Bone grafting in oblique versus prepared rectangular uncontained glenoid defects in reversed shoulder arthroplasty. A biomechanical comparison.

BACKGROUND: How the shape of the glenoid defect being reconstructed influences stability in reversed shoulder arthroplasty has never been evaluated. The purpose of this study was to compare the reconstruction of two different shaped defects in reversed shoulder arthroplasty. METHODS: Two groups (ten Sawbone scapulae each) of oblique- and rectangular-shaped glenoid defects were tested biomechanically. On the anterior half of the glenoid, bony defects (rectangular and oblique shaped) were prepared and reconstructed subsequently with a graft and reversed shoulder arthroplasty. As a control group, Sawbones without glenoid deficiency were used. In addition, these tests were reproduced in cadavers. FINDINGS: In Sawbones, no significant difference in initial stability was found between the two groups (p>0.05). Additionally, in the cadaver tests no significant difference was found between the groups with different defects (p>0.05). During the preparation, macroscopic loosening of the oblique bone grafts was found in three cases after the performance of the reversed shoulder arthroplasty due to the lack of medial support. The localization of the highest micromotion were measured primarily between the scapula bone and the graft compared to the measured micromotions between glenoid implant and the graft. INTERPRETATION: If the oblique-shaped bone graft was secured under the baseplate, the rectangular defect preparation did not show a significantly higher primary stability. However, the advantage of medial support in rectangular defects leads to more stability while placing the bone graft and baseplate during the surgical technique and should therefore be considered a preferable option.

Influence of different peg length in glenoid bone loss: A biomechanical analysis regarding primary stability of the glenoid baseplate in reverse shoulder arthroplasty.

BACKGROUND: There is no biomechanical basis to determine the influence of different length of the central peg of the baseplate anchored within the native scapula in glenoid defect reconstruction in cases of degenerative or posttraumatic glenoid bone loss in reversed shoulder arthroplasty. OBJECTIVE: The purpose of this study was to analyse the stability of different peg lengths used in glenoid bone loss in reversed shoulder arthroplasty. METHODS: Different lengths of metaglene pegs with different depths of peg anchorage performed with or without metaglene screws in sawbone foam blocks were loaded in vertical and horizontal directions for differentiating load capacities. Simulated physiological loadings were then applied to the peg implants to determine the limits of loading in each depth of anchorage. RESULTS: The loading capacity of the implant was reduced as less of the peg was anchored. The vertically loaded implants showed a significantly higher stability, in contrast to those loaded horizontally at a corresponding peg length and depth of anchorage (p < 0.05). The tests revealed that the metaglene screws are more essential for primary stability than is the peg particularly in the vertically directed loadings (2/3 anchored: peg contributed to 28% of the stability, 1/3 anchorage: peg contributed to 12%). Under the second test conditions, the lowest depth of peg anchorage (1/3) resulted in 322 Newtons [N] in the long peg with a vertical loading direction, and in 130 N in the long peg with a horizontal loading direction (p < 0.05). CONCLUSION: The pegs should be anchored as deeply as possible into the native scapula bone stock. The metaglene screws play a major role in the initial stability, in contrast to the peg, and they become more important when the depth of the peg anchorage is reduced. If possible, four metaglene screws should be used in cases of uncontained bone loss to guarantee the highest stability.

Computational fluid dynamics model of avian tracheal temperature control as a model for extant and extinct animals.

Respiratory evaporative cooling is an important mechanism of temperature control in bird. A computational simulation of the breathing cycle, heat and water loss in anatomical avian trachea/air sac model has not previously been conducted. We report a first attempt to simulate a breathing cycle in a three-dimensional model of avian trachea and air sacs (domestic fowl) using transient computational fluid dynamics. The airflow in the trachea of the model is evoked by changing the volume of the air sacs based on the measured tidal volume and inspiratory/expiratory times for the domestic fowl. We compare flow parameters and heat transfer results with in vivo data and with our previously reported results for a two-dimensional model. The total respiratory heat loss corresponds to about 13-19% of the starvation metabolic rate of domestic fowl. The present study can lend insight into a possible thermoregulatory function in species with long necks and/or a very long trachea, as found in swans and birds of paradise. Assuming the structure of the sauropod dinosaur respiratory system was close to avian, the simulation of the respiratory temperature control (using convective and evaporative cooling) in the extensively experimentally studied domestic fowl may also help in making simulations of respiratory heat control in these extinct animals.

Tensile trabeculae--myth or reality?

Understanding of the functional role of the trabecular bone is very important for the analysis and computer-aided simulations of bone remodelling processes. The aspired wide clinical applications remain a remote future despite a great number of developed up-to-date approaches and theories and collected data on both material properties of the trabecular bone and its reaction to various stimuli. It is widely accepted that the mechanical loading plays the major role for the structure of the cancellous bone. The in vivo loading conditions of the cancellous bone are not known. Hence, for the computer-aided analysis and modelling of the trabecular bone specimens, simplified loading conditions are used. Also for the analysis of the cancellous bone as a part of a whole bone simplified loading conditions are assumed based on previous research without questioning its accuracy or relevance to the real in vivo conditions. In particular, the bending loading of the bone, which originates from the well-known observations made more than a century ago that have evolved in the trajectorial theory or "tensile trabeculae tradition", is often assumed to reflect the physiological loading conditions of bones. Some studies show that the bending or tensile-compressive orthogonal loading conditions for the cancellous bone may lead to plausible results. However, some other research works suggest that the presence of the tensile trabecular structures (particularly in the proximal femur) is doubtful and the bending loading conditions in bone should be treated with caution. Moreover, the loading conditions with compensated (or minimised) bending also produce results that correlate with the material distribution in the bone. The purpose of this review is to analyse some of the data and ideas available in the literature and to discuss the question of the major factors that define the shape and structure of the trabecular bone during the process of functional adaptation.