Ultrasound-Based Spatial Frequency Analysis Differentiates Patellar Tendon Quality in Tendinopathic and Healthy Participants.

Introduction: Diagnostic ultrasound is valuable in identifying patellar tendon pathologies by assessing macrostructural features such as tendon thickness. Quantitative image analysis methods, like spatial frequency analysis (SFA), allow objective assessment of tendon microstructural organization. The purpose of this study was to examine patellar tendon microstructure in individuals with clinically diagnosed patellar tendinopathy and matched healthy controls using SFA. Methods: This case-control study evaluated regional differences in patellar tendon microstructure between NCAA Division I athletes with patellar tendinopathy (n=13) and age-, sex-, sport-, height-, and mass-matched healthy controls (n=13). B-mode ultrasound images were analyzed using SFA to extract Peak Spatial Frequency Radius (PSFR), P6, and MMax%. Tendon thickness was measured at proximal and distal regions using built-in ultrasound calipers. Group differences were assessed using independent t-tests or Mann-Whitney U tests as appropriate; effect sizes were calculated using Cohen's d. Results: Tendinopathic tendons demonstrated lower proximal PSFR (p<0.01, d=1.19) compared with controls. P6 values were higher in the tendinopathic group at both proximal (p=0.04, d=0.73) and distal (p=0.03, d=0.80) regions, while distal MMax% was lower (p=0.02, d=0.82). No differences were observed for distal PSFR (p=0.21) or proximal MMax% (p=0.10). Proximal tendon thickness was greater in the tendinopathic group (p<0.01), whereas distal thickness did not differ between groups (p=0.20). Conclusion: Spatial frequency analysis identified region-specific differences in patellar tendon microstructure between tendinopathic and healthy participants, with proximal PSFR demonstrating the most robust discrimination between groups. Differences observed in P6 and MMax% should be interpreted cautiously due to greater uncertainty in their effect size estimates. These findings support further investigation of SFA, particularly PSFR, as a quantitative approach for characterizing tendon microstructural organization in research settings.