Introduction

Congenital anatomic anomalies such as a prominent posterior talar process, an os trigonum or a talus bipartitus can facilitate occurrence of the posterior impingement syndrome. An os trigonum is estimated to be present in 1.7 - 7% and occurs bilateral in 1.4% people. These congenital anomalies in combination with a traumatic or overuse injury facilitate the occurrence of symptoms.

The posterior ankle impingement syndrome is by definition a pain syndrome and can be caused by trauma or overuse. The pain is present in the hindfoot during forced plantar flexion. Presence of an os trigonum is one of the many causes of the posterior impingement syndrome. 

In theory, it is not the os trigonum itself that causes pain, but soft tissue structures such as synovium, the posterior ankle capsule or one of the posterior ligamentous structures that get irritated because of presence of the os trigonum.

Anatomy

The os trigonum is an accessory bone that sometimes is present on the posterolateral talus. It is connected to the talus by a fibrous layer. Several structures are attached to it: the flexor retinaculum, the posterior talofibular ligament and the talocalcaneal ligament. The presence of a uni- or bilateral os trigonum is congenital. It becomes evident during adolescence when the posterior talar process does not fuse with the rest of the bone, creating the accessory os trigonum.

History & Physical Examination

Posterior ankle impingement syndrome is by definition a pain syndrome. Patients complain of pain during plantar flexion. Overuse or trauma are the main causes of the syndrome.

Diagnosis is clinical. The area is painful on palpation, sometimes swelling is present. The forced passive hyper plantar flexion test (see movie in section preparation) is positive when the patient experiences recognizable pain during the test. A negative test rules out any form of the posterior ankle impingement syndrome. A positive test can be followed by a diagnostic infiltration with xylocaine. Pain relief following infiltration confirms diagnosis.

Further differentiation to the diagnosis of an os trigonum is made using diagnostic imaging.

Differential diagnostically any other form of posterior impingement should be considered such as soft tissue impingement due to synovitis, fibrosis, or a ligamentous rupture; hypertrophic posterior talar process; ankle distortion, Achilles tendon problem or a fracture of the talus.

Diagnostic Imaging

Start with routine weight-bearing radiographs in an anteroposterior (AP) and lateral direction. In patients with a posterior ankle impingement the AP ankle view typically does not show abnormalities.

We recommend a lateral radiograph view with the foot in 25 degrees exorotation in relation to the standard lateral ankle view when presence of a hypertrophic or fractured posterior talar process or an os trigonum is clinically suspected. We named this view the 'pim' view, which is a dutch abbreviation of Posterior IMpingement- view (see pictures below).

Especially in posttraumatic cases, a spiral CT scan can be important to ascertain the extent and type of the pathology, and the presence and exact location of calcifications or fragments. A CT scan is recommended for preoperative planning.

1).   2). 

1). X-rays of a 22-year old professional dancer with complaints of posterior impingement on plantarflexion of the right ankle. Conventional lateral standing radiograph of the ankle.

2). Same patient with posterior impingement (pim) view, manufactured with foot in 25 degrees of external rotation in relation to conventional lateral radiographs. The os trigonum (arrow) is now clearly visible.

Surgical Treatment

Conservative treatment is always the first treatment of choice. If this fails, surgical intervention involves removal of the os trigonum.

Post-operative rehabilitation

The patient can be discharged the same day of surgery and weightbearing is allowed as tolerated. The patient is instructed to perform active dorsiflexion of the ankle to the neutral position (knee slightly bent). This exercise should be performed 2 or 3 days times per hour for the first days after surgery. The patient is instructed to elevate the foot when not walking to prevent edema. The dressing is removed 3 days post-operative. Normal walking without crutches at 4 - 5 days post surgery. Patients with limited range of motion are directed to a physiotherapist. Running can be started when range of motion is normal and in absence of swelling. This can on avarage be expected at 3 weeks post surgery. Sport resumption can on average be expected at 6 weeks post surgery. 

Recommended literature

Van Dijk CN, Fievez AWFM, Heijboer MP, et al. Arthroscopy of the ankle. Acta Orthop Scand 1993;64(suppl. 253):9.

Van Dijk CN, Scholte D. Arthoscopy of the Ankle Joint. Arthroscopy: The journal of Arthroscopic and Related Surgery, 1997;13(1):90-96.

Van Dijk, Verhagen RAW, Tol JL. Arthroscopy for problems after ankle fracture. J Bone Joint Surg (Br) 1997;79B(2):280-284.