Unstable kneecap

The kneecap (patella) is connected dynamically to the thighbone (femur) via a groove (trochlea). Both the patella and this femoral groove are coated in cartilage to reduce friction. The kneecap is a connective link between the thigh muscle (quadriceps) and the shinbone (tibia). As the quadriceps, along with other muscles, bend and extend your leg, the patella slides up and down accordingly. If the kneecap stays in this groove, then you hardly even notice that it’s there. You go about your life sitting, walking, running and squatting smoothly. If the kneecap should slip out of its groove, though, you become very aware that it is there.  If your kneecap becomes unstable, locomotion can be negatively affected.

Athletes who play very physically demanding sports are most at risk for unstable kneecap. The reason that many football players wear knee braces is to protect against such an injury. Without proper medical intervention and care, an unstable kneecap can cause permanent damage including severe arthritis.

Anatomy of the kneecap

The kneecap, while seemingly a small part of the human body, bears an outsized burden when it comes to balance and locomotion. It is not merely protection for the knee joint, it also is a key connection point between the tibia and quadriceps vital for leg bending and extending. This area of the body consists of the following:

  • Patella (the kneecap, which is a flat bone)
  • Patella tendon (attaches the kneecap to the tibia)
  • Femur (the thighbone, containing the groove which holds the patella)
  • Tibia (the shinbone, to which the patella is connected via the patella tendon)
  • Quadriceps muscles (located on the front of the thigh, they are connected to the kneecap)


An unstable kneecap can result from a traumatic injury or, if one is very predisposed to sustaining one, from normal, everyday movement. Causes of unstable kneecap include:

  • Intense impact – The kneecap can become dislodged from its groove because of a blow (most frequently associated with sports injuries).
  • Anatomical abnormality – If the femoral groove is not straight or is not sufficiently deep, the kneecap can slip out.


The symptoms of an unstable kneecap are consistent, though they vary in severity. Generally speaking, if you have an unstable kneecap, you will no longer be able to bear your own body weight, and bending your knee will cause you a lot of pain. Additionally, your knee will periodically lock up, swell, stiffen and make “popping” sounds. The following are common unstable kneecap symptoms:

  • Pain
  • Locking knee
  • Inability to walk without aid
  • Swelling
  • Stiffening
  • Knee “popping”

The symptoms of an unstable kneecap may be mistaken for the symptoms of other medical conditions such as kneecap dislocation or prepatellar bursitis. Make sure you consult a doctor to determine if you have an unstable kneecap, and get the appropriate treatment. 


Unstable kneecap can be divided into two main types:

  • Laterally unstable – When the kneecap moves out of its groove and toward the outside of the leg, you have a laterally unstable kneecap. This is the more common type.
  • Medially unstable – When the kneecap moves out of its groove and toward the inside of the leg, you have a medially unstable kneecap, which is less common.

The multidisciplinary team of knee experts at Northwell Health Orthopaedic Institute treats unstable kneecaps as well as a broad range of conditions that affect the joints.

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