Acute Knee Deformity
Radiology Cases in Pediatric Emergency Medicine
Volume 6, Case 7
Annabelle R. Mateo, MD
Kapiolani Medical Center For Women And Children
University of Hawaii John A. Burns School of Medicine
This is an 18 year old male with a history of
moderate psychomotor retardation and cerebral palsy,
who is brought to the ED by ambulance with a painful
and deformed left knee. As he attempted to stand up
from his chair, he turned his body and his knee
buckled. He heard a crack and fell onto the tile floor
landing on the lateral aspect of his left knee. The
patient kept his left knee in a flexed position and a
splint was applied by paramedics. There was no
breakage of the skin or prior history of any knee
trauma. He denies any numbness or tingling sensation.
Exam: VS T37, P86, R26, BP120/80. He is a
slender adolescent male in severe discomfort refusing
to move his left lower extremity. His left knee is visibly
deformed but no break in the skin or ecchymosis is
noted. His patella is displaced laterally. His left leg
and foot are warm with good distal pulses and intact
sensation. He is given morphine prior to obtaining AP,
lateral and sunrise views of his left knee.
View knee radiographs: Oblique, lateral and sunrise views.
View oblique view.
View lateral view.
View sunrise view.
An oblique view is taken instead of an AP because
of the patient's discomfort. The oblique and lateral
views are not very revealing, but the sunrise view
shows a laterally displaced patella.
The arrow points to where the patella should be.
The infrapatellar or "sunrise" view of the patella with
the knee at 20 degrees flexion isolates the patella.
This view is helpful in evaluating an older child or
adolescent since the patella ossifies after 5 years of
age. The patella is a sesamoid bone that articulates in
the groove between the femoral condyles [Arrow]. The
patella is stabilized by the quadriceps tendon
(superiorly), the patellar tendon (inferiorly), the vastus
medialis oblique (medially), and the iliotibial band and
vastus lateralis (laterally). The patella is rarely
fractured in childhood because the overlying cartilage
during early growth cushions it.
Patellar dislocations commonly occur between 16
and 20 years of age and tend to occur in females.
Acute patellar dislocations have occurred in simple falls
and sports injuries. Acute patellar dislocations almost
always occur laterally. These typically result from a
force displacing the patella while the foot is planted.
Commonly, patients feel like the knee cap is going to
pop out. This condition may spontaneously reduce or
Clinically, the swollen knee is kept in a flexed
position revealing a prominent medial femoral condyle.
Chronic recurrent subluxation or dislocation tends to
occur in patients with inherent mechanical
patellofemoral instability such as genu valgum
(knock-knees), shallow lateral femoral condyles,
elongated patellar tendon, deficient vastus medialis,
lateral insertion of the patellar tendon, shallow patellar
groove, ligamenous laxity and/or deformed patella.
Reduction of a Dislocated Patella
Nonoperative treatment is usually recommended.
Reduction of a laterally displaced patella requires
extension (preferably slight hyperextension) of the knee
while applying pressure directed medially to the
displaced patella which helps to guide the patella back
into position. Forceful manipulation is not needed and
should be avoided. Post reduction films should be
obtained to confirm the position of the patella and to
exclude the presence of an osteochondral fracture. A
posterior knee splint or knee immobilizer should be
placed for 4 to 6 weeks and an orthopedic referral is
recommended. Rehabilitation should be initiated as
soon as possible, starting with straight leg raises to
minimize quadriceps atrophy. Surgical repair is
considered in patients at risk for recurrent dislocation.
Approach to Acute Knee Pain
1) Medical history. Describe the quality of acute
knee pain and the location. Any previous knee injury or
pain should be documented.
2) Examination begins with inspection for
asymmetry, knee effusion, atrophy and congenital
anomalies. Palpate the knee to assess areas of
tenderness and crepitance. Muscle strength, joint
range of motion and gait should also be evaluated.
Palpation of the knee to ascertain tenderness and
crepitance should be done.
There are several tests for patellar instability. a)
The patella tilt test involves tilting the patella, which
normally should be less than 20 degrees. b) With the
knee flexed at 30 degrees, passive lateral and medial
movement of the patella evaluates for possible
dislocation or subluxation. This is referred to as the
patellar glide test. c) Similarly, the patellar
apprehension test is performed with the knee
flexed at 30 degrees while the examiner attempts to
sublux the patella laterally. It is positive if pain,
involuntary quadriceps contraction or sense of
"apprehension" is observed. This is indicative of a
previous patellar dislocation or subluxation.
3) Plain radiographs - AP, lateral at 30 to 45
degrees of flexion, and infrapatellar (sunrise) views
should be done in patients with recent knee trauma,
open physes, high energy contact sports, pain that
impairs activities of daily living, 3 or more months of
anterior knee pain and poor compliance. The AP view
identifies accessory ossification centers, osteochondral
fractures, and varus/valgus alignments. Patella alta
(high riding patella due to quadriceps contracture or an
avulsed patellar ligament) or patella baja (low riding
patella which may be congenital or due to ACL
reconstruction) may be diagnosed on lateral views.
