Introduction

　Knee kinematics was assessed in the same patients with different PS TKA systems using pattern matching method. The patients had Scorpio Superflex system (Stryker, USA) in one knee, and had Scorpio NRG system in the other (Fig. 1). Superflex has a rectangular tibial post and longer posterior flanges of the femoral component, and NRG has a round tibial post and shortened posterior flanges (Fig. 2).

Materials and methods

　This study consisted of 5 cases. The average period from TKA to this study was 5.7 years (range, 5.1-6.2) with Superflex and was 4.8 years (4.7-5.2) with NRG. One surgeon (R, N) performed all TKAs with modified gap control technique. The flexion angle was more than 135˚ in all knees, and one could sit straight. The knee society score (KSS) was 100 in all knees. Therefore, all knees had excellent function. The fluoroscopic images of the prostheses were taken during knee extension/flexion. Then, a torque of about 5Nm was applied to the lower leg, and the varus/valgus flexibility (Fig. 3) and external/internal rotational flexibility (Fig. 4) were assessed at 0˚, 45˚, 90˚ and 120˚ flexion using specially designed device. The pattern matching method was used to measure the 3D movements of the prostheses from the fluoroscopic images.

Results

　The mean varus/valgus angle during flexion was less than 1˚, and no difference was found between two groups (Fig. 5). The tibia rotated about 10˚ during flexion even with Superflex (Fig. 6). No significant difference was found in the varus/valgus flexibility (Fig. 7), or in the rotational flexibility (Fig. 8) at each flexion angle. Correlation of rotational flexibility between two groups (Fig. 9) was 0.96 at full extension, 0.83 at 45˚ flexion, 0.86 at 90˚ flexion and 0.90 at 120˚ flexion. Overall, correlation was 0.83 between two groups. Values of rotational flexibilities at 90˚ flexion are marked on Fig. 9. 　Overall correlation of varus/valgus flexibility (Fig. 10) was 0.64.

Discussion

In the well functioned implanted knees, the internal rotation angle of the tibia with the rectangular tibial post during flexion was the same with that with the round tibial post. Even with the longer posterior flange of the femoral component, the knees could have deep flexion angle (Fig. 1). The knees with NRG had larger flexibilities (Figs. 8 and Figs. 9). The results of the flexibilities showed that the flexibility after TKA depended mainly on the patient condition and partly on the components designs. 　Results of our previous 3D FEM analyses clearly showed that the rectangular tibial post had very high equivalent stress compared with the round tibial post when the tibia rotated 10˚ internally during flexion (Fig. 11).1 The tibial articular surface of Superflex with longer posterior flange of the femoral component also had high equivalent stress in deep flexion (Fig. 12). Therefore, the designs of components are very important. Because the tibia rotates internally during flexion regardless of the shape of the tibial post, the post should have round shape. The mobile tibial insert is the other option. Design modification of the posterior flange of the femoral component is necessary in order to reduce the contact stress on the tibial articular surface in order to obtain deep flexion angle safely. 　Evaluations of the implanted knees such as KSS only assess pain, ROM, stability and alignment. The results of this study clearly showed that the knees with poor components designs may have poor long-term clinical results due to wear of tibial articular surface or/and tibial post even though the implanted knees have high knee scores.

Conclusion

　The flexibility after TKA depended mainly on the patient condition and partly on the components designs.

References

1, Nagamine et al. 56th transaction 2009; 2115