ACL ganglion cysts (also commonly referred to simply as ACL cysts), along with ganglion cysts arising from the alar folds that cover the infrapatellar fat pad, make up the vast majority of intra-articular ganglion cysts of the knee. These are uncommon findings, with the prevalence of ACL ganglion cysts has been reported to be between 0.3 - 1.3% of knee MRIs, and are usually incidental findings. Knee pain, benign the most common presenting complaint, is usually attribuatable to other pathology.
ACL ganglion cysts are histologically identical to the ganglion cysts seen elsewhere, most commonly around the wrist. The pathogenesis remains controversial with two theories being favoured:
- sequelae of ACL mucoid degeneration
- herniation of synovial tissue through a defect in the joint capsule or tendon sheath
MRI is better at detecting these lesions than standard anterior portal approach arthroscopy as the surface of the ligament is often intact. In fact the ligament may appear entirely normal. A posterior approach will allow the arthroscopist to detect changes by probing the ligament, and will allow potential aspiration of the cysts.
Typically the cysts appear as regions of high T2 signal, often multiloculated, centred on the distal ACL. Often there is some irregulartiy to the bone subjacent to the tibial attachment.
Differential diagnosis of cyst around the knee joint includes popliteal cyst, meniscal cyst, ganglion cyst, cyst within tibial tunnel after ACL reconstruction and knee joint effusion
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