Archives of Orthopaedic and Trauma Surgery - 2026-09-11 - Journal Article
Are kneeling stress radiography and the posterior sag sign associated with subjective flexion instability after posterior-stabilized total knee arthroplasty?
Aşkın M, Çağlar Ö, Tokgözoğlu AM, Atilla B
Topics
Key Takeaway
In PS-TKA, kneeling stress radiography (Δ-TPFA AUC 0.68, OR 1.30/mm) and the posterior sag sign (specificity 82.6%, OR 3.26) independently predict subjective flexion instability, while the 90° anterior drawer test does not discriminate (p=0.41).
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Summary
This retrospective study evaluated whether kneeling stress radiography (Δ-TPFA) and physical examination findings identify subjective flexion instability in 145 PS-TKA knees, with 30 (20.7%) classified as unstable. Δ-TPFA was significantly higher in the unstable group (1.40 vs. -1.17 mm, p=0.001) and remained an independent predictor on multivariable regression (OR 1.30/mm), while the posterior sag sign showed 82.6% specificity (OR 3.26, p=0.002). The 90° anterior drawer test failed to discriminate between groups (56.7% vs. 47.0% positive, p=0.41), and unstable knees scored significantly lower on KSS, fKSS, and FJS (all p≤0.015).
Key Limitation
The AUC of 0.68 for Δ-TPFA reflects only modest discriminatory power, limiting its standalone utility as a diagnostic test and precluding its use as a sole criterion for surgical decision-making.
Original Abstract
INTRODUCTION
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.
MATERIALS AND METHODS
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.
RESULTS
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).
CONCLUSIONS
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.