A true os trigonum is a secondary ossification center that has not fully fused with the talus and is connected by fibrous or cartilaginous tissue. (yellow arrow) Frequently it is incorrectly described as a fracture of Stieda’s process which is an enlargement of the posterior process of the talus which is the foot bone that makes up the ankle joint, along with the tibia and fibula bones. This accessory bone lies in the posterior or back part of the ankle.
It is estimated that this accessory bone or ossicle occurs in 7-9 percent of the population and by and large it is asymptomatic.
Injury can occur to this bone when the foot is forced into severe plantarflexion (where the foot is bent down and away from the body). This can occur in sports or in dance such as point in ballet. This injury can also occur in repetitive lesser traumatic plantarflexion of the foot. An example would be someone who climbs up and down flights of stairs carrying heavy weights such as furniture movers or delivery men.
As the foot is being forced into plantarflexion the posterior process of the talus (Steida's process) as well as the os trigonum (depending on the pathology) is pressed against one of the tendons running in the back of the ankle (flexor hallucis tendon) causing excessive pressure and further exacerbated by the pressure of the posterior part of the ankle, thus resulting in irritation of the Steida's process or the os trigonum.
When seen in the office the doctor will attempt to reproduce this motion by plantarflexing the foot. If in doing so the patient experiences pain, the doctor should be suspicious of an irritation of the os trigonum or perhaps a fractured posterior talus .
Standard x-rays should differentiate between a fractured talar process or os trigonum syndrome. Typically x-rays are taken of both feet for comparison. When a clear diagnosis is difficult, an MRI may be ordered for better clarity.
Symptoms of os trigonum syndrome include a deep pain in the back of the ankle aggravated mostly when the foot is plantarflexed (toes bending downwards). An example would be walking down steep stairs or performing point in ballet. There may also be swelling and tenderness in the back of the ankle. This condition may be mimicked by fracture of Steida's process, as described, as well as achilles tendonitis or ankle sprain.
Conservative care is the treatment of choice in these situations and there is a very high success rate of os trigonum syndrome.
Surgical intervention should be considered in those individuals who do not respond to conservative care or those who seem prone to constant recurrence. This extra bone or in the case of Steida's process is not needed for everyday function. The procedure is done as an out patient and requires removal of either the accessory bone or the removal of the broken Steida's process. The patient will have to wear a cast for another few weeks during the healing process. The major complication of this procedure is irritation to the sural nerve which is a sensory nerve that runs down the posterior lateral side of the ankle and down the lateral side of the foot. This may result in sensory loss on the lateral side of the ankle joint and foot which can lead to disability.
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