[Radiographic measurements in total knee arthroplasty and their role in clinical practice]

The aim of the study was to present the use of digital sciagraphy and computed tomography for pre- and post-operative measurements in total knee arthroplasty. The authors were interested, in the first place, in the optimal adjustment of femoral component rotation and a valgus angle if extra-articular deformities of the femur and/or the tibia were present.

Digital sciagraphic examination was carried out on an AXIOM ARISTOS (Siemens) apparatus using the software designed by us. In group 1 comprising 269 knee joints, in a standing and weight-bearing position with lower extremity neutral rotation, the valgus angle was measured and the entry point for the intramedullary rod of a femoral cutting block was deter- mined. Subsequently, the mechanical axis and extra-articular deformities of the femur and/or the tibia were found and the patella position in 30-degree flexion of the knee joint was assessed on axial images. Based on radiographic evaluation, relevant treatment for different types of disorders, including extra-articular deformity, was proposed. In group 2 consisting of 204 knee joints, the values of a condylar twist angle were measured on axial sections, using a Siemens Somatom Sensation 64 CT Scaner.The method of condylar twist angle measurement was developed and the values for men and women were obtained.

In group 1, the mean values obtained for valgus knee deformity were: valgus angle, 5.4°; median, 5.5°; modus, 6.0°. Those for varus knee deformity were: valgus angle, 7.2°; median, 7.0; modus, 7.0. A normal knee joint alignment (mechanical axis of 0° to 5°) had the respective mean values of 6°; 6.0° and 6.0°. This group showed 76 extra-articular deformities (33.9 %). In group 2, for women the mean ± SD value of the condylar twist angle was 5.25° ± 1.68; and median and modus values were 5.0° and 4.0 °, respectively. For men, the respective values were 4.69° ± 1.33; 4.0° and 4.0°.

The mean values of valgus angle and CTA found in this study are in agreement with the literature data. In the pre-operative planning it is necessary to take extra-articular deformities in consideration, to respect the entry point for the intra- medullary rod and to take a compromise solution for adjustment of the valgus angle of the femur and for tibial deformities. Also, in severe valgus and varus deformities of the knee, the maintenance of a neutral mechanical axis should be strictly observed. The optimal adjustment of femoral component rotation is individual and depends on the type of deformity and femoro-patellar joint pathology. The external rotation of a femoral component should be set in the range of 0° to 7°.

Digital sciagraphy with suitable software and computed tomography contribute to radiographic measurements before and after total knee arthroplasty. They facilitate an accurate and quick measurement together with data storage. On examination in a standing weight-bearing position it is necessary to keep standard lower extremity neutral rotation. Computed tomography is recommended when more severe valgus and varus deformities and/or femoro-patellar pathology are present. The results of radiographic measurement analysis will allow the surgeon to plan the operative strategy and select a suitable type of implant.