Teaching Case

A 36-year-old male patient presented with knee pain and stiffness, progressively increasing over the last 6 months.

A 36-year-old male patient presented with right knee pain and stiffness, progressively increasing over the last 6 months. There was no history of trauma or any symptoms suggestive of infection. No history of locking or clicking sensation within the joint was reported. On clinical examination, no overt joint swelling or joint tenderness was seen. The anterior and posterior drawer test, Lachmans test and Mc Murray test were negative. There were no findings favouring a joint effusion. Patient could not completely flex or extend the knee.

Plain skiagram of the knee did not convey new findings. The patient was advised to have MRI performed for further evaluation. On MR, the ACL was thick, ill-defined and enlarged in size. On proton density weighted sagittal image there was homogeneous increased signal intensity within the enlarged and thickened ACL. The individual fibres appeared to be intact, but spread apart on the proton density and STIR sequence, similar in appearance to that of a "celery stalk". No intra-osseous signal alteration was seen at the tibial or femoral attachment of ACL. The PCL, collateral ligaments and menisci were normal. Mild joint effusion was also present. Based on the imaging findings, diagnosis of an ACL ganglion cyst (mucoid degeneration) was made.

Anterior and posterior cruciate ligament ganglion cysts are reported in the literature as being rare with an incidence of approximately 0.5% to 1%. The etiopathogenesis of cruciate ligament ganglion cysts is uncertain. Various theories which have been postulated include: [1] senescent mucinous degeneration of the ligament, [2] proliferation of congenitally entrapped synovium within the ligament, and [3] sequelae of remote ligamentous trauma.

Two characteristic MR appearances have been described for ganglion cysts of the ACL. The first presents as a discrete fluid signal cystic lesion within the substance or on the surface of the ACL. The second form has been compared to a “celery stalk” in appearance. The ACL demonstrates diffuse thickening containing ill-defined intraligamentous T2-hyperintensity, which splays the intact ACL fibres. This occurs due to accumulation of amorphous mucoid matrix which gets deposited between intact and almost parallel ACL fibres.

The thickening and increased signal intensity along the course of the ligament may be confused with the appearance of an ACL tear. Few important distinctions allow a confident diagnosis of an ACL ganglion cyst. In case of cruciate ligament ganglia, intact ACL fibres can often be visualised coursing through or around the cyst. Additionally, the ancillary MR findings of an ACL tear, such as bone bruises, buckling of PCL and the forward subluxation of tibia, are absent. Patient’s clinical history and clinical examination do not support an ACL injury. Lachman test for AC instability is negative.

Patients may be asymptomatic; and these ganglion cysts may be diagnosed incidentally on imaging. In symptomatic patients knee pain is the most common complaint at presentation. Larger ganglia may cause stiffness and restriction of movement.

Incidentally detected and asymptomatic cruciate ligament ganglion cysts do not need any intervention. In symptomatic patients arthroscopic decompression of the ganglion or resection is helpful in relieving the patient of the symptoms.