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 Dislocation
     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 
remain dislocated.
     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 
ligamentous injuries.
     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 
be evident.
     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.

Key Points:  
     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

     Nichols AW.  Anterior Knee Pain.  Atlas of Office 
Procedures 1998;1(2):297-317.
     Young G.  Reduction of Common Joint Dislocations 
(Chapter 108).  In:  Henretig, FM, King C (eds).  
Textbook of Pediatric Emergency Procedures.  
Baltimore, Williams & Wilkins, 1997, pp. 1075-1103.
     Bachman D, Santora S.  Orthopedic Trauma.  In:  
Fleisher GR, Ludwig S (eds).  Synopsis of Pediatric 
Emergency Medicine.  Baltimore, Williams & Wilkins, 
1996, pp. 674-691.
     The Knee, Fibular, and Patellar Dislocations 
(Chapter 28).  In:  Simon RR, Koenigsknecht SJ.  
Emergency Orthopedics:  The Extremities, second 
edition.  Norwalk, CT, Appleton & Lange, 1987, pp. 
     The Knee (Chapter 4G).  In:  Chung SMK.  
Handbook of Pediatric Orthopedics.  New York, Van 
Nostrand Reinhold Company, 1986, pp. 150-169.
     Busch MT.  Sports Medicine in Children and 
Adolescents (Chapter 34).  In:  Morissy RT (ed).  Lovell 
and Winter's Pediatric Orthopaedics, third edition.  J.B. 
Lippincott Company, 1990, pp. 1091-1120.

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Web Page Author:
Loren Yamamoto, MD, MPH
Professor of Pediatrics
University of Hawaii John A. Burns School of Medicine