Validation of High-Resolution peripheral Quantitative Computed Tomography-Derived Achilles Tendon Properties Against Diagnostic Ultrasound

Objectives: 1) To assess the precision of high resolution peripheral quantitative computed tomography (HR-pQCT)derived Achilles tendon (AT) cross-sectional area (HR AT-CSA) and density, and 2) to validate HR AT-CSA against ultrasound-derived AT-CSA (US AT-CSA). Methods: Women and men (>= 50 years) had HR-pQCT (0.082mm isotropic) and US scans (B-mode) performed on the non-dominant ankle. Linear regression and Bland-Altman analyses assessed systematic differences between HR-pQCT and US-derived AT-CSA. Precision measured by % root mean square coefficients of variation (%RMSCV) and agreement by type 2,1 intraclass correlation coefficients (ICC2,1), were determined for testretest US AT-CSA scans, and analysis-reanalysis of 30 HR-pQCT and US images. Results: Among 44 participants, HR and US AT-CSA were strongly correlated (R2=0.84, p<0.01, B=1.05[0.90-1.19]), with no differences between modalities (p=0.37). Bland-Altman analysis revealed minimal systematic bias (-0.7mm2[-10.7-9.3]; 1.3%) between HR-pQCT and US-derived AT-CSA with smaller AT-CSA values showing larger inter-modality differences (R2=0.098, B=-0.137 [-0.268--0.008], p=0.039). US AT-CSA demonstrated excellent test-retest precision (ICC2,1=0.998, %RMSCV=1.04%). Analysis-reanalysis of HR-pQCT AT-density and both HR-pQCT and US AT-CSA displayed ICC2,1 above 0.95 and %RMSCV within 3%. Conclusion: HR-pQCT can examine AT-morphometry with acceptable analytical precision. Future studies should explore these metrics' association with functional outcomes and ankle-bone structural and mechanical properties.