Patellar tracking is the most common complication observed following total knee arthroplasty. It may constitute a minor disorder or even frank dislocation. Its main cause is a rotation defect in the prosthetic components. CT is the most reliable instrumental test to assess this rotation defect, and an excellent aid for planning a possible revision operation.

From January 1999 to November 2003 we treated 33 patients with a total of 35 painful knees using TC prostheses. Ten patients were male and 33 female. Mean age was 66 years. We performed CT scanning using a modified Berger technique. The lower limbs were extended and scans were performed perpendicular to the mechanical axis of the knee from the supracondylar region of the femur to the plane passing under the distal end of the tibial component up to the anterior tibial tuberosity. Reference lines to assess the rotation of each prosthetic component were drawn electronically on the scan planes.

Adding together the rotation values of the prosthetic components, we observed that when total internal rotation was between 10° and 4°, there was subluxation or frank dislocation. When total internal rotation was between 1° and 4° the disorder was not severe, such as patellar tilt. When total internal rotation was less than 1° or when the components were externally rotated, no femorotibial compartment disorders were observed. The modified Berger technique enabled us to correlate the degree of prosthetic internal rotation with the severity of the disorder and further demonstrate the benefits of femoral component external rotation on patellar glide and ligament balance of the prosthesis.

The authors believe that using CT with the helicoidal technique, by modifying Berger’s technique, enables an accurate assessment of prosthetic component rotation, subsequent correlation with femoro-patella symptoms, and adequate pre-operative planning in case of revision surgery.