The patient presented with excruciating pain in the left leg, radiating from the knee to the lateral malleolus, decreased sensation over the dorsum of the left foot, weakness in extension of the left great toe and difficulty in walking. On clinical examination, a smooth and firm swelling was present over the anterolateral aspect of the left knee.
The patient presented with excruciating pain in the left leg, radiating from the knee to the lateral malleolus, decreased sensation over the dorsum of the left foot, weakness in extension of the left great toe and difficulty in walking.
On clinical examination, a smooth and firm swelling was present over the anterolateral aspect of the left knee (Fig. 1). There was atrophy of the left calf muscles, as compared to the right side; hypoaesthesia over the dorsum of the left foot, extending to the 2nd web space; and weak dorsiflexion of the great toe. On MR examination of the knee (Figs 2-4) a 4cm x 2cm x 2cm well-demarcated lobulated soft tissue mass was detected, localised at the anterior neck of the fibula starting below the tibio-fibular joint. The interior of the mass was homogenous hypointense on T1-weighted images and hyperintense on T2-weighted images. The fibula was normal without cortical destruction.
Ultrasound examination (Fig. 5) confirmed an elongated lesion which was cystic in nature. On aspiration of the cyst 1.5ml of thick clear jelly-like fluid was obtained.
Surgery revealed that the mass arose from the proximal tibio-fibular joint. It was tightly attached to the peroneal nerve, compressing it at the level of the fibular neck (Fig. 6). The mass, which was totally excised, was 4cm x 2cm x 2cm in size, lobulated, cystic in nature and contained mucinous material. Microscopic examination revealed mucous filled cavities.
There were no post operative complications. Three months after the surgery, pain and hypoaesthesia had resolved.
Peripheral nerve lesions due to ganglionic cysts are rare findings.
Synovial cysts compressing the peroneal nerve may be extra-neural or intra-neural (Ghossain et al.). Probably these are anatomical variants of the same entity. Many hypotheses have been put forward about the pathogenesis of these cysts, but synovial origin from proximal tibiofibular joint seems to be most likely. This hypothesis enables the origin of not only the extra-neural cysts to be explained but also the intra-neural ones. In effect, in the latter case, the cyst lifts and compresses the nerve then it produces adherence and fusion between its wall and the nerve sheath. The cyst from the upper tibio-fibular joint joins the peroneal nerve by means of a small recurrent articular branch. Most ganglionic cysts causing peroneal nerve compression described in the literature were of the intra-neural type. In this case, compression of the peroneal nerve was due to an extra-neural cyst.
Differential diagnoses should be made with L5 root pathology (Huaux et al.), a post-traumatic intraneural haemorrhage (Gurdjian et al.), a nerve compression near the tendinous arch located at the fibular insertion of the peroneal longus muscle (Sidey et al.) and a nerve sheath tumour.
Plain radiographs are of little value in the diagnosis of ganglia, but may be useful in excluding bony abnormalities or fracture of the neck of the fibula. Seeger and Bassett have described the characteristics of ganglia on magnetic resonance imaging (MRI). On T1-weighted images the signal density is low, whereas on T2-weighted images the signal density is high and they appear homogenous. It may be difficult to differentiate a ganglion from nerve sheath tumours on MRI as homogeneity of the lesion is also found in solid masses. Ultrasonography may be useful in showing the cystic nature of the mass and in distinguishing it from a solid tumour. A combination of MRI and ultrasonography may be helpful in differential diagnosis of doubtful cases. Takagishi et al. report the use of such a combination in the diagnosis of compression of the suprascapular nerve.
It is now well known that a permanent cure can be achieved by microsurgical removal of the cyst, respecting the adherent nerve fibres (Rondepierre et al.), together with ligation or electrocoagulation of both the pedicle stalk and the recurrent articular branch of the peroneal nerve.
