Ultrasonographic Assessment of Posterior Shoulder Capsule Thickness in Baseball Pitchers: A Validation Study

Abstract

Background:

A thickened posterior shoulder capsule may increase injury risk in baseball pitchers due to maladaptive shoulder biomechanics such as altered arthrokinematics, decreased internal rotation range of motion (ROM) and increased scapular upward rotation. While diagnostic ultrasound offers a reliable method of measuring posterior capsule thickness (PCT), it is currently unclear whether PCT thus measured is valid.

Hypothesis:

Diagnostic ultrasound is a valid method for measuring PCT compared with magnetic resonance imaging (MRI).

Study Design:

Cross-sectional.

Level of Evidence:

Level 4.

Methods:

Asymptomatic baseball pitchers recently drafted into professional baseball from a single organization were enrolled during the 2021 to 2024 seasons. Pitchers underwent both diagnostic ultrasound and shoulder MRI on the same day to minimize any acute changes in PCT. All shoulder ultrasounds were performed with a 15-MHz linear transducer. Posterior capsule was identified as the tissue immediately lateral to the tip of the labrum between the humeral head and rotator cuff. PCT was measured on axial MRI scans by a musculoskeletal radiologist blinded to the ultrasound measurements. Agreement between modalities was evaluated through Pearson correlations and Bland-Altman analysis.

Results:

Overall, 25 drafted pitchers were included. PCT obtained via diagnostic ultrasound had a mean of 2.4 ± 0.6 mm while PCT obtained via MRI scan had a mean of 2.4 ± 0.8 mm. Ultrasound PCT and MRI PCT were strongly positively correlated (R = 0.945, R² = 0.892, P < 0.001). The Bland-Altman plot demonstrated 95% limits of agreement of 0.55 mm between diagnostic ultrasound and MRI measurements of PCT.

Conclusion:

Measuring PCT with diagnostic ultrasound is a valid technique compared with the gold standard of MRI.

Clinical Relevance:

Clinicians who wish to evaluate PCT in baseball pitchers may use this validated technique to quantify PCT and evaluate for potential contributors to glenohumeral internal rotation deficit.

The posterior-inferior glenohumeral ligament (PIGHL) is a thickening of the posterior shoulder capsule that attaches at approximately 7-o’clock position on the glenoid (when looking at a right shoulder). The posterior capsule and PIGHL are thought to work synergistically with the rotator cuff muscles, specifically the infraspinatus and teres minor, to decelerate the throwing arm. ⁴ It has been suggested that, once the posterior rotator cuff muscles become fatigued during throwing, the posterior capsule becomes responsible for absorbing the majority of the deceleration forces at end internal rotation range of motion (ROM). ² Repetitive loading is believed to lead to a thicker and tighter posterior capsule, which can lead to maladaptive shoulder biomechanics.^3,6,10

In baseball players specifically, the posterior capsule on the dominant arm has been shown consistently to be thicker than the nondominant arm.^{1,5,14 -16} This is true across many different competition levels of baseball such as youth (8- to 12-year-olds), ¹ high school, ⁵ collegiate,^7,15,16 and professional baseball players. ¹⁴ Since posterior capsule thickness (PCT) has been correlated positively with scapular upward rotation, ¹⁶ and negatively with internal rotation (IR) ROM,^7,16 increased PCT may increase injury risk in baseball players due to the potentially harmful effects of these ROM changes. ⁸

Magnetic resonance imaging (MRI) serves as the gold standard for soft tissue imaging; however, MRI is costly and not readily accessible to the sports medicine staff. In contrast, diagnostic ultrasound provides a valuable tool for sports medicine clinicians due to its cost-effectiveness and mobility as long as measurement validity is affirmed. Therefore, the purpose of this study was to determine the validity of measuring PCT using diagnostic ultrasound in comparison with MRI (gold standard). The authors hypothesized that diagnostic ultrasound would be a valid method for measuring PCT compared with MRI.

