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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Article info
Publication history
Published online: October 25, 2022
Accepted:
October 7,
2022
Received:
March 8,
2022
Footnotes
See commentary on page 611
The authors report the following potential conflicts of interest or sources of funding: J.C., Z.Y., C.W., J.Z., and G.X. report National Key Research and Development Program of China, grants 2018YFC1106200 and 2018YFC1106202 and the Exploratory Research Program of Shanghai Sixth People’s Hospital Affiliated to Shanghai Jiao Tong University School of Medicine, grant no. YNTS202001, outside the submitted work. Full ICMJE author disclosure forms are available for this article online, as supplementary material.
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