Injuries to the flexor hallucis longus (FHL) tendon have classically been described in ballet dancers secondary to their constant repetitive plantar flexion. Hence, the injury is often called dancer's tendinitis. Such injuries have also been described in association with climbing, soccer, and running in relation to frequent push-off maneuvers of the forefoot in these activities. FHL tenosynovitis is infrequently seen in association with other conditions, such as diabetes, rheumatoid arthritis, lupus, and seronegative spondyloarthropathies. [1, 2, 3, 4, 5] The anatomic position and course of the FHL tendon are seen in the images below.
Pathology of the FHL tendon is commonly related to overuse. Direct trauma and, less commonly, inflammatory disease are other causes. Types of injuries with associated definitions include the following:
Tendinopathy: This is a degenerative lesion in tendon tissue without alteration of the tendon sheath.
Tenosynovitis: This is inflammation or infection in the vascular peritendinous tissue (demonstrated in the images below).Axial inversion-recovery image of the ankle shows prominent tenosynovitis of the sheaths of the flexor hallucis longus tendon and the adjacent posterior tibialis tendon, with normal-appearing, hypointense tendons. Courtesy of Stacy Smith, MD, University of Maryland.Axial inversion-recovery image of the ankle shows septic arthritis of the ankle with septic tenosynovitis of the flexor tendons of the ankle. Tendons otherwise appear unremarkable with regard to size and signal intensity. Courtesy of Stacy Smith, MD, University of Maryland.
Partial or complete tendon tears: Partial tears can be central or intrasubstance, or they can occur at the external margins of the tendon. Complete tears may occur with or without tendon retraction. The images below depict complete tears of the FHL tendon.Complete tear of the flexor hallucis longus (FHL) tendon. Axial T1-weighted image of the ankle depicts absence of the normal hypointense FHL tendon, with focal swelling and edema of the remaining muscle and deep soft tissues. Other flexor and peroneus tendons, as well as the Achilles tendon, are unremarkable. Courtesy of Stacy Smith, MD, University of Maryland.Complete tear of the flexor hallucis longus (FHL) tendon. Axial inversion-recovery image of the ankle depicts focal edema in the posterior soft tissues in the expected region of the FHL tendon. Courtesy of Stacy Smith, MD, University of Maryland.Complete tear of the flexor hallucis longus (FHL) tendon. Axial T1-weighted image of the foot confirms absence of the FHL tendon behind the sustentaculum tali, with associated edema. Courtesy of Stacy Smith, MD, University of Maryland.Complete tear of the flexor hallucis longus (FHL) tendon. Axial inversion-recovery image of the ankle and foot improves depiction of the edema posterior to the sustentaculum tali, where the FHL tendon should be present in this patient with a complete tear. Courtesy of Stacy Smith, MD, University of Maryland.Complete tear of the flexor hallucis longus (FHL) tendon. Sagittal inversion-recovery image shows the lack of continuity of the FHL tendon, which should run under the sustentaculum tali. Note the hyperintense edema in the ruptured area. Note also the bone marrow edema in the inferior talus. Courtesy of Stacy Smith, MD, University of Maryland.Complete rupture of the flexor hallucis longus (FHL) tendon. Axial T1-weighted image of the ankle at the level of the tibial metadiaphysis depicts the retracted FHL tendon superiorly. Courtesy of Stacy Smith, MD, University of Maryland.
Tendon entrapment or checkrein deformity: This is fixed tethering of the FHL tendon under or just proximal to the flexor retinaculum, which can be secondary to fracture.
Tendon dislocation: This can be complete or incomplete.
Physical examination is the initial evaluation for the patient with posteromedial ankle pain. To allow complete dorsiflexion at the ankle, the knee is flexed 90° to relax the gastrocnemius. The knee is then extended with plantar flexion of the ankle and great toe, with the clinician palpating behind the medial malleolus. These maneuvers can allow differentiation of tendinitis (diffuse tenderness or crepitation) from causes of trigger toe and hallux rigidus. Triggering occurs when a nodular thickening of the tendon snaps through the fibro-osseous tunnel. This is generally palpable. Hypertrophy of the FHL at the musculotendinous junction may produce hallux rigidus, with restriction of dorsiflexion of the hallux.
