Summary

Posteroinferior shoulder joint laxity in adolescent baseball players (age 19 and younger) was evaluated using zero position radiographs. The posteroinferior shoulder capsule was stiffer on the throwing side than on the nonthrowing side at all ages, influenced by years of experience.

Abstract

Introduction

The purpose of this study was to evaluate the relationship between posterior shoulder joint tightness and posteroinferior shoulder laxity using zero-position radiographs in adolescent baseball players.

Materials And Methods

Between September 2015 and December 2019, 134 baseball players (mean age 14.8±1.9 years. mean height 165.9±9.0cm, BMI 21±2.8 kg/m2) aged 19 years or younger with shoulder pain were enrolled. The average age of baseball start was 8.1 years old, with 94% of all players starting at age 10 or younger, and the average duration of baseball experience was 6.6 years. The degree of the posteroinferior capsule laxity of the shoulder joint was evaluated using the free surface of the humeral head (FSH) angle on the zero-position radiograph, which is the angle at which the glenoid surface is not in contact with the cartilage of the humeral head. When the FSH angle was less than 84° is taken as laxity negative. FSH angle of 85 ° or more was defined as laxity positive.
Posterior shoulder tightness was evaluated using the modified supine assessment of the posterior shoulder tightness test. For this test, the patient lies in a supine position with the shoulder placed in adduction and internal rotation. The test is positive if the position of the olecranon in this shoulder position is not exceeding the nose, with a positive test being indicative of posterior tightness of the shoulder. The study included a comparison of laxity-positive cases, mean FSH angle, with and without posterior tightness, laxity-positive cases with and without posterior tightness, and mean FSH angle by age on the throwing and nonthrowing sides. When the FSH angle was greater on the throwing side than on the non-throwing side, it was defined as the FSH+ group, and when it was smaller, it was defined as the FSH- group. The starting age and years of experience in baseball were compared in the FSH+ group and the FSH- group.

Results

The prevalence of shoulder joint laxity was positive on the throwing side and non-throwing side were 57 shoulders (40.7%), and 74 shoulders (53.7%), respectively (P=0.001). The average FSH angle was 82 .9±12.3 °on the throwing side and 89.0±12.8 ° on the non-throwing side (P =0.001). The posterior shoulder tightness was 127 shoulders (94.8%) on the throwing side and 40 shoulders (29.9%) on the non-throwing side. Of the laxity-positive cases, 55/57 cases (96.4%) on the throwing side and 20/74 cases (27%) on the non-throwing side had posterior tightness. A comparison of FSH angle by age showed that the throwing side was smaller than the non-throwing side across all age groups. The starting age of the FSH+ and FSH-groups were 8.5 and 8.1 years (P=0.12), with 6 and 6,9 years of experience, respectively (P=0.023).

Conclusion

The FSH angle of the throwing side was smaller than that of the non-throwing side at all ages, and the FSH angle was affected by years of experience. This suggested that throwing affected the laxity of the posteroinferior shoulder capsule.