Flexor Digitorum Accessory Longus: I’m not the only one

An 18-year-old girl presented with chronic lateral malleolar tarsal pain. Symptoms were triggered mostly during physical activity, with functional restrictions on her daily life. She was a tennis player with a history of repetitive ankle sprains. On physical examination, she had tenderness on her left ankle and minor foot oedema.

A left ankle magnetic resonance [MR] imaging examination was performed. The study revealed mild effusion on the subtalar joint and bone marrow oedema of the anterior margin of the posterior subtalar facet of the talus, suggesting prior trauma. Additionally, supernumerary muscles in the posterior ankle compartment were incidentally noticed. The most perceptible was the one coursing posterior to the neurovascular bundle in the tarsal tunnel, arising from the soleus muscle, consistent with the flexor digitorum accessory longus [FDAL] muscle (Fig. 1). This accessory muscle lies deep to the deep aponeurosis and flexor retinaculum and inserts distally on the quadratus plantae (Fig.2). Another two additional muscles were also noticed: a soleus accessory arising from the anterior surface of the soleus muscle with a tendinous insertion on the medial calcaneus tuberosity (Fig. 3); and a small peroneus quartus inserted directly in the retrotrochlear eminence of the calcaneus (Fig. 4).

Discussion

In the majority of the cases, accessory muscles of the ankle are asymptomatic and represent incidental findings at imaging. However, in some cases there may be clinical symptoms such as pain, instability, and rigid hindfoot deformities [1,2]. The detection of anomalous muscles on MR imaging is based on the recognition of its origin and insertion, on its relationship with adjacent structures such as other muscles and tarsal tunnel structures [3]. The FDAL is the most common accessory muscle in the posterior compartment of the ankle [4]. The prevalence of the FDAL is found in up to 6-8% of the lower extremities, and is usually unilateral [5]. It frequently arises from the medial margin of the tibia or the lateral aspect of the fibula, distally to the origin of the flexor hallucis longus [6,7]. The FDAL courses beneath the flexor retinaculum, through the tarsal tunnel, and it typically inserts on the quadratus plantae, as seen in this case, or into the flexor digitorum longus [FDL]. Its distal attachment allows the differentiation between the peroneocalcaneus internus and tibiocalcaneus internus, which insert directly into the calcaneus [6]. The FDAL is closely related to the neurovascular bundle and may compress or impinge upon the posterior tibial nerve, resulting in tarsal tunnel syndrome [TTS]. This condition has been reported as the most common complication of the presence of a FDAL [2,5,8], although it was not observed in our case. The FDAL has also been associated with flexor hallucis longus tenosynovitis due to its close relationship to the FHL tendon. Conservative treatment in the early stages of TTS may be successful, and the excision of the accessory muscle is only considered for patients whose conservative treatment fails [9,10]. The location of the FDAL within the tarsal tunnel and deep to the flexor retinaculum help differentiating it from the accessory soleus muscle. The latest usually arises from the anterior surface of the soleus muscle, as seen in this case and, although it resides outside the tarsal tunnel, it has been associated with TTS as well [11]. Surgical treatments for the symptomatic accessory soleus have been performed such as fasciotomy, tendon release, and accessory muscle excision. The peroneus quartus is a pronator and is normally asymptomatic. In some cases, it can cause thronging in the retinaculum, leading to subluxation or tears of the peroneal tendons [5].

Teaching Points

Accessory muscles are mainly incidentally noted on MR and CT and the vast majority of these are asymptomatic. However, in some cases, accessory muscles may result in entrapment neuropathies, such as tarsal tunnel syndrome.
MR examinations are essential for a correct and thorough interpretation of the posterior ankle, to make a differential diagnosis with tumours and guide surgical treatment.
The radiologist should be familiar with the typical anatomy of the ankle to avoid misdiagnosis of these anatomical variants.