Ultrasound is a method that allows the study of the peripatellar soft tissues and part of the trochlear groove,
in particular,
its hyaline cartilage and its relationship with the patellar tip.
Technique of study:
Patient lying in dorsal decubitus position,
with extension of both knees and feet in external rotation at 15 °,
with the US transducer placed axially on the thickest portion of the patellar tendon,
progressing cephalad until finding the tip of the patella (acoustic shadow) that allows us to assess its position in relation to the trochlear groove.
(Fig. 2, Fig. 3) Quantitative and qualitative echographic criteria have been developed for its evaluation.
Qualitative criteria:
- Trochlear dysplasia with quadriceps relaxation:
With the probe positioned just below the tip of the patella,
we visualize an axial image over the trochlear groove,
demonstrating the bone surface (hyperechogenic line) and the hypoechoic hyaline cartilage and its surface.
Classification of trochlear dysplasia (Dejour et al) (Fig. 4)
-Type A: the normal shape of the trochlea,
but a 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 and a vertical link between facets ('cliff pattern').
- Hyaline cartilage thickness:
There is a constitutional variability between patients regarding the thickness of the Hyaline cartilage,
which depends above all on age,
although it may be thinner in older patients.
In some situations,
we can find some defects or irregularities in the articular surface of the hyaline cartilage.
(Fig. 5)
Quantitative criteria:
- Cartilaginous trochlear angle:
The cartilaginous trochlear angle is valued by positioning the probe just below the tip of the patella,
with quadriceps relaxation,
at that level we evaluate the angle at the hyaline cartilage surface.
It is considered as positive for “femoropatellar dysplasia” when it is greater than 141°.
(Fig. 6)
- PTTG distance (Patellar tip-trochlear Groove):
At the subpatellar level,
in axial image over the trochlear groove,
we evaluate the acoustic shadow of the patella dynamically during relaxation and with maximum contraction of the quadriceps,
and we measure the distance between a vertical line passing over the sulcus of the trochlear groove,
considered the most rare point of the trochlear groove,
and through the patellar tip (the acoustic shadowing median line).
This measure is considered positive for patellar instability when the distance is above 10 mm.
(Fig. 7)