Lateral Synovial Joint Loading: How could they miss the height gain before?

Why didn't scientists see the possibilities earlier of height gain from lateral synovial joint loading?  Height gain was only alluded to with possible effects on the chondrocytes in the growth plate mentioned.

Joint loading-driven bone formation and signaling pathways predicted from genome-wide expression profiles. 

"Joint loading is a loading modality that induces anabolic responses by lateral loads applied to a synovial joint such as an elbow and a knee. The present study extended this loading modality to an ankle and addressed a question: does ankle loading promote bone formation in the tibia? If so, what signaling pathways are involved in the anabolic responses? Using C57BL/6 female mice as a model system, lateral loads of 0.5 N were applied to the ankle at 5 Hz for 3 min/day for 3 consecutive days and load-driven bone formation was evaluated at three tibial cross-sections (the proximal, middle, and distal diaphysis). [rNA was analyzed]. In all cross-sections ankle loading elevated the cortical area and thickness as well as the calcein-labeled surface. Signaling pathway analysis from microarray-derived whole-genome mRNA expression profiles and quantitative real-time PCR predicted that molecules in phosphoinositide 3-kinase (PI3K), ECM-receptor interactions, TGFbeta signaling, and Wnt signaling were involved in the joint-loading driven responses."

"The loading rod and the stator were in contact with the distal end of the tibia (lateral and medial malleoli), and 0.5 N force (peak-to-peak) was applied at 5 Hz"<-This is the same as the 5Hz used in the knee lengthening study.  A higher or lower frequency could induce different gene expression.

Both the ECM-receptor interacts and TGF Beta signaling suggests height increase possibilities via the growth plate at minimum.  ECM(Extracellular Matrix) helps height growth by increasing cell space.  TGF-Beta encourages chondrogenic differentiation of stem cells.

According to figure 7, BMP-2 was upregulated and BMP-2 IS pro-chondrogenic although also osteogenic as well.  According to figure 8 the endosteal bone formation was highest in the furthest part of the bone.  Since the endosteum is within the bone we can speculate that LSJL will induce chondrogenic differentiation most in the arrow farthest away the loading site.  So if you load the knee, chondrogenic differentiation will be highest in the ankle.

Osteogenic potentials with joint-loading modality. 

"Osteogenic potentials with a novel joint-loading modality were examined, using mouse ulnae as a model system. Load-induced deformation of rigid bone is known to generate interstitial fluid flow and stimulate osteogenesis. However, in most of the previous studies, loads were applied to cortical bone. In the current study, we addressed the question of whether deformation of the epiphysis underneath the joint would enhance bone formation in the epiphysis{New bone formation in the epiphysis can increase height} and the diaphysis. In order to answer the question, we applied lateral loads to a mouse elbow and conducted a bone histomorphometric analysis, as well as measurements of strains and streaming potentials. Compared to the no-loading control, 0.5-N loads, applied to the elbow at 2 Hz for 3 min/day for 3 consecutive days, increased the mineralizing surface (two- to threefold), the rate of mineral apposition (three- to fivefold), and the rate of bone formation (six- to eightfold) in the ulna. Strain measurements indicated that strains of around 30 microstrain}30 microstrain is extremely low according to mechanostat theory}, induced with the joint-loading modality, were under the minimum effective strain of around 1000 microstrain, which is considered necessary to achieve strain-driven bone formation. To evaluate the induction of fluid flow with the joint-loading modality, streaming potentials were measured in separate experiments, using mouse femurs ex vivo. The streaming potentials correlated to the magnitude of the load applied to the epiphysis, as well as the flow speed in the medullary cavity. Taken together, the findings of the current study support the idea of joint-loading driven osteogenesis, through a mechanism that involves the induction of fluid flow in cortical bone." 

Streaming potential is a measure of the pressure gradient of an electrolyte traveling through a channel or porous plug with charged walls(the channel is bone).  

In the study "Potential applications of pulsating joint loading in sports medicine" the authors speculate "This is probably because PJL[pulsating joint loading] effectively induces a pressure gradient throughout a long bone and this gradient establishes a remote communication link between the site of loading in the epiphysis and the site of bone formation in the metaphysis as well as in the diaphysis."   That study further states that "Several cell types are likely involved in the PJL distance effects: chondrocytes in the growth plate, bone marrow-derived cells in the bone cavity, osteoblasts and osteoclasts together with cells in muscles, and the vascular system."

