Background <p>Digital solutions for knee arthroplasty rehabilitation hold promise for improving outcomes, however, rigorous evaluation is essential. Guided by the Medical Research Council framework for complex interventions, we aimed to assess feasibility and user and clinician experience of a novel digital system – Grasp – which enables patients to report pain intensity by squeezing a handheld device and register other health status parameters through a mobile application (app) during knee arthroplasty rehabilitation.</p> Methods <p>For 6 postoperative weeks, participants used the Grasp device to report pain at rest and during exercise, while the app was employed to report sleep, appetite, activity, medication, and general condition. Pain and recovery trajectories were visualised in the app and informed physiotherapy evaluations. Feasibility was examined across recruitment, retention, adherence, and acceptability using logs, questionnaires, and participant and physiotherapist interviews in a convergent mixed-methods design. Quantitative data were analysed descriptively; qualitative data underwent reflexive thematic analysis. Findings were integrated through joint displays and narrative synthesis to explain adherence patterns, user experience, and inform refinements.</p> Results <p>Recruitment was 62%, with 34 of 55 eligible participants included (mean age 61.8 years; 68% female), and retention was 88%. Median daily pain reports declined from 9.0 (interquartile range (IQR) 3.8–14.8) at rest and 3.0 (IQR 1.0–6.0) during exercise in week 1 to 1.4 (IQR 0.6–4.9) and 0.5 (IQR 0.0–1.0) in week 5, respectively. Mean adherence declined across study weeks: rest ≥ 1 report/day from 88.9% to 60.4%, rest ≥ 5/day from 64.2% to 27.9%; exercise ≥ 1/day from 60.5% to 40.0%, exercise ≥ 3/day from 43.4% to 28.7%. App adherence was high in weeks 1–2 (85–87%), when daily reporting was scheduled. Early adherence and perceived usefulness were driven by feedback and self-monitoring, fostering motivation and awareness of pain and recovery trajectories. Declining adherence reflected recovery progress, intervention burden, and unclear exercise definitions. Physiotherapists valued data for overview and conversation but questioned workflow fit and clinical utility.</p> Conclusions <p>The Grasp system demonstrated moderate feasibility and potential to support pain and recovery monitoring and engagement after knee arthroplasty. However, declining adherence and usability challenges limited sustained use. Refinements are needed before progressing to effect studies.</p> ClinicalTrials.gov identifier <p>NCT06825988 – retrospectively registered on Feb 4^th, 2025</p>