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P.R.F. Saggin, P. Ferrua, P.G. Ntagiopoulos, D. Dejour
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CT and MRI have the ability to image the entire
trochlea in sequential cuts, allowing better
visualization of the dysplastic upper part.
Frequently, the dysplasia found in these
modalities is missed on X-ray axial views.
Comparing CT and MRI images, bony and
cartilaginous trochlear anatomy do not match
perfectly [8], but this looses importance in the
dysplasia setting, where flat or convex bone
will be covered by analogous cartilage.
Carillon
et al.
[9] investigated the lateral
trochlear inclination angle on axial MRI cuts
(LTI, calculated by means of a line tangential
to the subchondral bone of the posterior aspect
of the two femoral condyles crossed with a line
tangential to the subchondral bone of the lateral
trochlear facet) in healthy and patellar
instability patients. A significant difference
between groups was recorded. The mean value
in patellar instability patients was 6.17° while
in the control group it was 16.9°. Choosing 11°
as the threshold value for LTI, results were
excellent in discrimination between the two
groups with sensitivity of 93%, specificity of
87% and an accuracy of 90%.
Based primarily on X-ray lateral views, and
helped by CT or MRI axial cuts, one can
classify trochlear dysplasia in four types (D.
Dejour) [3, 4, 10]:
- Type A:
presence of crossing sign in lateral
true view. The trochlea is shallower than
normal, but still symmetric and concave.
- Type B:
crossing sign and trochlear spur. The
trochlea is flat or convex in axial images.
- Type C:
presence of crossing sign and the
double-contour sign on the lateral view,
representing the medial hypoplasic facet.
There is no spur, and in axial views, the lateral
facet is convex and the medial hypoplasic.
- Type D:
crossing sign, supratrochlear spur
and double-contour sign. In axial views, there
is clear asymmetry of the facets height, also
referred as a cliff pattern.
Lippacher
et al.
[11] analyzed intraobserver
and interobserver agreements of radiographic
and MRI-based D. Dejour’s classification.
They concluded that better overall agreement
was found for a 2-grade analysis on MRI scans
(type A, low grade, versus types B, C and D
combined, representing high grade dysplasia),
and that lateral radiographs tended to
underestimate the severity of dysplasia
compared with axial MRI views.
Patellaalta (Patellar
height)
Patella alta refers to an abnormally high riding
patella that engages the trochlear groove later
in flexion, increasing the patellar “free” arch of
movement and facilitating dislocation.
As in trochlear dysplasia, X-ray lateral views
are the key to the diagnosis of the patellar
height. Several methods of measurement (and
diagnosis) using the tibia as reference have
been described, the three main are (fig. 5):
- Caton and Deschamps
[12]: is the ratio
between the distance from the lower edge of
the patellar articular surface to the
anterosuperior angle of the tibia outline (AT),
and the length of the articular surface of the
patella (AP). A ratio (AT/AP) of 0.6 and
smaller determines patella infera, and a ratio
greater than 1.2 indicates patella alta.
- Insall and Salvatti
[14]: is the ratio between
the length of the patellar tendon (LT) and the
longest sagittal diameter of the patella (LP).
Insall determined that this ratio (LT/LP) is
normally 1. A ratio smaller than 0.8 indicates a
patella infera and greater than 1.2 patella alta.
- Blackburne-Peel
[15]: is the ratio between
the length of the perpendicular line drawn
from the tangent to the tibial plateau until the
inferior pole of the articular surface of the
patella (A) and the length of the articular
surface of the patella (B). The normal ratio
(A/B) was defined as 0.8. In patella infera it is
smaller than 0.5, in patella alta greater than
1.0. (fig. 2).
Patellar height using the tibia as reference can
also be measured on MRI. Miller
et al.
[16]
applied the Insall-Salvati method to 46 knees
comparing MRI and radiographs. Good-to-
excellent correlation between the values was
found, and they concluded that patellar height
can be reliably assessed on sagittal MR imaging