Patellofemoral instability

Last revised by Yuranga Weerakkody on 4 Jul 2023

Patellofemoral instability or maltracking is the clinical syndrome due to morphologic abnormalities in the patellofemoral joint where the patella is prone to recurrent lateral dislocation.

Most patients with patellar instability are young and active individuals, especially females in the second decade. Prevalence is 6-77 per 100,000 population 2.

Patellar instability might display the following different clinical presentations 3:

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.

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:

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

  • medial patellofemoral ligament reconstruction

  • trochleoplasty

  • medialisation of the tibial tuberosity

  • medial capsular plication

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Cases and figures

  • Figure 1: summary
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  • Case 1: patellar height ratio
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  • Case 2: trochear facet asymmetry
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  • Case 3: lateral trochlear inclination
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  • Case 4: lateral patellofemoral angle
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  • Case 5: 'Q' angle
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  • Case 6: trochlear depth
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  • Case 7: Caton-Deschamps index
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  • Case 8: transient patellar dislocation
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  • Case 9
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  • Case 10: TT-TG distance
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  • Case 11
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