Stability of the ankle mortise relies on the configuration of the osseus structures and the ligaments.
The main articulation is between the talus and the tibial plafond.
The saddle-shaped dome of the talus fits exactly into the tibial plafond and small disturbances in the congruity of this tibiotalar joint reduce the contact area and will overload the articular cartilage leading to arthrosis.

On the medial side the talotibial joint is firmly supported by the medial malleolus and the medial collateral ligament, which is stronger than its lateral counterpart.
On the lateral side there is a flexible support by the lateral complex which consists of fibula, syndesmosis and lateral collateral bands.
The syndesmosis is the fibrous connection between the fibula and tibia formed by the anterior and posterior tibiofibular ligament located at the level of the tibial plafond (French for ceiling) and the interosseus ligament which is the thickened lower portion of the interosseus membrane.
It is located 2cm above the tibial plafond where the superior recess of the joint ends.
The anterior and posterior tibiofibular ligaments are often referred to as anterior and posterior syndesmosis.
The lateral collateral ligaments connect the distal fibula to the talus and calcaneus.
The flexibility of the lateral complex allows talus and fibula to rotate and translate during normal ankle movements.
This fibular motion at the syndesmosis is an essential part of the normal ankle function.

A basic radiographic examination consists of a Mortise-view and a lateral view. Some add the AP-view.
The Mortise-view is an AP-view with 15-25° endorotation of the foot.
The technologist turns the foot inwards until the lateral malleolus is at the same height as the medial malleolus.
This view clearly demonstrates both lateral and medial joint spaces.
On a true AP-view the talus overlaps a portion of the lateral malleolus obscuring the lateral aspect of the ankle joint.
However the AP-view will give you a extra view on both malleoli from a different angle.
The lateral radiograph of the ankle should include the base of the fifth metatarsal because of the frequency of fractures at this side that clinically mimic a fracture of the ankle.

Ottawa Ankle Rules

These rules are used to determine the need for radiographs in patients with an injured ankle.

Ankle X-ray series are required only if there is:

Pain in the malleolar zone and any one of the following:

• Bone tenderness along the distal 6 cm of the posterior edge of the fibula or tip of the lateral malleolus.
• Bone tenderness along the distal 6 cm of the posterior edge of the tibia or tip of the medial malleolus.
• Inability to bear weight for 4 steps both immediately and in the emergency department.

Pull-off or Push-off fractures

Understanding how the ankle fractures is best done by studying the movements of the talus within the joint and looking at the forces applied to the collateral ligaments, the syndesmosis and the malleoli.
Injury to the ankle happens when the foot is fixed on the ground in supination (80%) or pronation (20%) and an exorotation or adduction force is applied.
Note: it is actually the leg and ankle fork that rotates upon the fixed foot, but these injuries are described as if it is the talus that rotates upon the leg.

It is important that you are able to determine if a fracture is the result of a pull off- or a push off force, because this tells you whether the foot was in pronation or supination and which rotatory or adduction force was applied.
The talus will either pull off a malleolus through tension on the ligaments or directly push off a malleolus (figure).
A transverse or horizontal fracture indicates a 'pull off', while an oblique or vertically oriented fracture indicates 'push off'.
So the form of the fracture indicates which forces were involved.

Ligamentous rupture or avulsion
Extreme tension in a ligament will either result in rupture of the ligament or an avulsionfracture at the insertion.
So each ligamentous rupture has its 'bony counterpart' or equivalent.
In supination extreme tension on the lateral side will either result in rupture of the lateral collateral bands, osseoligamentous avulsion or a transverse fracture of the lateral malleolus (pull off type).
Pronation will result in medial collateral band rupture or transverse fracture on the medial side.

Exorotation injury

Exorotation of the talus is the main deforming force in 75% of all ankle fractures and is seen in Weber B and C fractures.
These exorotation injuries follow a clockwise pattern from anterior via lateral to posterior and sometimes even medially.

1. Normal situation.
2. Exorotation of the talus.
3. Continuing exorotation of the talus will rupture the anterior tibiofibular ligament
4. Continuing force will fracture the fibula at the level of the joint in a Weber B-fracture or above the level of the syndesmosis in a Weber C-fracture.
5. Further posterolateral displacement of the lateral malleolus by the talus results in rupture of the posterior syndesmosis or avulsion of the malleolus tertius.
6. Finally the medial collateral band may rupture or the medial malleolus may avulse.

