This study was a repeated-measures analysis to assess changes in MJD during separate elbow loading conditions. The dependent variable was MJD, and the independent variables were the loading conditions and FDS contraction of each finger. The participants’ dominant and nondominant elbows were subjected to rest (unloaded), valgus load (loaded), and valgus load with individual FDS contraction (loaded-contracted). These conditions were tested by separate examinations of the index, middle, and ring fingers.
Seventeen physically active individuals aged 22–38 years (mean age, 27 ± 5 years) were recruited within our institute from December 2017 to November 2018. Only male participants were enrolled, to mimic the state of an adult male baseball player. Participants with any of the following were excluded: (1) current pain or injury in the upper extremities; (2) previous UCL tear or elbow dislocation; (3) previous surgery in the upper extremities; (4) previous participation in overhead sports. Institutional review board approval was obtained and written informed consent was obtained from all participants prior to enrollment.
All US scans were performed by an experienced orthopedic surgeon (SH) using a SNIBLE (KONICA MINOLTA, Chiyoda, Japan) scanner with an 11-MHz linear transducer. Each participant was placed in a sitting position on a chair with the shoulder at 60° abduction, the elbow at 30° flexion, and the forearm in supination, similar to that in previous research (Fig. 1a) [19,20]. All angles were measured using a goniometer, and the transducer was placed on the oblique coronal plane to visualize the MJD; this method is sensitive enough to identify an increase in the MJD when a valgus load is placed on the arm (Fig. 1b) [19,20,21].
Stress ultrasonographic examination across the 3 loading conditions
The stress US examination was used to assess the contribution of the FDS contraction of each finger to medial elbow joint stability. The MJD was measured as the distance between the distal-medial corner of the trochlea of the humerus and the proximal edge of the sublime tubercle of the ulna on the oblique coronal image (asterisks in Fig. 2). This was measured on the US screen with the use of electronic calipers with a precision of 0.1 mm. Fixed valgus stress (50 N) was applied using a standardized instrumented device (Telos, Marburg, Germany) to the lateral side of the elbow joint to strain the medial aspect of the elbow (Fig. 1b). This valgus stress of 50 N was selected for two reasons: (1) some participants were unable to tolerate stress of over 50 N and were uncomfortable, and (2) this amount of force has been suggested as appropriate for the Japanese population . Using these established methods of collecting images and applying valgus stress, we were able to collect the measurements (mm) of the unloaded, loaded and loaded-contracted testing conditions .
Under the unloaded condition, the participant was placed in the testing position with no valgus stress applied to the elbow joint. The participant was asked to relax completely before an image was taken. Under the loaded condition, the participant was asked to relax while the valgus load was applied, and another image was taken. As a transition took place between the loading conditions, the valgus load was fully removed from the elbow to prevent excessive stress. A one-minute transition period was allowed between the testing conditions.
In the loaded-contracted condition, the participant was asked to flex the specified finger and maintain it at a fixed angle of 90° at the proximal interphalangeal (PIP) joint (Fig. 3). To eliminate the effect of FDP contraction, we ensured that the finger was kept free of any tension at the distal interphalangeal (DIP) joint. With the finger in the flexed position, the FDS was activated, leading to its isometric contraction. The final image was taken while the load was applied during muscle contraction. No participants experienced elbow pain during the examination. Data for three trials of the separate loading conditions in each finger were collected and averaged for data analysis. We randomly tested the dominant and non-dominant arms, and tested the index, middle, and ring fingers in that order. Interrater reliabilities were established and maintained, with interclass correlation coefficients in the acceptable range for all measures (0.90–0.98).
A one-way repeated-measures analysis of variance (ANOVA) was performed to assess changes in the MJD for the separate loading conditions in each finger. When the ANOVA indicated statistical significance, a Bonferroni t-test was performed. In addition, a paired t-test was applied to compare the MJD between the dominant and non-dominant arms for each finger. A two-sided P-value of < 0.05 was considered to indicate statistical significance. All analyses were performed with IBM SPSS Statistics 18 software for Windows (IBM Japan Inc.). A power analysis for the detection of differences between separate loading conditions in each individual finger was conducted using an α value of 0.05, an effect size of 0.4 (which was determined according to the results of a preliminary study), and a power of 0.95. The power analysis suggested that 34 elbows were needed to assess the three separate loading conditions for each finger.