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.
On this page:
Epidemiology
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.
Clinical presentation
Patellar instability might display the following different clinical presentations 3:
- traumatic lateral patellar dislocation
- recurrent 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
- patella alta
- lateralization of the tibial tuberosity: femorotibial malrotation, genu recurvatum
Other factors contributing to patellar instability include 3,4:
- genu valgum
- increased femoral anteversion
- abnormal muscle tone, vastus medialis atrophy
- ligament or retinaculum injury or laxity (Ehlers-Danlos syndrome, Marfan syndrome)
- patellar dysplasia
- 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.
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
- medial patellofemoral ligament tear
- bone contusions or cartilage injuries of the medial patella facet and lateral femoral condyle
- edema/hemorrhage of vastus medialis muscle
- intra-articular fragments
Moreover, MRI can indicate risk factors such as trochlear dysplasia, patella alta and patellar translation for which all different measurements exist, such as:
- lateral trochlear inclination
- trochlear facet asymmetry
- trochlear depth
- ventral trochlear prominence
- Insall-Salvati ratio
- TT-TG distance (tibial tubercle to trochlear groove distance)
Treatment and prognosis
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
