Are kneeling stress radiography and the posterior sag sign associated with subjective flexion instability after posterior-stabilized total knee arthroplasty?

Flexion instability is a common but underdiagnosed cause of patient dissatisfaction after posterior-stabilised total knee arthroplasty (TKA). Standard clinical tests have limited reliability for this condition. We investigated whether kneeling stress radiography and selected physical examination findings can help identify it. We retrospectively reviewed 145 posterior-stabilised TKA knees in 119 patients, divided into two groups by the presence of subjective knee instability. The posterior sag sign, 90° anterior drawer test, and kneeling stress radiographic parameters—particularly the change in tibiofemoral translation under load (Δ-TPFA)—were compared between groups. Post-hoc analyses included ROC curve analysis, diagnostic accuracy with Wilson 95% confidence intervals, and multivariable logistic regression adjusted for age, sex, BMI, follow-up duration, implant type, and lumbar pathology. Thirty knees (20.7%) reported subjective instability. Mean Δ-TPFA was 1.40 ± 3.75 mm in the unstable group versus − 1.17 ± 3.28 mm in the stable group ( p = 0.001; AUC 0.68, 95% CI 0.56–0.80) and remained an independent predictor on multivariable analysis (OR 1.30 per mm, 95% CI 1.11–1.52, p < 0.001). The posterior sag sign had a specificity of 82.6% (95% CI 74.6–88.4%) and was independently associated with instability (OR 3.26, 95% CI 1.54–6.91, p = 0.002). The 90° anterior drawer test was positive in 56.7% of the unstable group and 47.0% of the stable group and did not discriminate between them ( p = 0.41). Patients with subjective instability also scored significantly lower on all three patient-reported outcome measures (KSS 84.8 vs. 91.6; fKSS 75.0 vs. 85.1; FJS 73.6 vs. 84.6; all p ≤ 0.015). On kneeling stress radiography, Δ-TPFA discriminated unstable from stable knees only modestly (AUC 0.68; Youden cut-off 0.30 mm, sensitivity 63%, specificity 73%). Kneeling stress radiography and the posterior sag sign add useful information when assessing flexion instability after posterior-stabilised TKA, although the limited discrimination of Δ-TPFA indicates it should complement rather than replace clinical assessment. The 90° anterior drawer test should be interpreted with caution in this setting.