Posterior Malleolus Fractures
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.
Mechanism of Injury
Anatomy"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." <a class="external" href="http://www5.aaos.org/oko/description.cfm?topic=TRA024&referringPage=mainmenu.cfm" rel="nofollow" target="_blank">(Orthopedic Knowledge Online, Treatment of Posterior Malleolus Fractures )</a>
The Soft Tissue Signs
The ankle has three important potential soft tissue signs.
1. Soft Tissue Symmetry
2. Ankle Effusion- The Teardrop SignAn 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
Misty Mountains Sign
normal ankle demonstrating a fat -like density (arrowed) abnormal ankle demonstrating a fluid-like density (arrowed)
3. Kager's Fat Pad
<a class="external" href="http://www.ajronline.org/cgi/content/full/182/1/147" rel="nofollow" target="_blank">Justin Q. Ly and Liem T. Bui-Mansfield Anatomy of and Abnormalities Associated with
Kager’s Fat Pad AJR:182, January 2004</a>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.
Misty Mountains Sign These 3 patients all have posterior malleolus fractures and misty mountains sign. Mountain Peaks in a mist
adapted from <a class="external" href="http://z.about.com/d/taoism/1/0/2/1/-/-/JadeDragonMountain12.jpg" rel="nofollow" target="_blank">http://z.about.com/d/taoism/1/0/2/1/-/-/JadeDragonMountain12.jpg</a>Coronal plane distal tibial metaphysis fractures have a plain film appearance which resembles mountain peaks faintly visible through mist. If this sign is not recognised , patients with tibial metaphysis fractures can have their fractures missed. Radiographers who are able to recognise this sign will be able to perform the necessary supplementary projection imaging required at the time of the acute presentation.
The Positioning Trap
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
Case 2
Case3
Case 4
Comment
The radiographer demonstrated considerable skill and judgement in performing supplementary views in a patient whose fracture had been demonstrated to the satisfaction of the referring doctor. It was within the radiographer's scope of practice to perform supplementary projections which proved to be pivotal in changing the patient's management from conservative to surgical. The mountain peak sign can be seen in any AP (ish) ankle projection of coronal plane distal tibial fractures... they are no always posterior malleolus fractures
Discussion
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.
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