Quantitative biomechanical evaluation of upper extremity-strain behavior of the humerus diaphysis under loading

Abstract

While mechanical behavior of the adult human lower extremity long bones under loading has been studied extensively, the same is not true for the adult human humerus. Mechanical data reported for cadaveric humeri and anatomic humerus models are limited to stiffness and rigidity. Strain characteristics of the humerus diaphysis as a function of loading provide a valuable addition to the currently limited knowledge. The objective of this dissertation was to accomplish this goal, using numerical/finite element (FE) methods applied to a standard anatomic humerus model (Reference-Humerus) that was developed from the NIH Visible Human Project for this purpose.

Four phases were defined, namely, (a) experimental strain (and stiffness and rigidity) characterization of structural properties of an existing humerus model, HS4 (Model 3404, Pacific Research Labs, USA), in four-point bending (under physiologic magnitude loads), (b) anatomic characterization of the Reference-Humerus model, and (c) development and experimental (four-point bending) validation of an FE model of the Reference-Humerus (under physiologic magnitude loads), followed by (d) study of strain characteristics of the humerus diaphysis under simplified physiologic loading, modeled using Deltoid and Supraspinatus action during shoulder abduction.

(a) The HS4 demonstrated linear mechanical behavior under physiologic magnitude loads. The bending stiffness, rigidity, and mean principal strain data pointed to a stiffer medio-lateral plane compared with the antero-posterior plane for this specimen. (b) The Reference-Humerus's measured osteoanatomic characteristics lay near/within respective ranges for cadaveric humeri, thus establishing anatomic validity. (c) Experimental validation of the Reference-Humerus FE model that incorporated the cortex-simulation material's experimentally-derived elastic modulus range established its validity for biomechanical applications. (d) Reference-Humerus FE modeling of simplified physiologic loading demonstrated changes in maximum and minimum principal strain magnitudes and distribution in the humerus diaphysis as a function of shoulder abduction, external load, and Supraspinatus weakness.

This dissertation provides novel insight into strain behavior of the humerus under loading as well as its surgical osteoanatomic characteristics. In addition, the anatomically characterized Reference-Humerus developed as part of this dissertation is a biomechanical tool with future biomechanical and research applications such as humeral fracture risk evaluation in musculoskeletal pathology, presurgical planning/surgical simulation, and implant design.