EFFECTS OF THE ALTERATIONS OF POSTERIOR SLOPE, JOINT LINE AND POSTERIOR CONDYLAR OFFSET ON CLINICAL OUTCOMES AFTER TKA WITH FOUR DIFFERENT IMPLANT TYPES

Abstract

Background: Previous studies reported that several kinematic parameters such as tibial posterior slope, joint line, and femoral posterior condylar offset influence clinical outcomes including maximum flexion after total knee arthroplasty (TKA). However, the effects of the kinematic factors may vary with the implant type. We aimed to determine whether implant type influence the associations between the three kinematic factors (posterior slope, joint line, posterior condylar offset) and clinical outcomes. We hypothesized that the associations between the kinematic factors and clinical outcomes would differ among four implant types [fixed bearing cruciate retaining (FB-CR), fixed bearing posterior stabilized (FB-PS), mobile bearing cruciate retaining (MB-CR), and mobile bearing posterior stabilized (MB-PS)]. Methods: A retrospective review of 1300 TKAs performed with one of the four implant types (FB-CR, FB-PS, MB-CR, MB-PS) was performed to select 50 TKAs for each implant type of which 1 year clinical outcomes (maximum flexion, AKS scores, patellofemoral scores, WOMAC, and SF-36) were available. Three radiographic parameters (posterior slope, joint line, and posterior condylar offset) were measured using pre- and post-operative lateral radiographs and postoperative alterations were calculated from the measurements. The correlations between the alterations in the radiographic parameters and the clinical outcomes were compared among the four groups by the implant type. Results: In 4 designs of implant (FB-CR, FB-PS, MB-CR and MB-PS), the mean increase in posterior condylar offset was +0.22, +0.67, +0.33 and +1.26, respectively. The mean joint elevation was −0.31, +1.34, −0.12 and +1.96, respectively. The mean posterior slope was 6.10, 5.64, 5.01 and 4.59, respectively. The mean maximum flexion was greater in the PS designs than in the CR designs (137.0° in FB-PS and 136.4° in MB-PS vs. 132.2° in MB-CR and 130.1° in FB-CR, p < 0.05). No significant correlations between the alterations in the radiographic parameters and maximum flexion. No significant correlations were found between the alterations in the radiographic parameters and the clinical outcomes in all implant types but the MB-CR type. In MB-CR type, the elevation of joint line was significantly associated with worse WOMAC stiffness and function scores (correlation Coefficient = 0.36 and 0.30, respectively) and the increase of posterior condylar offset was associated with a worse WOMAC pain score (correlation coefficient = 0.39). Conclusion: Our findings indicate that the effects of the alterations in the kinematic parameters on the clinical outcomes vary with the implant type. This study also indicates that implant type is more important in determining postoperative maximum flexion than the alterations in the kinematic parameters.