Appropriate sagittal femoral component alignment cannot be ensured by intramedullary alignment rods

The intraoperative sagittal cutting block alignment when intramedullary alignment rods are used was investigated. Its absolute orthogonal orientation in relation to the mechanical femoral axis should be analysed. It was hypothesized that (1) alignment rods do not ensure a desired deviation within a range between 0° and 3° flexion as it has been shown to be favourable for clinical and functional outcome. Further, the degree of flexion cannot be (2) foreseen by the surgeon or (3) estimated by clinical or radiological parameters.

Forty knees allocated to total knee arthroplasty were included. The distal femoral cutting block was aligned using an intramedullary rod. By means of a navigation device, the sagittal alignment of the cutting block in degrees of flexion was measured.

The mean measured flexion of the cutting block was 4.4° (3.6 SD). Twenty-five per cent (10/40) of the values were within a corridor between 0° and 3° of flexion. The mean difference between expected and measured flexion was -1.5° (-7.6 to 4.7 95 % limits of agreement). The dorsoventral diameter of the distal femur showed a significant influence on measured flexion (R (2) = 0.112, p = 0.035).

Intramedullary alignment rods used in the present study do not ensure a distal cutting block alignment between 0° and 3° of flexion in relation to the femoral mechanical axis. The extent of flexion could not be foreseen either by the surgeon. The dorsoventral diameter of the distal femur correlated with measured flexion. However, only limited data is available on influence of femoral component flexion on implant failure or clinical and functional outcome.

Diagnostic study, Level II.