Tendon Yaw (TY) Angle: Direct Measurement of the Quadriceps Vector Resolves the Rotational Enigma of Recurrent Patellar Instability

<i>Background and Objectives</i>: To validate a computed tomography (CT) measure of tendon yaw (TY) and determine its diagnostic specificity, precision, and clinical relevance in recurrent patellar instability (RPI) in comparison with standard imaging tests (SIT1, SIT2), femoral trochlear dysplasia (FTD), and vastus medialis obliquus (VMO) morphology. <i>Materials and Methods</i>: This retrospective cross-sectional study included 113 subjects (187 knees) examined using a standardized CT protocol for RPI following strict exclusion criteria. TY, SIT1, and SIT2 were measured using predefined axial landmarks. VMO cross-sectional area was assessed at three standardized levels. Diagnostic performance, measurement precision, and interrater agreement were evaluated. <i>Results</i>: TY significantly distinguished recurrent dislocators from nondislocators (<i>p</i> = 0.003) and was independent of age, sex, laterality, and femoral, tibial, or knee rotation (<i>p</i> ≥ 0.42). A threshold of ≥22° demonstrated high diagnostic specificity (92%; 95% CI, 85-97%), with a normal cutoff defined as ≤12°. Measurement precision was approximately 90%. SIT1 and SIT2 were influenced by femoral and knee rotation but not tibial rotation. All imaging tests were associated with FTD (<i>p</i> < 0.0001). No significant correlation was found between any imaging test and VMO area, although VMO was reduced in recurrent dislocators and in women. <i>Conclusions</i>: TY is a direct and highly specific CT measure of extensor mechanism yaw (<i>z</i>-axis rotation) that avoids indirect osseous and soft-tissue surrogates, supporting confirmatory diagnostic assessment and preoperative planning in RPI.