R.Du Hyun RoOrthopedics & Research
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posterior tibial slopeHTOACLTKAimage quality

2026 · Knee Surgery & Related Research

Can Changing the Radiographic Angle Change the Measurement of the Same Knee?

The ICC between AI and expert measurements was 0.78 (95% CI 0.73–0.82). PTS increased by approximately 0.2° per 1° of internal rotation (R² = 0.43).

Original Figure 3

A graph showing the degree of rotation in a sequence with other CT-based virtual side X-rays and rear cortex curvature measurementsEnlarge Figure

DRRs generated from one CT at different rotation angles, with automated PTS measurements.

Jaeseok Park, Andreas Persson, R Kyle Martin, Eivind Inderhaug, Sung Eun Kim, Sangyoon Kim, Donghyuk Kwak, Du Hyun Ro. Effects of axial malrotation on posterior tibial slope measurement: a digitally reconstructed radiograph study enabling automated quality assessment. Knee Surgery & Related Research (2026). https://doi.org/10.1186/s43019-026-00303-x

Evidence & Original SourceImage use terms

Participants and Methods

DRRs spanning −12° to +12° were generated from 55 CT scans. DEKR landmarks and segmentation measured PTS, while PCDR assessed rotation and error. PCDR is posterior-condylar AP displacement divided by distal-femoral AP width. 46 single-patient TKA scheduled patients 55knee; average age 73 years.

Main Findings

The ICC between AI and expert measurements was 0.78 (95% CI 0.73–0.82). PTS increased by approximately 0.2° per 1° of internal rotation (R² = 0.43). PTS error exceeded 1° when rotation exceeded ±6°. The R² between PCDR and rotation exceeded 0.98, and the AUROC for distinguishing errors greater than 1° was 0.77.

The point of reading on surgery

These findings support checking rotation and image quality on lateral radiographs before using PTS criteria in HTO, ACL or TKA assessment.

Limitations

This is a DRR-based proof of concept rather than a study of repeated clinical radiographs. Coronal tilt was fixed, and some measurement outliers occurred. Generalization to young ACL patients is limited. The analysis included 55 knees from 46 patients. Statistical descriptions differ between the abstract and the main text, so values from the two sources should be read separately.

Let's put it briefly.

What did this study confirm?

The ICC between AI and expert measurements was 0.78 (95% CI 0.73–0.82). PTS increased by approximately 0.2° per 1° of internal rotation (R² = 0.43).

Can it be applied directly to individual patients' treatment decisions?

This is a DRR-based proof of concept rather than a study of repeated clinical radiographs. Coronal tilt was fixed, and some measurement outliers occurred. Generalization to young ACL patients is limited.

Keywords

Abstract · Original text

Jaeseok Park, Andreas Persson, R Kyle Martin, Eivind Inderhaug, Sung Eun Kim, Sangyoon Kim, Donghyuk Kwak, Du Hyun Ro. Effects of axial malrotation on posterior tibial slope measurement: a digitally reconstructed radiograph study enabling automated quality assessment. Knee surgery & related research. 2026;38(1):3. doi:10.1186/s43019-026-00303-x.

BACKGROUND

Accurate measurement of posterior tibial slope (PTS) is highly sensitive to axial malrotation of the knee during acquisition, but its impact has not been systematically quantified across different anatomical variations. This simulation study aimed to quantify the effect of axial malrotation on PTS using digitally reconstructed radiographs (DRRs) and suggest a practical marker for filtering out low-quality images with excessive malrotation.

MATERIALS AND METHODS

A total of 55 preoperative computed tomography (CT) scans from January 2021 to December 2024 in a single, tertiary hospital were retrospectively reviewed. DRRs were generated from those scans to simulate lateral knee radiographs with malrotation ranging from -12° to +12° relative to an anatomically aligned baseline. An artificial-intelligence (AI)-based tool automatically measured PTS on each DRR, with agreement evaluated using intraclass correlation coefficient (ICC). PCDR was calculated from femoral contours and analyzed for correlation with malrotation angles and resulting PTS measurement error.

RESULTS

AI-based PTS measurements on DRRs showed good agreement with expert annotations (ICC = 0.78, 95% CI 0.73-0.82). PTS increased linearly with internal rotation, with each 1° of rotation resulting in approximately 0.2° change in PTS (R2 = 0.43, p < 0.01). Errors exceeded 1° when malrotation surpassed ±6°. PCDR was strongly correlated with malrotation angle (R2 > 0.98, p < 0.001) and achieved fair discriminative performance as a binary classifier for > 1° PTS error [area under the receiver operating curve (AUROC) = 0.77].

CONCLUSIONS

CT-derived DRRs combined with AI analysis showed that PTS measurement error proportionately increased with axial malrotation. Identifying and excluding radiographs with excessive rotation improves the reliability of slope-based assessments and supports more accurate surgical planning.

LEVEL OF EVIDENCE

III, retrospective cohort study.

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© The Author(s) 2026

The original paper.

Effects of axial malrotation on posterior tibial slope measurement: a digitally reconstructed radiograph study enabling automated quality assessment

Jaeseok Park, Andreas Persson, R Kyle Martin, Eivind Inderhaug, Sung Eun Kim, Sangyoon Kim, Donghyuk Kwak, Du Hyun Ro
Knee Surgery & Related Research · 2026

DOI · 10.1186/s43019-026-00303-xPubMedBased data
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