The Effect of the Loading Condition Corresponding to Functional Shoulder Activities on Trabecular Architecture of Glenoid

The adaption of the bony microstructure of the human glenoid cavity as a result of long-term biomechanical loading

Structural arrangements of the bony microstructure of a joint through adaptational processes are thought to be determined by the biomechanical demands and its changes. Pursuing this theory of "form follows the biomechanical function", the load distribution of the glenoid cavity, as it is mirrored in its mineralization pattern, should link not only to its thickness distribution, but also will have an impact onto the trabecular network below. To prove and confirm this hypothesis, we analysed the mineral distribution in correlation to the subchondral bone plates thickness and the distribution of architectural parameters of the trabecular network below. Our findings clearly state an inhomogeneous but regular and reproducible mineral distribution pattern in respect to the biomechanical demands and a thickness of the subchondral bone plate which shows a significant correlation (78-93%). As for the trabecular network below, the distribution of the analysed parameters also revealed an inhomogeneous distribution with a regular pattern in correlation to the biomechanical impact. We found distinctive maxima of material distribution and stability (bone volume 79%, plate-like architecture 77%) situated below areas of high long-term load intake. With increasing depth, the trabecular network administers the expression of each structural parameter following the fact that the strain energy gets more and more evenly distributed and changes from a high degree of differentiation just beneath the SBP to a more equal distribution within the deeper areas. After all, the biomechanical situation of a joint directly influences the bony formation of the subchondral bone plate and the trabecular network below.

Effects of rotator cuff tears on muscle moment arms: a computational study

Rotator cuff tears cause morphologic changes to cuff tendons and muscles, which can alter muscle architecture and moment arm. The effects of these alterations on shoulder mechanical performance in terms of muscle force and joint strength are not well understood. The purpose of this study was to develop a three-dimensional explicit finite element model for investigating morphological changes to rotator cuff tendons following cuff tear. The subsequent objectives were to validate the model by comparing model-predicted moment arms to empirical data, and to use the model to investigate the hypothesis that rotator cuff muscle moment arms are reduced when tendons are divided along the force-bearing direction of the tendon. The model was constructed by extracting tendon, cartilage, and bone geometry from the male Visible Human data set. Infraspinatus and teres minor muscle and tendon paths were identified relative to the humerus and scapula. Kinetic and kinematic boundary conditions in the model replicated experimental protocols, which rotated the humerus from 45 degrees internal to 45 degrees external rotation with constant loads on the tendons. External rotation moment arms were calculated for two conditions of the cuff tendons: intact normal and divided tendon. Predicted moment arms were within the 1-99% confidence intervals of experimental data for nearly all joint angles and tendon sub-regions. In agreement with the experimental findings, when compared to the intact condition, predicted moment arms were reduced for the divided tendon condition. The results of this study provide evidence that one potential mechanism for the reduction in strength observed with cuff tear is reduction of muscle moment arms. The model provides a platform for future studies addressing mechanisms responsible for reduced muscle force and joint strength including changes to muscle length-tension operating range due to altered muscle and tendon excursions, and the effects of cuff tear size and location on moment arms and muscle forces.