Head


chapter 10: Patellar Dislocation

Radiologic Features

CONGENITAL FORM

Before the appearance of the ossific nucleus of the patella at approximately age 4 years, the radiograph is of no help in establishing the diagnosis. Once the ossific nucleus has formed, the patella will be seen (even on the AP view) to be lying lateral to its normal position (Fig. 10.8A). The associated valgus may also be evident on the AP film. Ficat (41) has reported an associated hypoplasia of the medial tibial plateau and corresponding medial femoral condyle in his cases. Later, when the patella is more fully ossified, it may be seen to be smaller than normal and more proximally situated. An associated external torsion of the tibia may give the appearance of dissociation of the femoral and tibial outlines on the AP view.

The lateral radiograph (Fig. 10.8B), when taken in a few degrees of flexion, suggests absence of the patella, because it is hidden by the femoral condyles. In addition, the condyles themselves often appear flattened anteriorly, particularly the lateral condyle.

The axial view (Fig. 10.9) shows the lateral position of the patella, with its articular surface sitting against the lateral border of the lateral condyle. The patella itself is generally smaller than the contralateral side, and its articular surface is either flat or slightly convex. The sulcus of the trochlea appears shallow, particularly in the 30‑degree flexion axial view, where a groove is sometimes barely evident and occasionally absent. CT or MRI study can be helpful in defining the trochlear morphology in detail. The shallow trochlea in these patients is presumably due to failure of modeling of the sulcus during growth due to lack of pressure from the patella in its normal anterior position. Brattstrom (2) has shown this to cause the shallow sulcus in recurrent dislocation of the patella, and it would seem logical that this same situation exists in permanent congenital dislocation.

ACQUIRED FORM

Because the presentation of this form is later, the radiograph corroborates the physical findings. In this form, the AP view may be considered normal because the patella more frequently returns to near the normal position. There is usually only a relatively short period of time between the onset of the condition and the presentation of the patient for evaluation so that secondary deformities such as external tibial torsion and valgus de­formities are much less common. The axial view will show the patella to be dislocated (Fig. 10.10), but both sulcus and patellar form are much more likely to be normal un­less the condition has been neglected for a long time. The precise lateral will show the lateral facet edge posterior to the central ridge (Fig. 10.11) in milder cases. Computerized tomography illustrates how chronic lateral dislocation will destroy the normal trochlea morphology (Fig. 10.12).

Because age of presentation for the acquired form is older, the patella is easily palpable and visualized. It is usually more nearly in the midline with the leg fully extended. Neither fixed flexion contracture nor quadriceps lag is present. The remainder of the physical examination is nearly the same as for the congenital form.

        

Inside Chapter 10: