Vertebral trabecular bone mechanical properties vary among functional groups of cetaceans

Since their appearance in the fossil record 34 Mya, modern cetaceans (dolphins, whales, and porpoises) have radiated into diverse habitats circumglobally, developing vast phenotypic variations among species. Traits such as skeletal morphology and ecologically-linked behaviors denote swimming activity; trade-offs in flexibility and rigidity along the vertebral column determine patterns of caudal oscillation. Here, we categorized 10 species of cetaceans (Families Delphinidae and Kogiidae; N = 21 animals) into functional groups based on vertebral centra morphology, swimming speeds, diving behavior, and inferred swimming patterns. We quantified trabecular bone mechanical properties (yield strength, apparent stiffness, and resilience) among functional groups and regions of the vertebral column (thoracic, lumbar, and caudal). We extracted 6 mm3 samples from vertebral bodies and tested them in compression in three orientations (rostrocaudal, dorsoventral, and mediolateral) at 2 mm min−1. Overall, bone from the pre-fluke/fluke boundary had the greatest yield strength and resilience, indicating that the greatest forces are translated to the tail during caudal oscillatory swimming. Group 1, composed of five shallow-diving delphinid species, had the greatest vertebral trabecular bone yield strength, apparent stiffness, and resilience of all functional groups. Conversely, Group 3, composed of two deep-diving kogiid species, had the least strong, stiff, and resilient bone, while Group 2 (three deep-diving delphinid species) exhibited intermediate values. These data suggest that species that incorporate prolonged glides during deep descents in the water column actively swim less, and place relatively smaller loads on their vertebral columns, compared with species that execute shallower dives. We found that cetacean vertebral trabecular bone properties differed from the properties of terrestrial mammals; for every given bone strength, cetacean bone was less stiff by comparison. This relative lack of material rigidity within vertebral bone may be attributed to the non-weight bearing locomotor modes of fully aquatic mammals.

Separate and Combined Effects of Geometrical and Mechanical Properties Changes Due to Aging on the Femoral Strength in Men and Women

Aging, from 40 to +80 years old, causes geometrical and mechanical properties changes in the proximal femur. The subperiosteal width expands faster in men compared to women during aging, while the cortical thickness varies unequally in each sector and differently between men and women. Another change which occurs during aging is bone mechanical properties such as stiffness and ultimate strains. Numerical analysis allows us to study the potential effects of each of the age-dependent changes on the fracture forces separately and combined. We investigated the effects of the geometrical and bone mechanical properties changes due to aging on the femoral strength during a common falling scenario using a transverse isotropic continuum damage model. First, the femur model was adapted from a previously developed human body model named THUMS v4.02. Then, three sets of models were developed to address each of the changes separately and combined for both sexes. We found that the fracture forces in women are on average 1500 N less than in men of the same age. The age-dependent geometrical changes increased the fracture forces in men (25 N/decade), whereas it reduced the fracture forces by 116 N/decade in women. The mechanical properties changes reduced the fracture forces in men more than in women (354.5 N/ decade vs. 225.4 N/decade). When accounting for both geometrical and mechanical properties changes due to aging, the fracture forces decreased by 10.7% of the baseline in women per decade compared to 7.2% per decade in men.

Characterization of Baboon Cortical Bone Microstructural Changes by Low Field NMR and Correlation of Bone Mechanical Properties

A NMR spin-spin (T2) relaxation technique has been described for determining the porosity, mobile and the bound water distribution in baboon cortical bone and correlate to their mechanical properties. The technique of low-field proton NMR involves spin-spin relaxation and free induction decay (FID) measurements, and the computational inversion methods for decay data analysis. The advantages of using NMR T2 relaxation techniques for bone water distribution are illustrated. The CPMG T2 relaxation data can be inverted to T2 relaxation distribution and this distribution then can be transformed to a pore size distribution with the longer relaxation times corresponding to larger pores. The FID T2 relaxation data can be inverted to T2 relaxation distribution and this distribution then can be transformed to bound and mobile water distribution with the longest relaxation time corresponding to mobile water and the middle relaxation time corresponding to bound water. The technique is applied to quantify apparent changes in porosity, bound and mobile water in cortical bone. Overall bone porosity is determined using the calibrated NMR fluid volume from the proton relaxation data divided by overall bone volume. The NMR porosity, bound and mobile water components are determined from cortical bone specimens obtained from baboon donors of different ages, and the results are correlated to bone mechanical properties.