This was a retrospective study. Of 191 players (83 fielders and 108 pitchers) who were members of a single Japanese professional baseball team between January 2013 and August 2022 and included full and partial participation, there were 8 players (3 fielders and 5 pitchers) with subscapularis muscle strain during the 10-year period, thus affecting 4.2% of all players. Those who developed subscapularis muscle strain were enrolled in this study. There were no players who were affiliated with the team for less than one season. The details of the mechanism of injury, such as hitting, throwing, and pitching, the injured side, the site of the injury, and the time to return to play, were investigated in these cases. All medical records were from the baseball team, and there were no uncounted players.
Diagnosis of Subscapularis Muscle Strain
In the present study, a diagnosis of subscapularis muscle strain was made on the basis of medical history and local shoulder pain, as well as the magnetic resonance imaging (MRI) findings with fat suppression showing high signal intensity in the same area in all cases.10 MRI was performed when the player could not play for more than one day due to the shoulder pain. During the observational period, 44 players had the pain in the shoulder, so MRI was performed in all 44 players; 14 of the 44 players had periarticular shoulder muscle strain, and 8 of those 14 players were diagnosed with subscapularis muscle strain (Figure 1). Although MRI findings were assessed by experienced orthopedic surgeons and radiologists, the reading results shown in the present study were basically those of the radiologist.
Classification of the Muscle Injury Site on MRI
The site of the subscapularis muscle injury was assessed using MRI of the axial, coronal, and sagittal planes with respect to the glenoid fossa. MRI was conducted using a 3.0-T scanner (MAGNETOM Lumina; Siemens Health Care, Erlangen, Germany). Settings for axial and coronal spectral adiabatic pulse inversion recovery (SPAIR) imaging were as follows: repetition time, 4000 ms; echo time, 65 ms; echo train length, 13; slice thickness, 3.0 mm; field of view, 180×180 mm2; matrix size, 320×259; and number of excitations, 1. Settings for sagittal SPAIR imaging were as follows: repetition time, 5170 ms; echo time, 62 ms; echo train length, 14; slice thickness, 3.0 mm; field of view, 180×180 mm2; matrix size, 320×259; and number of excitations, 2. The subscapularis muscle originates from the subscapular fossa and inserts into the lesser tubercle of the humerus. The site of muscle injury was classified into the myotendinous junction or the muscle belly, as well as whether it was in the superior or inferior half of the muscle (Figure 2).
On axial MRI with fat suppression, the site of muscle injury was classified as the myotendinous junction and the muscle belly.
Injured Side
The injured side was assessed in the case of injuries caused by batting, throwing, or pitching. In the present study, it was more straightforward to use the dominant batting side or the dominant arm as a reference point. The dominant batting side in this study was defined as the side of the batting box used.
Return to Play
For the present study, return to play was defined as participation in an official game. The time to return to play was defined as the period from injury to return to official games, even in the case of players who were able to play for a while after their injury. If the season ended before a player returned to official games, the time to return to play could not be measured, and the player was therefore excluded from this evaluation.
Statistical Analysis
The frequency of subscapularis muscle strain and the time to return to play were compared between fielders and pitchers using Fisher’s exact probability test and Student’s t-test, respectively. The level of significance was set at p<0.05. The statistical analyses were performed with BellCurve for Excel (Social Survey Research Information Co., Ltd., Tokyo, Japan).
This study was approved by our institutional review board (file no. blind for review).