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Flexor Hallucis Longus Tenosynovitis: Ultrasound Diagnosis and Sonoanatomy of the Medial Ankle
What Is Flexor Hallucis Longus Tenosynovitis?
Flexor Hallucis Longus Tenosynovitis is an inflammatory condition involving the tendon sheath of the flexor hallucis longus (FHL) tendon. It can present with pain around the posteromedial ankle and may be associated with repetitive loading, overuse, sports activities, trauma, or other ankle and foot pathology.
Because several important tendons, nerves and vessels pass through the medial ankle, accurately identifying the pain-generating structure can sometimes be challenging on clinical examination alone.
Musculoskeletal ultrasound (MSK USG) provides a dynamic and readily accessible method for evaluating the FHL tendon and detecting abnormalities around the tendon sheath.
Ultrasound Anatomy of the Medial Ankle
When performing an ultrasound examination of the medial ankle, it is important to systematically identify the structures passing behind the medial malleolus.
A useful approach is to identify the structures in sequence and then trace the FHL tendon distally and proximally.
The major structures encountered include:
Tibialis posterior (TP)
Flexor digitorum longus (FDL)
Posterior tibial neurovascular bundle
Flexor hallucis longus (FHL)
The Flexor Hallucis Longus Tenosynovitis demonstrated in this case is particularly interesting because the tendon can be followed dynamically with ultrasound while assessing the surrounding tendon sheath.
Flexor Hallucis Longus Tenosynovitis: Identifying the FHL Tendon
The FHL tendon lies posterior to the ankle and travels toward the great toe. On ultrasound, the tendon can be identified in relation to the other medial ankle tendons and the tibial neurovascular structures.
Careful probe positioning and dynamic scanning are useful for distinguishing the FHL from the adjacent FDL and tibialis posterior tendons.
Ultrasound Findings in Flexor Hallucis Longus Tenosynovitis
One of the important ultrasound findings in Flexor Hallucis Longus Tenosynovitis is fluid surrounding the FHL tendon within its tendon sheath.
In the presented case, ultrasound demonstrated fluid accumulation around the FHL tendon. Importantly, the area also corresponded with maximum probe tenderness, providing useful clinical correlation.
Depending on the severity and cause, ultrasound assessment may also evaluate for:
Peritendinous or tendon-sheath fluid
Tendon thickening
Alteration of the normal fibrillar tendon pattern
Hypervascularity on Doppler examination
Focal tenderness during dynamic examination
Associated abnormalities of adjacent structures
The ultrasound findings should always be interpreted together with the patient's symptoms and physical examination.
Why Ultrasound Is Useful for Flexor Hallucis Longus Tenosynovitis
Ultrasound offers several advantages when evaluating suspected Flexor Hallucis Longus Tenosynovitis.
Dynamic Assessment of Flexor Hallucis Longus Tenosynovitis
Unlike static imaging, ultrasound allows the examiner to assess the tendon dynamically. The FHL can be followed during movement of the great toe and ankle, which can help demonstrate tendon excursion and its relationship with surrounding structures.
Real-Time Clinical Correlation
Another important advantage is the ability to reproduce the patient's symptoms by applying probe pressure over the suspected pathology.
In this case, the region demonstrating fluid around the FHL tendon was also the area of maximal probe tenderness. Such correlation can be particularly useful when multiple abnormalities are present on imaging.
Flexor Hallucis Longus Tenosynovitis and Other Ankle Pathology
Patients with ankle pain may have more than one abnormality.
In the case demonstrated by Dr. Debjyoti Dutta, MRI also showed osteoarthritic changes in the ankle. However, the clinical examination and ultrasound findings indicated that Flexor Hallucis Longus Tenosynovitis was an important potential contributor to the patient's current symptoms.
This highlights an important principle in pain medicine:
An imaging abnormality does not necessarily mean that it is the primary source of the patient's pain.
