Presentation
Ongoing knee pain, ? insufficiency fracture
Patient Data
The ACL is intact but markedly thickened and edematous throughout in keeping with mucoid degeneration. The PCL is intact but also thickened.
The middle third of the medial meniscus is extruded, causing bowing of the MCL, with associated complex tears.
Extrusion of the lateral meniscus middle third with radial tears and areas of truncation.
Moderate medial patellofemoral joint osteoarthritis. Small joint effusion. Small medial gastrocnemius/semimembranous cyst.
Focal fusiform lesion of the distal tibial nerve proximal to the popliteal fossa, which returns high T2 signal and heterogeneously enhances, in keeping with a peripheral nerve sheath tumor.
No osteonecrosis or insufficiency fracture.
Case Discussion
MRI knees in patient's over 50 are less frequently performed, due to the increasing prevalence of osteoarthritis and meniscal tears, and confirming such pathologies are unlikely to change the clinical management. In older patients, MRI is still performed if there is severe knee pain and osteonecrosis and/or insufficiency fracture are suspected.
This case excellently demonstrates the celery stalk sign of an ACL with mucoid degeneration. In younger patients with an ACL which has edematous expansion and there is an associated pivot-shift bone contusion pattern, high grade partial thickness tearing of the ACL can have a similar appearance. Therefore, care should be taken not to attribute all ACLs with this appearance as just mucoid degeneration.
The case also displays a good example of a peripheral nerve sheath tumor, likely a schwannoma, with fusiform thickening, a split fat sign and a perineural tail.
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