A Direct Comparison of Node and Element-Based Finite Element Modeling Approaches to Study Tissue Growth.

Finite element analysis is a useful tool to model growth of biological tissues and predict how growth can be impacted by stimuli. Previous work has simulated growth using node-based or element-based approaches, and this implementation choice may influence predicted growth, irrespective of the applied growth model. This study directly compared node-based and element-based approaches to understand the isolated impact of implementation method on growth predictions by simulating growth of a bone rudiment geometry, and determined what conditions produce similar results between the approaches. We used a previously reported node-based approach implemented via thermal expansion and an element-based approach implemented via osmotic swelling, and we derived a mathematical relationship to relate the growth resulting from these approaches. We found that material properties (modulus) affected growth in the element-based approach, with growth completely restricted for high modulus values relative to the growth stimulus, and no restriction for low modulus values. The node-based approach was unaffected by modulus. Node- and element-based approaches matched marginally better when the conversion coefficient to relate the approaches was optimized based on the results of initial simulations, rather than using the theoretically predicted conversion coefficient (median difference in node position 0.042 cm versus 0.052 cm, respectively). In summary, we illustrate here the importance of the choice of implementation approach for modeling growth, provide a framework for converting models between implementation approaches, and highlight important considerations for comparing results in prior work and developing new models of tissue growth.

Location of brachial plexus birth injury affects functional outcomes in a rat model.

Brachial plexus birth injury (BPBI) results in shoulder and elbow paralysis with shoulder internal rotation and elbow flexion contracture as frequent sequelae. The purpose of this study was to develop a technique for measuring functional movement and examine the effect of brachial plexus injury location (preganglionic and postganglionic) on functional movement outcomes in a rat model of BPBI, which we achieved through integration of gait analysis with musculoskeletal modeling and simulation. Eight weeks following unilateral brachial plexus injury, sagittal plane shoulder and elbow angles were extracted from gait recordings of young rats (n = 18), after which rats were sacrificed for bilateral muscle architecture measurements. Musculoskeletal models reflecting animal-specific muscle architecture parameters were used to simulate gait and extract muscle fiber lengths. The preganglionic neurectomy group spent significantly less (p = 0.00116) time in stance and walked with significantly less (p < 0.05) elbow flexion and shoulder protraction in the affected limb than postganglionic neurectomy or control groups. Linear regression revealed no significant linear relationship between passive shoulder external rotation and functional shoulder protraction range of motion. Despite significant restriction in longitudinal muscle growth, normalized functional fiber excursions did not differ significantly between groups. In fact, when superimposed on a normalized force-length curve, neurectomy-impaired muscle fibers (except subscapularis) accessed regions of the curve that overlapped with the control group. Our results suggest the presence of compensatory motor control strategies during locomotion following BPBI. The clinical implications of our findings support emphasis on functional movement analysis in treatment of BPBI, as functional and passive outcomes may differ substantially.

Preganglionic and Postganglionic Brachial Plexus Birth Injury Effects on Shoulder Muscle Growth.

PURPOSE: Brachial plexus birth injury can differ in presentation, depending on whether the nerve ruptures distal to, or avulses proximal to, the dorsal root ganglion. More substantial contracture and bone deformity at the shoulder is typical in postganglionic injuries. However, changes to the underlying muscle structure that drive these differences in presentation are unclear. METHODS: Seventeen Sprague-Dawley rats received preganglionic or postganglionic neurectomy on a single limb on postnatal days 3 and 4. Muscles crossing the shoulder were retrieved once the rats were sacrificed at 8 weeks after birth. External rotation range of motion, muscle mass, muscle length, muscle sarcomere length, and calculated optimal muscle length were measured bilaterally. RESULTS: Average shoulder range of motion in the postganglionic group was 61.8% and 56.2% more restricted at 4 and 8 weeks, respectively, compared with that in the preganglionic group, but affected muscles after preganglionic injury were altered more severely (compared with the unaffected limb) than after postganglionic injury. Optimal muscle length in preganglionic injury was shorter in the affected limb (compared with the unaffected limb: -18.2% ± 9.2%) and to a greater extent than in postganglionic injury (-5.1% ± 6.2%). Muscle mass in preganglionic injury was lower in the affected limb (relative to the unaffected limb: -57.2% ± 24.1%) and to a greater extent than in postganglionic injury (-28.1% ± 17.7%). CONCLUSIONS: The findings suggest that the presence of contracture does not derive from restricted longitudinal muscle growth alone, but also depends on the extent of muscle mass loss occurring simultaneously after the injury. CLINICAL RELEVANCE: This study expands our understanding of differences in muscle architecture and the role of muscle structure in contracture formation for preganglionic and postganglionic brachial plexus birth injury.

Influence of Brachial Plexus Birth Injury Location on Glenohumeral Joint Morphology.

