Cyclops lesion | Radiology Signs

Answer:

The cyclops lesion is a focal nodule of arthrofibrosis lying anterior to the intercondylar portion of an anterior cruciate ligament graft, seen on sagittal MRI as a rounded mass of low to intermediate T1 and heterogeneous T2 signal in the anterior intercondylar notch. It develops in a minority of patients after reconstruction and is a recognised cause of loss of terminal extension, sometimes with an audible or palpable clunk, usually presenting within the first year. The distinction that matters on imaging is between the symptomatic nodule and asymptomatic cyclops-like scar tissue, which is common and requires no treatment; correlation with a fixed extension deficit is therefore essential before the lesion is blamed. The MRI report should also assess the graft itself for tear or laxity, the position of the tibial tunnel, since an anteriorly placed tunnel predisposes to both roof impingement and cyclops formation, tunnel widening, and the presence of generalised arthrofibrosis or infrapatellar contracture. Treatment of a symptomatic lesion is arthroscopic excision, which reliably restores extension, and recurrence after resection is uncommon but described. Prevention centres on correct tunnel placement, avoiding graft impingement against the intercondylar roof, and early rehabilitation aimed at restoring full extension.

Why is it called so?

At arthroscopy the nodule is a rounded mass with a central area of bluish discoloration that resembles a single eye, so it was named after the one-eyed Cyclops of Greek myth.

Pathophysiology

After reconstruction, debris from the drilled tunnels, fibrous tissue from the torn native ligament and repetitive impingement of the graft against the intercondylar roof provoke a localised fibroproliferative response. Fibroblasts and disorganised collagen accumulate at the anterior aspect of the graft, sometimes with ossification or cartilaginous metaplasia, forming a discrete nodule tethered to the graft. Because the nodule occupies the anterior intercondylar notch, it becomes mechanically trapped between the tibia and the intercondylar roof as the knee approaches full extension, which produces the block to extension and, over time, further impingement and enlargement of the lesion in a self-reinforcing cycle.

Alternative names: Cyclops syndrome; localised anterior arthrofibrosis

Other associated named signs: Empty notch sign of the acute anterior cruciate ligament tear and the deep lateral femoral notch sign

References

1. Recht MP, Piraino DW, Cohen MA, Parker RD, Bergfeld JA. Localized anterior arthrofibrosis (cyclops lesion) after reconstruction of the anterior cruciate ligament: MR imaging findings. AJR Am J Roentgenol. 1995;165(2):383-5. PMID: 7618562.