KNEE Mucoid degeneration of the anterior cruciate ligament: MRI, clinical, intraoperative, and histological findings Arturo Makino • Cecilia Pascual-Garrido • Alejandro Rolo ´n • Mariana Isola • Domingo L. Muscolo Received: 3 December 2009 / Accepted: 28 July 2010 / Published online: 1 September 2010 Ó Springer-Verlag 2010 Abstract Purpose To describe the clinical, MRI, intraoperative, and histological features of mucoid degeneration of the anterior cruciate ligament (ACL). Methods This study included 10 patients (average age 47 years, range 19–69 years) who underwent surgery due to clinical diagnoses of mucoid degeneration of the ACL. All patients underwent preoperative magnetic resonance imaging (MRI) and were arthroscopically treated. Results All 10 patients reported preoperative posterior knee pain. MRI indicated a thickened ACL with increased signal on all sequences. Postoperatively, only six of 10 patients were confirmed by biopsy to have a mucoid degeneration of the ACL. An associated intraosseous tibial cyst or ganglion was observed only in patients with a definitive diagnosis of mucoid degeneration. Conclusions Mucoid degeneration of the ACL can be suspected in patients with posterior knee pain associated with a thickened ACL. Associated findings such as an intraosseous tibial or ganglion cyst help to rule out dif- ferential diagnosis. Keywords ACL Á Mucoid Á Intraosseous tibial cyst Á Ganglion Introduction Mucoid degeneration of the anterior cruciate ligament (ACL) has been suggested in patients with back knee pain and limitation of knee flexion associated with a bulky ACL as shown by magnetic resonance imaging (MRI) [3, 6]. However, this association is not sufficient to establish an accurate diagnosis, as its presentation and appearance can mimic other pathologies such as partial ACL tears, ganglion cysts, and synovial chondromatosis [7, 12]. Mucoid degeneration of the ACL, with a reported prevalence of 0.43% [9], affects middle-aged patients and is thought to be secondary to ACL degeneration. Patients characteristically do not complain of knee instability, and the Lachman test is typically negative. When mucoid degeneration is present, MRI reveals an ill-defined but intact ACL with increased girth and normal orientation of the ligament. Histologically, mucoid degeneration results in a mucoid substance in the connective tissue containing glycoproteins and mucoproteins [9]. Mucoid degeneration has been associated with ganglions and intraosseous femoral and tibial cysts, suggesting the presence of a global mucoid degeneration of the connective tissue of the joint [1]. The objective of this study is to describe the clinical, MRI, and histological features of mucoid degeneration of the ACL in a series of patients with a clinical diagnosis of mucoid degeneration. Materials and methods Inclusion criteria consisted of those patients who under- went surgery with a clinical diagnosis of mucoid degen- eration of the ACL. A. Makino (&) Á C. Pascual-Garrido Á D. L. Muscolo Orthopedics Department, Hospital Italiano de Buenos Aires, Potosi 4247, C1199ACK, Buenos Aires, Argentina e-mail: makinoresearch@gmail.com A. Rolo ´n Department of Radiology, Hospital Italiano de Buenos Aires, Buenos Aires, Argentina M. Isola Department of Pathology, Hospital Italiano de Buenos Aires, Buenos Aires, Argentina 123 Knee Surg Sports Traumatol Arthrosc (2011) 19:408–411 DOI 10.1007/s00167-010-1239-5