Summary

Mobile-bearing TKA allowed higher degree of rotation when walking, stepping up stairs and standing up from a chair and had higher functional outcomes compared to fixed-bearing TKA at one-year follow-up. However, no difference in in vivo kinematics nor in clinical outcomes was observed between fixed- and mobile-bearing prostheses at two-year follow-up.

Abstract

Introduction

Fixed-bearing TKA, despite excellent clinical results, still exhibits polyethylene wear-related problems, such as osteolysis around the implants, especially in younger patients. Meanwhile, the main interest of using mobile-bearing TKA is to reduce polyethylene wear and osteolysis by allowing freedom of the tibial polyethylene insert to rotate around a central post, generating less stress between components.
Three-dimensional (3D) in vivo knee kinematics analysis is a valuable method to evaluate physiological and pathological processes that may occur in a prosthetic knee. Changes in knee kinematics during gait and daily living activities can develop after knee arthroplasty, thus a better understanding of the in vivo knee kinematics after these procedures is critical for a better understanding of how these implants behave postoperatively and may help determine the diagnosis of implant failures.
Therefore, the objective of the present study was to determine in vivo knee kinematics and clinical outcomes of patients who underwent fixed- and mobile-bearing TKA at one- and two-year follow-up. Considering the theoretical advantages of mobile-bearing TKA, it was hypothesized that mobile-bearing TKA would have higher axial rotation during gait, stepping up and stepping down stair steps, getting up from a chair and sitting on a chair, and higher range of motion and patient reported outcome measures compared to fixed-bearing TKA at one- and two-year follow-up.

Methods

This prospective double-blinded randomized controlled trial was carried out from November 2011 to December 2012 in accordance with the Consolidated Standards of Reporting Trials (CONSORT) guideline for clinical trials. Sixty-four patients were randomized to fixed- and mobile-bearing TKA groups (32 patients in each group). All patients were evaluated with: three-dimensional in vivo knee kinematics analysis during gait, stepping up and stepping down stair steps, getting up from a chair and sitting on a chair; knee range of motion and patient reported outcome measures [Knee Outcome Survey-Activities of Daily Living Scale (KOS-ADLS) and pain visual analogue scale (VAS)] at one- and two-year follow-up. The Kolmogorov-Smirnov test was used to test if variables were normally distributed. A Student's t test was utilized to compare continuous variables between patients in the two groups. Chi-square test was used to compare the groups with respect to categorical variables. The alpha level for statistical significance was set at p<0.05.

Results

The mean axial tibiofemoral rotation in patients with mobile-bearing TKA was significantly higher during gait (13.3 vs. 10.7), stepping up (12.8 vs. 10.0) and getting up (16.1 vs. 12.1) from a chair compared to fixed-bearing TKA patients at one-year follow-up (p<0.05). KOS-ADLS function score was significantly higher in the mobile-bearing compared to the fixed-bearing TKA group (32.0 vs. 27.7) at one-year follow-up (p<0.05). No significant difference in kinematics and clinical outcomes between fixed- and mobile-bearing TKA groups was observed at two-year follow-up (p>0.05).

Discussion And Conclusion

Mobile-bearing TKA allowed higher degree of rotation when walking, stepping up stairs and standing up from a chair and had higher functional outcomes compared to fixed-bearing TKA at one-year follow-up. However, no difference in in vivo kinematics nor in clinical outcomes was observed between fixed- and mobile-bearing prostheses at two-year follow-up. The greater axial rotation in patients with mobile-bearing TKA compared to fixed-bearing TKA in the present study at one-year follow-up but not at two-year follow-up could be explained by a hardening effect, which at certain period of time postoperatively would cause the tibial insert of mobile-bearing prostheses to behave kinematically similar to a fixed tibial insert. This also explains the higher functional scores at one-year follow-up in the mobile-bearing TKA group but no better clinical outcomes after two-year follow-up.