Intratendinous Ganglion Cyst of the Achilles Tendon: A Rare Clinical Finding

As specialists in musculoskeletal and sports medicine, we routinely assess chronic Achilles tendon pain, degeneration, and partial ruptures. However, encountering a cyst within the midsubstance of the Achilles tendon is exceptionally rare and until recently, I had never seen one in clinic. When I scanned a patient with chronic mid-portion Achilles symptoms and discovered a well-defined fluid-filled lesion within the tendon, my initial thought was that it couldnt be a ganglion. Yet, the imaging and clinical picture matched precisely what has been documented in isolated cases in the literature: an intratendinous ganglion cyst.

What Is It?

An intratendinous ganglion cyst is a benign, mucin-filled lesion that forms within the substance of a tendon. While such cysts are most commonly seen in the hand and wrist particularly in extensor tendons their occurrence in lower limb tendons is exceedingly rare. Within the Achilles tendon specifically, only a handful of published case reports exist. This condition should not be confused with paratenon or retrocalcaneal bursitis or with partial tears, although chronic tendinopathic change may coexist.

Pathophysiology

The exact etiology remains unclear, but chronic mechanical stress and repetitive microtrauma are suspected to play a central role, particularly in weightbearing tendons under constant loading. Mucoid degeneration within the tendon fibers may act as a precursor to cyst formation, leading to localized accumulation of viscous fluid within a well-circumscribed, non-vascular capsule. Some case reports suggest that longitudinal splits in the tendon may predispose to or accommodate cyst development.

Diagnostic Clues

MRI remains the gold standard for diagnosis, revealing a well-defined, hyperintense lesion on T2-weighted images within the midsubstance of the tendon. Ultrasound, however, may also demonstrate an anechoic or hypoechoic intratendinous structure with posterior acoustic enhancement, as was the case in my clinic. Care must be taken not to confuse this with chronic degeneration, fluid in the paratenon, or small partial ruptures.

Management Considerations

Most intratendinous ganglia are asymptomatic and do not require intervention. However, symptomatic lesions may cause discomfort, swelling, or a palpable mass, and management ranges from conservative offloading and physiotherapy to ultrasound-guided aspiration or surgical excision. Due to the anatomical location and function of the Achilles tendon, surgery carries higher risks than in upper limb tendons, and recurrence is a known complication.

Clinical Takeaway

This is a lesion most of us will rarely if ever see in practice. Prior to this case, I had regarded the concept as nearly theoretical. However, awareness of its existence can help refine diagnosis and avoid mislabeling such cysts as tendon tears or simple tendinosis. For clinicians using diagnostic ultrasound, careful scanning and correlation with history and symptom location are essential. In this case, I will focus on treating the coexisting tendinopathy, viewing the cyst as a symptom of the overall generative change rather than a cause of symptoms in and of itself own right.

References

1. Yalcin, B., et al. (2017). Intratendinous ganglion cysts: Case report and review of literature. World Journal of Clinical Cases, 5(10), 404–408. https://www.wjgnet.com/2307-8960/full/v5/i10/404.htm

2. Oztekin, H.H., et al. (2017). Intratendinous ganglion cyst of the Achilles tendon: MRI findings. Radiopaedia.org. https://radiopaedia.org/cases/intratendinous-ganglion-cyst-of-the-achilles-tendon

3. Finnoff, J.T., et al. (2010). Sonographically guided aspiration of intratendinous ganglion cysts: A report of two cases. American Journal of Physical Medicine & Rehabilitation, 89(2), 164–169.

4. Firoozabadi, R., et al. (2016). Ganglion cyst within the peroneus tertius tendon: An unusual location. American Journal of Case Reports, 17, 318–321. https://amjcaserep.com/abstract/full/idArt/938498

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