"Grand piano sign," a marker for proper femoral component rotation during total knee arthroplasty

A malpositioned femoral component is an established risk factor for patellar instability and pain after total knee arthroplasty (TKA). In the assessment of femoral rotation, several axes, including the transepicondylar axis, the posterior condylar axis, and the anteroposterior axis, are useful. However, these axes are not always easily applicable, particularly when significant deformity exists. An anecdotal method used by some surgeons involves assessing the shape of the anterior femoral surface osteotomy. Our observations from saw bone models and TKA led to our hypothesis that proper femoral component placement is indicated by a bimodal peak on the anterior femur, approximately twice as high on the lateral side than on the medial side. We use the term "grand piano sign" to describe the shape of the trochlea after the osteotomy is correctly completed. To our knowledge, this common observation has not been studied either in the laboratory or in vivo. Our cadaveric models demonstrated that the grand piano sign correlated with proper femoral rotation during TKA. Surgeons who are knowledgeable about this marker should find it helpful when orienting components during knee replacement surgery.