When The Presence Of A Flexor Hallucis Accessory Longus Contributes To Chronic Foot Pain With Standing

In a case involving a 34-year-old patient who had left foot pain for four years, these authors discuss the rare muscular anomaly of flexor hallucis accessory longus and emphasize including accessory musculature in the differential diagnosis for foot and ankle pain. 

In many cases, accessory muscles are asymptomatic and represent incidental findings on imaging. The flexor hallucis accessory longus (FHAL) is a rare muscular anomaly, which is infrequently discussed in the literature. However, in some cases, symptoms present when this extra musculature creates a mass effect and compresses adjacent structures in confined spaces and fibro-osseous tunnels.¹ 

The prevalence of the flexor hallucis accessory longus is unknown. In their research describing anatomical variations of the flexor hallucis longus (FHL) muscle and tendon, Vega and colleagues did not find any cases of flexor hallucis accessory longus being present in a review of 355 magnetic resonance imaging (MRI) studies.² In our review of the literature, we found that various articles discussing accessory muscles of the foot and ankle did not include a discussion of flexor hallucis accessory longus.^1,3

In contrast, the prevalence of the flexor digitorum accessory longus (FDAL) was 4.5 percent in the aforementioned MRI review by Vega and colleages.² The variability of the origin, insertion, size and location of the flexor digitorum accessory longus muscle can result in complex presentations with varying symptoms.⁴ Reported cases show that the flexor digitorum accessory longus may present as tarsal tunnel syndrome and posterior ankle impingement, or have symptoms similar to flexor hallucis longus dysfunction.^4,5 We found only one reported case by Lin and Zaw that clinically describes the flexor hallucis accessory longus.⁶ The patient in that report presented with tarsal tunnel syndrome, likely due to the space-occupying nature of the muscular anomaly, and underwent resection of the accessory muscle with complete resolution of symptoms. At eight months post-resection, the patient was pain-free and training for a marathon.⁶ The presence of a flexor hallucis accessory longus or any accessory musculature within the tarsal tunnel may also correlate with pathologic processes resulting in tarsal tunnel syndrome, posterior ankle impingement and symptoms mimicking flexor hallucis longus dysfunction, just as one may see with the presence of flexor digitorum accessory longus. 

A review of the recent case that we are presenting below may prompt foot and ankle surgeons to rule out flexor hallucis accessory longus and other muscular anomalies in foot and ankle pain recalcitrant to provided therapies.

A Closer Look At The Patient’s Treatment History And Current Diagnostic Workup

The patient in question is a 34-year-old male seeking a third opinion regarding his left foot pain, which he has had for four years. The pain is sharp and generally localized to the left plantar foot at the level of the calcaneal tubercle. The pain occurs with weightbearing activity and is prominent with standing. He does not experience post-static dyskinesia. He reported slight instability of his ankle but the main reason for seeking medical attention was the continued foot pain. The pain also radiates down into the arch of his foot and up the back of his leg intermittently. He did not experience any trauma or any other clear inciting event to his foot or ankle. 

In 2018, the patient had surgery with another provider to treat the described pain. Although his pain was medial in the foot and ankle, the procedures included a left ankle arthroscopy with synovectomy, excision of peroneal synovitis, repair of a peroneal brevis tendon tear, excision of a low lying peroneal brevis muscle belly, application of an amniotic membrane allograft to the peroneal tendons, excision of a distal fibula avulsion fracture and repair of the anterior talofibular and calcaneofibular ligaments. The patient reports that although the procedures stabilized his ankle, he continued to experience the same foot pain postoperatively that he experienced preoperatively.   

Clinical examination demonstrates swelling of the posterior medial ankle. Pain is elicitable with palpation posterior to the medial malleolus that follows along the long plantar flexor tendons distally to the level of the plantar medial arch.  Plantarflexion of the ankle, palpation of the flexor hallucis longus and dorsiflexion of the hallux simultaneously elicits increased pain. There are also positive Tinel and Valleix signs with percussion of the posterior tibial nerve posterior to the medial malleolus.  

We determined a preliminary diagnosis of flexor hallucis longus dysfunction based on the clinical examination, although, we considered tarsal tunnel syndrome and posterior ankle impingement in the differential diagnosis. Radiographs were unremarkable.  We ordered an MRI to evaluate the integrity of the flexor hallucis longus and evaluate any intrinsic pathology regarding the tarsal tunnel. The MRI results revealed the presence of a flexor hallucis accessory longus muscle and tendon located adjacent to the normal flexor hallucis longus muscle and tendon with no other evidence of any tarsal tunnel mass. The anomalous flexor hallucis accessory longus is located posterior and lateral to the normal flexor hallucis longus (see photos above). If the MRI results had shown no abnormalities, we would have ordered flexor hallucis longus tenography to evaluate for stenosing tenosynovitis.  

