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Radiol Case Rep
Radiol Case Rep
Radiology Case Reports
1930-0433
Elsevier

S1930-0433(24)00692-7
10.1016/j.radcr.2024.07.106
Case Report
Novel presentation of rigid flat foot flexor hallucis longus passing through the subtalar joint: A comprehensive case report
Alkhreisat Mohammad MD m.alkhreisat@bau.edu.jo
a⁎
Battah Khairat MD b
Aldahamsheh Osama MD a
Alananzh Mohammad MD a
Ababneh Omar MD c
Farouji Iyad MD d
a Department of Special Surgery, Faculty of Medicine, Al-Balqa Applied University, Al Salt, Jordan
b Department of Pathology and Laboratory Medicine, Faculty of Medicine, Al-Balqa Applied University, Al Salt, Jordan
c Department of Orthopedics and Traumatology, Al Husein Hospital, Al Salt, Jordan
d Department of Cardiology, Saint Michael's Medical Center, New York Medical College, Newark, USA
⁎ Corresponding author. m.alkhreisat@bau.edu.jo
23 8 2024
11 2024
23 8 2024
19 11 51875190
6 7 2024
17 7 2024
18 7 2024
© 2024 The Authors. Published by Elsevier Inc. on behalf of University of Washington.
2024

https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
This case report presents a novel cause of rigid flatfoot caused by the entrapment of the Flexor Hallucis Longus (FHL) tendon within the subtalar joint. A 19-year-old male with chronic right ankle and foot pain diagnosed with rigid flatfoot deformity. MRI identified the FHL tendon entrapped within the subtalar joint, a condition to our knowledge never previously reported. This report highlights the importance of thorough clinical evaluation and advanced imaging techniques in diagnosing rare causes of rigid flatfoot and suggests that surgical options may still be valid for such rare presentations.

Keywords

Subtalar joint
Sustentaculum tail
Flat foot
Pes cavus deformity
Flexor hallucis longus
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pmcIntroduction

Flat foot, or pes planus, is characterized by a lateral axis deviation of the calcaneus in relation to the tibia, often associated with loss of the medial arch and foot abduction. This condition is categorized into flexible and rigid subtypes, each with different underlying pathologies and treatment approaches [1,2]. Flexible flatfoot is commonly seen in children and usually asymptomatic, often managed non operatively [1,2]. In contrast, rigid flatfoot, though less common, is usually associated with more serious underlying issues such as tarsal coalition or neuromuscular disorders, often necessitating surgical intervention [2,3]. Accurate diagnosis through history, clinical examination, and imaging (weight-bearing X-rays, MRI, and CT) are crucial in determining the pathology in tendons, soft tissue, involved joints, or coalition as well as guiding the appropriate treatment [[3], [4], [5]].

The Flexor Hallucis Longus (FHL) tendon plays a critical role in hindfoot alignment and subtalar movement due to its unique anatomical path. The tendon passes behind the ankle, through a groove posterior to the talus, under the sustentaculum tali, and crosses the Flexor Digitorum Longus (FDL) at the knot of Henry. This pathway is vital for maintaining the medial arch and subtalar motion [6,7].

This case report presents a novel cause of pes planus related to the entrapment of the FHL within the subtalar joint, a phenomenon not previously documented.

Case presentation

A 19-year-old male presented with right ankle and foot pain persisting since childhood. Initially treated for flatfoot with insoles and shoe modifications, he continued to experience difficulties, especially with shoe wear and sports participation. Referred to the orthopedic department, he was diagnosed with rigid flatfoot, exhibiting hindfoot valgus, inability to perform a single heel raise, and a collapsed medial arch. Examination revealed significantly reduced subtalar joint range of motion with mild tenderness, but normal and pain-free movements in the talonavicular and calcaneocuboid joints. Tibialis posterior strength was rated at 5/5, and the first metatarsophalangeal joint (MTPJ) exhibited good range of motion without pain.

