Histopathology of synovial cysts of the spine

Aims: Cystic lesions derived from the synovial and ligamentous structures of the spine have varied histological appearances. Not uncommonly, there is discrepancy between the clinicoradiological diagnosis and histology. Therefore, we sought to characterise the histological features of tissue submitted as 'synovial cysts' of the spine.

Methods and results: Resected specimens of the spine labelled 'synovial cysts' and 'lumbar cysts' were histologically evaluated and classified on the basis of histopathological features. Seventy-five histological samples of spinal cysts were identified. Thirty-one were classified as synovial cysts (definite synovial lining), 28 showed pseudocystic degeneration of the ligamentum flavum, seven showed pseudocyst formation without evidence of a synovial lining or degeneration of the ligamentum flavum, and eight showed cyst contents only or no histological evidence of cyst wall for evaluation. Twenty-five cases (33%), especially those showing pseudocystic degeneration of the ligamentum flavum, were associated with very characteristic tumour calcinosis-like calcium deposition with a surrounding foreign-body giant-cell reaction.

Conclusion: Histology of 'synovial cysts' of the spine shows varied types of cyst; a large proportion are not synovial-lined cysts, but rather show pseudocystic degenerative changes of the ligamentum flavum, often associated with very characteristic finely granular calcifications and a foreign-body giant-cell reaction. This may have implications not only for understanding the pathogenesis of these lesions, but also for their varied responses to non-surgical interventions.