Previously, the patient had unsuccessful attempts to treat the foot pain with various conservative therapies, including over-the-counter arch supports, custom orthotics, corticosteroid injection and physical therapy. We planned for surgical excision of the flexor hallucis accessory longus due to inadequate management of the symptoms with non-operative treatments and failure of the prior surgeries to address the pathologic structure discovered by MRI.

Emphasizing Appropriate Imaging In Cases Involving Nerve Complaints And Overlapping Symptoms Akin To Flexor Hallucis Longus Dysfunction

In this case study, the patient suffered from many symptoms including arch pain, nerve complaints and tendon pain with motion. The overlapping symptoms create a complex clinical picture. However, this presentation is similar to what clinicians see in cases of flexor hallucis longus dysfunction.⁷ Oloff and Schulhofer coined the term “flexor hallucis longus dysfunction” to describe the overlapping signs and symptoms of flexor hallucis longus tendinitis, distal plantar fasciitis and tarsal tunnel syndrome that frequently present in the face of a primary diagnosis of flexor hallucis longus stenosing tenosynovitis.^7,8 Researchers have suggested that flexor hallucis longus dysfunction represents chronic repetitive injury with resultant inflammation and stenosing tenosynovitis, or tearing of the tendon as it courses from the ankle to the midfoot.^4,7 

This pathology may occur in athletes who forcibly plantarflex the ankle such as dancers that go en pointe.^7,8,9 As we noted previously, if the MRI showed no abnormality, the next exam that we recommend is flexor hallucis longus tenography. Flexor hallucis longus tenography under fluoroscopy effectively demonstrates abnormalities of the tendon sheath. Additionally, the procedure provides the opportunity to administer diagnostic anesthesia and injectable corticosteroid. Pain relief with the anesthetic may confirm the sheath as the pain generator and the steroid may serve to lessen fibrous adhesions.¹⁰

Positive Tinel and Valleix signs indicated that there may be an element of compression within the tarsal tunnel. The nerve complaints demonstrated in flexor hallucis longus dysfunction and in the presence of flexor hallucis accessory longus are both results of this compression. The tendons of the posterior tibialis, flexor digitorum longus (FDL) and flexor hallucis longus muscles all pass through a fibro-osseous tunnel accompanied by the posterior tibial artery, vein and nerve. 

Tarsal tunnel syndrome is an entrapment neuropathy associated with compression of that nerve. Accessory musculature and space-occupying lesions create a mass effect within the confined space.¹¹ As we noted earlier, Lin and Zaw reported a case of flexor hallucis accessory longus complicated by tarsal tunnel syndrome.⁶ After surgical resection of the flexor hallucis accessory longus, all symptoms resolved. Similarly, there are reported cases of flexor digitorum accessory longus excision that successfully resolve any accompanying symptoms of tarsal tunnel syndrome.^5,12,13

Magnetic resonance imaging was critical to the diagnosis of this patient.  Recognizing accessory musculature is not possible with plain radiographs. Magnetic resonance imaging provides clear delineation of accessory muscles and tendons. It is critical to become familiar with normal anatomy as well as common muscle variants and the pathology that they may cause. In this case, the patient saw two other providers that did not appreciate the flexor hallucis accessory longus and the patient ultimately had operative treatment directed at ankle structures that were not contributing to his symptoms. Careful review of ankle MRI would direct surgical treatment at the pathologic structure causing his pain. For patients who experience ankle pain gratuitously and fail treatments for more common pathologies, careful evaluation of MRI and recognition of accessory muscles will aid in successful diagnosis and treatment.

Final Thoughts

Flexor hallucis accessory longus is a rare muscular anomaly, which may contribute to the development of tarsal tunnel syndrome and symptoms that are similar to flexor hallucis longus syndrome. Careful review of MRI was critical to the diagnosis of flexor hallucis accessory longus in this case. Podiatrists should include accessory musculature in the differential diagnosis for foot and ankle pain until this practice is proven otherwise.

Dr. Griffin is a third-year podiatric medicine and surgery resident at Saint Mary’s Medical Center in San Francisco.

Dr. Oloff is the Director of the Podiatric Medicine and Surgery Residency at Saint Mary’s Medical Center in San Francisco and a staff surgeon at the Palo Alto Medical Foundation in Burlingame, Calif.

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