Computed tomography (CT) is best for precisely depicting the ossific anatomy and the gross size and location of the tendon. However, because of its multiplanar and multisequence capabilities, MRI helps in clarifying tendinous findings, such as coexistent tenosynovitis, tendinopathy, partial or complete tears, and associated fluid. In addition, magnetic resonance imaging (MRI) permits characterization of the anatomy of the fibrous retinaculum. Bone marrow edema is best evaluated with MRI and may help in identifying an os trigonum syndrome or contusions of adjacent bones.
In a patient with a history of posteromedial ankle injury, careful assessment of the ossific structures and soft tissues of the ankle and foot is required.
For highly sensitive and specific assessment of the soft tissues of the ankle, radiography should be performed first to assess for fractures, an os trigonum, or foreign bodies. This examination should then be followed by MRI if an injury to a tendon is suspected.
CT for ossific evaluation can be performed if clinically indicated (eg, in a patient with previous fractures or a known os trigonum) and before MRI to evaluate the callus and extent of the fracture.
Radiographic findings include soft-tissue swelling at the posteromedial aspect of the ankle. This finding is usually nonspecific, and an associated joint effusion may be present. Fractures of the calcaneus, distal medial malleolus, or os trigonum may suggest an abnormality of the FHL tendon, as all 3 have been reported in association with FHL tendinopathy, partial and complete tears, entrapment, and dislocation.
Degree of confidence
Radiography provides a low degree of confidence for definitive diagnosis because the tendon itself is not visualized. However, radiography offers a moderate degree of confidence in identifying the ancillary findings described above that may raise the suspicion for FHL injury and can identify the presence of an os trigonum.
False-positive results may involve a focal area of soft-tissue swelling secondary to hematoma rather than tendon injury because soft tissues are not well delineated with this modality. Lack of findings on the radiograph do not preclude further advanced imaging, such as CT or MRI, as these are best suited for fine analysis of the soft tissues.
CT can be useful in evaluating the presence, location, size, and anatomy of the FHL tendon, as well as in eliciting any fluid in the synovial sheath.
The normal tendon is round or slightly oval, with intermediate homogeneous density. About 20% of healthy patients may have fluid in the tendon sheath, because the FHL tendon sheath communicates with the ankle joint.
Absence of the tendon or complete discontinuity of the tendon suggests tear with retraction. Thickening or heterogeneous attenuation suggests either tendinopathy or partial tear or healing remote injury. Increased abnormal fluid in the tendon sheath is highly suggestive of FHL entrapment. Ossific anatomy, such as associated fractures (particularly calcaneal or posterior talar or tibial), an os trigonum, and foreign bodies, are best evaluated with this modality.
Degree of confidence
CT offers a moderate degree of confidence because an abnormal size and attenuation in the tendon with associated fluid are seen in injuries of the FHL. However, the fine detail, such as delineation of tendinopathy versus partial tendon tear, is best assessed with MRI. Complete tears can be detected on CT if the torn ends of the tendon are retracted.
Focal thickening of tendons such as xanthomas or fibrous nodules can mimic focal areas of tear or tendinopathy.
Magnetic Resonance Imaging
In conjunction with physical examination and clinical history taking, MRI can be helpful in evaluating FHL tendon injuries. MRI may easily show abnormal signal intensity in tendons, the intactness of fibers, and the size of the tendon, as well as any surrounding fluid. Adjacent bone marrow edema, contusion, inflammation, or nondisplaced fractures are also best visualized with this modality. [6, 7, 8, 9]
Axial proton density (PD)– and T2-weighted fat-saturated fast spin-echo (FSE) sequences, as well as sagittal inversion-recovery (IR) or T2-weighted fat-saturated FSE and T1-weighted sequences of the ankle (which includes the foot to the base of the distal phalanx of the great toe), are best for evaluating the FHL tendon. The FHL is best seen just lateral to the posterior tibialis tendon as it curves around the sustentaculum. Coronal images can help evaluate adjacent bone, soft tissue, or tendons.
