Purpose: Mucoid pseudocysts are infrequent benign tumours affecting all peripheral nerves near joints. Reported for the first time in 1891, these cysts raise an unresolved question as to their nature. Identifying the origin of these cysts could have an impact on management and the risk of recurrence.

Material and methods: Twenty-three patients (21 men two women), mean age 38 years, were followed for a mean six years. The mucoid cyst was located at the level of the common fibular at the neck of the fibula in 16 cases, the tibial nerve at the knee in one, the medial nerve in one, the ulnar nerve in three and the supra-scapular nerve in two. Pain was local in 18 patients, irradiated to the concerned territory in 20. In 17 patients, the nerve lesion was discovered due to development of motor deficit. An EMG was performed in all cases, an ultrasound exploration in 15, a CT scan in seven and an MRI in ten. All patients underwent surgery with neurolysis under microscopy for intraneural cysts. A pedicle communicating with the joint was search for during each intervention.

Results: An articular communication was found in 17 cases. Mean delay to recovery of motor force was five months with recovery of normal sensitivity in 16 cases at seven months. Total lack of recovery was noted in one patient. There was one recurrence requiring tibiofibular arthrodesis.

Discussion: Three theories have been put forward. Besides the cystic degeneration of certain schwannomas, degeneration of the nerve sheath connective tissue, the joint theory appears to be the most probable. The presence of a pedicle linking the cyst to the joint in more than 60% of the cases, the periarticular situation of the nerves involved and sometimes the migration along an articular nerve as well as the mucoid content of the cysts is in favour of an articular origin. The notion of recurrrence after complete minute resection is also in favour of a joint disease.

Conclusion: A mucoid cyst is a likely diagnosis for neruological lesions lying near a joint. Search for an articular communication both before and during the surgical procedure is important to limit the risk of recurrence.