In conclusion, compression of the peroneal nerve by an extra-neural ganglion is a rare and often misleading condition. MRI is useful for accurate diagnosis of this condition and in doubtful cases ultrasound guided aspiration may confirm the cystic nature of the swelling. It should be treated by microsurgical exploration as early as possible.
Ganglionic cyst
1. Plain Radiograph (X-ray): No obvious bony abnormalities or damage to the proximal tibia/fibula and joint structures, primarily used to rule out fractures and significant bone destruction.
2. Magnetic Resonance Imaging (MRI): A round or round-like lesion is visible on the lateral side of the left knee joint, near the fibular head; low signal on T1WI and high signal on T2WI, suggesting a fluid-filled or mucoid component; the lesion margins are relatively clear, with uniform or slightly uniform enhancement, indicating a likely cystic lesion.
3. Ultrasound (US): A fluid-filled hypoechoic area is observed in the same region, with no obvious internal echoes and a relatively smooth contour, consistent with a cystic lesion.
4. Lesion Location: The lesion is located in the anterolateral aspect of the left knee joint, adjacent to or in close contact with the course of the common peroneal nerve, showing evident signs of nerve compression.
Based on the patient’s lateral knee mass, sensory deficit in the distribution of the common peroneal nerve, and motor dysfunction (e.g., weakness in dorsiflexion of the foot and toes), possible diagnoses include:
1. Joint cyst/tendon sheath cyst (synovial cyst) compressing the common peroneal nerve: MRI and ultrasound findings suggest a cystic lesion in a location corresponding to the course of the common peroneal nerve, consistent with the typical presentation of a “lateral cyst compressing the common peroneal nerve” described in the literature.
2. Schwannoma or neurofibroma: Although these can present as well-demarcated soft tissue lesions on MRI, they are typically solid, with more complex internal signals compared to cysts. While T2WI may also show high signal, it is often more heterogeneous.
3. L5 nerve root lesion: A lesion affecting the L5 root due to lumbar spine pathology can cause foot dorsiflexion weakness; however, in this case, there is a palpable cystic mass locally, and clinical symptoms and signs are concentrated around compression near the fibular head, making local nerve compression more likely.
4. Other soft tissue masses (e.g., hematoma, inflammatory lesion, or acquired cyst): A hematoma following trauma can compress a nerve, but here the lesion is well-defined, and there is no clear history of trauma or imaging evidence of an old hematoma.
Taking into account the patient’s severe left leg pain radiating to the lateral malleolus, decreased sensation on the dorsum of the foot, inability to extend the great toe, and MRI and ultrasound findings of a cystic lesion, the most likely diagnosis is:
An extra-neural synovial cyst compressing the common peroneal nerve.
If final confirmation is required, pathological examination during surgery can be performed to verify the nature of the cyst fluid and the histological characteristics of the cyst wall.
Treatment Strategy:
1. Surgical Treatment: For patients with significant symptoms and nerve dysfunction, prompt microsurgical exploration is recommended. During surgery, the cyst should be completely excised while preserving nerve fibers, and any connection to the joint or tendon sheath (e.g., cauterization or ligation of the cyst stalk) should be addressed to reduce recurrence risk.
2. Conservative Treatment: If symptoms are mild, protective bracing, oral nonsteroidal anti-inflammatory drugs (NSAIDs), and physical therapy may be attempted for observation; however, for cases with evident motor and sensory dysfunction from nerve compression, the success rate of conservative treatment is limited.
3. Ultrasound- or CT-Guided Aspiration and Injection: In some medical centers, aspiration of the cyst followed by injection of a sclerosing or similar agent under imaging guidance can be tried. However, without addressing the underlying communication with the joint or synovium, recurrence is likely.
Rehabilitation/Exercise Prescription (FITT-VP Principle):
The above report is based on the provided medical history and imaging data for reference and does not replace an in-person consultation or professional medical advice. A specific treatment plan should be developed with consideration of the patient’s overall condition, as well as laboratory and pathological findings. If further concerns arise or symptoms worsen, please seek medical attention promptly.
Ganglionic cyst
