How to Interpret Knee Films

It’s just a simple hinge joint, but with many complex problems!

Positioning

• So your patient comes in with a knee looking like this…

• So you want to order some imaging for them…• Well, what will radiology have to do?

Positioning

• There are 3-views, 4-views, wt-bearing, non-wt-bearing, sunrise, etc.

• It is important to consider the mechanism when considering the tests because the pt might not be able to get into a position to have the pictures taken…

Wt-bearing P/A Standard A/P Standard lateral Tunnel view Sunrise view Merchant view

Frank ED, Merrill's Atlas of Radiographic Positioning and Procedures, 2007

Views

• Sunrise: Best at evaluating the patella• Tunnel: Best for evaluating intercondylar

notch• Lateral: Best at identifying fat-fluid levels

(lipohemarthrosis) suggesting intra-articular fractures

Know Your Rules

Ottawa Knee Rules• Age 2-55• No fibular head TTP• No isolated patellar TTP• Able to flex 90 degrees• Able to weight bear for 4

steps after injury and in ED (regardless of limping)

Validated in children age 2-16 (Annals EM 42:1, 2003)

Pittsburgh Knee Rules• No fall or blunt knee trauma• Age 12-50yo• Able to walk 4 weight

bearing steps in the ED

More specific than Ottawa (Ann Emerg Med 32:8 1998)

You DON’T need to get an Xray if…

Standard A/P View

• A/P and Lat (standard 2 view) is 79% sensitive for fxs• Adding 2 oblique views (4-view) increases sensitivity to 85%

Standard Lateral View

Poor Image AcquisitionA good lateral film should have…• Overlapping femoral

condyles (unlike here, red arrows)

• Fibula behind tibia (unlike here, yellow arrow)

• Patella should have two hyperlucencies on anterior and posterior aspects (here it just looks weird)

Improved View

• This is the more ideal lateral

• Note the overlapping condyles (red arrows)

“DOH”!!!Common DOH findings…1. Knee dislocations are

either there or they’re not…

2. The occult fractures are most common: along the tibial plateau, to the patella, or to the proximal lateral tibia (Segond)

3. The only half pathology is the Maissoneuvre fx (see ankle radiology)

The knee is the perfect joint to apply the “DOH” pneumonic

• Dislocations?• Occult fractures?• Half pathology?

There is plenty of minutia, but we are responsible for the big stuff…

Fracture DataRelative frequencies of fractures to the knee in adults

1. Patella (40%)2. Tibial plateau (32%)3. Fibular head (9%)4. Distal femur (8%)5. Tibial spine (7%)6. Tibial tuberosity (2%)7. Osteochondral junction

(1%)--Stiell 1996, Weber 1995, Bauer 1995

Most frequently overlooked fractures in an ED

1. Tibial plateau (16%)2. Radial head (14%)3. Elbow – child (14%)4. Scaphoid (13%)5. Calcaneus (10%)6. Patella (6%)7. Ribs (4%)

--Data from Freed and Shields 1984

The Patella• Most common bony

element of the knee injured (account for 1% of ALL bony fractures)

• Most common in pts 20-50yo, men>women 2:1

• Fracture usually following direct trauma or forceful quads contraction

• When evaluating for TTP, avoid performing the patellar grind test (is diagnostic of chondromalacia pattelae, not fracture)

Trochlear groove

Patellar Fracture ClassificationsFrom Hohl M, Johnson EE, Wiss DA. Fractures of the knee, in Rockwood CA Jr, Green DP, Bucholz RW (eds): Fractures in Adults, 3d ed, vol. 2. Philadelphia, Lippincott, 1991, p.

1765.

• Transverse most common

Obvious Fractures

Transverse fractures commonly result in wide fragment separation due to strong ligamentous traction

The A/P view often makes visualization difficult, but should still be reviewed

Patellar Fractures

Interrogate the cortical borders for any irregularities (blue arrow), circle the patella like clockwork (red arrow)

The sunrise view is the best way to isolate the patella to evaluate for injury

Management

• Non-displaced– Intact extensor function: knee immobilizer, rest, ice,

analgesia, encourage WBAT– Diminished extensor function: immobilize, rest, ice,

analgesia, NWB status, Ortho referral 3-5d for ORIF• Displaced >3mm– Knee immobilizer, NWB status, ice, analgesia, early

Ortho referral for ORIF• Severely comminuted or open– Admit for OR, empiric ABx if open

Sunrise View

• This is only indicated for patients in which you suspect a vertical fracture

• If you have a patient with an obvious transverse fracture, flexion of the knee could cause further separation

Merchant’s ViewModified sunrise, requires the angle to be 30°

1. The more prominent condyle (blue arrow) denotes the side being imaged (i.e. if it is prominent on the left, it is the left femur)

2. A normal patella has a degree of tilt to it (lower right image)

3. The upper right image demonstrates patellar subluxation as it is rotated lateral to the trochlear groove

Trochlear groove

Pathologic Vertical Fracture

The fracture line extends from the cortical margin, is incomplete

Patellar ZebraBipartite Patella

• Normal anatomic variant, commonly misinterpreted as vertical fracture

• Note the clean borders and lack of cortical margin disruption

• Most often located superolateral

• If in doubt, get other knee (is bilateral in 50% of cases)

Patellar Positioning

• Patella “alta” and “baja” denote a high-riding and low-riding patella, respectively, and can be identified by using Blumensaat’s Line

• This is a line drawn by the oblique hyperlucent shadow of the distal femur (see left)

Patellar Sleeve Fracture• Unique to children• M>F 3:1, peak age 12.7yrs• Avulsion fracture of the

distal patellar pole• MOI: Forceful quadriceps

contraction against a fixed lower leg or high impact jumping

• PE: Look for hemarthrosis, decreased ability to extend leg, local pain and TTP

• Tx: Knee immobilizer and ortho f/u for ORIF 1. Patella alta (relationship to Blumensaat’s line)

2. May see small fragments of avulsed bone (blue arrows), but this is not always presentBates DG, Hresko MT, and Jaramillo D. Patellar sleeve fracture:

Demonstration with MR imaging. Radiology 1994;193:825-827.Hunt D and Somashekar N. A review of sleeve fracture of the patella in children. The Knee 2005;12:3-7.

