Patellar instability

Patellar instability is the morphologic abnormality in patellofemoral joint, which leads to recurrent patellar dislocations.

Epidemiology

Most patients with patellar instability are young and active individuals, especially females in the 2nd decade. Prevalence is 6-77 per 100,000 population².

Pathology

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.

Most common mechanism of first time patellar dislocation is a flexed position of knee with internal rotation of planted foot with a valgus component.

Anatomical variants of patellar stabilizers are trochlear dysplasia, patella alta and lateralization of tibial tuberosity. Secondary factors contributing to patellar instability are femorotibial malrotation, genu recurvatum and ligament laxity (Ehler-Danlos syndrome, Marfan syndrome).

Trochlear dysplasia

Trochlear joint surface is flattened.

Plain film 

The "crossing sign" is a line represented by deepest part of trochlear groove, crossing the anterior aspect of condyles, assesed on lateral radiographs. "Double contour sign" is a double line at anterior aspect of condyles, and seen if medial condyle is hypoplastic. Skyline view can show decreased  trochlear depth and large sulcus angle (>150 degree).

MRI

Lateral trochlear inclination, trochlear facet asymmetry and trochlear depth, are used to judge trochlear dysplasia (See images of Case 1).

Classification of trochlear dysplasia (Dejour et al)³

• type A : normal shape of trochlea, but shallow trochlear groove
• type B : markedly flattened or even convex trochlea
• type C : trochlear facet asymmetry, with too high lateral facet, and hypoplastic medial facet
• type D : type C features + vertical link between facets ('cliff pattern')

Lateral trochlear inclination

The most superior section showing trochlear cartilage is selected for this measurement. The inclination is the angle formed between the plane of the lateral trochlear facet subchondral bone and a tangential line through posterior femoral condyles (See images from Case 1). An angle of less than 11 degrees is considered abnormal.

Trochlear facet asymmetry

This is the ratio of medial trochlear facet width to lateral trochlear facet width and measured in the axial plane (As described in images from Case 1). A ratio of less than 0.4 is considered abnormal (ie. it is abnormal if the medial facet is less than 40% the width of the lateral facet).  

Trochlear depth

This measures the inset depth of the trochlear groove (sulcus) relative to the mean of the medial and lateral femoral condyle outsets.  It is determined on axial imaging at the same level as trochlear facet asymmetry. A trochlear depth of less than 3mm is considered abnormal. 

Patella alta

Distance from tibial tubercle to trochlear groove

It is being advocated as one of the accurate measures of patellar instability. For this measurement, axial images at these levels are superimposed by increasing the MIP thickness. The distance from the tibial tuberosity to the trochlear groove is measured parallel to the tangential line through posterior femoral condyles. A distance of less than 15mm is considered normal, 15 to 20mm is considered borderline and a distance greater than 20mm is considered abnormal.

Associated radiological features of patellar instability

• knee joint effusion
• medial patellofemoral ligament tear
• bone contusion in patella and lateral condyle
• osteochondral defects in patella
• edema/hemorrhage of vastus medialis
• intra-articular loose bodies
• internal derangement of knee

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. 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
• medialization of tibial tuberosity
• medial capsular plication