Efficacy of a smartphone digital therapeutic (re-R) for post-total knee arthroplasty rehabilitation: study protocol for a single-center randomized controlled trial in the Republic of Korea

Rehabilitation after total knee arthroplasty (TKA) is challenged by limited supervised contact, clinic-visit burden, and variable adherence to unsupervised home exercise. re-R is a smartphone-delivered digital therapeutic that individualizes knee range-of-motion (ROM) practice and strengthening with real-time feedback and remote logging. This trial aims to determine whether adding re-R to usual care improves active knee flexion ROM at 6 weeks after primary TKA compared with usual care alone, to compare pain, function, health-related quality of life, and satisfaction, and to assess safety. This is a single-center, parallel-group, assessor-blinded, superiority randomized controlled trial (1:1) conducted at Dongguk University Ilsan Hospital, a tertiary academic hospital in Goyang, Republic of Korea. Trained clinical research coordinators and investigators will recruit adults aged 50 years or older undergoing rehabilitation after primary unilateral TKA for knee osteoarthritis who routinely use a smartphone; key exclusion criteria are cognitive impairment, revision TKA, inflammatory arthritis, and neurological or muscular disease affecting motor function. Participants are randomized using permuted blocks without stratification, with allocation concealed by sequentially numbered, opaque, sealed envelopes; baseline assessments are performed before randomization. The intervention group receives re-R plus usual care; the control group performs standardized pamphlet-guided exercises at the same frequency and under identical supervision (3–5 sessions per week for 6 weeks). The primary outcome is the change in active knee flexion ROM from baseline to week 6, measured by blinded goniometry and analyzed by analysis of covariance under the intention-to-treat principle. Secondary outcomes (baseline, week 2, week 6) include pain (visual analogue scale), the Western Ontario and McMaster Universities Osteoarthritis Index, the 36-item Short Form Health Survey, and week-6 satisfaction. Adverse events and serious adverse events will be collected from randomization to week 6. The planned sample size is 32 participants (16 per group). This exploratory trial will provide a preliminary estimate of the efficacy and safety of a ROM-targeted digital therapeutic as an adjunct to usual post-TKA rehabilitation and will inform the design of an adequately powered multicenter trial. Clinical Research Information Service (CRIS), Republic of Korea, KCT0010360. Registered on 28 March 2025, https://cris.nih.go.kr/cris/search/detailSearch.do?seq=33633 . Date of first enrollment: 1 January 2026.