Summary

A 2mm worn-unworn difference needs to be re-evaluated in order for true KA to be performed.

Abstract

Background

Kinematic alignment (KA) in total knee arthroplasty (TKA) aims to resurface the articular surface of the pre-arthritic knee. One cornerstone of KA TKA and the basis for conventional instrumentation (CI) is a 2 mm cartilage thickness of the healthy distal femur. This study of image-based (CT) robotic-assisted surgery (RAS) TKA determined 1) the accuracy of measuring cartilage thickness using the preoperative MRI and the RAS registration probe and 2) the percentage of knees with a > 2 mm thickness of the unworn cartilage.

Methods

Twenty-two patients undergoing a TKA using an image-based RAS were included. Preoperative long-leg radiographs provided the limb phenotype. The four zones of interest for cartilage thickness were the distal worn (DW), distal unworn (DU), posterior worn (PW) and posterior unworn (PU). The preoperative MRI provided cartilage thickness values for all zones. Intraoperatively, the surgeon identified the zones without cartilage wear (healthy cartilage) and measured the distance between the cartilage and bone with the RAS registration probe. After resecting the condyle with healthy cartilage, a ruler provided the cartilage thickness reference measurement.

Results

Mean patient age was 68.7±4.4 years, gender was evenly distributed. In terms of HKA, three patients were NEUHKA0°, eight patients were VARHKA3°, seven patients VARHKA6°, two patients VARHKA9°, respectively. The difference between measurement techniques was statistically significant for all zones except PW (DU p=0.004, DW p=0.007, PU p=0.003, PW=0.196). The range of differences was 0.083 - 0.173 mm, depending on the zone. In 3/20 cases (15%), the difference in cartilage thickness distally was >2.5mm measured with any of the three techniques.

Conclusion

In the present study, 15% of cases had a worn/unworn difference of >2.5mm. If KA is applied, additional measurements, such as preoperative MRI or intraoperative use of robotic technology is recommended, to avoid the risk of femoral component malpositioning.