In our institution we use a Siemens Magnetom Avanto 1.5 T MRI system,
equipped with a dedicated knee coil.
Acquisition starts with the knee placed within a quadrature standard knee coil.
The foot is placed 15 degrees in external rotation.
Pads are placed under the heel,
to raise the foot and place the knee close to zero degrees extension (Fig. 1 and Fig. 2).
Afterwards a routine MRI knee study is performed (sagittal,
axial and coronal proton density,
fat saturated,
images and sagittal T1 and T2* images - Fig. 3).
Next,
the knee coil is removed and both knees rest on the table (in extension,
with the lower limb axis parallel to the longitudinal axis of the table,
feet 15 degrees in external rotation - Fig. 4).
Fast bilateral axial proton density images are planed (Field of view = 30cm; interval = 5mm; slice thickness = 4mm),
the first without quadriceps contraction and the second with quadriceps contraction (from the base of the patella to the tibial tuberosity - Fig. 5 and Fig. 6).
Total time to perform the study is around 20-25minutes (CT protocol of the extensor apparatus takes around 10minutes).
On the reviewing workstation (software - Osirix MD 7.03 or Syngo VE31H),
the following measurements are taken:
- Tibial tuberosity–trochlear groove distance (Fig. 7) - The distance between two lines,
both perpendicular to the posterior intercondylar line,
one intersecting the deepest area of the trochlear groove and the second the tip of the tibial tuberosity (which corresponds to the midpoint of the patellar tendon distal insertion).
Normal value - < 15mm (a distance between 15 and 20mm is considered borderline; greater than 20mm is abnormal).
- Patellar tilt,
at rest and after quadriceps contraction (Fig. 8 and Fig. 9) - The angle between a line joining the extremities of medial and lateral patellar facets and a line tangent to the posterior femoral condyles.
Normal value - < 20º.
- Tibial tuberosity-posterior cruciate ligament distance (Fig. 10) - The distance between the center of the patellar tendon insertion and the medial border of the posterior cruciate ligament,
using a line parallel to the dorsal tibial condylar line.
Normal value - < 24mm.
- Lateral trochlear inclination (Fig. 11) - Angle between a line parallel to lateral trochlear facet and a line connecting the posterior aspects of femoral condyles.
Normal value - > 11º.
- Trochlear facet asymmetry (Fig. 12) - The ratio of the length of medial trochlear facet to the length of the lateral facet.
Normal value - > 40%.
- Trochlear depth (Fig. 13) - The distance between a line intersecting the anterior aspects of femoral condyles and the deepest point of the sulcus.
Normal value - > 3mm.
- Trochlear groove angle (Fig. 14) - The angle between two lines connecting the highest point of femoral condyles to the deepest point of trochlear groove.
Normal value - < 145º.
- Patellotrochlear index (Fig. 15) - The ratio between the craniocaudal diameter of the trochlea and patella,
on sagittal images. Normal value - > 12%.
- Insall-Salvati ratio (Fig. 16) - The ratio between patellar tendon length and patella length,
on sagittal images.
Normal value - between 0.8 and 1.2 .
Tips:
Bilateral axial proton density images are used to determine the tibial tuberosity–trochlear groove distance (TT-TG) and patellar tilt.
Unilateral images are used to obtain the remain measurements.
Lateral trochlear inclination,
trochlear facet asymmetry,
trochlear depth and trochlear groove angle all use the same axial image (the image with the “Roman arch” appearance of the posterior intercondylar fossa,
where both epicondyles are seen - Fig. 17).
Patellotrochlear index should use the sagittal image with the largest contact between patellar and trochlear cartilage.
Bony landmarks are used to avoid variability related to the cartilage thickness.