Radiographic detection of posterior malleolus fractures of the distal tibia can be difficult. Consideration of: the mechanism of injury; clinical presentation; radiographic soft tissue signs; pattern recognition; and positioning traps can assist in avoiding missed fractures.

"Posterior malleolus fractures may occur in isolation or, more commonly, in association with bimalleolar or trimalleolar fracture patterns. These fractures result from posterior tibiofibular ligament avulsion, or bony impaction from the talus." (Orthopedic Knowledge Online, Treatment of Posterior Malleolus Fractures )

The ankle has three important potential soft tissue signs.

1. Soft Tissue Symmetry

The thickness of the soft tissues overlying the medial malleolus and lateral malleolus should be approximately equal. This patient has increased soft tissue thickness over the lateral malleolus associated with a largely undisplaced spiral fracture of the distal fibula	Compare with this normal ankle

2. Ankle Effusion- The Teardrop Sign

An ankle effusion suggests a significant injury to the ankle joint. The anterior and posterior juxta-capsular region of a normal ankle joint should appear as a fat-like density. In the presence of an ankle effusion, the capsule can become distended and may appear to have a more fluid-like density

normal ankle demonstrating a fat -like density (arrowed)	abnormal ankle demonstrating a fluid-like density (arrowed)

3. Kager's Fat Pad

It is not uncommon for ankle injuries to involve Kager's fat pad. A careful examination of the density, shape and borders of Kager's fat pad can provide indicators of bony injury to the ankle. An abnormal Kager's fat pad does not indicate definite bony injury to the ankle.

Justin Q. Ly and Liem T. Bui-Mansfield Anatomy of and Abnormalities Associated with Kager’s Fat Pad AJR:182, January 2004

A poorly positioned lateral ankle can either hide a posterior malleolus fracture or reveal a posterior malleolus fracture. The following case studies demonstrate both types of cases

Case 1

Patient History

This patient presented to the Emergency Department with a painful and swollen ankle. The mechanism of injury was unknown. The referring doctor requested an ankle x-ray examination to rule out any bony injury.

Case3

Patient History

Twisted right ankle. Not weight-bearing

Images

	The AP ankle image demonstrates soft tissue thickening laterally. There is no evidence of an associated displaced fracture of the fibula
	The oblique ankle image demonstrates no displaced fractures
	The lateral ankle image demonstrates the following ankle effusion (lower white arrow) abnormal Kager's fatpad (black arrow) very subtle defect in cortex of posterior malleolus of tibia (top white arrow)

Discussion

The radiographer assessed the routine views in the context of the patient's mechanism of injury and clinical features. She considered there was a reasonable possibility that this patient had sustained a fracture of the posterior malleolus of the tibia and there was a fair chance of demonstrating such a fracture by performing off-lateral views of the ankle.

This is an under-rotated lateral. No displaced fracture is demonstrated

bingo! the over-rotated lateral position demonstrates a minimally displaced fracture of the posterior malleolus. Interestingly, this a very similar position to that which failed to demonstrate the posterior malleolus in case 1 and case 2. It is noteworthy that when the radiographer returned to the patient to perform these supplementary views, the referring doctor was discharging the patient with an ankle sprain. Following the demonstration of the fracture on the supplementary views, the patient had a plaster cast applied to his ankle and a follow-up appointment was made for an orthopaedic clinic review in 2 weeks. If you think you can't make a difference to patient outcomes as a radiographer...I would beg to differ!

Posterior malleolus fractures of the distal tibia are easily missed, particularly when they exist in isolation and there is minimal displacement of the fragment. There is no guarantee that a perfectly positioned lateral ankle will demonstrate a patient's fracture. Indeed, a perfect lateral ankle position may hide the fracture. A consideration of the patient's mechanism of injury, clinical features, and radiographic soft tissue signs will decrease the likelihood of a missed fracture. It is noteworthy that case three was performed by a newly graduated radiographer. The level of proficiency demonstrated in this case is achievable by junior radiographic staff.