Residual cysts o develop from residual periapical infection or from cyst fragments left following extraction of a non-vital tooth similar features to the radicular cyst o radiographically are found as isolated circumscribed. unilocular radiolucent lesions in the alveolar process but without an obvious causative toothResidual cysts ⚫ develop from residual periapical infection or from cyst fragments left following extraction of a non-vital tooth ⚫ similar features to the radicular cyst ⚫ radiographically are found as isolated, circumscribed, unilocular radiolucent lesions in the alveolar process but without an obvious causative tooth