservatively with specific exercises and braces. The exercises help strengthen the thigh muscles so that the kneecap stays aligned.
Cycling is often recommended as part of physiotherapy. In addition, special soft braces can help with the patient's recovery. The goal is for the patient to return to daily activities within one to three months.
If the kneecap remains unstable, surgery may be needed. During the operation the kneecap is realigned, either by repairing and reinforcing the medial retinacular ligaments or by releasing the ligaments that pull the kneecap out of place (release).
Kneecap dislocation
The kneecap articulates with the thigh bone (femur), and specifically with the trochlea. This is a groove in which the kneecap glides as the knee bends and straightens. Sometimes, after an injury, the kneecap can come out of place – that is, dislocate – either partially or completely.
When the kneecap dislocates, it causes pain and an inability to bend or straighten the knee. Even when it goes back into place by itself – without the help of an orthopaedic surgeon – further treatment is needed.
Causes
There are several ways in which the kneecap can dislocate. Often very little force is needed for it to dislocate because of abnormalities in the structure of the child's knee.
- Underdevelopment (hypoplasia) of the femoral condyles, leading to an abnormal, “shallow” trochlea, a condition that predisposes to instability or dislocation of the kneecap.
- Some children have looser ligaments than normal, which also makes the knee joint lax and predisposes to kneecap dislocations. This is more common in girls during adolescence and often affects both knees.
- Children with cerebral palsy or Down syndrome have kneecaps that dislocate more easily, mainly because of generalised muscle weakness and poor balance.
- Very rarely, some children have congenital instability of the kneecaps, leading to painless dislocations from a very young age.
In healthy children with normal knees, a kneecap dislocation is often the result of a direct blow or a fall to the ground during sport. However, the kneecap can also dislocate without a direct blow, for example after a sudden twisting movement.
Symptoms
A patient's symptoms after a kneecap dislocation depend on whether it was a complete or partial dislocation and on whether it caused damage to the kneecap itself.
Some of the most common symptoms include:
- Pain
- A feeling that the kneecap is unstable. The patient feels the kneecap “slipping” out of place
- A characteristic sound at the moment of dislocation
- Swelling
- Visible deformity of the joint – a change in the appearance of the knee.
Clinical examination
A kneecap dislocation is treated in the emergency department of the hospital on duty, as it needs to be reduced immediately. In most cases, however, it has already gone back into place before the patient sees an orthopaedic surgeon.
During the examination, the doctor asks for a detailed description of how the injury happened and of the child's symptoms. They then examine the range of motion and tenderness of the knee, taking care not to cause the patient unnecessary pain.
Imaging
X-rays: These give a detailed view of the bones of the knee. Special (tangential) views show the relationship between the kneecap and the trochlea.
Magnetic resonance imaging (MRI): This shows the soft tissues of the knee, such as the ligaments, in detail. It is not a “first-line” test, as it is not needed to diagnose the dislocation. Nevertheless, it can rule out other associated injuries, such as loose fragments of the kneecap's articular cartilage inside the joint.
Immediate treatment
If the kneecap dislocates, the young patient should be treated in the emergency department. Before any manipulation, the orthopaedic surgeon gives painkillers and then gently moves the kneecap back into place.
Non-surgical treatment
IMMOBILISATION: The doctor fits a brace for 3 to 4 weeks, which stabilises the knee until the pain subsides.
OFFLOADING THE LEG: Putting weight on the affected leg can cause pain and slow down recovery. For this reason, crutches are sometimes recommended.
PHYSIOTHERAPY: After the period of immobilisation, the orthopaedic surgeon recommends physiotherapy so that the child regains strength and range of motion. Specific exercises help strengthen the thigh muscles. Sticking to the physiotherapy programme is important for returning to sport, which is achieved within 3 to 6 weeks.
A kneecap dislocation injures the ligaments that hold the kneecap in place, so it becomes increasingly unstable after repeated injuries. As a result, the kneecap dislocates with injuries of less and less force. Recurrences are more common in children who have anatomical abnormalities of the knee. Regular knee exercise, such as cycling, can strengthen the quadriceps muscle and help prevent kneecap dislocations.
Surgical treatment
If the child's knee shows marked patellofemoral instability despite conservative treatment, surgical repair is recommended, and the type of operation will depend on the causes of the instability.
Surgical repair involves reconstructing the ligaments that hold the kneecap in place. This operation is often performed arthroscopically, using a small camera and special surgical instruments inserted into the joint through small incisions.
Recurrent kneecap dislocations caused by anatomical variations and abnormalities may require more complex surgery.
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