This patient presented to the Emergency Department with a painful distended left knee. A knee x-ray examination was performed and the resultant images are shown above. What abnormal findings are demonstrated?

There is abnormal bone growth adjacent to the medial condyle of the knee on the AP view (arrow 1). This is known as Pellegrini-Stieda Disease and is a sequela of an old injury (or injuries) to the medial collateral ligament (MCL) of the knee. The abnormal bone growth is a form of myosotis ossificans or soft tissue calcification and is located within the superior attachment of the medial collateral ligament of the knee. Pellegrini-Stieda Disease is commonly associated with sporting injuries. The calcification is variable in its appearance but sufficiently characteristic in most cases to make a firm diagnosis. In some cases of Pellegrini-Stieda Disease the calcification can be extensive, involving most of the MCL. Comparison views should be strongly discouraged.

Some mild osteoarthritic marginal osteophyte formation is evident particularly at the medial femoral aspect of the knee(arrow 2).

The double contour seen along the medial femoral articular surface is caused by the largely obscured fabella(arrow 3).

There is a moderately sized knee joint effusion. This is evidenced by the increase in size and density of the suprapetallar pouch of the knee joint (dotted red line). In addition, there is a suggestion of additional irregular fluid density seen in Hoffa’s Triangle (1). Increased density within Hoffa’s Triangle/fatpad is associated with knee joint effusion. Hoffa’s fat pad is the triangular infrapatella portion of the joint and should be an area of relative radiolucency on the lateral view (ie fat density rather than fluid density)

The distention of the suprapatellar pouch by the knee effusion is anteriorly displacing the quadriceps femoris muscle/suprapatellar tendon. This is causing some anterior displacement and angulation of the patella.

The articular surface of the patella is irregular and demonstrates osteophytic changes typical of osteoarthritis(2).

There is a fabella(3). There appears to be some clothing artifact.

There is Pelligrini-Stieda Disease. There is a moderately large knee effusion. There is evidence of osteoarthritis. A fabella is noted. It is likely that this patient has been an active sportsman in the past. The cause of the knee effusion is unknown.

Recognition of Pellegrini-Stieda disease is a useful pattern recognition skill for radiographers who work in the Emergency Department. It is often a source of confusion for the referring doctor who may request unnecessary comparison views. Equally, recognition of a knee effusion either from a distended suprapatellar pouch or from fluid density in Hoffa's Triangle can be useful. Once you are able to recognise a knee effusion confidently, you will be able to recognise the absence of a knee effusion and this can be important in cases where there is equivocal evidence of bony injury.