Static compression on growth plates

Static compressive loading reduces the mRNA expression of type II and X collagen in rat growth-plate chondrocytes during postnatal growth.

"This in vitro study investigated the effects of loading on the mRNA expression pattern of key molecular components of the growth-plate related to the extracellular matrix (type II and type X collagen) and the PTH-PTHrP feedback loop. Short-term static compressive loading was applied to rat proximal tibial growth-plate explants. Four age groups at specific developmental stages were investigated. The spatial variation in the mRNA expression was compared among loaded explants, their contralateral sham controls, and uncultured growth plates from normal animals. Basic cell metabolism (18S rRNA) was unaffected by load. Results indicated a narrower spatial distribution of mRNA expression of type II collagen throughout the growth plate; similarly, a narrowed distribution of expression of type X collagen was noted in the lower hypertrophic zone of the growth-plate. Mechanical compression influences chondrocytes of the hypertrophic zone to alter their expression of specific genes encoding proteins of the extracellular matrix, while PTH-PTHrP receptor mRNA, a regulatory protein, remained unaffected by loading. The effects of compression were similar at the different stages of growth, suggesting that additional factors may be involved in the clinical progression of skeletal deformities observed during growth spurts."

"Age group (days old) Body weight (g) Growth-plate thickness (μm) Static load (N) Plug area (mm2) Stress (kPa)

21 50.2 ± 5.0   741.9 ± 69.0 0.27 ± 0.03 22.4 ± 3.2 12.2 ± 1.3

35 143.3 ± 8.1 634.4 ± 76.6 0.77 ± 0.04 39.9 ± 3.9 19.5 ± 2.0

56 203.6 ± 9.4 470.9 ± 89.6 1.10 ± 0.05 44.1 ± 5.4 25.2 ± 3.4

80 251.8 ± 20.0 207.5 ± 20.5 1.36 ± 0.11 46.0 ± 5.2 29.8 ± 4.1"

Here's an image of the growth plates with and without load:

"load did not appreciably perturb the basic metabolism of chondrocytes and cell viability was preserved"

A study in vivo of the effects of a static compressive load on the proximal tibial physis in rabbits.

"Static compressive loads (10 N or 30 N) were applied for two or six weeks across one hind limb proximal tibial physis of thirteen-week-old female New Zealand White rabbits (n = 18). The contralateral hind limb in all rabbits underwent sham surgery with no load to serve as an internal control. Harvested physes were divided into portions.

Compared with unloaded shams, physes loaded at 10 N or 30 N for two weeks and at 10 N for six weeks showed histological changes in cells and matrices. Physes loaded at 30 N for six weeks were decreased in thickness and had structurally disorganized chondrocyte columns, a decreased extracellular matrix, and less intense type-II and X collagen immunohistochemical staining. Quantitative reverse transcription-polymerase chain reaction analysis of loaded samples compared with unloaded shams yielded a significantly decreased gene expression of aggrecan and type-II and X collagen and no significant changes in the matrix metalloprotease-13 gene expression with increasing load.

Compressed rabbit physes generate biochemical changes in collagens, proteoglycan, and cellular and tissue matrix architecture. Changes potentially weaken overall physeal strength, consistent with the Hueter-Volkmann principle, and lend understanding of the causes of pathological conditions of the physis."

"Compared with unloaded shams, six-week, 30-N-loaded physes were decreased in thickness and stained less intensely for collagen. The 30-N specimens also had structurally altered cells, lacunae, and chondrocyte columns (chondrons) as well as decreased extracellular matrix areas between cells and chondrons in proliferating and hypertrophic tissue zones. On enlargement compared with unloaded shams, structural changes in 30-N physes were marked by apparently fewer chondrocytes in all physeal regions, enlarged cell lacunae, irregularly aligned chondrons shortened in height, and disorganized cell arrangements within chondrons. The latter included individual chondrocytes in 30-N samples that varied in size and shape and failed to stack in an orderly, linear manner, one on the other, as did chondrons of unloaded shams"

This provides evidence that the growth plates physically push the bone apart as they are affected by load on it.  Although epiphyseal distraction helps the bones push apart and does not result in adult height gain.  Although epiphyseal distraction cannot help the growth plates with the ossification stage.

Chronic in vivo load alteration induces degenerative changes in the rat tibiofemoral joint.

"A varus loading device was attached to the left hind limb of mature rats to apply increased compression to the medial compartment and decreased compression to the lateral compartment of the tibiofemoral joint of either 0% or 100% body weight for 0, 6 or 20 weeks.

