Charles MD, Haloman S, Chen L, Ward SR, Fithian D, Afra R.
Plain films and computed tomography (CT)-based imaging were the first
to establish measurements that evaluated patellar instability. Limited research
has shown the efficacy of magnetic resonance imaging (MRI) in evaluating these
To identify morphological differences between normal knees and those
with patellofemoral instability on MRI to determine what measurements are
significant and how MRI-based means differ from historical means based on
radiograph and CT imaging.
Case control study; Level of evidence, 3.
Eighty-one controls and 40 patients with recurrent patellar instability
between 2006 and 2010 were reviewed. The control patients had a history and an
examination negative for patellofemoral symptoms. Patients with patellar
instability had a history of at least 2 frank patellofemoral joint dislocations
(PFJDs). The MRI images were obtained on the nonweightbearing knee in full
extension. Measurements of patellar tilt, trochlear morphologic characteristics,
and tibial tuberosity-trochlear groove (TTTG) distance were evaluated on axial
slices, and patellar height was measured on sagittal images. Trochlear shape was
assessed at the proximal and distal trochlea.
All measurements of patellar tilt (mean ± SD) were found to be
significantly different between the 2 groups. For patellar height, the
Insall-Salvati ratio (control, 1.08 ± 0.02; PFJD, 1.26 ± 0.03) and
Caton-Deschamps ratio (control, 1.13 ± 0.02; PFJD, 1.29 ± 0.03) proved to be
significantly different. Trochlear morphologic characteristics had numerous
measurements prove to be significantly different proximally and distally. These
included classic measurements such as sulcus angle (control, 148.48° ± 0.94°;
PFJD, 165.57° ± 2.65°) and lateral trochlear inclination (control, 21.27° ±
0.66°; PFJD, 13.31° ± 1.36°) proximally and less established measurements such as
the ratio of external (lateral) trochlea to internal (medial) trochlea (control,
1.51 ± 0.05; PFJD, 2.11 ± 0.17), a measurement of facet asymmetry.
The MRI-based patellar tilt measures proved to be an excellent group
of measurements for delineating between controls and those with instability.
Patella alta ratios, such as Insall-Salvati and Caton-Deschamps, demonstrated a
statistically significant difference between normal and recurrent dislocators.
Trochlear measurements proved significantly different at the proximal and distal
trochlea. Our findings demonstrate that MRI is appropriate to help discern
recognized pathologic abnormalities that characterize patellofemoral instability.
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