 

Since lateral synovial joint loading can influence chondrocytes in the growth plate, it is logical to think that it can also influence the stem cells in the surrounding bone marrow. 

As for the mechanisms of how lateral synovial joint loading affects the bone:

"First, mitogen-activated protein kinases (MAPKs) such as extracellular signal-regulated kinase 1/2 (ERK1/2)[ERK 1/2 may be a way that myostatin negatively regulates muscle and likely bone growth], p38 MAPK[MAPK plays a role in chondrogenesis], and Jun N-terminal kinase (JNK) are activated in osteoblasts in response to shear stress induced by fluid flow. Osterix, one of the critical transcription factors for bone development is induced by p38 MAP kinase. Furthermore, upregulation of type I collagen is mediated by ERK1/2 and JNK[type I collagen is the elastic part of bone]. Consistent with the predicted role of interstitial fluid flow, the Wnt/β-catenin pathway in osteocytes has been proposed to mediate load-driven bone formation by crosstalk with the prostaglandin pathway and a suppression of negative regulators such as Sclerostin (Sost) and Dickkopf 1(Dkk1).

Second, expression of peroxisome proliferator-activated receptor gamma (PPARγ<-PPAR-gamma is bad, senescence accelerated mice showed high levels of PPAR-lambda mRNA), is downregulated by mechanical stimulation[Since PPAR-gamma is present in high levels in senescent mice downregulating PPAR-gamma may increase height]. PPARγ is a stimulator of adipocyte proliferation and a suppressor of differentiation of osteoblasts. Therefore, its down regulation seems to favor promotion of osteogenesis over adipogenesis. Our recent in vitro experiment using mouse C57BL/6 (MC3T3 E1) osteoblast-like cells, as well as primary mesenchymal stem cells isolated from mice, shows that fluid flow at 10 dyn/cm² for 1h significantly reduces the messenger ribonucleic acid (mRNA) level of PPARγ.

Expression of bone morphogenetic proteins (BMPs) and insulin-like growth factors (IGFs) are elevated in response to mechanical loading. The molecules are anabolic and their expression is at least in part mediated by transforming growth factor β (TGFβ) and/or phosphoinositide 3-kinase (PI3K) pathways. Taken together, molecular signaling pathways, which are potentially involved in PJL, include MAPK, PPAR, BMPs, IGFs, TGFβ, and PI3K[BMP-2 and TGF-Beta are two compounds that can induce chondrogenesis and hydrostatic pressure may be able to do so on it's on without any chemical mediation]."

"Flow in the medullary cavity is thought to be induced in response to a bone wound that acts as a pressure sink. This flow is assumed to promote migration of bone-marrow derived cells including hematopoietic stem cells, mesenchymal stem cells, and endothelial progenitor cells. Their load-driven recruitment towards the site of the wound is proposed as the basis of accelerated healing."  <-it's likely that much the same occurs in the epiphysis causing chondrogenesis allowing for height growth

"First, trabecular bone in the epiphysis is less stiff in the lateral direction than in the axial direction and, therefore, lateral loads to the elbow may effectively deform the epiphysis of the ulna.
Deformation of the epiphysis may then induce fluid flow in the ulnar diaphysis in cortical bone, and load-induced fluid flow may enhance bone formation in the epiphysis and the diaphysis."

14 week old mice were used.  "3min per day for 3 consecutive days. The loading force was sinusoidal, at 2Hz, with a peak-to-peak amplitude of 0.5N."

"The measured intramedullary streaming potential (f1, in mV) correlated to the magnitude of the applied force (Fig. 5A), according to the equation: f1 = 7.3 ¥ F (r2 = 0.92)"

"These data indicate that the magnitude of the streaming potential in the medullary cavity is proportional to the lateral load applied to the joint and the speed of fluid flow."

I think the reason that scientists missed the height gain applications is that they were looking for a mechanical signaling way of how lateral synovial bone joint loading improves bone growth rather than looking at it's effect on stem cells or chondrocyte proliferation.  The reason they investigated height gain in the first place was they saw increased chondrocyte proliferation in the growth plate. 

It seems unlikely that increased fluid flow in the diaphysis can increase height unless somehow the pressure gets to be so much that the bone expands like a balloon.