Biomechanical Measurements of Stiffness and Strength for Five Types of Whole Human and Artificial Humeri

`The human humerus is the third largest longbone and experiences 2-3% of all fractures. Yet, almost no data exist on its intact biomechanical properties, thus preventing researchers from obtaining a full understanding of humerus behavior during injury and after being repaired with fracture plates and nails. The aim of this experimental study was to compare the biomechanical stiffness and strength of "gold standard" fresh-frozen humeri to a variety of humerus models. A series of five types of intact whole humeri were obtained: human fresh-frozen (n = 19); human embalmed (n = 18); human dried (n = 15); artificial "normal" (n = 12); and artificial "osteoporotic" (n = 12). Humeri were tested under "real world" clinical loading modes for shear stiffness, torsional stiffness, cantilever bending stiffness, and cantilever bending strength. After removing geometric effects, fresh-frozen results were 585.8 +/- 181.5 N/mm(2) (normalized shear stiffness); 3.1 +/- 1.1 N/(mm(2) deg) (normalized torsional stiffness); 850.8 +/- 347.9 N/mm(2) (normalized cantilever stiffness); and 8.3 +/- 2.7 N/mm(2) (normalized cantilever strength). Compared to fresh-frozen values, statistical equivalence (p >= 0.05) was obtained for all four test modes (embalmed humeri), 1 of 4 test modes (dried humeri), 1 of 4 test modes (artificial "normal" humeri), and 1 of 4 test modes (artificial "osteoporotic" humeri). Age and bone mineral density versus experimental results had Pearson linear correlations ranging from R = -0.57 to 0.80. About 77% of human humeri failed via a transverse or oblique distal shaft fracture, whilst 88% of artificial humeri failed with a mixed transverse+oblique fracture. To date, this is the most comprehensive study on the biomechanics of intact human and artificial humeri and can assist researchers to choose an alternate humerus model that can substitute for fresh-frozen humeri.