A 35-year-old female suffered from Rt posterior ankle pain with activities, especially during forefoot push-off for 2 months. There was no resting pain. She denied any previous trauma on the affected ankle. Physical examination revealed tenderness over the posteromedial ankle. Resisted great toe fl exion and passive stretch into full dorsiflexion of the hallux and ankle reproduced her symptoms. Plain radiograph showed no evidence of Stieda process nor Os Trigonum. Ultrasound examination over the right ankle showed mild hypoechoic change without fi ber disruption over fl exor hallucis longus (FHL) tendon. Sonopalpation of the FHL showed tenderness. Besides, an extra tendon is present superfi cial and posterior to the FHL tendon (Figure 1). Under the impression of FHL tendinopathy, we injected 2 mL hyaluronic acid into the tendon sheath of FHL under ultrasound guidance. Her symptoms totally subsided a few weeks after injection.
an anomalous tendon, positioned closely to the flexor hallucis tendon.

FHL tendinopathy has traditionally been associated with individuals in the dance and sports communities, particularly those engaged in activities involving repetitive forefoot push-off, such as running and jumping. In ballet dancers, they frequently change from a fl at-foot stance to the en pointe position, which demands extreme plantarflexion. This repetitive movement places significant strain on the FHL tendon and is a common cause of tendinopathy in this population. Additionally, wearing excessively large shoes that necessitate the athlete to grip with their toes can also lead to the development of FHL tendinopathy.

Pain is typically concentrated in the posteromedial ankle region, which is located adjacent to the Achilles tendon. Swelling in this area may also be evident. During a physical examination, common findings may involve tenderness specifically in the posteromedial ankle area. Pain may be aggravated with resisted great toe flexion or passive stretch into full dorsiflexion. This condition is frequently linked with posterior impingement syndrome because the FHL tendon lies in a fibro-osseous tunnel between the lateral and medial tubercles of the posterior process of the talus. Posterior ankle impingement syndrome encompasses a range of clinical conditions characterized by posterior ankle pain during plantar flexion.

The accessory flexor digitorum longus muscle has been previously documented mostly as an anatomical anomaly, with a prevalence of up to 6% [1]. Its origin and insertion points are known to exhibit considerable variation. The typical course of the accessory flexor digitorum longus muscle in the tarsal tunnel is tendinous and positioned behind the FHL. However, there are reports in the literature suggesting that the accessory flexor digitorum longus muscle can be anterior to the FHL, with a low-lying muscle belly potentially leading to compression of the posterior tibial nerve. In this patient, the accessory flexor digitorum longus tendon is posterior to the FHL tendon and seems to share the fibro-osseous tunnel with the FHL.

The accessory flexor digitorum longus has been identified as the underlying cause of tarsal tunnel syndrome [2]. Eberle et al. [3] reported the case of a 15-year-old female who had posterior ankle pain for one and one-half years. MRI scan showed accessory flexor digitorum longus muscle presenting superficial and lateral to the FHL muscle and tendon. Due to persistence of disabling symptoms despite conservative treatment, surgical exploration and excision of the accessory flexor digitorum muscle were performed.

We present this case to remind the physicians of this accessory variant muscle as a cause of FHL tendinopathy. FHL tendinopathy may develop due to recurrent friction of the FHL, resulting from the presence of the accessory tendon within the tarsal tunnel. The accessory tendon may be visualized through an ultrasound, which would depict