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Introduction
Diagnosis of patellofemoral instability can be
difficult because patellar instability, patello
femoral pain, meniscal and cruciate ligament
insufficiencies can produce a similar presen
tation of nonspecific pain and knee instability.
Careful examination and imaging are helpful in
making the correct diagnosis. In addition to the
history, evaluation should include a search for
abnormal limb alignment, patellar tilt, crepitus,
patellar tracking, tenderness, apprehension,
and laxity [1]. Imaging is another component
to the diagnosis of patellofemoral instability.
Fithian
et al.
demonstrated that recurrent
dislocators had specific factors such as smaller
angles of Laurin. A smaller angle of Laurin
demonstrates lateral rotation of the patella
along in its cephalad-caudal axis and larger
lateral patellar overhang on imaging at the time
of their first dislocation [2]. Historically, plain
radiographic imaging and subsequently CT
have dominated the clinician’s attention
regarding the evaluation of the patellofemoral
joint. More recently, MRI has gained a more
prominent role in the imaging armamentarium
for patellofemoral instability because it gives
the clinician the ability to visualize the
cartilaginous articular surfaces and ligaments
in addition to the bony alignment observed on
X-ray and CT.
Patellofemoral instability is a problem
orthopedic surgeons have tried to address as
early as Albee in 1915 [3]. Merchant helped
visualize the patellofemoral joint in 1974 with
patient’s knee flexed at 45° and the camera
angled at 30° [4]. Dejour
et al
. utilized lateral
radiographs to analyze osseous femoral
morphology (i.e. trochlear dysplasia) to classify
trochlear dysplasia into 4 types (I-IV) based on
radiographic findings of the crossing sign,
trochlear bump >3mm, and of trochlear depth
<4mm [5, 6]. Radiographic and CT based
standard values for the patellofemoral joint are
well established. Measurements of trochlear
morphology like lateral trochlear inclinination
<11°, sulcus angle >145°, and trochlear groove
depth <4mm have been well documented
cutoffs [1, 5-7]. Limb geometry evaluation
with measurements like Tibial Tuberosity-
Trochlear Groove (TTTG) distance, which was
first established by Judet
et al.
, have proven
useful for evaluating patients with instability. A
TTTG value greater than 20mm is considered
pathologic [5, 8]. Standards for Caton-
Deschamps and Insall-Salvati ratios indicative
of
patella alta
, are based in either plain films or
CT imaging [5, 9, 10]. Even the analysis of
dynamic stabilizers, as measured by their
effects on patellar tilt, also have their roots in
CT and plain films [5, 11-14].
Magnetic Resonance Imaging
in Patellofemoral Instability
M. Charles, R. Afra, D.C. Fithian