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Case of the Week #16: A symptomatic os trigonum in a 32yo lady.


Mrs S is a delightful 32 year old lady who had a symptomatic os trigonum in her left ankle. She was referred by her general practitioner for posterior ankle pain and loss of range of motion, without a history of trauma.


She had seen a Podiatrist in the past who prescribed some orthotics but unfortunately these did not help her posterior ankle pain. She had one ultrasound guided steroid injection around the os trigonum which relieved her pain for several months.


On examination, she was a lady of slim build with an obvious ankle effusion. She was unable to perform a single leg heel raise due to pain. She was very tender to palpation of the posterior ankle just either side of the achilles, and a palpable lump was apparent (a palpable lump is not always as obvious in heavier patients). Her pain was worse with ankle plantarflexion. Occasionally, the flexor hallucis longus tendon can be irritated by an os trigonum but this was not the case in this instance as mobilising the great toe did not exacerbate her pain.


A lateral ankle x-ray clearly demonstrated a moderate sized os trigonum (see below):


An MRI confirmed extensive oedema around the os trigonum, as well as some cystic change at the synchondrosis (fibrous bridge) with the talus:


Mrs S had exhausted her non-operative management options so surgery was discussed. The os trigonum can be removed via a posterior ankle arthroscopy and the risks and benefits of the procedure was discussed in detail. In particular, there is risk of injury to the posterior tibial nerve or flexor hallucis longus tendon with a posterior ankle arthroscopy, as well as risk of incomplete excision. Mrs S was very symptmoatic as she was struggling to walk or look after her young children, so was keen to proceed with surgery.


The procedure, performed as a day case, was uncomplicated. She was encouraged to weight bear and mobilise her ankle immediately after the procedure, although did require crutches for support for a few days. Her Image Intensifier images taken intra-operatively demonstrated complete excision of the os trigonum (see below).


One year after surgery, Mrs S is pain free and able to run again. She has occasional stiffness in her ankle and pain when twisting her ankle suddenly, but otherwise had a good result from surgery.


DISCUSSION

An os trigonum is a failure of fusion of the secondary ossification centre of the talus. It normally forms between aged 8 and 13 years and is therefore a normal finding in this age group. If the ossicle fails to fuse, it joins the talus through a synchondrosis (fibrous bridge). 10-25% of people have an os trigonum, although only a very small proportion of people have symptoms from it. It can be irritated by movement that requires extensive plantarflexion of the ankle such as ballet dancing, horse riding and netball, or occasionally the synchondrosis can fracture after trauma (Nault et al, 2014).

An os trigonum should initially be managed non-operatively. This would include non-steroidal anti-inflammatory medication, rest, ice and activity modification as well as an ultrasound guided injection of local anaesthetic and steroid. Taping techniques such as plantarflexion block taping can also be effective. The injection can be repeated, but repeated injections can lose their effectiveness over time. An arthroscopic excision of an os trigonum via a posterior ankle arthroscopy is a reliable, minimally invasive procedure for this condition should non-operative measures fail.

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