Brachial plexus birth injury (BPBI) is a peripheral nerve injury occurring during childbirth. It affects 1-3 of every 1,000 births and causeslifelong arm impairment in 30-40% of those affected. Impairments include joint contracture, muscle shortening, osseous deformities, and arm disuse. Clinically, impairment severity depends on whether injury occurred as a nerve rupture or nerve avulsion. Nerve rupture, represented in animal models by a postganglionic neurectomy, results in both contractures and limb disuse, while nerve avulsion, represented in animal models by a preganglionic neurectomy, results in limb disuse without contractures. Past studies revealed altered bone morphology and muscle architecture with injury. Postganglionic injury resulted in a declined and flattened scapula and shorter, smaller muscles surrounding the glenohumeral joint. Preganglionic injury resulted in a smaller humeral head and even shorter and smaller muscles than with postganglionic injury. Only a few studies have characterized bone microstructure after BPBI, showing fewer, thinner, and more sparse trabeculae in the humeral head after postganglionic injury. Trabecular microstructure in the scapula, or following preganglionic injury in humerus or scapula, remain unknown. Additionally, underlying muscle composition and bone metabolism have not been characterized, which would further knowledge for how deformities develop.

The goal of this dissertation was to examine the effects of BPBI on glenohumeral muscle and bone at microstructural and cellular levels and identify possible underlying factors in deformity. We experimentally isolated direct biological effects of BPBI from concomitant effects of high passive force due to shortened muscles using our rat neurectomy models, in which nerve injury location controls presence or absence of contracture. The primary deformity drivers are muscle contracture, denervation, and limb disuse in postganglionic injury, and denervation and limb disuse in preganglionic injury. Our disarticulation model further isolates reduced active functional loading without nerve injury, representing limb disuse. To mimic injury timing, interventions occurred at postnatal day 3. The first aim was to determine differential effects of denervation, muscle contracture, and limb disuse following BPBI on parallel changes in glenohumeral bone macrostructure and microstructure. Bone macrostructure and microstructure in the scapula and humerus were analyzed with standard trabecular metrics. Morphologically, postganglionic injury affected the scapula, preganglionic injury affected the humerus, and disarticulation affected the scapula and humerus. Bone microstructure was affected after postganglionic injury in the scapula and humerus, after preganglionic injury in the scapula and humerus (most severely), and after disarticulation in the scapula and some in the humerus. The second aim was to determine impact of denervation, muscle contracture, and limb disuse following BPBI on bone metabolism, muscle composition, and muscle-bone crosstalk. Bone metabolism in the humerus was characterized using standard dynamic histomorphometry. Postganglionic injury reduced diaphyseal bone growth on the endosteal surface, and preganglionic injury reduced growth on the periosteal surface;​ preganglionic injury and disarticulation reduced longitudinal growth at the growth plate. Muscle composition was assessed with collagen deposition (fibrosis). Fibrosis was present in the biceps after postganglionic injury but more so after preganglionic injury, and in subscapularis after preganglionic injury. Muscle-bone crosstalk was determined via FGF-2 quantity in muscles. Only preganglionic injury resulted in upregulation of FGF-2 in the upper subscapularis.

This work highlights factors contributing to deficits in bone and muscle following postganglionic injury, preganglionic injury, and limb disuse. Results show postganglionic and preganglionic BPBI cause different musculoskeletal deficits from a different combination of underlying drivers. Postganglionic injury produces greater morphological changes and includes muscle composition changes. Preganglionic injury produces greater microstructural deficits and includes muscle composition, bone metabolism, and crosstalk changes. These results will allow further investigation into other cell-signaling factors, muscle composition changes, and the timeline in which they occur, ultimately informing more effective treatment targets.