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Objective:
Ultrasonographic (US) localization of the radial collateral ligament (RCL) is important to determine the presence of RCL tears in the setting of lateral epicondylosis. The conventional position of examining the lateral elbow under US is a slightly flexed position. However, because the RCL is more deeply attached on the lateral epicondyle than the common extensor tendons, an anisotropic artifact of the RCL could be observed by US in the conventional slightly flexed position, making it difficult to fully visualize RCL to determine the presence of RCL tears. The hypothesis of this study was that there is a certain elbow position, in which the two bony structures under RCL, the capitellum (CPT) and RH, come in line with each other, thus rendering the course of the RCL as straight as possible and providing increased echogenicity without anisotropic artifacts. To test this hypothesis, we conducted a study in two parts. First, we compared the relative depth between RH and CPT among different elbow positions to determine the best positions for visualizing RCL fibers in a straight course under US. Moreover, the relevant landmarks that help to visualize RCL were explored using US to establish the frequency of successful identification in healthy elbows. In the second part of the study, a consensus process was undertaken for determination of the optimal elbow flexion angle, or angles, that facilitate accurate visualization of RCL using US.

Materials and Methods:
Healthy individuals without history of elbow pain were recruited. RCL was evaluated using ultrasonography with six different elbow flexion positions (0°, 30°, 60°, 90°, 120°, and 140°). The relative depth, defined as the depth of the capitellum subtracted by the depth of the radial head under ultrasonography, was measured at each angle. The rates of successful identification of ultrasonographic landmarks for localizing the RCL were calculated. After ultrasonography, the optimal elbow position for identifying the RCL was determined by the group consensus.

Results:
A total of 40 healthy elbows of 10 men and 10 women were evaluated by ultrasonography. The average of age and a maximal angle of elbow flexion were 30.1 (SD 2.9) years, and 142.4° (3.2), respectively. The relative depth significantly decreased with an increase in the flexion angle (p for trend <.001), approaching zero at the angles of 90° and 120°. The rates of successful identification of the superior tubercle, hyperechogenic line, and anterior and posterior tubercles were 100%, 100%, 90%, and 80%, respectively. In the group consensus, the 90° and 120° flexion angles were selected with the highest frequency (90%; 36/40).

Conclusion:
The present study demonstrates that the optimal elbow positions to visualize the RCL with the least possibility of anisotropism are 90° and 120° of flexion of elbow under US, implicating that the elbow should be flexed far more than the conventional “slightly flexed” position. In the optimal elbow positions, the ultrasonographic landmarks to identify the RCL such as the hyperechogenic line and tubercles are distinctively observed.