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Patella-Posterior Turning Point of the Distal Femur Distance Is a Potential Indicator for Diagnosing Patella Alta in Recurrent Patellar Dislocation Population

  • Author Footnotes
    ∗ J.C., Z.Y., and C.W. contributed equally to this article.
    Jiebo Chen
    Footnotes
    ∗ J.C., Z.Y., and C.W. contributed equally to this article.
    Affiliations
    Department of Sports Medicine, Shanghai Jiao Tong University Affiliated Sixth People’s Hospital, Shanghai, China
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  • Author Footnotes
    ∗ J.C., Z.Y., and C.W. contributed equally to this article.
    Zipeng Ye
    Footnotes
    ∗ J.C., Z.Y., and C.W. contributed equally to this article.
    Affiliations
    Department of Sports Medicine, Shanghai Jiao Tong University Affiliated Sixth People’s Hospital, Shanghai, China
    Search for articles by this author
  • Author Footnotes
    ∗ J.C., Z.Y., and C.W. contributed equally to this article.
    Chenliang Wu
    Footnotes
    ∗ J.C., Z.Y., and C.W. contributed equally to this article.
    Affiliations
    Department of Sports Medicine, Shanghai Jiao Tong University Affiliated Sixth People’s Hospital, Shanghai, China
    Search for articles by this author
  • Jinzhong Zhao
    Correspondence
    Address correspondence to Jinzhong Zhao, M.D. and Guoming Xie, M.D., Department of Sports Medicine, Shanghai Jiao Tong University Affiliated Sixth People’s Hospital, 600 Yishan Road, Shanghai, 200233, China.
    Affiliations
    Department of Sports Medicine, Shanghai Jiao Tong University Affiliated Sixth People’s Hospital, Shanghai, China
    Search for articles by this author
  • Guoming Xie
    Correspondence
    Address correspondence to Jinzhong Zhao, M.D. and Guoming Xie, M.D., Department of Sports Medicine, Shanghai Jiao Tong University Affiliated Sixth People’s Hospital, 600 Yishan Road, Shanghai, 200233, China.
    Affiliations
    Department of Sports Medicine, Shanghai Jiao Tong University Affiliated Sixth People’s Hospital, Shanghai, China
    Search for articles by this author
  • Author Footnotes
    ∗ J.C., Z.Y., and C.W. contributed equally to this article.
Published:October 25, 2022DOI:https://doi.org/10.1016/j.arthro.2022.10.008

      Purpose

      To introduce a simple patellar height measurement method (patella-posterior turning point of the distal femur [P-PTP] Distance) independent of patellar anatomy with standardized patient position, and tested the reliability, validity, and diagnostic accuracy compared with commonly used ratios in knee extension.

      Methods

      We retrospectively reviewed 418 computed tomography (CT) images of the knee joint in a group of patients who were diagnosed recurrent patellar dislocation (RPD). With the three-dimensional (3D) CT reconstructed knee, patellar height was qualitatively assessed by the patellar engagement with the femoral trochlea in terminal knee extension to divide RPD population into case (patella alta) and control group. With digitally reconstructed lateral radiographs, patellar height was measured with P-PTP distance (perpendicular distances between the distal edge of patella articular surface and posterior turning point of distal femur), and four commonly used ratios: Caton-Deschamps index, Modified Insall-Salvati index, Blackburne-Peel index, and Insall-Salvati index. An unpaired t-test was conducted to determine significant differences between groups. Correlation coefficient, intra- and inter-observer reliability, receiver operating characteristic (ROC) curves and the area under the ROC curve (AUC) were also calculated.

      Results

      198 knee images (198 patients) were included for final evaluation. Patella alta was present in 72 knees (36.3%) with RPD. The effect size was relatively large between the case and control group for P-PTP distance (d = −1.619; 95% CI, −1.948 to −1.286). P-PTP distance correlated moderately to strongly with four commonly used ratios (P < .001). Intraobserver and interobserver reliability was good for P-PTP distance. The AUC of the ROC curve was categorized as excellent for P-PTP distance, better than other measurements (P < .001), and the cutoff value was 4.2 mm with the highest sensitivity (86.11%) and specificity (84.92%).

      Conclusions

      The measurement method, P-PTP distance, showed good intra-observer and inter-observer reliability, well correlated with commonly used ratios, and presented best diagnostic accuracy among commonly used ratios for predicting RPD. P-PTP distance might be a potential indicator for identifying patella alta in RPD patients when supine and knee extended.

      Clinical Relevance

      The measurement reported in this study may help in advancing clinical evaluation of patella alta, providing an alternative and simple method to measure patellar height. Standing or weight-bearing plain lateral radiographs obtained from the routine practice should be further assessed in the next step to further validate the method.
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