LIVRE JDG 2012

Imaging in patellofemoral instability must

identify the 4 classic factors implied in the

genesis of the instability – trochlear dysplasia,

patella alta, abnormal tibial tubercle – trochlear

groove distance (TT-TG) and patellar tilt

(excessive patellar tilt with medial ligamentous

disruption) – specially in the chronic setting. In

the acute cases, imaging is sometimes the only

element to provide the diagnosis.

Trochlear dysplasia

Trochlear dysplasia is the most important factor

implied in the genesis of patellar instability [1]

since the femoral sulcus is not sufficient to

provide the osseous restraint to the patella.

Standard lateral X-ray films with perfect

superimposition of the posterior medial and

lateral femoral condyles are the key to the

diagnosis of dysplasia. The crossing sign is

typically found in this projection and represents

the point where trochlea becomes flat (the

bottom of the groove reaches the height of the

facets) [1, 2]. Additional findings include the

double-contour sign (representing the

hypoplastic medial facet found posterior to the

lateral one) and the supratrochlear spur (found

in the superolateral aspect of the trochlea) [3,

4] (fig. 1).

Axial X-ray views performed in 45° of knee

flexion allow the measurement of the sulcus

angle [5]. Normal mean value is 138° (SD±6)

[6]. Angles above 150° are found in trochlear

dysplasia. An important issue when analyzing

axial views is that X-rays obtained with higher

flexion angles image the lower part of the

trochlea, frequently missing the dysplasia

present in its upper portion [7]. For this reason,

images obtained at 30° of flexion are

preferred.

Imaging in patellofemoral

instability

P.R.F. Saggin, P. Ferrua,

P.G. Ntagiopoulos, D. Dejour

Fig. 1: Trochlear dysplasia

features on the lateral view.