This study would only be relevant to people with existing growth plates as it only studies load effects on chondrocytes. But, regardless, since we are studying mechanical loading methods to study height growth any mechanical loading study may be relevant.
“The physis, or growth plate, is a complex disc-shaped cartilage structure that is responsible for longitudinal bone growth. In this study, a multi-scale computational approach was undertaken to better understand how physiological loads are experienced by chondrocytes embedded inside chondrons when subjected to moderate strain under instantaneous compressive loading of the growth plate. Models of representative samples of compressed bone/growth-plate/bone from a 0.67 mm thick 4-month old bovine proximal tibial physis were subjected to a prescribed displacement equal to 20% of the growth plate thickness. At the macroscale level, the applied compressive deformation resulted in an overall compressive strain across the proliferative-hypertrophic zone of 17%. The microscale model predicted that chondrocytes sustained compressive height strains of 12% and 6% in the proliferative and hypertrophic zones, respectively, in the interior regions of the plate. This pattern was reversed within the outer 300 μm region at the free surface where cells were compressed by 10% in the proliferative and 26% in the hypertrophic zones, in agreement with experimental observations. While the current model is relevant to fast dynamic events, such as heel strike in walking, we believe this approach provides new insight into the mechanical factors that regulate bone growth at the cell level and provides a basis for developing models to help interpret experimental results at varying time scales. ”
“The physis, or growth plate, is a complex disc-shaped cartilage structure that is responsible for longitudinal bone growth. This growth is modulated by many systemic and local factors including those arising from mechanical loading”
“Potential tissue level signals for modulating endochondral bone formation [include] hydrostatic and octahedral shear stresses and principal stresses”
“chondrocytes, are the active agents of growth and contribute to bone growth through cell proliferation, hypertrophy and extracellular matrix secretion in highly specialized, highly cellular and organized anatomic structures known as chondrons. It is likely that tissue level strains and stresses are experienced differently by chondrocytes embedded within such structures, possibly providing signals of varying intensity and type depending on location.”
“Mechanotransduction is more readily related to cell deformation than to tissue level stresses. In the context of bone growth, there is evidence that cell proliferation and differentiation can be regulated through activation of stretch-activated ion channels in the cell membrane following changes in cellular shape and size”
“Recent observational studies of physeal samples under compression have revealed regional variations in cellular strains and suggest that global physeal strains may be amplified at the cellular level”<-This statement may be applicable to other cells like the enthesis of ligament attachments or mesenchymal stem cells.
“The prescribed compressive displacement which was equivalent to 20% of the initial uncompressed combined initial thicknesses (0.67 mm) of RZ and P/H zones resulted in a -14.8% growth plate engineering strain across the combined layers of the reserve, proliferative and hypertrophic zones, while the combined proliferative and hypertrophic zone received -17.2% strain. The reserve zone experienced -9.5% and the calcified cartilage zone -3.3% compressive strain.”<-Effects on the reserve zone are interesting to us because those contain stem cells so would show any interesting effects on non-growth plate cells.
“Chondrons, the primary functional and structural units of the growth plate, deformed by buckling near the free surfaces where the cartilage bulged outward while remaining straight in the interior regions”
“chondrocytes in the interior of the growth plate undergo about 50% less strain than the prescribed overall growth plate strain.”
“Younger growth plates that are thicker (3 mm) also had thicker reserve zones, which may affect the transverse outward bulging at the free surface differently.”
“During compression of cells at short times scales (~0.5 s) intracellular water is redistributed within the cell[but not necessarily more compression]”
” At long time scales constant external pressure induces cell shrinkage, but cell shrinkage takes an order of magnitude (~10 s,) longer than the transient impact of heel strike or even the 0.2 s duration of the stance phase of a gait cycle and as long as an hour to reach equilibrium”
“growth plate stresses reached values of 0.1 to 0.6 MPa in compression during activities such as sitting. From the stress relaxation experiments on explants it may be estimated that 0.2 MPa corresponds to the stress levels attained after stress relaxation for displacements of the epiphysis equal to 20% of the initial growth plate thickness (defined as the combined thicknesses of the reserve, proliferative and hypertrophic zones).”
“Chondrons appear to be oriented along the minimum principal strain directions, becoming roughly perpendicular to the epiphyseal bone plate, whereas the hypertrophic portion along with the calcified cartilage bars and primary spongiosa align more with the primary compressive load direction along the tibial long axis”
Some interesting info but the significance is unclear yet.
