Inter‐Session Repeatability of a Smartphone‐Based Markerless System to Measure Segment Length and Lower‐Limb Kinematics in Patients With End‐Stage Knee Osteoarthritis

ABSTRACT Background While kinematic analysis is crucial to characterize multi‐planar functional deficits in end‐stage knee osteoarthritis (OA), traditional laboratories impose severe physical burdens on patients. In‐field markerless systems like OpenCap offer an accessible alternative, yet their repeatability in this population remains underexplored. The objective of this study was to assess the inter‐session repeatability of OpenCap for quantifying segment lengths and lower‐limb kinematics during gait and sit‐to‐stand tasks in end‐stage knee osteoarthritis (OA) patients. Methods Twenty‐three patients with end‐stage knee OA scheduled for total knee arthroplasty performed gait and sit‐to‐stand tasks while wearing street clothes across two visits. Segment lengths obtained from static calibration were compared between sessions, while waveform repeatability of lower‐limb joint angles was assessed using root mean square deviation (RMSD) and intraclass correlation coefficient (ICC). Additionally, the standard error of measurement (SEM) and minimal detectable change (MDC) were calculated for discrete kinematic parameters. Results The segment lengths exhibited moderate to excellent repeatability between sessions (mean ICC : 0.79). Measurement bias was generally minimal (< 0.5 cm, except for the upper arm), with the overall 95% limits of agreement ranging from −4.51 to 3.98 cm. For gait, the average RMSD and ICC values were 4.0° ( SD : 0.9) and 0.82 ( SD : 0.07), respectively, whereas for the sit‐to‐stand task, these values were 5.2° ( SD : 1.1) and 0.69 ( SD : 0.08), respectively. The average SEM and MDC values for gait were 2.9° and 8.0°, and for sit‐to‐stand, 3.0° and 8.4°, respectively. Conclusions OpenCap demonstrates moderate to good inter‐session repeatability for static anatomical measurements and gait kinematics in patients with end‐stage knee OA, showing promise as a low‐burden clinical assessment tool. However, the higher variability observed during the sit‐to‐stand task necessitates cautious, task‐specific interpretation, underscoring the need for further improvements before expanding its use across broader functional assessments.