Weight-bearing X-rays indicated severe right flatfoot with heel valgus and some medial arch loss and talar peaking, suggesting a possible talocalcaneal coalition Fig. 1.Fig. 1 Lateral Weight bearing xray. Xray with talar beaking (white arrow).

Fig 1

MRI of the right ankle revealed a novel finding: the FHL tendon was entrapped within the subtalar joint, with a hypoplastic sustentaculum tali and hypertrophied FHL muscle belly, while the knot of Henry was anatomically preserved Fig. 2, Fig. 3, Fig. 4, Fig. 5.Fig. 2 Coronal MRI. FHL passing through the subtalar joint (white arrow).

Fig 2

Fig. 3 Axial MRI. Axial MRI with FHL muscle hypertrophy (white arrow).

Fig 3

Fig. 4 Axial MRI. FHL seen in subtalar joint space (white arrow).

Fig 4

Fig. 5 Sagittal MRI: FHL tendon over the hypoplastic Sustentaculum tali (white arrow).

Fig 5

The left foot, though asymptomatic, showed a flexible flatfoot upon examination, negating the need for further imaging. Given the unique and previously undocumented pathology, treatment options were explained to the patient. These included conservative management with orthotics and activity modification or surgical interventions, such as calcaneal medial slide osteotomy and tendon transfer, or triple arthrodesis. The patient opted for nonoperative treatment.

Discussion

Rigid flatfoot deformities are well-documented in the literature, with various management strategies being explored. Backus JD [8] highlights tendon transfers as a treatment for adult flatfoot, offering insights into surgical interventions that can address the biomechanical dysfunctions associated with this condition. Raj et al. [9] provide a comprehensive overview of pes planus, emphasizing both nonsurgical and surgical management options in the latest edition of StatPearls. Cass and Camasta [10] discuss the intricacies of tarsal coalition and pes planovalgus, focusing on clinical examination, diagnostic imaging, and surgical planning. Michaudet et al. [11] further contribute to the understanding of foot and ankle pathologies, with a specific section on rigid flatfoot deformities and their impact on patient mobility.

In the context of flexor hallucis longus (FHL) entrapment, the literature primarily documents cases within the sheath, particularly among ballet dancers, as noted by Tudisco and Puddu [12] who reported stenosing tenosynovitis in a classical ballet dancer. Arachchige et al. [13] delve into the biomechanical implications, assessment, and management of flat feet, providing a broad perspective that includes FHL involvement. Coughlin et al. [14] also explore FHL tenosynovitis in their comprehensive surgical text, although they do not specifically address entrapment within the subtalar joint .

In the presented case, the FHL was anomalously located within the subtalar joint, contributing to the rigid flatfoot deformity. This unusual presentation contrasts with previously documented cases of FHL entrapment, which are typically confined to the sheath [15,16]. Despite the chronic nature of the entrapment, the FHL maintained good muscle bulk and function, an unexpected finding that suggests a degree of muscle resilience [17]. Additionally, the presence of a hypoplastic sustentaculum tali complicates the clinical picture, raising questions about the congenital versus acquired nature of this condition [18]. The absence of prior imaging or a detailed childhood history further obscures the etiology, underscoring the need for a thorough diagnostic approach [19].

The diagnostic limitations of this report, due to the lack of surgical intervention, prevent a comprehensive intraoperative assessment of the FHL and subtalar anatomy [20]. Future cases with similar presentations may benefit from early surgical exploration to provide more detailed anatomical and functional insights [21]. The role of advanced imaging techniques, such as MRI or CT scans, should also be emphasized in identifying subtle anatomical anomalies that may not be apparent on standard radiographs [22].

Conclusions

This case emphasizes the importance of proper clinical evaluation and use of MRI as an imaging modality in diagnosing rigid flatfoot, as clinical judgment alone may overlook rare causes like FHL entrapment within the subtalar joint. Surgical options for flatfoot treatment may still be applicable for this novel presentation.

Patient consent

Informed consent for publication of their case was obtained from the patient(s).

Competing Interests: All authors declare no competing conflict of interest or financial support.
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