Methods

Study Design

This cross-sectional study was Institutional Review Board approved (Thomas Jefferson University) with written informed consent obtained from all study participants. Baseball pitchers who were drafted recently by a single professional baseball organization were enrolled in the study during the 2021 to 2024 seasons. Data collection was performed before their initial medical evaluation with the team, typically 3 to 5 days after the annual Major League Baseball draft in July. Ultrasound and MRI scans were performed on the same day during the initial postdraft pitcher evaluation. The researchers responsible for measuring PCT via ultrasound and MRI scan were blinded to each other’s measurements.

Inclusion/Exclusion Criteria

Asymptomatic pitchers were included only if they were ≥18 years of age, underwent both dominant shoulder ultrasound imaging and shoulder MRI on the same day, were healthy and eligible for participation in team activities, and had not undergone upper extremity surgery within the past year. Pitchers who were not eligible for full participation in team activities or had undergone upper extremity surgery within the past year were excluded.

Shoulder Ultrasound

All included pitchers underwent ultrasound evaluation of the shoulder with previously published ultrasound techniques.^16,17 Ultrasound imaging of the dominant shoulder PCT was performed by a team research consultant with >15 years of experience in musculoskeletal ultrasound imaging. All shoulder ultrasounds were performed with a 15-MHz linear transducer (Sonosite M-Turbo; Sonosite, Inc). Reliability of PCT measurement via diagnostic ultrasound have been established previously using a 10-MHz transducer, with previously published ICC values for intratester and test-retest reliability of 0.88 (standard error of measurement [SEM], 0.02 cm) and 0.72 (SEM, 0.02 cm), respectively. ¹⁶

To enable visualization of the posterior capsule, participants were seated upright in a chair with their forearm resting on their thigh. This position standardized the amount of glenohumeral rotation of the shoulder during testing. In addition, this position is similar to the shoulder position during standard MRI acquisition. In a transverse plane at the posterior aspect of the shoulder, the transducer was placed just inferior to the scapular spine and moved until the humeral head, glenoid labrum, and rotator cuff were visualized. The posterior capsule was identified as the tissue immediately lateral to the tip of the labrum between the humeral head and rotator cuff. ¹⁶ Once the posterior capsule was clearly visible, a standard gray scale image was saved for later image processing of PCT (Figure 1).

Figure 1.

PCT (yellow line) measured with diagnostic ultrasound. D, deltoid; HH, humeral head; IN, infraspinatus; L, labrum; PCT, posterior capsule thickness.

Shoulder MRI

Measurements of the posterior capsule were made by a musculoskeletal radiologist with >15 years of experience using MRI scans from either noncontrast MRI or, in some cases, MRI arthrogram performed on the same day as the ultrasound. To ensure measurements of the posterior capsule were made in the same location as the ultrasound images, the axial slice at the level of the spinoglenoid notch was identified, deep to the infraspinatus muscle/tendon, and PCT was measured specifically on this slice. Furthermore, similar to the measurements made on ultrasound, the posterior capsule was identified as the tissue immediately lateral to the tip of the labrum between the humeral head and rotator cuff (Figure 2).

Figure 2.

(a) Coronal T2 fat-saturated MRI view with the horizontal blue line indicating the level of the SG notch, which was used to standardize visualization of the posterior capsule. (b) Axial PD view of the PCT (vertical yellow line). MRI, magnetic resonance imaging; PD, proton density; PCT, posterior capsule thickness; SG, spinoglenoid.

Statistical Analysis

Agreement between PCT as measured via diagnostic ultrasound versus MRI scan was evaluated through both Pearson correlation coefficients and a Bland-Altman analysis. Correlation coefficients (R) of 0.00 to 0.19 were considered inexistent, 0.20 to 0.39 as weak, 0.40 to 0.59 as moderate, 0.60 to 0.79 as strong, and 0.80 and 1.00 considered as very strong. Statistical significance was set at 0.05. All statistical analyses were done using R Studio (Version 3.6.3).

Results

Overall, 25 drafted pitchers underwent postdraft medical evaluation during the 4-year study period and all 25 pitchers were included in this study. Players averaged 21.1 ± 1.5 years of age, a height and weight of 189 ± 4 cm and 93 ± 5 kg, and 22 (88%) were right-hand dominant.