After clinical examination, an MRI can be performed to confirm or further delineate the exact location and extent of the abnormalities. Three areas of tendinopathy are (1) posterior to the talus in the region of the fibroosseous tunnel (classic type), (2) the knot of Henry under the base of the first metatarsal, and (3) the head of the first metatarsal where the tendon passes between the sesamoids.
MRI can help identify tendinopathy in the classic location due to an anatomic variant (short tendinous region of FHL with long distal muscle belly) or due to direct irritation. Direct irritation is seen on MRI as irregular fraying, fusiform enlargement, cystic changes, and/or nodule formation in the tendon.
Tenosynovitis may be present either alone or in combination with FHL tendinopathy. Isolated fluid in the synovial sheath may indicate tenosynovitis. However, this finding (seen in the image below) can be a normal variant in up to 20% of patients, but increased fluid is suggestive of tenosynovitis.
Partial tendon tears show intrinsic longitudinal marked hyperintensity and fusiform tendon thickening on T2-weighted fat-saturated or IR MRI.
The MRI findings of complete tendon tear include fluid signal intensity between the torn ends of the tendon or between the musculotendinous junction and the tendon, with minimal or marked retraction and or undulation of the tendon ends. Soft-tissue edema and swelling may also be seen. If the tear is due to inflammatory changes, other associated findings (eg, synovial thickening due to rheumatoid arthritis) may be seen. Complete entrapment with acute tears may show associated bone marrow edema.
MRI signs of entrapment include the following: tendon entrapment or tethering by bone or soft tissue, abrupt cutoff of the synovial fluid of the FHL tendon sheath with differential volumes of fluid (hyperintensity on T2-weighted or IR MRIs) above and below the site of entrapment, and alteration of the size or signal intensity of the tendon morphology. Prominent areas of persistent hypointense callus around a fracture or fractures that have been treated with surgery can be noted as the cause of entrapment; these are most often complete (checkrein deformity). Os trigonum syndrome is one cause of partial entrapment or tethering of the FHL tendon (see below).
An unfused lateral tubercle of the posterior talus found just lateral to the fibro-osseous tunnel may be seen in 14-25% of the population. The size of the tubercle is not usually a predictor of clinical symptoms. However, increased bone marrow edema or fluid around the posterior talus, an os trigonum, adjacent calcaneal and synovial sheath fluid, or tendon signal change suggests os trigonum syndrome as the cause of posteromedial ankle pain, especially when heterogeneous signal changes are noted in the FHL tendon. Correlative clinical reproduction of pain in extreme plantar flexion or maximum dorsiflexion of the great toe with resistance is usually present and helps support this diagnosis.
Tendon dislocation is a straightforward diagnosis on MRI, as the tendon lies outside the posterior intertubercular groove. However, if the patient has a voluntary snap of the posterior medial ankle triggered by maximal ankle dorsiflexion and interphalangeal plantar flexion of the toes, axial PD-weighted and T2-weighted fat-saturated FSE MRI in static and dynamic positions can help make the diagnosis of the rare intermittent dislocation of the FHL tendon.
Degree of confidence
MRI offers a high degree of confidence with respect to FHL tendon injuries. The multiplanar and multisequence capabilities of MRI allow the distinction to be made between tendinopathy partial tear and complete tear and can demonstrate abnormal intrinsic signal intensity versus fluid in the tendon sheath (tenosynovitis). This modality also allows visualization of possible alternative anatomic causes of the clinical symptoms that may mimic injuries of the FHL tendon.
In rare cases, abscesses or masses of the tendon or tendon sheath may mimic injuries to the FHL tendon. However, the degree of signal intensity usually helps in differentiating a true abscess around a tendon from true tendinopathy or partial tear (acute or chronic).
