Tibial tunnel cyst
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Tibial tunnel cysts, including pretibial cysts, are occasional complications of autologous or synthetic anterior cruciate ligament (ACL) reconstruction surgeries. They are benign ganglion cysts that develop in or around the osseous tibial tunnel made during ACL reconstruction using the transtibial technique 1-2. They are not to be confused with ACL ganglion cysts.
Tibial tunnel and pretibial cysts may present as palpable, painful masses along the anterior surface of the proximal tibia, but are often completely asymptomatic incidental findings. These cysts usually occur between 1 and 5 years after surgery 1-3.
The exact cause of tibial tunnel cysts is unclear. Existing theories suggest excess motion of the ligament graft within the tunnel, graft fixation in close proximity of the joint, early overuse and mechanical overload of the graft. An infectious causative factor cannot be proven, and tunnel cyst formation also could not be linked to knee instability or graft failure.
Tunnel cysts are divided into two broad categories 1-2:
- maintain a connection with the knee joint space, thus require a different therapeutical approach
- presumably caused by the incomplete attachment of the graft to the bony tunnel or graft microtrauma
- non-communicating cyst: thought to be caused by a sterile foreign-body reaction against fixation devices
Large cysts can be indirectly diagnosed by the cystic lucency caused by bone resorption, and dilatation of the tibial tunnel 1.
Only pretibial cysts can be identified using ultrasound, where they usually appear entirely anechoic or very hypoechogenic and avascular similarly to ganglion cysts encountered elsewhere.
Focal tunnel dilatation filled by low density material can signal the presence of these cysts. CT is also useful for evaluating the bone stock around tunnel 1.
MRI is the modality of choice for evaluating tibial tunnel and pretibial cysts. These cysts can appear uni- or multilocular and are typically hyperintense on fluid-sensitive sequences (fat-suppressed T2, PD, STIR). Reactive bone marrow edema around the tunnel may also be present. They do not enhance after administration of IV contrast 2-3.
Treatment and prognosis
Non-communicating tibial tunnel cysts can be treated with simple excision, whilst for cysts communicating with the joint space bone grafting and removal of the hardware may also be necessary 1.
Isolated fluid within the graft fibers is considered normal within 18 months after surgery. Foreign body granulomas may mimic cysts but these masses enhance after the administration of IV gadolinium. The possibility of infection, abscess formation and migration of hardware (screw extrusion) should be also considered 1-3.