BackgroundFlexion instability is a common cause for revision after total knee arthroplasty (TKA); however, little objective criteria exist to determine excessive laxity in flexion. This study sought to determine the reliability of stress radiographs for flexion laxity using manual stress as well as a commercially available flexion stress device, with the hypothesis that a commercially available force device would provide increased translation compared to manual stress, and radiographic measurements would be reproducible.MethodsTen patients who previously underwent TKA with non-hinged components were prospectively and consecutively enrolled at a single center to undergo stress radiographs. Three lateral radiographs with the knee at 90° of flexion were obtained for each patient: rest, commercial stress device at 150N, and manual stress. Calibrated radiographs were evaluated by two raters, and inter-rater and intra-rater reliability were determined using intraclass correlation coefficients (ICC).ResultsTen patients (seven female) with mean age 72 (range 55–82) years and average duration from surgery 36 (range 12–96) months were evaluated. The commercial stress device provided significantly less anterior translation than manual stress (− 0.3 mm vs. 3.9 mm;p< 0.01). Two patients reported pain with use of the stress device. Inter-observer reliability of measurements was good for commercial stress (ICC = 0.86) and excellent for manual stress (ICC = 0.94). Eighty-five percent of measurements were within 1 mm between observers. Intra-observer reliability of measurements was good to excellent for both the stress device and manual stress.ConclusionsLateral stress radiographs may assist in the objective evaluation of flexion instability. A commercially available product provided less translation than manual stress; however, measurements were reliable and reproducible between observers. Further research is required to correlate translation with stress radiographs to patient outcomes following revision arthroplasty.