Brachial plexus birth injury (BPBI) occurs when the nerve bundle innervating the shoulder is damaged due to excessive stretching of an infant’s neck during childbirth. It occurs in about 0.9 of every 1,000 births, with 30-40% of those resulting in lifelong shoulder impairment, including musculoskeletal deformities, reduced range of motion, and muscle paralysis. These deficits vary by injury location (preganglionic vs. postganglionic), but the effect on underlying muscle composition is unknown. We hypothesize diminished muscle growth from BPBI is related to increased collagen content (fibrosis), impairing muscle function. We will examine the effects of altered passive muscle loading and active functional loading on collagen buildup between muscle fibers using four groups.

Sprague Dawley rats underwent surgery on one forelimb at postnatal day 3-6: preganglionic or postganglionic neurectomy, forelimb disarticulation (n=8/group/timepoint), or sham surgery (n=6/timepoint). Contralateral limbs served as added controls. Biceps, subscapularis, and supraspinatus muscles were dissected at 2, 3, 4, 8, or 16 weeks post-injury, snap-frozen, cryosectioned longitudinally, and stained using Masson’s trichrome. Muscle sections were imaged and analyzed as a ratio of collagen to muscle tissue. Fibrosis was compared across groups and timepoints using repeated measures ANOVA and Tukey’s tests for multiple comparisons (GraphPad Prism 10, α=0.05). Comparisons between injured and uninjured limbs in each group were assessed using paired t-tests.

Preliminary data indicate greater fibrosis in postganglionic injuries than preganglionic. Collagen content peaked at 4 weeks in all muscles for postganglionic injured limbs and in the subscapularis and supraspinatus for disarticulation injured limbs. No significant differences have been found across timepoints in sham or preganglionic injured limbs. This is the first study characterizing fibrosis development and progression in glenohumeral muscles following BPBI. Understanding the progression of altered muscle composition throughout development following BPBI may inform treatment planning.