Occasionally, intermittent dislocation of the FHL may occur and may not be evident at the time of the MRI, giving rise to a potential atypical false negative. In such cases, ultrasound may be of benefit in better evaluating for potential intermittent dislocation of the FHL under real-time imaging evaluation with dynamic maneuvers. 
Although ultrasonography is commonly used to evaluate injuries of the Achilles tendon, the posterior medial location of the FHL tendon makes evaluation with sonography more problematic. However, the utility of sonography is in the ability to examine the patient at rest and in dynamic flexion and extension. 
Transverse and longitudinal scans at the level of the medial malleolus posteriorly allow visualization of the size, echogenicity, and location of the tendons with relation to one another. Associated fluid collections can also be seen as lobular areas of lower echogenicity surrounding the tendon or in the ankle joint. A thickened, heterogeneous tendon suggests tendinopathy.
Complete absence of the tendon, disappearance of the tendon or retraction of the ends of the FHL tendon between rest and dynamic flexion studies of the tendon in the prone position, and a fluid-filled gap between the ends of the tendon are consistent with a tendinous tear.
The medial flexor group lies adjacent to the hyperechogenic cortical rim of the medial malleolus. The FHL tendon is normally slightly smaller and located medial to the FDL tendon just posterior to the malleolus. The FHL tendon and muscle are posterior to the posterior tibial border and are separated from the Achilles tendon by the Kager triangle.
Degree of confidence
With skilled and trained operators, sonography offers a moderate-to-high degree of confidence in diagnosing abnormalities of the FHL tendon, providing detailed images of the tendon and surrounding soft-tissue structures throughout most of the course of the tendon itself.
Adjacent ossific structures are noted but are better depicted on radiographs, CT scans, and MR studies.
Subtle changes in the position of the transducer during the examination may cause artifacts. Subtle anatomic abnormalities previously unknown to the examiner may also obscure or preclude complete imaging of the area.
The normal anatomy and course of the tendon are fairly straightforward. However, the distal pathway between the sesamoid bones may be difficult to image if the appropriate high-resolution transducer is not available.
At present, nuclear medicine has a limited role in the evaluation of tendon injuries other than showing areas of increased tracer uptake in the area of soft-tissue injury. It may be of benefit in depicting other abnormalities of bone (infection or fracture). However, MRI is more sensitive and specific than nuclear medicine studies for this purpose. WBC nuclear scans may be sensitive to infection of the tendon or adjacent bone and direct treatment with regard to antibiotics, rather than just physical therapy for traumatic injury.
Angiography is not needed to assess or diagnose FHL tendon injuries.
Although no radiologic intervention is generally required, steroid injection under ultrasonographic guidance may alleviate pain and discomfort in some cases for short-term management of chronic tendinopathy or tenosynovitis. 
Reach et al investigated the degree to which ultrasonographically guided injections could accurately reach common foot and ankle injection sites. Using ultrasonographic guidance, the authors injected a methylene blue–saline mixture into 10 fresh cadaver feet, targeting the FHL sheath, as well as the first and second metatarsophalangeal joints, the tibiotalar joint, the Achilles peritendinous space, the posterior tibial tendon sheath, and the subtalar joint. They reported that the injections for all but the subtalar joint were 100% accurate, with this last being 90% accurate. 
Injection of the tendon sheath under fluoroscopy or ultrasound may be of clinical benefit for those patients with stenosing tenosynovitis. Stenosing tenosynovitis refers to the development of tendon adhesions within the tendon sheath that can preclude the normal sliding pattern of the tendon and can contribute to a syndrome known as trigger toe, particularly if this occurs at the level of the sesamoids, giving rise to loss of flexion. [12, 2] Tenography under fluoroscopic guidance has proven useful for confirmation of the diagnosis of stenosing tenosynovitis, characterized by lobulated contrast filling of the tendon sheath, as well as providing therapeutic treatment via fluoroscopic-guided injection of steroid, typically dexamethasone, and long-acting anesthetic such as bupivacaine. [13, 3]