@article{howe_dixit_saul_fisher_2022, title={A Direct Comparison of Node and Element-Based Finite Element Modeling Approaches to Study Tissue Growth}, volume={144}, ISSN={["1528-8951"]}, DOI={10.1115/1.4051661}, abstractNote={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 results of initial simulations, rather than using the theoretically-predicted conversion coefficient (median difference in node position 0.042 cm vs. 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.}, number={1}, journal={JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME}, author={Howe, Danielle and Dixit, Nikhil N. and Saul, Katherine R. and Fisher, Matthew B.}, year={2022}, month={Jan} }
 @article{dixit_mccormick_cole_saul_2021, title={Influence of Brachial Plexus Birth Injury Location on Glenohumeral Joint Morphology}, volume={46}, ISSN={["1531-6564"]}, DOI={10.1016/j.jhsa.2020.10.019}, abstractNote={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.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.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.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.This study expands understanding of the altered glenoid and humeral head morphologies after preganglionic BPBI and its comparisons with morphologies after postganglionic BPBI.}, number={6}, journal={JOURNAL OF HAND SURGERY-AMERICAN VOLUME}, author={Dixit, Nikhil N. and McCormick, Carolyn M. and Cole, Jacqueline H. and Saul, Katherine R.}, year={2021}, month={Jun} }
 @article{doshi_reid_dixit_fawcett_cole_saul_2021, title={Location of brachial plexus birth injury affects functional outcomes in a rat model}, volume={9}, ISSN={["1554-527X"]}, url={https://doi.org/10.1002/jor.25173}, DOI={10.1002/jor.25173}, abstractNote={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.}, journal={JOURNAL OF ORTHOPAEDIC RESEARCH}, publisher={Wiley}, author={Doshi, Raveena M. and Reid, Monique Y. and Dixit, Nikhil N. and Fawcett, Emily B. and Cole, Jacqueline H. and Saul, Katherine R.}, year={2021}, month={Sep} }