CASE REPORT Metastatic adenocarcinoma of the lung mimicking mucoid degeneration of the anterior cruciate ligament M. J. Shelly Æ S. Dheer Æ E. C. Kavanagh Received: 6 January 2009 / Accepted: 23 February 2009 / Published online: 14 March 2009 Ó Royal Academy of Medicine in Ireland 2009 Abstract We report a case of a 63-year-old male with known adenocarcinoma of the lung who presented with knee pain which was initially diagnosed as mucoid degeneration of the anterior cruciate ligament (ACL) on magnetic resonance imaging (MRI). Due to persistent knee pain an interval MRI was performed, followed by image guided biopsy which showed metastatic adenocarcinoma of the lung infiltrating the ACL. This is the first reported case in the literature of an intra-articular metastasis mimicking mucoid degeneration of the ACL. Keywords Magnetic resonance imaging Á Knee Á Anterior cruciate ligament Á Mucoid degeneration Á Intra-articular metastasis Introduction Mucoid degeneration of the anterior cruciate ligament (ACL) is an uncommon cause of knee pain [1] and is often mistaken for a cruciate ligamentous tear [2]. Intra-articular metastases from a solid tumour are a rare cause of joint pain and carry a very poor prognosis [3, 4]. We report a case of knee pain in a patient with known metastatic ade- nocarcinoma of the lung that was initially thought to be due to mucoid degeneration of the ACL with ganglion cyst formation but was subsequently found to be due to an intra- articular metastasis. Case report A 63-year-old male with known metastatic adenocarcinoma of the lung presented with right lower limb pain. Clinical examination of the right hip and knee was unremarkable. MRI of the right knee at that time demonstrated an oede- matous and thickened ACL with increased signal intensity on all sequences (Fig. 1). Specifically, the anteromedial and posterolateral bundles of the ACL could not be identified. In the clinical setting of knee pain, a diagnosis of mucoid degeneration with early ganglion formation at the femoral origin of the ACL was made. The patient’s knee pain per- sisted and an interval MRI was performed which showed an intra-articular metastatic lesion that likely originated in the intercondylar notch (Fig. 2). The metastasis had eroded through the cortex of the distal femur and followed the path of least resistance into the joint using the ACL as a scaffold. CT guided biopsy of the lesion was performed to confirm the diagnosis. Pathologic evaluation revealed a lesion with elements of well differentiated and moderately undifferen- tiated adenocarcinoma of lung origin. The patient was transferred to an oncology unit for radiotherapy for symp- tomatic relief of right knee pain. After an initial treatment response, the patient’s general condition deteriorated rap- idly and he died with widespread metastatic disease. M. J. Shelly Á E. C. Kavanagh Department of Radiology, Mater Misericordiae University Hospital, Eccles Street, Dublin 7, Ireland e-mail: eoin.kavanagh@mac.com S. Dheer Department of Radiology, Thomas Jefferson University Hospital, Philadelphia, PA 19107, USA e-mail: sachdheer@yahoo.com M. J. Shelly (&) 59 Belgrove Park, Clontarf, Dublin 3, Ireland e-mail: martinshelly@gmail.com 123 Ir J Med Sci (2010) 179:309–311 DOI 10.1007/s11845-009-0311-y