The final clinical diagnosis should integrate history, physical examination, imaging findings and targeted symptom reproduction.
A Practical Ultrasound Approach to Flexor Hallucis Longus Tenosynovitis
When evaluating a patient with medial or posteromedial ankle pain, a systematic ultrasound examination can be helpful.
Position the patient comfortably with access to the medial ankle.
Identify the medial malleolus as an anatomical landmark.
Identify the tibialis posterior and FDL tendons.
Identify the tibial nerve and accompanying vessels.
Locate the FHL tendon.
Follow the FHL tendon along its course.
Examine the tendon sheath for fluid or other abnormalities.
Use Doppler when appropriate to assess vascularity.
Apply gentle probe pressure to correlate ultrasound abnormalities with tenderness.
Compare with the opposite side when clinically useful.
This systematic approach can improve recognition of Flexor Hallucis Longus Tenosynovitis and help differentiate it from other causes of medial ankle pain.
Clinical Pearls
Flexor Hallucis Longus Tenosynovitis should be considered in patients presenting with appropriate medial or posteromedial ankle symptoms, particularly when tenderness corresponds to the FHL tendon.
Key Points to Remember
Know the medial ankle sonoanatomy before looking for pathology.
Identify the tibialis posterior, FDL, neurovascular bundle and FHL systematically.
Look for fluid surrounding the FHL tendon.
Assess tendon morphology and Doppler vascularity when appropriate.
Correlate the ultrasound finding with the patient's point of maximum tenderness.
Remember that coexisting MRI abnormalities may not necessarily represent the primary pain generator.
Use dynamic ultrasound to understand tendon movement and surrounding anatomy.
Watch the Ultrasound Demonstration
In this educational video, Dr. Debjyoti Dutta of Asian Pain Academy demonstrates the ultrasound anatomy of the medial ankle and shows the sonographic appearance of Flexor Hallucis Longus Tenosynovitis, including the surrounding fluid collection and clinical correlation with probe tenderness.
The demonstration is designed for clinicians interested in MSK ultrasound, pain medicine, sports medicine, rehabilitation, orthopaedics and ultrasound-guided procedures.
Learn MSK Ultrasound and Ultrasound-Guided Pain Procedures
If you want to develop practical skills in musculoskeletal ultrasound and ultrasound-guided pain procedures, explore the educational programs offered by Asian Pain Academy.
Asian Pain Academy: Visit Asian Pain Academy
MSK Ultrasound Course / Fellowship: Explore the MSK USG Course
Frequently Asked Questions About Flexor Hallucis Longus Tenosynovitis
What is Flexor Hallucis Longus Tenosynovitis?
Flexor Hallucis Longus Tenosynovitis refers to inflammation or irritation involving the tendon sheath of the FHL tendon and may cause pain around the posteromedial ankle.
Can ultrasound detect Flexor Hallucis Longus Tenosynovitis?
Yes. Ultrasound can demonstrate abnormalities such as fluid surrounding the FHL tendon, tendon thickening, altered tendon architecture and increased Doppler vascularity when present.
What is the main ultrasound finding?
A commonly recognized finding is fluid surrounding the FHL tendon within its tendon sheath, although the complete ultrasound assessment should include tendon morphology, vascularity and clinical correlation.
Why is probe tenderness important?
Reproduction of the patient's familiar pain while examining the abnormal structure can help establish a relationship between the imaging finding and the patient's symptoms.
Conclusion
Flexor Hallucis Longus Tenosynovitis is an important potential cause of medial and posteromedial ankle pain. A systematic MSK ultrasound examination allows clinicians to identify the FHL tendon, assess its tendon sheath and correlate sonographic abnormalities with the patient's symptoms.
The combination of sonoanatomy, dynamic assessment and clinical correlation makes ultrasound a valuable tool in the evaluation of patients with suspected FHL pathology.
Learn. Scan. Diagnose. Apply. — Asian Pain Academy
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