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Femorotibial angle scan-rescan reproducibility: A high-precision calculation on a large cohort
Thomas A G Hall1, Gareth G Jones2, Richard J van Arkel1
1Department of Mechanical Engineering, Biomechanics Group, Imperial College London, London, UK.
Summary
Scan-rescan reproducibility for femorotibial angle (FTA) is nearly double that of single-image measurements. While not precise enough for surgery, FTA is reliable for predicting knee osteoarthritis incidence and progression.
Area of Science:
- Orthopedic biomechanics
- Radiographic assessment
- Osteoarthritis research
Background:
- Femorotibial angle (FTA) is a key measure of coronal knee alignment derived from knee radiographs.
- Short knee radiographs offer a radiation-sparing alternative to full-leg views for FTA assessment.
- Previous studies confirmed intra- and inter-reader reproducibility from single images, but scan-rescan reliability remained unquantified.
Purpose of the Study:
- To evaluate the scan-rescan reproducibility of femorotibial angle (FTA) measurements.
- To compare FTA scan-rescan reliability with existing intra- and inter-reader reliability data.
- To assess the implications of FTA reproducibility for clinical applications and research.
Main Methods:
- Utilized 4589 femorotibial angle (FTA) measurement pairs from 2586 subjects in the Osteoarthritis Initiative, acquired one year apart.
- Excluded subjects with radiographic osteoarthritis progression or conditions affecting coronal knee alignment.
- Analyzed measurement pairs using paired-samples t-tests for differences and correlated with symptomatic changes (pain, stiffness, function) using WOMAC scores.
Main Results:
- The 95% limit of agreement for FTA scan-rescan reproducibility was calculated as [-1.76°, +1.78°], with a paired-samples correlation of 0.938.
- This scan-rescan reproducibility was found to be considerably worse than intra- and inter-reader reliability from a single image.
- The observed error showed no significant relation to symptomatic or radiographic changes in knee condition.
Conclusions:
- Scan-rescan reproducibility of femorotibial angle (FTA) is almost double the reliability of single-image measurements.
- With an accuracy of approximately ±2°, FTA is deemed inappropriate for surgical decision-making.
- However, FTA demonstrates sufficient reproducibility for correlative studies predicting knee osteoarthritis incidence and progression.