The ankle can be considered as a ring in which bones as well as ligaments play an equally important role in the maintenance of the stability of the joint.
If the ring is broken in one position the ring remains stable.
When it is broken in two positions the ring is unstable because a portion of it can be or is dislocated.

Ligamentous rupture in combination with a fracture can lead to instability, because the ring is broken in two places.
Ligamentous rupture can sometimes be assumed on the basis of the Lauge-Hansen classification.
Widening of the ankle mortise is another sign of ligamentous rupture.
The medial clear space should not exceed 4 mm and usually measures the same as the distance between the tibial plafond and the talus.
Widening of the medial joint space up to 6 mm or more requires disruption of the medial collateral ligament.

Lateral clear space
The lateral clear space is measured from the medial border of the fibula to the lateral border of the posterior tibia 1cm above the tibial plafond.
It is less well defined because it's width varies with positioning.
Evident widening of the lateral clear space indicates syndesmotic rupture.
Some state that a width of 5.5mm is abnormal.
It is very important to realize, that a normal lateral or medial clear space does not exclude ligamentous rupture.
It simply means that there is no dislocation, but there still can be instability.
The case on the left shows a Weber B fracture.
On these images the ankle fork is normal.
Both medial and lateral clear space are prominent, but within normal limits.
We can conclude that there is no dislocation, but we do not know if the ankle is unstable.
Continue with the images post surgery.

In the same case after osteosynthesis there is obvious widening of medial and lateral clear space.
This indicates that there is a syndesmotic rupture and medial collateral band rupture.
The ankle joint is unstable and dislocated.
A re-operation was necessary with placement of a syndesmotic screw to stabilize the ankle joint.

Another case on the left shows a Weber B fracture.
Both medial and lateral clear space are widened indicating instability.
The talus is displaced laterally.
Patient was sceduled for osteosynthesis of the fibula fracture and placement of a syndesmotic screw if necessary.
After osteosynthesis of the fibula, the ankle was tested in the operating room and found to be stable.
So there was no indication for placement of a syndesmotic screw.
It was concluded that the syndesmosis was only partially ruptured (as is usually the case in Weber B fractures).

Weber classification system
This system dates from 1966 and is based upon the level of the fibula fracture.
Type A is below the level of the syndesmosis, type B is at the level of the syndesmosis and in type C the fracture of the fibula is above the level of the syndesmosis.
The higher the level of the fracture the more chance for syndesmotic rupture and instability.

Lauge-Hansen system
This system dates from 1950 and is based upon the position of the foot at the time of injury and the direction of the force on the talus resulting in the following fracture-types: supination-adduction , supination exorotation and pronation exorotation.
These basic fracture types of Lauge-Hansen are futher divided into stages reflecting the sequential disruption of bone and ligaments.
The Lauge-Hansen system is essentially the same as the Weber system, but netter defines the ligamental injury.

Weber A - Supination-Adduction

20-25% of all ankle fractures.
The foot is fixed on the ground in supination and an adduction force is applied to the talus.
The first injury will occur on the lateral side which is under tension.

Stage 1.
Supination results in a tear of the lateral collateral ligament or avulsion fracture of the lateral malleolus below the level of the tibial plafond.

Stage 2.
More talar tilt results in the medial malleolus being pushed off in a vertical or oblique way .

Weber B - Supination-Exorotation

50-60% of all ankle fractures.
The foot is fixed on the ground in supination and an exorotation force is applied to the talus.
The first injury will occur on the lateral side which is under tension.

Stage 1
As the talus exorotates first the anterior tibiofibular ligament is ruptured.

Stage 2
Since the foot is in supination, the lateral malleolus is held tight by the lateral collateral ligaments and cannot move away without breaking.
As a result, more rotation of the talus will fracture the fibula in an oblique or spiral fashion because the lateral malleolus is pushed off from anterior to posterior.
The fracture starts at the level of the ankle joint and extends proximally.

Stage 3
Posterior displacement of the lateral malleolus fragment by the talus results in rupture of the posterior tibiofibular ligament or avulsion of the malleolus tertius.

Stage 4
More posterior movement of the talus will result in extreme tension on the medial side and the deltoid ligament will either rupture or pull off the medial malleolus in a transverse way.

Weber C - Pronation-Exorotation

20% of ankle fractures.
The foot is fixed on the ground in pronation and an exorotation force is applied to the talus.
The first injury will occur on the medial side which is under tension.

Stage 1
Tension on the medial complex leads to rupture of medial collateral band or a medial malleolar avulsion fracture.

