os trigonum syndrome
Os trigonum syndrome, sometimes called posterior ankle impingement syndrome (PAIS), is a potentially underrecognized condition and cause of posterior ankle pain. This has been increasing in clinical practice secondary to awareness, increased interest in physical fitness and the use of exercise machines. The name comes from an accessory ossicle and how it is oriented between the medial navicular bone and posterior tibial tendon. In 1804, Rosenmuller first described the os trigonum as an accessory ossicle. The name originated from the trigonal contours of its three surfaces: the anterior surface adjacent to the talus, the posterior surface with its ligamentous attachment, and the inferior surface adjacent to the calcaneus.
An elongated lateral tubercle to the talus (Stieda process) may cause symptoms similar to os trigonum syndrome. An os trigonum syndrome originates from a stress fracture of the Stieda process or failed fusion of a secondary ossification center of the lateral tubercle, creating a synchondrosis with the talus. It can also originate from an acute fracture of the Stieda process (posterior process fracture of the talus) . Laterally, the os trigonum and the trigonal process receive fibers from the posterior talofibular ligament whilst immediately medially lies the thick, fibrous tendon sheath of flexor hallucis longus, both of which may contribute to the clinical picture in os trigonum syndrome. In radiologic studies of normal feet and ankles, the prevalence of os trigonum syndrome ranges from 14% to 25% in patients with an ununited lateral tubercle . Peace et al. reported a prevalence rate of 30% in ballet dancers, which may be attributed to repetitive forced plantar flexion in ballet training during the skeletal maturation phase of development that precedes the closure of the ossification center .
The prevalence is higher in sports and activities with repetitive plantarflexion including ballet, soccer, downhill running. Any ankle trauma can initiate or exacerbate the condition. Almost three quarters of patients report an acute ankle injury as the initiating cause of symptoms . Symptoms of os trigonum syndrome include stiffness, particularly while in plantar flexion, chronic pain, and swelling behind the ankle. Pain occurs during specific training, in particular pointe (dancing performed on the tip of the toes with the ankle in maximal plantar flexion) and demipointe (dancing performed on the balls of the feet with the ankle in maximal plantar flexion) positions in ballet, or any sports movement involving forced plantar flexion or push-off. Symptoms typically diminish initially with rest from the triggering activity.
Physical examination reveals posterolateral tenderness on palpation, typically between the Achilles and peroneal tendons. Passive maximal plantar flexion may reproduce the patient’s symptoms. The clinician must also be aware that os trigonum syndrome often coexists with FHL tenosynovitis in the same patient population. Repetitive plantar flexion leads to constant pressure exerted on the os trigonum by the FHL tendon, leading to tenosynovitis. In the setting of concomitant FHL tenosynovitis, patients may report episodes of the ankle giving way, triggering of the hallux, pain on palpation of the posteromedial ankle between the Achilles tendon and the medial malleolus, or pain over the FHL tendon at the level of the ankle/ hindfoot with active contraction or passive stretching . A creaking or grating sensation over the posterior tibial tendon may be appreciated by the examiner with range of motion of the ankle if serious posterior tibial tendinitis is present.
Imaging normally begins with plain radiographs with the lateral view being most revealing. The presence of an os trigonum on radiographs is not clinically relevant without associated symptoms. Calcification may be present around the region due to prior injury or inflammation. If os trigonum syndrome is suspected, lateral radiographs while in full plantarflexion. Bony contact may be present on this view. However, it isn’t necessary for diagnosis because there is a cartilaginous layer that often surrounds the os trigonum [7,8]. If the diagnosis is unclear, an MRI can reveal bone marrow edema in the os trigonum or inflammation and scarring of the adjacent soft tissue. It is especially useful when a bony os trigonum is absent on plain radiography.
MRI can also help a surgeon determine whether the os trigonum has a fibrous, fibrocartilaginous, or cartilaginous attachment to the talus and can detect FHL tenosynovitis or chondral injury. MRI or ultrasonography can be used to assess for FHL tenosynovitis, which can present with fluid in the tendon sheath or nodules. Ultrasound can also be used for dynamic evaluation of the ankle. Dynamic ultrasound assessment of the ankle in active or passive plantar flexion may reveal impingement of the os trigonum between the posterior tibia and calcaneus.
Initial management is almost always nonsurgical. This normally consists of rest, avoidance of the aggravating activity, NSAIDs and possibly anesthetic or corticosteroid injections. A diagnostic anesthetic can also confirm the diagnosis. Mouhsine et al. reported on 19 athletes with os trigonum syndrome. First-line nonsurgical treatment failed. To confirm diagnosis, the patients received an anesthetic injection administered under fluoroscopic control. Ten patients had complete resolution of symptoms after one injection, 6 patients after two injections, and the remaining 3 patients underwent open excision of the os trigonum . Some clinicians will immobilize in a short leg cast or CAM walking boot for one to four weeks and it has been shown to be more beneficial for acute injuries .
If there is a failure of a three to six month course of nonsurgical management, it may be beneficial to proceed with surgery. Each case will likely be unique, but athletes looking to get back to training or sport are more likely to proceed with resection of the os trigonum. van Dijk et al. first described the use of posterior endoscopy to manage posterior ankle pathology. This technique is performed with the patient positioned prone . This was an attempt to minimize complications and hasten return to sport. Previously, arthroscopic resection of the os trigonum had been performed through the subtalar joint, with the patient in the supine or lateral decubitus position.
Ahn et al. compared outcomes between the subtalar and posterior endoscopic approaches in twenty eight patients. All patients were amateur athletes and both groups had substantially improved VAS and AOFAS scores postoperatively, with no significant difference between the groups. They authors recommended the use of posterior endoscopy for patients with an os trigonum .135 mm2 and for those with concomitant FHL tendinitis or other posterior ankle conditions. In patients with concomitant talar osteochondritis dissecans, anterior ankle impingement, or sinus tarsi syndrome, the subtalar arthroscopic technique is preferred .
A third approach is the more traditional open approach. Guo et al. compared the outcomes of open os trigonum excision with those of endoscopic excision. Of 41 patients, 16 underwent open excision with a posterolateral approach and 25 underwent posterior endoscopy. The end results were comparable with pain and function. However, the return to sport was roughly 12 weeks in the open group and 6 weeks in the endoscopy group .
In summary, the os trigonum can be a source of pain during forced plantar flexion of the foot, in which position an impingement may occur between the posterior edge of the tibia and the superior surface of the calcaneus, leading to posterior ankle impingement syndrome (PAIS). it can become symptomatic following an overuse injury or ankle trauma in athletes and dancers performing repetitive forced plantarflexion. Clinical presentation includes pain in the posterior ankle, particularly with forced plantar flexion. Nonsurgical management is typically first line for most athletes. Surgical management is reserved for cases that fail nonsurgical treatment and arthroscopic surgery has been shown to have a faster return to sport in the limited studies available when compared to the open approach.
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