Infrapatellar view (also called horizon view, Merchant's
view, tangential or sunrise view) assesses patellar
subluxation. The lateral patellar tilt, the sulcus angle,
and congruence angle are measurements that can be
made on the sunrise view which help to assess patellar
Acute Knee Injuries
1) A knee dislocation is considered an orthopedic
emergency since neurovascular compromise is
common. Lower extremity infarction is the
complication if vascular integrity is not restored.
The popliteal artery is involved in 30% to 40% of
dislocated knee injuries while the peroneal and tibial
nerves are frequently involved in traction injuries. Knee
dislocations are classified as anterior, posterior, medial,
lateral or rotary (anterolateral, posteromedial and
Anterior dislocation results from hyperextension and
is the most common type of knee dislocation. This
occurs when a person walking briskly, steps into a hole
hyperextending the lower extremity. This usually
results in a posterior capsule tear followed by a rupture
of the anterior cruciate and a partial tear of the posterior
cruciate. Popliteal arterial injuries occur secondary to
traction or laceration.
Posterior dislocation results when there is a direct
posterior force applied to the anterior tibia when the
knee is slightly flexed. The posterior capsule and
cruciates rupture as the tibia is displaced posteriorly.
Arterial injuries are not commonly seen.
Lateral dislocations typically involves damage to the
medial collateral, both cruciates and the medial
posterior capsule. This injury is occurs when there is a
violent abduction force on the tibia against the femur.
Arterial injuries are not common.
Medial dislocation occurs when there is a significant
adduction force on the tibia against the femur resulting
in damage to the lateral collateral ligaments, both
cruciate ligaments, the posterior capsule and
commonly, the peroneal nerve.
Rotary dislocations result from anteromedial or
anterolateral force on the anterior tibia.
2) Patellar fractures commonly occur in adolescents
and present as avulsion fractures from dislocations,
osteochondritis dessicans due to overuse, symptomatic
bipartite conditions, avulsion or "sleeve" fractures and
transverse displaced fractures. Clinically, patients with
direct tenderness over the superolateral patella should
be suspicious for bipartite patellar stress fracture and
fibrous nonunion. In nondisplaced fractures, a cast
should be placed for 4 to 6 weeks. Open reduction and
internal fixation (ORIF) is used to treat fractures that
are displaced more than 3 to 4 mm. Knee stiffness,
quadriceps atrophy, extensor lag and persistent pain
are possible complications of patellar fractures.
3) Most ligamentous injuries are commonly due to
direct knee trauma as seen in motor vehicle accidents
or during vigorous sports activities when the knee is
subjected to marked valgus and/or varus stresses. The
most frequently injured are the medial collateral and
anterior cruciate ligaments. Avulsion of the tibial spine
is associated with anterior cruciate ligament injury.
Radiographs should be routinely ordered to rule out
fractures since knee injuries in children less than
14 years of age are more commonly seen than
Avulsion of the tibial spine results from a
hyperflexion of the knee (as would occur during a fall
from a bicycle). Clinically, there is significant pain and
the patient refuses to bear weight. Hemarthrosis may
Avulsion fracture of the tibial tuberosity is
uncommon but is seen in adolescence. Males between
12 to 17 year old who actively engage in sporting
activities such as basketball or high jumping are at risk.
This injury usually occurs during jumping when the
quadriceps is strongly contracted that the tibial
tuberosity can be torn.
4) Patellar tendonitis is commonly seen in running
or jumping athletes who train or play on hard surfaces.
Clinically, pain and tenderness localizes at the patellar
tendon, inferior or superior aspects of the patella or
tibial tuberosity. Tendon thickening and dystrophic
calcification may be detected on lateral view
radiographs. Initial treatment modalities include rest,
ice, NSAIDS, local modalities and rehabilitation
programs to strengthen the muscles. Surgery is
reserved for complete tendon rupture and considered in
patients with refractory pain.
1) An unstable knee after traumatic injury is a
reduced dislocation until proven otherwise.
2) An acutely traumatized unstable knee with absent
distal pulses, an ability to move the toes or absent
sensation to touch and pinch requies emergent surgical
3) PE: inspection, palpation and distal
neurovascular examination. Consider obtaining
radiographs before a more stressful examination in
patients with extreme pain, swelling or inability to bear
weight. An effusion may not be evident because a tear
in the joint capsule will allow the blood to dissect into
the surrounding tissues. Hyperextension places an
unnecessary traction on the peroneal nerve and should
be avoided. Swelling in the popliteal fossa may be
indicative of popliteal artery injury.
4) AP and Lateral views are adequate. Infrapatellar
or "sunrise" view is recommended if patellar dislocation
or fracture is suspected.
5) Treatment : Reduction, immobilization,
assessment of vascular injuries and emergent referral.
6) All reductions should be confirmed
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