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Related Experiment Video

Updated: Sep 24, 2025

Video Movement Analysis Using Smartphones ViMAS: A Pilot Study
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Knee Flexion Angle Measurement Using Virtual Assessment Tools: Correct Procedure and Potential Pitfalls.

Gerard A Sheridan1, Gráinne Keenan1, David E Beverland1

  • 1Department of Orthopaedics, Musgrave Park Hospital, Belfast, Ireland.

Arthroplasty Today
|May 5, 2022
PubMed
Summary

Smartphone apps can measure knee range of motion, but observer angle matters. Maintaining a perpendicular view ensures accurate remote knee flexion measurements, crucial for virtual patient assessments.

Keywords:
RehabilitationTKATelemedicineTotal knee replacementVirtual

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Area of Science:

  • Biomedical Engineering
  • Orthopedics
  • Digital Health

Background:

  • Remote patient assessment is increasingly reliant on smartphone technology.
  • Accurate measurement of knee range of motion (ROM) is essential for virtual patient evaluations.
  • Existing methods may be susceptible to measurement errors based on device positioning.

Purpose of the Study:

  • To analyze the impact of observer position on knee flexion angle measurements using a smartphone application.
  • To determine the optimal viewing angle for accurate remote knee ROM assessment.
  • To quantify measurement variability in virtual patient assessments.

Main Methods:

  • An experiment was conducted using an iPhone application (Dr. Goniometer) to measure knee flexion angles.
  • Two observers recorded measurements from 7 different positions and 3 heights relative to the knee joint.
  • Intraclass correlation coefficients (ICCs) were calculated to assess intraobserver and interobserver reliability.

Main Results:

  • Interobserver reliability was excellent, with an ICC of 0.804.
  • Measurements deviated significantly from the true 90° knee flexion when the observer angle exceeded 15° from the perpendicular.
  • Observed angles ranged from 95°-121° (proximal) and 92°-108° (distal) when not perpendicular.

Conclusions:

  • Observer position significantly influences smartphone-based knee ROM measurements.
  • A camera angle perpendicular to the long axis of the lower limb is ideal for accuracy.
  • Maintaining the camera within 15° of perpendicular minimizes perceived angle overestimation in virtual assessments.