Threshold values for stress radiographs in unstable knees after total knee arthroplasty.

Purpose: The primary aim of this study was to investigate the potential benefit of stress radiographs for diagnosis of unstable total knee arthroplasty (TKA) and to identify clinically relevant cut-off values to differentiate between unstable and stable TKAs.

Methods: Data of 40 patients with 49 cruciate retaining (CR) TKA who underwent stress radiographs as part of the diagnostic algorithm in a painful knee clinic were prospectively collected. Anterior and posterior stress radiographs were done in 90° and 15° flexion, varus-valgus stress radiographs in 0° and 30° knee flexion. Knee laxity was measured in mm and degrees by two independent observers using standardized landmarks. Intra- and inter-observer single measure intraclass correlations were between 0.92 to 1 and 0.89 to 1, respectively. For evaluation and investigation of the potential cut-off values, two groups of patients with and without revision surgery due to instability were compared. Radiographic measures of standardized z values according to the group without revision due to instability were used to calculate average and maximum laxity z-scores.

Results: Knees undergoing revision TKA due to instability showed significantly (p < 0.001) lower (KSS) pain/function scores (94 ± 6.3, range 80-100; control group: 112 ± 19.2, range 80-148) and total KSS scores when compared to the control group. The laxity values of patients with instability were significantly higher in terms of mean values (p < 0.01) when compared to the control group. The maximum laxity z-score showed the strongest difference between the groups (R = 0.26, p < 0.001). The following cut-off values indicating need of revision due to instability were established: in 90° (15°) flexion-anterior translation 5.2 mm (22.4 mm), posterior translation 16.6 mm (13.2 mm); varus stress in 0° (20°-30°) flexion-inlay gap 5.2 mm (6.1 mm) or joint angle 6.1° (6.8°); valgus stress in 0° (20°-30°) flexion-inlay gap 4.6 mm (5.7 mm) or joint angle 5.2° (7.1°).

Conclusion: Standardized stress radiographs are helpful tools for diagnosis of instability after TKA. The established cut-off values help to guide decision making in this challenging group of patients. However, laxity values should not be considered as the only criteria for diagnosis of unstable TKA.

Level Of Evidence: IV.