May 2004 Case of the Month - Proscan Imaging

History: Dull non-focal knee pain.

Compiled by: Steven Harper, MD

Technique: Sagital T1-W (Image 1) and axial T1-W datasets (Image 2).

Findings: Diffuse tumefactive swelling/edema is seen throughout the anterior cruciate ligament on both the Sagital T1-W (Image 1) and Axial T1-W datasets (Image 2).

Technique: Sagital T2-W FS (Image 3) and coronal STIR dataset (Image 4).

Findings: The fibers of the ACL are taunt and intact but have a “celery stalk” like appearance on the sagital T2-W FS (Image 3) and coronal STIR dataset (Image 4).

Discussion: The “ACL synovial inflammatory pseudomass” (ASIP) is a termed coined by Pomeranz, et. al., in 1996. ASIP is also known as intercondylar notch cyst, ACL mucoid degeneration, ACL cyst, and ACL ganglion. This pseudomass is seen in up to 1% of knee MR studies.

One recent paper by Bergin et al. suggested that anterior cruciate ligament ganglia (synovial inflammatory pseudomass) and mucoid degeneration commonly coexist on MRI. However, we believe that the ACL synovial inflammatory pseudomass likely reflects a spectrum of disease. This disease process begins with non-cystic mucoid degeneration of the cruciate ligament. The mucoid degeneration may or may not progress to loculated or multiloculated cystic formation- the so called ACL ganglion.

The etiology of the ACL synovial inflammatory pseudomass is thought to be either post-traumatic or related to a defect in the cruciate ligament synovial lining. On gross examination, the cyst contains thick, sticky, jelly-like material.

Clinically, the ACL synovial inflammatory pseudomass often presents as an incidental finding on MR, particularly if small. Larger lesions, especially if associated with erosions, can present with dull non-focal pain. Large lesions can also contribute to notch impingement. ASIP are typically not associated with ligament instability.

On MR, mucoid degeneration begins with homogenous increased signal intensity within an ill-defined enlarged ACL. The individual fibers are intact, but spread apart, similar in appearance to that of a “celery stalk.” Eventually, aloculated or multiloculated cyst can develop. This cyst, similar to a meniscal cyst, can be intraligamentous, periligamentous or both. Cystic enlargement of the ACL has been likened to the appearance of a “drumstick.” As the cyst enlarges, scalloping of the intercondylar notch can occur from benign pressure erosions. Associated subchondral cysts (pseudocysts) are common. On post-contrast images, enhancement of the pericruciate synovium can be identified.

One should avoid the pitfall of confusing an ASIP with a cruciate ligament tear or neoplasm. One report in the surgical literature describes the removal of an intact ACL due to misinterpretation of this benign pseudomass as a neoplasm.

Treatment of ASIP begins with NSAID. Five lesions which fail conservative treatment can be arthroscopically aspirated or resected. Interventional symptomatic relieve has also been obtained through the use of CT guided drainage.

References:
Antonacci V. Foster T. Fenlon H. Harper K. Eustace S. Technical report: CT-guided aspiration of anterior cruciate ligament ganglion cysts. Clin Radiol. 1998 Oct; 53(10):771-3.
Bergin D. Morrison W. Carrino J. Nallamshetty S. Bartolozzi A. Anterior Cruciate Ligament Ganglia and Mucoid Degeneration: Coexistence and Clinical Correlation. AJR 2004; 182:1283-1287.
Kaplan P. Helms C. Dussault R. Anderson M. Major N. Musculoskeletal MRI. W.B. Saunder Co. 2001.
McIntyre J. Moelleken S. Tirman P. Mucoid degeneration of the anterior cruciate ligament mistaken for ligamentous tears. Skeletal Radiol. 2001 Jun;30(6):312-5.
Stoller D. Tirman P. Bredella M. Beltran S. Branstetter R. Blease S. Diagnostic Imaging Orthopaedics. Amirsys. 2004.
Pomeranz S. Orthopedic MRI: A Teaching File; J.B. Lippincott