Abstract
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.
The abstracts were prepared by Mr Tim Briggs. (Editoral Secretary 2003/4) Correspondence should be addressed to him at Lane Farm, Chapel Lane, Totternhoe, Dunstable, Bedfordshire LU6 2BZ, United Kingdom
