Cyclops lesion

What it is

The name comes from its appearance at arthroscopy: a round, reddish nodule with blood vessels on its surface that resembles an eye, like the one-eyed Cyclops of Greek myth. It sits at the front of the knee where the ACL graft enters the shin bone.

It develops in a small proportion of patients after ACL reconstruction, usually in the first months, and is more likely if the knee did not get fully straight early in rehabilitation.

Why it matters

When the knee straightens, the nodule gets pinched between the graft and the thigh bone. This causes a loss of the last few degrees of extension, pain at the front of the knee and sometimes an audible clunk. Walking with a slightly bent knee places extra load on the quadriceps and kneecap.

A loss of extension that is not improving with physiotherapy should always be checked, because the earlier it is addressed, the easier recovery is.

How it is diagnosed and treated

Examination shows the block to straightening, and MRI usually confirms the nodule. Physiotherapy alone rarely moves a true mechanical block. Arthroscopic removal is a short keyhole procedure, after which most people regain full extension with focused rehabilitation. It does not usually affect the graft itself.