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CT Evaluation of Patellar Instability
Poster No.:
C-2157
Congress:
ECR 2014
Type:
Educational Exhibit
Authors:
R. Ruef, C. Edgar, C. Lebedis, A. Guermazi, A. Kompel, A.
Murakami; Boston, MA/US
Keywords:
Epidemiology, Dysplasias, Congenital, Surgery, Physiological
studies, Education, CT, Conventional radiography,
Musculoskeletal joint, Extremities, Anatomy
DOI:
10.1594/ecr2014/C-2157
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Learning objectives
To review the pathophysiology, physical exam, imaging findings, and treatment of
patellar instability. In this poster, we will focus on the role of CT in dynamically tracking
patellar alignment, more definitively characterizing trochlear dysplasia, and diagnosing
subluxation and malalignment.
Background
Maintaining the stability of the patellofemoral joint is biomechanically complex,
with contributions from the bony architecture of the patella and trochlea, muscles,
and ligaments. The articulation is dynamic, and characterized by varied levels of
patellofemoral engagement with varied degrees of flexion.
The patella is most vulnerable to lateral dislocation in extension and early flexion. Several
anatomic factors contribute to this: The trochlear groove is most robust along its lateral
aspect, proximally. This contributes maximally to stability in early flexion. In full extension,
the patella is completely disengaged from the trochlea. At extension, the quadriceps
exerts its maximal lateral force on the patella (measured by the q angle). This lateral force
vector decreases with flexion. The quadriceps also exerts a posterior force vector, which
helps pull the patella into the trochlea. This vector is maximal in flexion, and minimal in
extension.
In patella alta, the patella engages later in flexion. The maximal lateral force vector and
minimal posterior force vector seen in extension, combined with delayed engagement
with the trochlea predisposes patients with patella alta to lateral dislocations. In
some patients, the trochlea has a shallow, dysplastic groove resulting in reduced
osseous constraint of the patella, and decreased stability of th articulation. The medial
patellofemoral ligament also restrains the patella, primarily in extension and early flexion.
This is often injured in lateral dislocations, contributing to chronic instability. This may
be a candidate for repair in some cases to prevent recurrent dislocations. A more lateral
tibial tubercle, relative to the patella, results in a higher lateral force vector on the patella
throughout its course, predisposing patients to dislocation.
Findings and procedure details
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In patients with patellar instability, on examination, the patella may be seen to move
laterally with full extension (the J sign). Pain with compression of the patella as it moves
through the trochlea is suggestive of articular cartilage injury, which often occurs with a
subluxation event. Patients with instability, often also show apprehension when lateral
pressure is exerted on the patella in early flexion.
CT is an important modality in quantifying the severity of trochlear dysplasia and patellar
malalignment. Patellar tracking CT is preformed at our institution as follows: The patient
is placed supine. 4 contiguous 1.5 mm axial images are obtained through the mid patella,
perpendicular to the long axis of the patella at 0, 10, 20, 30, 40, 50, and 60 degrees of
flexion. Following this, an acquisition through the entire knee is preformed at 0 degrees
of flexion. These images can be used assess the trochlear sulcus angle and patellar
tilt angles at varying degrees of flexion, as well as the trochlear depth. A reconstructed
image is used to measure the tibial tubercle-trochlear groove distance. This measures
the laterality of the tibial tubercle relative to the trochlear groove, on superimposed axial
CT images, with >20 mm considered abnormal. A more lateral tibial tuberosity may be
surgically medialized to help reduce the lateral force vector on the patella. The patellar
height ratio, also known as the Insall-Salvati ratio is used to diagnose patella alta on the
lateral radiograph. This is the ratio of the length of the patella, to the length of the patellar
tendon, with a ratio >1.3 considered abnormal.
Measurement
Vaules
Trochlear depth
<3 mm indicative of a shallow, dysplastic
trochlea
Patellar tilt angle
>5 degrees indicative of lateral patellar tilt
Patellar height ratio
>1.3 indicative of patella alta
Trochlear sulcus angle
>145 degrees indicative of a shallow,
dysplastic trochlea
Tibial tubercle - trochlear groove
>20 mm associated with patellar
instability
Table 1
Images for this section:
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Fig. 1: Patellar height ratio measurement. The ratio is measured as the length of the long
axis of the patella, to the length of the patellar tendon. >1.3 is considered abnormal.
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Fig. 2: CT acquisitions acquired from 0 to 60 degrees of flexion allow dynamic
assessment of the patellofemoral articulation.
Fig. 3: The trochlear sulcus angle is measured using lines crossing through both condylar
eminences, and the trochlear sulcus.
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Fig. 4: The patellar tilt angle is measured using a line through the long axis of the patella,
and a line parallel to a line through the posterior aspect of the femoral condyles. Angles
>5 degrees are considered abnormal.
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Fig. 5: Trochlear groove depth is measured as the distance between a line drawn
tangential to the anterior margin of the medial and lateral femoral condyles and the
deepest part of the femoral notch, with less than 3 mm considered abnormal.
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Fig. 6: The tibial tubercle - trochlear groove distance is measured on superimposed axial
images.
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Fig. 7: A shallow, dysplastic trochlea.
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Fig. 8: This patient's trochlear depth is at the lower limit of normal, and the patellar tilt
angle of 16 degrees is abnormal.
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Fig. 9: This patient had previously dislocated her patella. The tibial tubercle - trochlear
groove ratio of 25 mm is abnormal. This increases the lateral force vector seen by the
patella, and predisposes patients to lateral dislocation.
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Fig. 10: The femur on the left is normal. The femur on the right is dysplastic. The facets
appear more prominent, because the are flatter and pointed coronally.
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Conclusion
Patellar instability is a predisposing factor for patellar dislocation. Patellar stability
is maintained by various stabilizing muscles and ligaments, particularly the medial
patellofemoral ligament. It is further affected by bony anatomic variations of the trochlear
groove, and the relative location of the tibial tuberosity. Evaluation for the etiology of
instability predisposing patient's towards dislocation is primarily preformed with CT. The
ability to dynamically characterize the tibiofemoral articulation confers an advantage to
CT, and consequently, helps guide treatment approach.
Personal information
References
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Novicoff, Shahin Sheibani-Rad, Khaled J. Saleh; Patellar Instability. The Journal of Bone
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