Ultrasound can assess soft tissue laxity in osteoarthritic knees and after total knee arthroplasty.

Purpose: Peripheral knee laxity is commonly assessed through clinical examination and stress radiographs, both before and after total knee arthroplasty (TKA). This study aimed to evaluate the efficacy of ultrasound in assessing peripheral knee laxity compared to radiographic stress imaging. Methods: This retrospective bicentric study included 68 patients (mean age: 71.2 ± 5.8 years; body mass index [BMI]: 28.7 ± 7), with 32 osteoarthritic knees (47.1%) and 36 post-TKA knees (52.9%). Laxity was measured in millimetres using manual stress manoeuvres applied during both ultrasound and radiographic examinations at 0° and 30° of flexion, under varus and valgus conditions. Results: Under ultrasound, the unstressed medial joint space measured 10.6 ± 4.5 mm at 0° and 10.9 ± 4.2 mm at 30°; under valgus stress, it increased to 13.0 ± 4.9 and 14.0 ± 5.1 mm, respectively. The unstressed lateral joint space measured 11.8 ± 4.5 mm at 0° and 11.6 ± 4.0 mm at 30°, increasing to 14.8 ± 5.1 and 15.5 ± 5.9 mm under varus stress. No statistically significant differences were observed between ultrasound and radiographic measurements of lateral laxity (2.9 ± 2.3 vs. 2.96 ± 1.74 mm, Δ = 0.04 mm, p = 0.91) or medial laxity (2.35 ± 2.16 vs. 2.44 ± 1.55 mm, Δ = 0.1 mm, p = 0.78). Correlation coefficients between the modalities ranged from 0.5 to 0.7 (p < 0.001). At 30° of flexion, lateral laxity was significantly greater in the TKA group (4.9 vs. 2.8 mm, p < 0.01). Conclusion: Stress ultrasound is a reliable tool for analysing medial and lateral peripheral knee laxities in both osteoarthritic knees and after total knee arthroplasty. While it could requires trained personnel, this technique represents a viable alternative to conventional stress radiographs. Level of Evidence: Level II.