Plain radiographs are a poor indication of the overall coronal, sagittal and axial alignment of a total knee arthroplasty (TKA). We describe a new CT method that allows the mechanical axis in both planes to be defined and seven alignment characteristics to be defined.

A GE Light Speed multislice CT scanner performed a high-speed helical scan from the acetabular roof to the talus in 100 patients following TKA. The knees were scanned in a supine position with the legs in a neutral position. The images were reformatted in coronal, sagittal and axial planes and the mechanical and anatomical axes identified. The femoral component (varus/ valgus, flexion/extension, rotation) as well as the tibial – (varus/valgus, posterior slope and rotation) are measured. Coupled femoro-tibial rotational alignment was assessed by superimposition of the femoral and tibial axial images. The accuracy of this technique has been checked by using a mechanical FARO-arm.

The technique has a low intraobserver error rate of 9% (in each case less than 1 degree) and an accuracy of 3mm in a three-dimensional plane, as determined against an independent FARO arm technique. The CT analysis of 100 patients shows normal tibial baseplate rotation to be 8–12 degrees from the tibial tuberosity.

Conclusion: The CT protocol is the first single radiographic investigation that characterizes all the alignment parameters of a TKA. It sets an excellent standard in planning revision knee surgery and provides a valuable tool in assessing alignment of painful knee replacements as well as in outcome measures of TKA.

Introduction: We describe a CT method that allows the seven alignment characteristics of a knee arthroplasty to be defined in a single investigation.

Method: A multislice CT scanner, scans in 2.5mm slices from the acetabular roof to the dome of the talus with the legs in a standard position.

The mechanical and anatomical axes are identified, from 3 dimensional landmarks, in both AP and lateral planes. The coronal and sagittal alignment of the pros-theses is then measured against the axes.

The rotation of the femoral component is measured relative to the transepicondylar axis. Tibial rotation was measured with reference to the posterior tibial condyles and the tibial tuberosity. Coupled femorotibial rotational alignment was assessed by superimposition of the femoral and tibial axial images.

The results of 100 scans show a low inter and intra observer error rate whilst independent assessment shows a mean measurement error of 3mm in a three dimensional plane. The radiation dose is 2.7mSV.

Conclusions: The technique provides the only currently available measure of all the alignment characteristics required to assess the quality of a knee arthroplasty. It will become a gold standard in planning revision surgery and provide a valuable tool in assessing alignment of painful knee replacements.

Introduction: The epicondylar axis is often cited as a guide to rotation of the femoral component in total knee arthroplasty. Our aimwas to accurately identify with digital palpation, the epicondyles in 14 cadaveric knees Method: Each cadaveric knee had a midline parapatellar approautil ch and the patella was everted. The epicondyles where palpated and the position of the epicondyles was marked by inserting a pin in each epicondyle. All cadavers had a CT scan to identify the position of the epicondyles and pins. The angular difference was calculated with computer-guided measurements. Results: Of the seven right knees, þve had perfect epicondylar identiþcation, whilst two had inaccurate placement of pins. In both cases of error the medial epicondyle had a sulcus conþguration as opposed to a prominent ridge. This resulted in internal rotation of 2 degrees and 3 degrees. Of the seven left knees, þve had perfect epicondylar identiþcation, whilst two had inaccurate pin placement. In both cases this was inaccurate placement of the medial epicondylar pin in a sulcus conþguration. In both this resulted in extra external rotation of the component to 6 degrees. Overall four out of 14 knees had inaccurate placement and in each the medial epicondyle had a sulcus conþguration.