Description
Background
As opposed to conventional technique, robotic system for TKA can simulate postoperative soft-tissue balance based on 3D positional information of the prosthesis and preoperative soft tissue balance data. Based on this simulation data, surgeon can manipulate the implant position to avoid the medio-lateral or flexion-extension soft-tissue imbalance before bone resection (Figure 1). Although this simulation data is critical for ligament balancing in robotic-assisted TKA, the accuracy and reproducibility of the soft-tissue balance data has not been examined (Figure 2). To clarify this point, we conducted a cadaveric study to compare the evaluation method of soft-tissue balance. The purpose of this study is to establish the best way to measure the soft tissue balance in robotics by comparing different evaluation methods.
Methods
This study was approved by our institutional review board (No.2068). Seven fresh frozen cadavers were included in this study. Three examiners (A: high volume surgeon, B: low volume surgeon, C: resident doctor) performed gap assessment by applying the continuous varus/valgus load to the knee throughout range of motion in using an image-free robotics (NAVIO). The evaluation methods were 1) manual mild stress (mild), 2) manual maximum stress (max), and 3) using Z-retractor (Z). Each examiner performed the procedure twice, and intra- and inter-rater reliability was evaluated using the intraclass correlation coefficient (ICC). The mean difference between examiners at each flexion angle were also recorded and compared.
Results
For the medial gap assessment, the intra-rater reliability was mild (A=0.95, B=0.88, C=088), max (A=0.94, B=0.89, C=0.95), Z (A=0.93, B=0.89, C=0.94), and the inter-rater reliability was mild=0.88, max=0.87, Z=0.92. Mean difference between examiner was mild=0.6 mm, max=0.6 mm, z=0.4 mm. For the lateral gap assessment, intra-rater reliability was mild (A=0.66, B=0.37, C=0.42), max (A=0.86, B=0.84, C=0.33), Z (A=0.62, B=0.62, C=0.96), and the inter-rater reliability was mild=0.41, max=0.24, and Z=0.57. Mean difference between examiner was mild=1.3 mm, max=1.6 mm, z=1.2 mm.
Conclusion
The medial gap assessment showed high intra- and inter-rater reliability in each method. However, Z-retractor could not be applicable after implantation because it is so bulky for postoperative medial laxity. Therefore, to acquire consistent pre and postoperative data, the maximum stress method was considered as an optimal method. On the other hands, the results for the lateral gap assessment were less reproducible in each method. Maximum stress method was highest intra-rater reliability in these three methods, but mean difference between examiner was relatively large. Thus, there is no recommended method as reliable as the medial gap assessment.
