Tactile Incentiviser Improves Partial Weight Bearing After Lower-Limb Surgery

Background and Rationale Partial weight-bearing (PWB) restrictions (10-30% of body weight) are routinely prescribed for 2-6 weeks following hip or knee arthroplasty, fracture fixation, and pelvic or ankle reconstruction. Failure to adhere to these limits may overload internal fixation devices, delay bone healing, or impair implant osseointegration. While electronic smart-insole systems can enhance load accuracy, they often cost over €600, require batteries and smartphone connectivity, and are not readily available in low-resource settings. Therefore, a purely mechanical "tactile incentiviser" was developed to provide an immediate plantar cue when the load threshold is exceeded, aiming to offer a low-cost alternative.

Device Design and Calibration The incentiviser has a 3-mm rubber insole incorporating four spring-steel snap-buttons (15 mm diameter, 4 mm height) aligned along the heel-midfoot axis. Three interchangeable polyurethane-foam pads (35 / 45 / 60 Shore A, 6 mm thickness) allow adjustment according to participant body mass. Each insole is statically calibrated using a custom hydraulic press with a 0-500 N load cell (accuracy ± 0.5 N), increasing in 10 N increments until the buttons collapse at a target of 20% ± 1% of body weight. Calibration is performed in triplicate to ensure repeatability.

Study Design This is a single-centre, assessor-blinded, parallel-group pilot randomised controlled trial. Ethical approval was granted by the Institutional Review Board of the University of Insubria (protocol number 0026262).

Participants Eligible participants are adults aged 18-85 years undergoing hip or knee arthroplasty, femoral-shaft fracture intramedullary nailing, or pelvic fracture fixation who can ambulate ≥ 5 meters with assistive devices. Exclusion criteria include neurological gait disorders, pre-existing non-weight-bearing status, and foot sizes outside the 35-47 EU range.

Randomisation and Allocation Concealment All eligible admissions are listed chronologically each calendar week and assigned an incremental study ID. An independent researcher uses Random.org to randomly allocate three participants to the intervention arm and three to the control arm via simple random sampling without replacement. The allocation list is disclosed to ward staff only after baseline assessment to maintain allocation concealment. Outcome assessors and data analysts are blinded to group assignment. Due to the nature of the intervention, participants and therapists are not blinded.

Interventions Intervention Arm: Participants wear the calibrated incentiviser in the surgical-side shoe during every supervised walk starting from post-operative day 1 until discharge (approximately 14 days). Instructions are provided to encourage load reduction in response to the tactile cue.

Control Arm: Participants receive standard physiotherapy, including verbal instruction to limit loading to 20% of body weight and assisted practice using a walker or crutches, without any feedback device.

Outcome Measures Primary Outcome: Absolute deviation in kilograms from the prescribed 20% body weight load, measured using a validated wireless in-shoe pressure-sensor system (GeBioM-MobilData) during a 10-meter walk at baseline (T0) and at discharge (T1).

Secondary Outcomes: (i) Pain intensity using a 0-10 Numerical Rating Scale (NRS); (ii) Walking distance during the Six-Minute Walk Test (6MWT); (iii) Usability of the device assessed using the System Usability Scale (SUS) at T1.

Sample Size Justification A priori sample size calculation was based on an expected mean difference of 3 kg with a standard deviation of 5 kg, alpha = 0.05, and 80% power, yielding a required sample of 26 participants. The total sample was increased to 34 participants (17 per arm) to accommodate potential attrition.

Statistical Analysis Plan Between-group comparisons are planned using independent-samples t-tests (two-tailed, α = 0.05). Effect sizes will be reported using Cohen's d and corresponding 95% confidence intervals. Additional exploratory analyses will be conducted using two-way ANOVA, including diagnosis-stratified comparisons and interaction effects. All analyses will be performed using SPSS v19 and R v4.3.

Safety Monitoring The device is expected to be well tolerated, with no anticipated adverse effects. Skin condition will be monitored daily. Any adverse events will be recorded and reported to the IRB according to institutional guidelines.