Increased compression in the medial compartment produced significant degenerative changes consistent with the development of osteoarthritis (OA) including a progressive decrease in cartilage aggregate modulus (43% and 77% at 6 and 20 weeks), diminished cellularity (38% and 51% at 6 and 20 weeks), and increased histological degeneration. At 20 weeks, medial compartment articular cartilage thickness decreased 30% while subchondral bone thickness increased 32%{consistent with endochondral ossification} and subchondral bone modulus increased 99%. Decreased compression in the lateral compartment increased calcified cartilage thickness, diminished region-specific subchondral bone thickness and revealed trends for reduced cellularity and decreased articular cartilage thickness at 20 weeks."

"Chronic increased load of 44% BW applied to the rabbit knee for 12 h/day over 12 weeks resulted in increased articular cartilage thickness and permeability with minimal fibrillation of the articular surfaces"

"9-month-old, male, Sprague–Dawley rats (weight: 666 ± 32 g)"


Strain-dependent recovery behavior of single chondrocytes.

"In this study, the compressibility and recovery behaviors of single chondrocytes were determined as a function of compressive strains from 6 to 63%. Bovine articular chondrocytes from the middle and deep zones were subjected to this range of strains, and digital videocapture was used to track changes in cell dimensions during and after compression. The normalized volume change, apparent Poisson's ratio, residual strain after recovery, cell volume fraction after recovery, and characteristic recovery time constant were analyzed with respect to axial strain. Normalized volume change varied as a function of strain, demonstrating that chondrocytes exhibited compressibility. The mean Poisson's ratio[the negative ratio of transerse to axial strain] of chondrocytes was found to be 0.29 +/- 0.14, and did not vary with axial strain{a value below 0.5 means the cell is compressible}. In contrast, residual strain, recovered volume fraction, and recovery time constant all depended on axial strain. The dependence of residual strain and recovered volume fraction on axial strain showed a change in behavior around 25-30% strain, opening up the possibility that this range of strains represents a critical value for chondrocytes."

"The average height and diameter of chondrocytes were found to be 8.4 ± 1.8μm and 11.1 ± 0.8μm, respectively. Chondrocytes were either elliptical (n = 16) or hemi-elliptical (n = 18) in appearance. All parameters determined in this study were insensitive to cell shape. The average cell volume was calculated to be 430 ± 251μm^3."

"the recovery behavior of single chondrocytes, but not their compressibility, depended on axial strain."

"At low strain levels (< 20%), cells recovered relatively quickly, and recovery was generally over 90% of the cell’s original height. At intermediate strains (20–30%), some cells exhibited similar recovery behavior, while others did not recover as quickly and showed significant residual strain"

"Above [25-30%] strain level, chondrocytes showed an impaired ability to recover, as evidenced by increased residual strain and decreased volume fraction after recovery, which differed significantly from chondrocytes compressed less than 25%. Below 25% axial strain, residual strain and recovered volume fraction showed weaker or non-existent dependence on axial strain. This suggests that 25–30% compressive strain may represent a threshold akin to the yield strain, above which some plastic deformation may occur."<-Maybe above 30% tensile strain produces similar changes.  Are these plastic changes good or bad for growth.

"Chondrocytes decreased in volume with increasing axial strain"<-could they increase in volume with increasing tensile strain?

"Though the individual components that comprise the cell, e.g., cytoskeletal filaments, cytoplasm, and plasma membrane, may be intrinsically incompressible, fluid exudation through the plasma membrane would result in a net volume loss."

It is important to note that these chondrocytes were not in the ECM.

The effects of mechanical loading on the mRNA expression of growth-plate cells.

"This in vitro study investigated the effects of mechanical loading on the mRNA expression pattern of key molecular components of the growth-plate. Short-term static loading was applied to rat proximal tibial growth-plate explants. Various age groups at specific developmental stages were investigated. In situ hybridization was used to assess the mRNA expression of the cells in different zones of the growth-plate. Four key components were investigated: 18s (basic cell metabolism), type II collagen (major extracellular matrix component), type X collagen (matrix component in hypertrophic zone) and PTH-PTHrP receptors (pre-hypertrophic chondrocytes). The spatial variation in the mRNA expression between loaded explants and their contralateral controls was compared to establish: -the sensitivity of the different growth-plate zones to mechanical loading; -the sensitivity of the different developmental stages to loading. Preliminary results indicated that static loading on the growth plate of 80 d.o. rats affects type II and X collagen gene expressions while PTH-PTHrP remains insensitive to static loading."

Couldn't get full study.