Poster P13 - Relationship between functional forelimb usage and musculoskeletal tissue changes in a rat model of brachial plexus birth injury

Introduction: Brachial plexus birth injury (BPBI) is a common neonatal neuromuscular injury that occurs during a difficult childbirth. Injury occurs when the head and neck are excessively stretched, resulting in damage to the brachial plexus nerve bundle and ultimately shoulder and elbow impairment. Musculoskeletal detriments include upper-limb motor dysfunction, muscle paralysis, and osseous deformities rendering children unable to complete tasks of daily living. Detriments vary by injury location, with postganglionic injuries having more restricted shoulder passive external range of motion (ROM) than preganglionic injuries. Clinical consensus is lacking for which BPBI treatment is optimal: physical therapy, surgical intervention, or waiting for spontaneous nerve recovery. Further, clinical knowledge is limited on quantitative changes in functional arm usage and tissue-level changes in bone and muscle following BPBI. While our previous studies in rodent models of BPBI have assessed injury location-specific impairments in gait and shoulder ROM throughout development, we have not yet examined degree of weight bearing or how these functional and joint-level metrics may relate to underlying musculoskeletal tissue changes following BPBI. Our objective is to examine the relationship between functional forelimb usage during gait and spontaneous exploration and changes in bone and muscle tissue following BPBI .

Materials and

Methods: Sprague Dawley rats were divided into 3 surgical groups: postganglionic neurectomy, preganglionic neurectomy, or sham surgery. Rats received surgery on one forelimb on postnatal day 3-6 and were sacrificed at 3, 4, 8, or 16 weeks post-surgery. Prior to sacrifice, rat walking gait (5 m/min) was recorded and scored 1 to 5 for weight-bearing ability over three gait cycles (1=no steps, 2=single-limb weight bearing only, 3=both limbs with prompting to walk, 4=both limbs with minimal prompting, 5=completed cycles with no assistance ). Bilateral passive external ROM was also measured prior to sacrifice; with rats under anesthesia the forelimb was stabilized to a rotating lever in 90° shoulder abduction and 90° elbow flexion, a weight was applied to the lever to apply a constant external rotation force to the shoulder, and degree of rotation was measured with a built-in protractor. To assess spontaneous limb usage, rats were placed in a clear cylinder, recorded for up to 5 minutes, and forelimb movements were counted for pushing off from the table, resting on the cylinder wall, and landing back on the table. Following sacrifice, tissues were fixed, various shoulder bones and muscles were dissected , and humeral bone length and biceps brachii length for injured limbs were measured. For each surgical group and timepoint, correlation analyses were used to assess relationships between 1) weight-bearing score vs. humeral or biceps length and 2) spontaneous limb usage metrics (push-off, wall resting, landing) vs. external ROM.

Results, Conclusions, and Discussions: Weight-bearing score: Weight-bearing score was greatest in the sham group (average score=5 across timepoints), followed preganglionic injury (3.3-5 across timepoints) and postganglionic injury (2.8-4.1 across timepoints). For sham, no significant relationship was found between weight-bearing score and either humeral or biceps length. For postganglionic injury, across all timepoints weight-bearing score had a moderate negative correlation with biceps length (r=-0.42), and at the 16-week timepoint only, weight-bearing score had a strong negative correlation with biceps length (r=-0.89). For preganglionic injury, overall weight-bearing score had a strong positive correlation with both humeral and biceps lengths (r=0.88-0.96), and at the 3-week timepoint only, weight-bearing score had a moderate positive correlation only with humeral length (r=0.55). The relationships between weight-bearing score and bone and muscle lengths need to be examined further in other muscles surrounding the shoulder to better understand their impact on limb usage.
Spontaneous forelimb usage: External passive ROM was greatest in the sham group (injured=41-55 °), followed by preganglionic (injured=30-39°) and postganglionic (injured=23-39°) injury. For sham and postganglionic injury, no significant relationships were observed between spontaneous forelimb usage and ROM. For preganglionic injury, across all timepoints wall resting was negatively correlated with ROM (r=-0.38), and at the 4-week timepoint only, push-off was positively correlated with ROM (r=0.60). Our previous work showed that preganglionic injury caused shorter muscles with less mass compared to postganglionic injury, with differences observed by 3-weeks post-injury. Since muscle function contributes to joint ROM, muscle tissue changes likely also contribute to the altered spontaneous limb usage observed here. We also previously found that the amount of BPBI-related growth disruption varied across different muscles and differed between postganglionic and preganglionic injury, so analyses are ongoing to assess other muscles and other muscle architecture metrics for a broader look at these relationships of tissue-level changes to shoulder function.
Future assessments will include the relationship between functional limb usage and other tissue-level metrics including trabecular bone architecture in the humeral head and scapula and dynamic bone growth rates in the humerus. Understanding these relationships more broadly could provide insight into potential therapeutic targets to examine in future BPBI studies.

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