PURPOSE: Patient presentation after brachial plexus birth injury (BPBI) is influenced by nerve injury location; more contracture and bone deformity occur at the shoulder in postganglionic injuries. Although bone deformity after postganglionic injury is well-characterized, the extent of glenohumeral deformity after preganglionic BPBI is unclear. METHODS: Twenty Sprague-Dawley rat pups received preganglionic or postganglionic neurectomy on a single forelimb at postnatal days 3 to 4. Glenohumeral joints on affected and unaffected sides were analyzed using micro-computed tomography scans after death at 8 weeks after birth. Glenoid version, glenoid inclination, glenoid and humeral head radius of curvature, and humeral head thickness and width were measured bilaterally. RESULTS: The glenoid was significantly more declined in affected compared with unaffected shoulders after postganglionic (-17.7° ± 16.9°) but not preganglionic injury. Compared with the preganglionic group, the affected shoulder in the postganglionic group exhibited significantly greater declination and increased glenoid radius of curvature. In contrast, the humeral head was only affected after preganglionic but not postganglionic injury, with a significantly smaller humeral head radius of curvature (-0.2 ± 0.2 mm), thickness (-0.2 ± 0.3 mm), and width (-0.3 ± 0.4 mm) on the affected side compared with the unaffected side; changes in these metrics were significantly associated with each other. CONCLUSIONS: These findings suggest that glenoid deformities occur after postganglionic BPBI but not after preganglionic BPBI, whereas the humeral head is smaller after preganglionic injury, possibly suggesting an overall decreased biological growth rate in this group. CLINICAL RELEVANCE: This study expands understanding of the altered glenoid and humeral head morphologies after preganglionic BPBI and its comparisons with morphologies after postganglionic BPBI.

Integrated iterative musculoskeletal modeling predicts bone morphology following brachial plexus birth injury (BPBI).

Brachial plexus birth injury (BPBI) is the most common nerve injury among children. The glenohumeral joint of affected children can undergo severe osseous deformation and altered muscle properties, depending on location of the injury relative to the dorsal root ganglion (preganglionic or postganglionic). Preganglionic injury results in lower muscle mass and shorter optimal muscle length compared to postganglionic injury. We investigated whether these changes to muscle properties over time following BPBI provide a mechanically-driven explanation for observed differences in bone deformity between preganglionic and postganglionic BPBI. We developed a computational framework integrating musculoskeletal modeling to represent muscle changes over time and finite element modeling to simulate bone growth in response to mechanical and biological stimuli. The simulations predicted that the net glenohumeral joint loads in the postganglionic injury case were nearly 10.5% greater than in preganglionic. Predicted bone deformations were more severe in the postganglionic case, with the glenoid more declined (pre: -43.8°, post: -51.0°), flatter with higher radius of curvature (pre: 3.0 mm, post: 3.7 mm), and anteverted (pre: 2.53°, post: 4.93°) than in the preganglionic case. These simulated glenoid deformations were consistent with previous experimental studies. Thus, we concluded that the differences in muscle mass and length between the preganglionic and postganglionic injuries are critical mechanical drivers of the altered glenohumeral joint shape.

Computational analysis of glenohumeral joint growth and morphology following a brachial plexus birth injury.

Children affected with brachial plexus birth injury (BPBI) undergo muscle paralysis. About 33% of affected children experience permanent osseous deformities of the glenohumeral joint. Recent evidence suggests that some cases experience restricted muscle longitudinal growth in addition to paralysis and reduced range of motion at the shoulder and elbow. It is unknown whether altered loading due to paralysis, muscle growth restriction and contracture, or static loading due to disuse is the primary driver of joint deformity after BPBI. This study uses a computational framework integrating finite element analysis and musculoskeletal modeling to examine the mechanical factors contributing to changes in bone growth and morphometry following BPBI. Simulations of 8 weeks of glenohumeral growth in a rat model of BPBI predicted that static loading of the joint is primarily responsible for joint deformation consistent with experimental measures of bone morphology, whereas dynamic loads resulted in normal bone growth. Under dynamic loading, glenoid version angle (GVA), glenoid inclination angle (GIA), and glenoid radius of curvature (GRC) (-1.3°, 38.2°, 2.5 mm respectively) were similar to the baseline values (-1.8°, -38°, 2.1 mm respectively). In the static case with unrestricted muscle growth, these measures increased in magnitude (5.2°, -48°, 3.5 mm respectively). More severe joint deformations were observed in GIA and GRC when muscle growth was restricted (GVA: 3.6°, GIA: -55°, GRC: 4.0 mm). Predicted morphology was consistent with literature reports of in vivo glenoid morphology following postganglionic BPBI. This growth model provides a framework for understanding the most influential mechanical factors driving glenohumeral deformity following BPBI.

Evaluation and correlation of stress scores with blood pressure, endogenous cortisol levels, and homocysteine levels in patients with central serous chorioretinopathy and comparison with age-matched controls.

CONTEXT: Stress had been associated with the development of central serous chorioretinopathy (CSC). The study was designed to evaluate the effect of stress on other risk factors of CSC such as serum cortisol levels, serum homocysteine levels, and blood pressure (BP) in CSC patients. AIMS: To compare stress scores, serum cortisol and serum homocysteine levels, and BP of CSC patients with that of control population and to correlate stress scores of CSC patients with BP, serum cortisol levels, and serum homocysteine levels. MATERIALS AND METHODS: Stress scores, serum morning and evening cortisol levels, serum homocysteine levels, systolic and diastolic BP of 54 CSC patients were measured and compared with that of 54 age- and sex-related controls using Student's t-test. Stress scores of CSC patients were correlated with systolic and diastolic BP, serum morning and evening cortisol levels and serum homocysteine levels and Pearson correlation coefficient (r) were calculated. RESULTS: Stress scores, serum homocysteine levels, serum morning and evening cortisol levels, and systolic and diastolic BP were all elevated in CSC patients as compared with age- and sex-related controls (P < 0.05). Stress scores of CSC patients were found to correlate strongly with serum homocysteine levels, serum morning and evening cortisol levels, and systolic and diastolic BP, with r values 0.82, 0.8, 0.8, 0.8, and 0.81, respectively (P < 0.0001). CONCLUSIONS: Stress scores were elevated in CSC patients and were strongly correlated with serum homocysteine and cortisol levels and BP.