PCT obtained via diagnostic ultrasound had a mean of 2.4 ± 0.6 mm while PCT obtained via MRI scan had a mean of 2.4 ± 0.8 mm. There was a very strongly positive correlation between ultrasound PCT and MRI PCT (R = 0.945, R² = 0.892; P < 0.001) as shown in Figure 3. The Bland-Altman plot in Figure 4 demonstrates the measurement bias and 95% limits of agreement (±0.55 mm) between diagnostic ultrasound and MRI measurements of PCT.

Figure 3.

Linear correlation between PCT as measured via diagnostic ultrasound and MRI (gold standard). MRI, magnetic resonance imaging; PCT, posterior capsule thickness.

Figure 4.

Bland-Altman plot demonstrating measurement bias (blue dashed line) and 95% limits of agreement (red dashed lines) between PCT as measured via diagnostic ultrasound and MRI (gold standard). MRI, magnetic resonance imaging; PCT, posterior capsule thickness.

Discussion

The primary purpose of this study was to determine the validity of measuring PCT via diagnostic ultrasound in comparison with PCT measurement via MRI (gold standard). The authors’ hypothesis was supported as evidenced by the very strongly positive correlation observed between PCT measured by both ultrasound and MRI.

Confirming diagnostic ultrasound as a valid measurement technique for PCT in the shoulder is vital as research continues to emerge supporting the influence of the PCT on glenohumeral IR ROM in baseball players. To determine which shoulder adaptations contributed most to IR ROM, Paul et al ¹¹ used ultrasound to evaluate humeral retroversion (HR), PCT, and the posterior rotator cuff muscles of 49 professional pitchers at the beginning of Major League Baseball spring training. The authors found that HR and PCT were the primary mechanisms responsible for IR ROM in professional pitchers (R = 0.535; P < 0.001), while the posterior rotator cuff was not related significantly to preseason IR ROM measures. ¹¹ These findings suggest that PCT may be a better representation of chronic adaptations to the throwing shoulder while the posterior rotator cuff is more dynamic and instead related to acute changes in IR ROM.^9,11

A recent systematic review highlights the importance of accounting for HR when assessing glenohumeral internal rotation deficit (GIRD) in baseball pitchers, suggesting that measuring soft tissue GIRD (calculation: clinical GIRD plus the bilateral difference in HR) is a better assessment of true soft tissue restrictions. ¹³ The difference in PCT between the throwing and nonthrowing shoulders (termed posterior capsule hypertrophy) has been found to be related positively and moderately to soft tissue GIRD (R = 0.40; P = 0.007). ¹² While further research is needed to clarify what other structural changes relate to soft tissue GIRD, it is clear that thickening of the posterior capsule is one of the major contributors.

The relationship of the posterior capsule to maladaptive shoulder mechanics has also been highlighted in cadaveric research. Huffman et al ⁶ used a cadaveric thrower’s shoulder model to compare glenohumeral kinematics between intact, after anterior stretching, and after posterior-inferior capsular plication. They found that, at the late cocking phase of throwing (maximum external rotation), the humeral head shifted posteriorly due to anterior stretching and posterior-inferior capsular plication. During the deceleration/follow-through phases of pitching (maximum internal rotation) the humeral head apex shifted inferiorly and anteriorly due to posterior-inferior capsular plication. ⁶ Furthermore, other cadaveric studies have also shown that posterior capsular plication causes increased compressive loads toward the articular cartilage, ³ increased anterior-superior forces on the humeral head, ³ and increased subacromial contact pressure during shoulder flexion. ¹⁰ The current study confirms the validity of ultrasound to readily evaluate posterior shoulder capsular thickness and thereby allowing practitioners in a timely, straightforward manner to implement rehabilitation techniques that may mitigate any adverse effects of that thickening.

Limitations

The authors recognize this study has several limitations. First, only professional baseball pitchers were included, potentially limiting the external validity of the study findings. In addition, ultrasound assessment and MRI scan analysis were performed by clinicians with many years of experience in these techniques. The learning curve of PCT measurement via ultrasound has not yet been studied, and measurement validity may be decreased in those with less experience.

Conclusion

Measuring PCT with diagnostic ultrasound is a valid technique compared with the gold standard of MRI. Clinicians who wish to evaluate PCT in baseball pitchers may use this validated technique to quantify PCT and evaluate for potential contributors to GIRD.