Patellar Sleeve Fracture

• Hemarthrosis and physical exam findings are more predictive than radiographic evidence

• There is a high morbidity associated with this injury, so a low index of suspicion should be held

Avulsed fragment

Hemarthrosis

Patella alta

Dislocations

Not very subtle…

Patellar Dislocations• Most common knee

injury in children• MOI: Pivoting on a

planted leg• Presentation: Patella

laterally located and knee held in flexion

• Associated fracture: Lateral femoral condyle or medial patellar margin

Tibial Plateau

• Tibia bears 85% of knee wt

• Fxs to articular surface (plateau) often have high morbidity if undiagnosed

• Common fx mechanisms…– Direct valgus/varus force

(lateral/medial blow)– Compressive force (fall)

The most important area to thoroughly interrogate!

Fxs are 2/2 direct impaction of femoral condyles onto tibia

Tibial Plateau Fractures: ClassificationsBased on the Schatzker scheme…1. Lateral condylar split2. Split-compression3. Pure lateral

compression 4. Medial condylar split5. Bicondylar split6. Split with

metadiaphysial extension

Difficult to See

• Most TPFs are minimally displaced, making their visualization difficult– In addition, they most commonly occur along an

oblique plane and are not parallel to the x-ray beam in any view

– Moreover, the tibial plateau surface slopes inferiorly from anterior to posterior, meaning the cortical surface of the plateau is never parallel to the x-ray beam

Subtleties of the Tibia• The normal (blue

arrow) tibial trabeculae are more dense medially (this is where most of the weight cephalad is bore)

• If the lateral plateau is more radiopaque, consider a compression fracture

Hemarthrosis

• Sometimes, all you get is a history, physical, and some subtle radiology findings and we are expected to make the diagnosis.

• Look to the suprapatellar bursa for signs of a lipohemarthrosis that would indicate an underlying TPF (blue arrow)

Type I: Lateral Split

• Ensure knee stability on physical exam (especially MCL/ACL)

• Tx: – Undisplaced/

displaced, stable knee: Immobilize, NWB status 6-8wks

– Displaced w/ condylar widening or unstable exam: Immobilize, NWB, will need surgery

Type I• Closely evaluate the

plateau for any disruptions in the cortical margin (blue arrow)

• Note the increased trabecular density laterally as compared to medial (yellow circle)

Type II: Split-Compression• Commonly associated

with…– Fibular head fxs– Ligamentous injury

(19%)• LCL most commonly

• Depression of >4mm is clinically significant

• From the ED, immobilize and NWB status until ortho f/u for surgery

Split

Depression

Type II: Split-Compression

Note the fracture line (red arrow) and slightly depressed articular surface (blue

arrow)

Loss of the cortical rim of the lateral fragment (red arrows) and a subtle

depression (blue arrow) give this away

Type III: Pure Compression• No associated lateral wedge

fracture but apparent central or peripheral depression

• More common in the elderly (osteoporotic)

• Seldom causes instability• Position of knee at time of

injury usually dictates severity of compression (flexed 5x worse than extended)

• Most treated non-operatively:– Immobilize and strict

NWB for 8-12wks

Type III

Note the cortical depression (yellow arrows) without wedge component.

Note the increased trabecular markings (blue circle) drawing your attention to the region

Type III (Lateral)

Note the cortical

findings on the A/P and the obvious

depression is only visualized on the lateral

Type IV: Medial Split• Indicates a higher

force of injury than types I-III

• Beware of underlying vascular and ligamentous damage (consider arteriography)

• Intercondylar eminence prone to fracture as well

Immobilize and NWB status w/ Ortho referral to decide on need for OR

Type V: Bicondylar

• Occasionally, can have an “upside-down Y” appearance • 50% have meniscal detachment, 33% have ACL avulsions

Type VI: Metaphysis Extension

All that needs to be said about these is …”Ouch”

Bicondylar w/ metaphyseal extension

Suprapatellar Bursa

The suprapatellar bursa is bounded by the quadriceps tendon anteriorly and should measure less than 5mm

<5mm

Effusions• These are often the only clues to a more significant underlying injury• Best seen on lateral radiographs in the suprapatellar bursa, posterior to

the quadriceps tendon

LipohemarthrosisBlood and fat do not mix, with the fat (radiolucent)

layering on top of the blood (radiodense)

Can Use Ultrasound if Unclear

Fat is hyperechoic (light) and blood is hypoechoic (dark) on ultrasound

Segond Fracture

• Proximal lateral tibial avulsion fracture 2/2 a rupture from the lateral capsular ligament

• Associated with ACL (>75%) and meniscal (67%) injuries

• Immobilizer, NWB status, ortho f/u

Segond Fracture

Occasionally, there can be a “mirror” Segond where the

same process occurs to the proximomedial

aspect and is associated with MCL and PCL injuries as well as the medial

meniscus. (shown is a typical

Segond, not mirror)