This study aimed at investigating the effects of static and dynamic compression applied on growth plate explants using matched compressive strains. Growth plate explants from 4-week-old swine ulnae were submitted to in vitro static (10% strain) or dynamic (oscillating between 7% and 13% at 0.1 Hz) unconfined compression for 48 h. The total growth plate height, the combined proliferative and hypertrophic thickness and the resulting ratio between these two thicknesses were evaluated. Standard immunohistochemistry was used to analyze the protein expression of key components of the extracellular matrix: aggrecan, type II collagen, type X collagen, and MMP13. In the statically loaded samples, the columnar organization of the cells was preserved but with slight columns deviation from the growth axis. Decreases in all histomorphological parameters were important and a notable loss of aggrecan, type II and type X collagens expressions was denoted. In the dynamically loaded samples, a severe loss of columnar arrangement was observed in the proliferative and hypertrophic zones. However, dynamic compressive loads preserved the proliferative and hypertrophic zones ratio and contributed to the synthesis of aggrecan and type II collagen in the extracellular matrix. The exact response of the growth plate to mechanical stresses along with optimal loading parameters could help improve the current treatment approaches or develop new treatment approaches for the underlying progressive musculoskeletal deformities. ß

Aging and degeneration are associated with changes in mechanical properties in the intervertebral disc, generating interest in the establishment of mechanical properties as early biomarkers for the degenerative cascade. Magnetic resonance elastography (MRE) of the intervertebral disc is usually limited to the nucleus pulposus, as the annulus fibrosus is stiffer and less hydrated. The objective of this work was to adapt high-frequency needle MRE to the characterization of the shear modulus of both the nucleus pulposus and annulus fibrosus. Bovine intervertebral discs were removed from fresh oxtails and characterized by needle MRE. The needle was inserted in the center of the disc and vibrations were generated by an amplified piezoelectric actuator. MRE acquisitions were performed on a 4.7-T small-animal MR scanner using a spin echo sequence with sinusoidal motion encoding gradients. Acquisitions were repeated over a frequency range of 1000-1800 Hz. The local frequency estimation inversion algorithm was used to compute the shear modulus. Stiffness maps allowed the visualization of the soft nucleus pulposus surrounded by the stiffer annulus fibrosus surrounded by the homogeneous gel. A significant difference in shear modulus between the nucleus pulposus and annulus fibrosus, and an increase in the shear modulus with excitation frequency, were observed, in agreement with the literature. This study demonstrates that global characterization of both the nucleus pulposus and annulus fibrosus of the intervertebral disc is possible with needle MRE using a preclinical magnetic resonance imaging (MRI) scanner. MRE can be a powerful method for the mapping of the complex properties of the intervertebral disc. The developed method could be adapted for in situ use by preserving adjacent vertebrae and puncturing the side of the intervertebral disc, thereby allowing an assessment of the contribution of osmotic pressure to the mechanical behavior of the intervertebral disc.