The femoral component alignment resulting from spacer block technique is not worse than after intramedullary guided technique in medial unicompartimental knee arthroplasty

Purpose

Although the spacer block technique has been recommended for the implantation of unicompartmental knee arthroplasty (UKA), there is still a lack of data concerning the resulting component positioning.

Methods

This retrospective study included 193 consecutive patients who had undergone medial UKA using the spacer technique. On the basis of the postoperative long standing radiographs, the coronal component alignment was determined in relation to the mechanical axes and the sagittal component alignment in relation to the anatomical axes of the tibia and femur. The coronal alignment of the femoral component was determined through post hoc 3D planning with the CAD data projected onto the radiograph.

Results

The angle of the tibial component was on the average 2.3° ± 2.8° in varus, the femoral component on the average 2.6° ± 3.7° in varus. Only 4 implants (2%) were outside an assumed tolerance range of 10° varus–10° valgus. A tilting from the femoral to the tibial component of more than 10° was observed in 8 cases (4%). A valgus positioning of the tibial component was followed by a valgus alignment of the femoral component (R = − 0.194, p = 0.007). An increased posterior slope of the tibial component led to an extended positioning of the femoral component (R = − 0.230, p = 0.001).

Conclusions

The spacer block technique produces results comparable to the intramedullary guided technique. However, the precision is low and outlier frequent. Due to the possibility of transferring a tibial malalignment to a femoral malalignment, even greater attention should be paid to the precision of tibial resection.