Stage 2
The talus rotates externally and moves laterally because it is free from its medial attachment.
Due to the pronation of the foot the lateral complex is completely relaxed and the talus will easily force the fibula to twist and move laterally resulting in rupture of the anterior syndesmotic ligament.

Stage 3
The interosseus membrane will rupture up to a point where the fibular shaft will fracture.
This level depends on the amount of external rotation.
More exorotation will result in a high fibular fracture or Maissoneuve fracture. Less exorotaion and more abduction will result in a fracture only just above the talotibial joint.
Stage 4
Finally the posterior syndesmotic ligament ruptures or avulses the posterior malleolus.

Start with a basic interpretation and look for fractures and signs of ligamentous rupture.
This interpretation will direct you to both a Weber as well as a Lauge-Hansen classification.
The Lauge-Hansen classification will give you the fracture mechanism and the preliminary stage of the ankle injury.
Now re-examine the films to make sure that you do not overlook a higher stage ankle injury.
After this re-examination you can make a final report.
In the final report the fracture is described according to Weber and according to Lauge-Hansen.
Describe the number of malleoli involved and if there are signs of instability or dislocation.

Example 1

• Basic interpretation
  There is a medial malleolar fracture.
  You interprete this as an avulsion fracture.
  
• Classification
  Not possible to classify according to Weber, but according to Lauge Hansen a medial avulsion fracture indicates that the foot probably was in pronation at the moment of injury.
  So this injury is probably a pronation exorotation injury (PER) stage 1 or higher.

• Re-examination
  You re-examine the x-rays to look for stage 2 (rupture or avulsion of the anterior syndesmosis), stage 3 (high fibular fracture = Weber C) or even stage 4 (rupture or avulsion of posterior syndesmosis).
  So at second look you notice a subtle widening of the lateral clear space on the original films, which could indicate but is definitely no proof of a syndesmotic rupture.
  Although the patient is already in a cast you order additional films to look for a possible stage 3.
  These films show a high fibular fracture and a subtle malleolus tertius avulsion.
• Final report
  Weber C fracture or a PER stage 4 according to Lauge-Hansen. This is an unstable fracture that needs surgical repair.
  The ankle circle is interrupted at two places i.e. the medial malleolus and the syndesmosis.
  A syndesmotic screw was inserted.

Example 2

• Basic interpretation
  Transverse lateral malleolar fracture.
• Classification
  Weber A and Supination Adduction stage 1.
• Re-examination
  No sign of SA stage 2 (medial malleolar fracture)
• Final report
  Stable Weber A or SA stage 1 fracture.
  Patient will be treated conservatively.

Example 3

• Basic interpretation
  Dislocated bimalleolar fracture. Avulsion fracture of the medial malleolar.
  The lateral malleolus is 'pushed off' from anterior to posterior.
  
• Classification
  The fracture starts at the level of the ankle joint and extends proximally, i.e. a Weber B fracture.
  According to Lauge Hansen the oblique fibular fracture indicates Supination Exorotation injury stage 2 or higher.
• Re-examination
  Look for stage 3 (posterior syndesmotic rupture or avulsion of the posterior malleolus) and stage 4 (rupture of the deltoid ligament or medial malleolar avulsion).
  Only now you notice the posterior malleolar fracture on the lateral view.
• Final report
  Trimalleolar fracture. Weber B. SER stage 4 (Lauge-Hansen).
  This is an unstable fracture with dislocation that needs surgical repair.
  The size of the posterior malleolar fragment is probably less than 25% of the articular AP-diameter and will need no separate repair.
  Sometimes CT is needed to get a better impression of the size of the fracture fragment of the posterior malleolus.

Example 4

• Basic interpretation
  Fracture of the lateral malleolus starting anteriorly at the level of the joint extending proximally posteriorly.
• Classification
  The fracture is classified according to Weber as a type B fracture.
  
  According to Lauge Hansen the oblique fibular fracture indicates that this is a Supination Exorotation injury stage 2 or higher.
• Re-examination
  Look for stage 3 and stage 4.
  On the lateral view the posterior cortex of the tibia is interrupted indicating a fracture of the malleolus tertius, i.e.stage 3.
  There is a widened medial clear space, which indicates a rupture of the medial collateral band, i.e. stage 4.
• Final report
  Weber B fracture. According to Lauge-Hansen this is a SER stage 4.
  This is an unstable fracture with dislocation that needs surgical repair.