Morphological Changes to the Glenohumeral Musculoskeletal System Following Brachial Plexus Birth Injury (BPBI)

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URL: https://repository.lib.ncsu.edu/handle/1840.20/36740

Abstract

This dissertation evaluates musculoskeletal abnormalities occurring due to preganglionic and postganglionic BPBI. BPBI paralyzes muscles responsible for shoulder and elbow movement with severe bone deformities at the glenohumeral joint. Currently, there is a lack of understanding about the influence of nerve injury location on muscle changes and osseous deformities.

Aim 1 was to understand the contributions of altered loading due to static loading and muscle growth restriction to joint deformity after BPBI. This study used a computational framework to examine the mechanical factors contributing to changes in bone growth and morphology. Simulations predicted that static loading is primarily responsible for joint deformation. Under dynamic loading, glenoid version angle (GVA), glenoid inclination angle (GIA), and glenoid radius of curvature (GRC) (-1.3°, 38.2°, 2.5mm) were similar to the baseline values (-1.8°, -38°, 2.1mm) but increased in magnitude in the static case (5.2°, -48°, 3.5mm). More severe joint deformations were observed in GIA and GRC when muscle growth was restricted (GVA: 3.6°, GIA: -55°, GRC: 4.0mm).

Aim 2 sought to identify the changes to underlying muscle structure following preganglionic and postganglionic BPBI. Twenty-six Sprague Dawley rat pups received preganglionic or postganglionic neurectomy on a single limb at postnatal days 3-4. Range of motion was more restricted in the postganglionic group (4-weeks: -40.7 ± 21.8%; 8-weeks: -29.6 ± 49.1%) compared to preganglionic (4-weeks: 21.1 ± 41.0%; 8-weeks: 26.6 ± 45.8%), but muscles following preganglionic injury were more degenerated relative to the unaffected limb than following postganglionic injury. Compared to unaffected limb, optimal muscle length was shorter in the affected limb remarkably more for the preganglionic group (-18.2 ± 9.2%) than the postganglionic group (-5.1 ± 6.2%), and muscle mass was on average lower in the affected limb for the preganglionic group (-57.2 ± 24.1%) compared to the postganglionic group (-28.1 ± 17.7%).

While the extent of bone deformity after postganglionic injury is well characterized, the nature of glenohumeral deformation following preganglionic BPBI is unclear. Aim 3 characterized bone changes in the same animals studied in Aim 2. MicroCT scans of the joint showed after postganglionic injury, the glenoid was significantly more declined on the affected limb compared to the unaffected limb (-17.7 ± 16.9°, p = 0.021). Compared to the preganglionic group, the affected shoulder in the postganglionic group exhibited significantly higher declination (p = 0.0010) and higher glenoid radius of curvature (p = 0.0070). After preganglionic injury, humeral head radius of curvature, thickness, and width were significantly smaller on the affected side compared to the unaffected side (curvature: -0.5 ± 0.5 mm, p = 0.0085; thickness: -0.5 ± 0.5 mm, p = 0.011; width: -0.4 ± 0.4 mm, p = 0.0076) and tended to be linearly correlatedinterlinked.

Finally, Aim 4 investigated whether changes to muscle mass and optimal muscle length over time following BPBI provide a mechanically-driven explanation for observed differences in postural and bone deformity between preganglionic and postganglionic BPBI. A computational framework was developed to simulate bone growth in response over time. The simulations predicted the net glenohumeral joint loads in the postganglionic injury case were nearly 11% greater than the preganglionic. Bone deformations were more severe in the postganglionic case, with the glenoid more declined (pre: -43.8°, post: -48.9°), flatter with higher radius of curvature (pre:3.0mm, post:3.7mm), and retroverted (pre: -13.7°, post: -16.1°) than the preganglionic case.

This dissertation provides new information about the influential mechanical factors driving glenohumeral joint deformity. The work found that the reduced muscle mass and restriction in longitudinal muscle growth were more severe due to preganglionic injuries than postganglionic injuries. However, the restriction in ROM and the osseous deformities were more severe in postganglionic injuries than preganglionic injuries. The alterations in bone morphology can be predicted from the mechanical contributions of the altered muscle structure

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