A forty-seven year old female presented with the complaints of progressive right knee pain for 6 months, aggravated by activities. On clinical examination, lateral joint line tenderness and swelling were noted. There was no history of trauma or fever.
Imaging workup of the patient was performed. Knee radiograph showed notching of the lateral femoral condyle. USG showed a cystic lesion with few thin smooth septations, without any internal vascularity, abutting the lateral femoral condyle.
CT and MRI showed a well-defined lobulated, partly exophytic, smooth rim enhancing cystic lesion with few thin smooth enhancing septations, adjacent to the lateral femoral condyle at the site of popliteus tendon attachment, measuring 2.8 x 2.0 x 1.8 cm.
Small extension along the popliteus tendon sheath with mucoid degeneration of the popliteus tendon was seen. The lesion caused indentation of the adjacent soft tissues and smooth scalloping of the lateral condyle with a thin sclerotic rim. No extension to the articular surface was seen. No diffusion restriction, haemorrhage, fluid level or enhancing soft tissue component were noted within the lesion.
Background
Ganglion cysts are benign cystic lesions that occur in collagenous structures due to mucinous degeneration produced by persistent irritation, recurrent damage and ischemia. They are filled with gelatinous fluid rich in hyaluronic acid and other mucopolysaccharides, surrounded by dense connective tissue, and lack synovial epithelial layer [1]
Ganglion cysts are classified into four types viz. intra-articular, extra-articular, intraosseous and (rare) periosteal. [2]
The common locations of ganglion cyst in the knee are: joint capsule, anterior cruciate ligament, posterior cruciate ligament, Hoffa fat pad and menisci. It may also occur in atypical locations like tendon sheaths, within muscle bundles, or adjacent to tibiofibular joints. [3] However, ganglion cyst involving the popliteus tendon is a rare occurrence. [4]
Clinical Perspective
Ganglion cyst of the popliteus tendon is an unusual cause of posterolateral knee pain.[4] The popliteus muscle offers posterolateral stability to the knee by internally rotating the tibia. It takes origin from the posterior surface of proximal medial tibia, extends via the popliteus hiatus, and inserts into the posterior horn of lateral meniscus and lateral margin of lateral femoral condyle. [5]
Imaging Perspective
On imaging, ganglion cyst appears as a well-demarcated rounded or lobulated fluid collection, often with sharply defined internal septations (“bunch of grapes” appearance). MR imaging shows high signal intensity on T2-weighted images and variable signal intensity in T1-weighted images depending on the protein content. Adjacent bone scalloping and pericystic oedema have also been demonstrated. [2] Ganglion cyst of tendon can resemble parameniscal cyst or intra-articular ganglion cyst in appearance, but it lacks the associated internal derangements of the knee which are common in the latter entities.
Outcome
Excision biopsy of the lesion was done which revealed a pearly white cyst over the popliteus tendon. Histopathological examination showed a cyst with proliferating capillaries, focal myxoid change, fibrin deposits and calcified debris, with a dense collagenous wall without lining epithelium; which was consistent with ganglion cyst.
Take-Home Message / Teaching Points
Ganglion cyst of the popliteus tendon is a rare lesion, the diagnosis of which requires a multimodality approach for differentiating from other lesions of the knee. MRI is a valuable tool for identifying typical patterns that aid in determining the accurate diagnosis, thereby guiding specific therapy and reducing unnecessary interventional procedures.
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Mucoid degeneration of popliteus tendon with ganglion cyst
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Based on the provided X-ray, ultrasound, CT, and MRI images of the right knee joint, the following findings are noted:
This type of cyst often appears as a T2 hyperintense, well-demarcated cystic lesion, commonly found within soft tissues or around tendon sheaths, and closely related to local ligaments or tendons.
If the lesion is near the joint capsule or associated with a meniscal tear, it could be a parameniscal cyst. However, this case shows no clear sign of meniscal injury, making this less likely.
Usually located in the posteromedial aspect of the knee, communicating between the semimembranosus and gastrocnemius tendons. In this case, the lesion is in the posterolateral area (close to the popliteus tendon), which is atypical for a Baker’s cyst.
Generally, the MRI signal characteristics differ from this lesion. Also, the patient has no fever, trauma history, or systemic symptoms to suggest acute infection or malignancy.
Considering the patient is a middle-aged female with chronic pain and no trauma history, along with MRI findings of a typical cystic lesion, and surgical plus pathological confirmation of a cystic structure without an epithelial lining, we conclude:
Most likely diagnosis: A tendon cyst (ganglion cyst) related to the popliteus tendon.
The pathology result also indicates that the cyst is consistent with a ganglion cyst (gel-like mucus without epithelial lining).
During postoperative or conservative treatment, rehabilitation exercises should be gradually intensified based on the patient’s condition. The FITT-VP principle is recommended:
If significant pain, swelling, or other discomfort appears during rehabilitation, seek medical advice or consult a physiotherapist promptly.
This report is a reference analysis based on the available imaging data and medical history. It does not replace in-person consultation or professional medical advice. In case of any uncertainties or changes in condition, please consult a specialist promptly.
Mucoid degeneration of popliteus tendon with ganglion cyst
