Mucoid degeneration of the ACL is a rare cause of knee pain. - This lesion of the ACL was first reported by Kumar et al. in 1999. They reported that a biopsy indicated mucoid degeneration and a relief of symptoms could be obtained through arthroscopic partial resection of the ligament.t was thought that this patient had knee pain due to mucoid degeneration of the PCL as well as the ACL. MRI is the most useful examination for differentiating between mucoid degeneration of the ACL and intraligamentous ganglion, as well as other lesions in the knee joint. McIntyre et al.