Patellofemoral instability is the where the patella does not remain in the trochlear groove during normal knee range of motion.
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Epidemiology
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.
Clinical presentation
Patellofemoral instability might display the following different clinical presentations 3,12:
patellar subluxation
recurrent symptomatic patellar instability
permanent patellar dislocations
Pathology
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:
trochlear dysplasia: the trochlear joint surface is flattened
lateralization of the tibial tuberosity: femorotibial malrotation, genu recurvatum
Other factors contributing to patellar instability include 3,4:
increased femoral anteversion
abnormal muscle tone, vastus medialis atrophy
ligament or retinaculum injury or laxity (Ehlers-Danlos syndrome, Marfan syndrome)
changes of Osgood-Schlatter disease 8
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 subtyped into 10:
-
lateral instability
in early flexion (<45 degrees): most common
in late flexion (>45 degrees)
medial instability: uncommon, mostly iatrogenic
multidirectional
Radiographic features
Plain radiograph
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).
MRI
In the acute and subacute setting MRI displays features of lateral patellar dislocation as:
knee joint effusion
bone contusions or cartilage injuries of the medial patella facet and lateral femoral condyle
edema/hemorrhage of vastus medialis muscle
Moreover, MRI can indicate risk factors such as trochlear dysplasia, patella alta and patellar translation for which all different measurements exist, such as 12:
patellar height using Insall-Salvati ratio, Caton-Deschamps index, patellotrochlear index 9
tibial tuberosity lateralization using TT-TG distance, TT-PCL distance
Treatment and prognosis
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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