Biomechanical analysis of anterior bone graft augmentation with reversed shoulder arthroplasty in large combined glenoid defects compared with total bony joint line reconstruction (modified bony-increased-offset reversed shoulder arthroplasty).

BACKGROUND: The aim of this biomechanical study was to compare 2 surgical techniques for the reconstruction of large, combined, uncontained glenoid defects with reversed shoulder arthroplasty (RSA). METHODS: Three groups of scapulae with RSA were tested by the application of a physiological combination of compressive/shear loads in Sawbones (Pacific Research Laboratories, Inc., Vashon Island, WA, USA) and cadavers. Two of the groups (both Sawbones and cadaveric specimens) consisted of anterior combined defects (14 mm in depth), and the third group served as a control group (only Sawbones specimens). The first group with an anterior combined defect was reconstructed with anterior bone grafts to contain the defect and cancellous bone to fill the central defect before RSA with partial bony joint line reconstruction (p-BJR). In the second group with an anterior combined defect, the dorsal rim was reamed and the joint line was reconstructed with a bone disc fully covering the peg. This total BJR (t-BJR) corresponds to the technique of bony-increased-offset-RSA (BIO-RSA). RESULTS: At 150 µm of displacement, the loadings in the inferior-superior (IS) direction were significantly more stable than those in the anterior-to-posterior (AP) direction within both reconstructed defect groups (P ≤ .002). In contrast, no significant differences were found between the partial BJR and t-BJR group in either direction (Sawbones: AP: P = .29; IS: P = .44; cadavers: AP: P = .67; IS: P = .99). The control group revealed significantly higher values in all loadings of the IS direction and significantly higher loadings at 40 µm and 150 µm in the AP direction. CONCLUSION: Both techniques could be applied for such complex defects provided that there is sufficient medial bone stock for a t-BJR. Significantly greater stability was found in the IS direction than in the AP direction within each group, which could be explained by the longer screw anchoring within the superior and inferior columns. Both defect groups were less stable than the group of intact glenoids.

The functional role of the ischiopubic membrane for the mechanical loading of the pubis in the domestic fowl (Gallus gallus).

Soft tissues other than muscles are supposed to be of mechanical importance, yet they are rarely integrated into finite element models. Here, we investigate the functional role of the ischiopubic membrane for the loading of the pubis of the domestic fowl using 2D finite element analysis. For this purpose, a specimen of the domestic fowl was dissected and soft tissues attaching to the pubis were studied in great detail. Muscles were removed and measurements taken. For the 2D finite element model, the outline was taken from the dissected specimen. Two 2D finite element models were generated: one without and one with ischiopubic membrane. The same muscular loading based on own measurements and electromyographic data was applied to both models. The model without ischiopubic membrane shows anteroventral bending deformation of the scapus pubis, resulting in high compressive and tensile principal stresses at the level of ultimate bone stress values. The model with ischiopubic membrane shows low compressive principal stresses in the pubis consistent with the levels of steady state remodelling of bone. Based on these results, the ischiopubic membrane of the domestic fowl potentially establishes a physiological loading of the pubis and therefore might be of great mechanical significance for the loading of the bone.

Boundary conditions for heat transfer and evaporative cooling in the trachea and air sac system of the domestic fowl: a two-dimensional CFD analysis.

Various parts of the respiratory system play an important role in temperature control in birds. We create a simplified computational fluid dynamics (CFD) model of heat exchange in the trachea and air sacs of the domestic fowl (Gallus domesticus) in order to investigate the boundary conditions for the convective and evaporative cooling in these parts of the respiratory system. The model is based upon published values for respiratory times, pressures and volumes and upon anatomical data for this species, and the calculated heat exchange is compared with experimentally determined values for the domestic fowl and a closely related, wild species. In addition, we studied the trachea histologically to estimate the thickness of the heat transfer barrier and determine the structure and function of moisture-producing glands. In the transient CFD simulation, the airflow in the trachea of a 2-dimensional model is evoked by changing the volume of the simplified air sac. The heat exchange between the respiratory system and the environment is simulated for different ambient temperatures and humidities, and using two different models of evaporation: constant water vapour concentration model and the droplet injection model. According to the histological results, small mucous glands are numerous but discrete serous glands are lacking on the tracheal surface. The amount of water and heat loss in the simulation is comparable with measured respiratory values previously reported. Tracheal temperature control in the avian respiratory system may be used as a model for extinct or rare animals and could have high relevance for explaining how gigantic, long-necked dinosaurs such as sauropoda might have maintained a high metabolic rate.

Principles of determination and verification of muscle forces in the human musculoskeletal system: Muscle forces to minimise bending stress.

While there are a growing number of increasingly complex methodologies available to model geometry and material properties of bones, these models still cannot accurately describe physical behaviour of the skeletal system unless the boundary conditions, especially muscular loading, are correct. Available in vivo measurements of muscle forces are mostly highly invasive and offer no practical way to validate the outcome of any computational model that predicts muscle forces. However, muscle forces can be verified indirectly using the fundamental property of living tissue to functional adaptation and finite element (FE) analysis. Even though the mechanisms of the functional adaptation are not fully understood, its result is clearly seen in the shape and inner structure of bones. The FE method provides a precise tool for analysis of the stress/strain distribution in the bone under given loading conditions. The present work sets principles for the determination of the muscle forces on the basis of the widely accepted view that biological systems are optimized light-weight structures with minimised amount of unloaded/underloaded material and hence evenly distributed loading throughout the structure. Bending loading of bones is avoided/compensated in bones under physiological loading. Thus, bending minimisation provides the basis for the determination of the musculoskeletal system loading. As a result of our approach, the muscle forces for a human femur during normal gait and sitting down (peak hip joint force) are obtained such that the bone is loaded predominantly in compression and the stress distribution in proximal and diaphyseal femur corresponds to the material distribution in bone.