Patellofemoral instability

Last revised by Henry Knipe on 20 Oct 2023

Patellofemoral instability is the where the patella does not remain in the trochlear groove during normal knee range of motion.

Most patients with patellar instability are young and active individuals, especially females in their second decade 2. Incidence is ~25 (range 6-77) per 100,000 person-years 2,9,12.

Patellofemoral instability might display the following different clinical presentations 3,12:

The normal patellofemoral joint has two kinds of stabilizers - active stabilizers (extensor muscles) and passive stabilizers (bones and ligaments):

  • bone stabilizers: deep femoral sulcus, high lateral trochlea

  • ligament stabilizers: medial patellofemoral retinaculum, medial patellofemoral ligament

Three major morphologic abnormalities that predispose to patellar instability:

Other factors contributing to patellar instability include 3,4​:

The most common mechanism of first-time patellar dislocation is internal rotation of the femur relative to the knee (i.e. exorotation of the tibia relative to the femur) while the foot is planted and the knee is flexed.

Patellofemoral instability can be classified into 10:

  • lateral instability

    • in early flexion (<45 degrees): most common

    • in late flexion (>45 degrees)

  • medial instability: uncommon, mostly iatrogenic

  • multidirectional

The "crossing sign" represents an abnormally elevated floor of the trochlear groove rising above the top of the wall of one of the femoral condyles, assessed on lateral radiographs. "Double contour sign" is a double line at the anterior aspect of condyles and is seen if the medial condyle is hypoplastic. The skyline view can show decreased trochlear depth and a large sulcus angle (>144º).

In the acute and subacute setting MRI displays features of lateral patellar dislocation as:

Moreover, MRI can indicate risk factors such as trochlear dysplasia, patella alta and patellar translation for which all different measurements exist, such as 12:

Nearly half of the patients with first-time dislocation will sustain recurrent dislocation after conservative management. Chronic/repeated patellofemoral instability, if not treated, may lead to severe arthritis and chondromalacia patellae 11. The aim of the surgery is to repair the knee damage caused by patellar dislocation and to correct the primary anatomical anomaly. Surgical treaments include 9,10:

  • medial patellofemoral ligament reconstruction +/- lateral retincular release

  • medialisation of the tibial tuberosity

  • trochleoplasty

and often a combination of procedures will be performed 9.

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