Clinical History
61-year-old male patient with non-specific left knee pain of 18 months duration.
There were no clinical signs or symptoms consistent with instability.
MRI of the knee was performed.
Imaging Findings
T2 weighted fat suppressed fast spin echo image showed complex anterior cruciate ligament ganglion cyst arising from superior aspect of ligament. The ACL presented diffuse increased signal (figures 1 and 2).
Intraosseous cysts were shown in distal femur and proximal tibia (figures 1 and 2).
These changes were consistent with diffuse ACL mucoid degeneration.
When mucoid degeneration is present, an amorphous mucoid matrix increases the signal intensity of the ACL (figures 3 and 5) and is focally collected between intact, almost parallel fibres. This appearance bears similarity to a stalk of celery (“Celery Stalk Sign") [1] (figures 4 and 6).
Images did not show secondary signs of ACL injury.
Discussion
The pathogenesis of mucoid degeneration of the ACL remains unknown. One theory holds that this lesion may simply represent a continuum of senescent degeneration of the ligament (age related degeneration). The second theory considers congenital or acquired synovial tissue entrapment between ACL fibres. [2]
Criteria for mucoid degeneration include ligament bundles poorly seen on T1 weighted but with both bundles intact on T2 or PD weighted images (figures 4 and 6). Both anterior cruciate ligament ganglion cysts and mucoid degeneration have a high association with intraosseous cysts [3] (figures 1 and 2).
The lesion may manifest as an elongated cyst along the long axis of the ACL, or as an enlarged ACL. This appearance can mimic acute or chronic interstitial partial tears of the ligament. [2]
Anterior cruciate ligament mucoid degeneration and ganglia, commonly coexist on MRI, and are typically not associated with ligament instability (secondary signs of ACL injury are usually absent). It is postulated that mucoid degeneration may be a predisposing factor to the formation of ACL ganglion cysts. [3]
Patients may be symptomatic or asymptomatic. Pain is the most common complaint at presentation. Patients may also be unable to fully flex or extend the knee. The source of pain is unclear and may be either mechanical irritation due to the bulk of the ligament or a chemically mediated reaction. [1]
Clinical presentation associated with ganglion cysts of the anterior cruciate ligament is variable in the literature. Most reported cases are incidental findings without contributory symptoms. Some case reports have ascribed symptoms, including pain at the medial joint line, mechanical locking, clicking and swelling, to the presence of these cysts. Knee pain was the most common symptom reported. This, however, was a nonspecific symptom and was attributed to other intraarticular causes in most patients. [1]
Intraosseous ganglia, at the femoral and tibial attachments, are observed in 77% of the cases with mucoid degeneration of the ACL. [3]
Mucinous degeneration of the anterior cruciate ligament was described as a potential pitfall for the diagnosis of a ligament tear. [3]
Along with tears and anterior cruciate ligament ganglion cysts, mucoid degeneration is a relatively common cause of increased signal within the ACL. [3]
The absence of clinical signs and symptoms consistent with instability also helps to avoid misdiagnosis. [1, 4]
Differential Diagnosis List
Anterior cruciate ligament mucoid degeneration
Anterior cruciate ligament complete tear
Anterior cruciate ligament interstitial partial tear
Final Diagnosis
Anterior cruciate ligament mucoid degeneration
