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Patellar luxation, patellar instability

How does patellar luxation develop?

The kneecap (lat. patella) is a bone that is embedded in the tendon of the four-headed thigh muscle (quadriceps) and facilitates movement in the knee joint. On the back surface, the patella is covered with cartilage that allows low-friction sliding with the thigh bone (femur).

Due to its nature or due to an injury, the patella can deviate outwards from its sliding bearing (lateralisation of the patella or subluxation of the patella) or completely jump out of its sliding bearing (patella luxation). This often results in injuries to the cartilage layer (cartilage damage). Young women are most often affected by patella luxation.

The following factors favour patella luxation:

  • Congenital malformation of the patellar gliding bearing (trochleadysplasia)
  • Congenital malformation of the patella (patella dysplasia)
  • Valgus malalignment (knock knees)
  • Rotational malalignment of the knee joint
  • Raised patella (patella alta)

Symptoms of patellar luxation

Patellar luxation is usually very painful and associated with knee joint effusion. After popping out, the patella usually does not remain fixed on the outside, but spontaneously springs back into its sliding bearing. This can result in shearing of cartilage/bone fragments. If the fragments are larger, this can lead to symptoms of entrapment and blockage of the knee joint.

Diagnosis for patellar luxation

Based on a detailed description of the mechanism of injury and the symptoms of discomfort as well as a detailed clinical examination, the diagnosis of patellar luxation can be made. X-rays in 2 planes serve to exclude bony injuries. For further therapy planning, it must be determined whether there are any factors that favour a patellar luxation. Special X-rays (Defilée images), computer tomography (CT) and magnetic resonance imaging (MRI) examinations are used for this purpose. MRI is also a very good way of assessing possible cartilage damage and other concomitant injuries.

Therapy of patellar luxation

The choice of therapy depends on whether the patellar dislocation is congenital or purely traumatic and whether there are concomitant injuries such as cartilage damage. In the case of a traumatic initial dislocation without further risk factors and without concomitant injuries, a non-surgical (conservative) therapy can be carried out.  For this purpose, the knee is immobilised with a splint.

If there are risk factors for a new patellar luxation or if a luxation has already occurred several times, surgery is necessary. The type of surgery depends on the exact cause of the patellar luxation. For this reason, a detailed diagnosis is essential. Possible surgical procedures are: Reconstruction of the medial patellofemoral ligament (MPFL-plasty), tuberosity dislocation, conversion osteotomy, trochleaplasty.

If a sheared bone-cartilage fragment is present, the fragment can be refixed during surgery or, in the case of smaller fragments, removed to prevent further cartilage damage. You can read in detail about the treatment of cartilage damage in the menu item Cartilage damage.

Aftercare for patellar luxation

The aftercare depends very much on the type of surgical treatment and is often associated with splint immobilisation and/or relief or partial weight-bearing.