Wednesday, March 17, 2010

Compression versus Distraction in Growth Plates

Many have wondered whether weight lifting can stunt growth.  Now it's logical to think that an immature cell type like a growth plate chondrocyte would be less compression resistant than a mature osteon.  Intervertebral discs are made up of chondrocytes and can compress radically during the day.  It'd be extremely difficult to compress the osteons within the bone themselves.  More likely the bone would bend.  It's unlikely that the bone would compress from physiologically normal loads so we're only looking at compression in terms of the growth plate chondrocytes. 

Enlargement of growth plate chondrocytes modulated by sustained mechanical loading.

"Compression or distraction force of nominally 60% of body weight was maintained for four weeks on a caudad vertebra of growing rats by an external apparatus attached, by means of transcutaneous pins, to the two vertebrae cephalad and caudad to it. Growth of the loaded and control vertebrae was measured radiographically. After four weeks, the animals were killed and histological sections of the loaded and control vertebrae were prepared to measure the height of the hypertrophic zone (average separation between zonal boundaries), the mean height of hypertrophic chondrocytes, and the amount of increase in cell height in the growth direction. Over the four weeks of the experiment, the growth rates of the compressed and distracted vertebrae averaged 52% and 113% of the control rates, respectively. The reduction in the growth rate of the compressed vertebrae was significant (p = 0.002). In the compressed vertebrae, the height of the hypertrophic zone, the mean chondrocyte height, and the amount of increase in cell height averaged 87%, 85%, and 78% of the control values, respectively, and all were significantly less than the corresponding control values. In the distracted vertebrae, these measurements did not differ significantly from the control values. The height of the hypertrophic zone and the mean chondrocyte height correlated with the growth rate (r (2) = 0.29 [p = 0.03] and r (2) = 0.23 [p = 0.06], respectively), when each variable was expressed as a proportion of the control value. The percentage changes in the measurements of the chondrocytic dimensions relative to the control values were smaller than the percentage changes in the growth rates, a finding that suggested that the rate of chondrocytic proliferation was also modulated by the mechanical loading. Mechanical loading of tail vertebrae in rats modulated their growth rate, which correlated with changes in the height of hypertrophic chondrocytes. The effects of compression were greater than those of distraction" 

"the distracted and compressed vertebrae grew at an average (and standard deviation) of 74.9 ± 20.5 and 35.4 ±14.3 µm/day, respectively"  Control grew at "67.3 ± 18.4 µm/day".

"three patients with idiopathic scoliosis who had been treated with long-term immobilization in a plaster cast had increased vertebral height and thinner discs."

"a "vicious circle" develops in the progression of scoliotic deformity. Assuming that the concave side of a curve is subjected to greater compression load, he proposed that this decelerates growth of the vertebral bodies compared with that on the convex side."

The loading in this study was continuous.  Intermittent compressive loading may have different effects.

It's unknown whether the hypertrophic chondrocytes ossify at their smaller state or if the hypertrophic chondrocytes undergo a catch up growth phase.  If the hypertrophic chondrocytes do ossify at 87% of their size then compression would reduce height.  If the hypertrophic chondrocytes ossified at 113% then the distraction would increase height.  There's definitely catch up growth for chondrocyte proliferation but is there catch up growth for chondrocyte hypertrophy?

This study suggests that compression growth plate trauma is permanent:

Weight loading young chicks inhibits bone elongation and promotes growth plate ossification and vascularization

"In this work, chicks were loaded with bags weighing 10% of their body weight during their rapid growth phase. The increased load reduced the length and diameter of the long bones. The average width of the bag-loaded group's growth plates was 75 +/- 4% that of the controls, and the plates showed increased mineralization. Northern blot analysis, in situ hybridization, and longitudinal cell counting of mechanically loaded growth plates showed narrowed expression zones of collagen types II and X compared with controls, with no differences between the relative proportions of those areas. An increase in osteopontin (OPN) expression with loading was most pronounced at the bone-cartilage interface. This extended expression overlapped with tartarate-resistant acid phosphatase staining and with the front of the mineralized matrix in the chondro-osseous junction. Moreover, weight loading enhanced the penetration of blood vessels into the growth plates and enhanced the gene expression of the matrix metalloproteinases MMP9 and MMP13 in those growth plates. the mechanical strain on the chondrocytes in the growth plate causes overexpression of OPN, MMP9, and MMP13. The MMPs enable penetration of the blood vessels, which carry osteoclasts and osteoblasts. OPN recruits the osteoclasts to the cartilage-bone border, thus accelerating cartilage resorption in this zone and subsequent ossification which, in turn, contributes to the observed phenotype of narrower growth plate and shorter bones."

"the Bag groups had lower BWs as a result of reduced feed consumption rather than energy loss or other metabolic problems"<- so that could've been the cause of the height reduction.

"Weight loading the chick did not significantly alter the expression levels of Col II, a characteristic marker for proliferating chondrocytes, or Col X, which is typical of differentiated chondrocytes"

Note that they only measured at the end of four days so there was no opportunity to measure the catch-up growth phenomena.  The growth plate height did go down significantly in the 4 day bag loading group, both the hypertrophic and the proliferative cell count decreased.  The ratios remained the same in groups meaning that the compressive forces likely only had an effect on growth rate.  Weight loading increases MMP-13 which is bad for height growth. However, there are MMP-13 inhibitors available that can counteract the MMP-13 negative effects.

Response of the growth plate to distraction close to skeletal maturity. Is fracture necessary?

"In the patient study, both slow distraction rates and low constant distraction loads were applied. For all the distraction regimens, it was not possible to lengthen the limb significantly without evidence of fracture as demonstrated by a sudden decrease in distraction force. Growth plate failure was observed from 600 to 800 N, these levels being lower than those recorded from the in vitro tests. In the animal study, three distraction regimens (0.13, 0.26, and 0.53 mm/day) were applied across the upper tibial growth plate of New Zealand white rabbits close to skeletal maturity. Distraction was applied and force measured using a strain-gauge dual-frame external fixator. The force-time results revealed two distinct patterns. One pattern, in which the forces rapidly increased to maximum values of approximately 25 N and then suddenly decreased, indicated fracture of the growth plate, which was confirmed histologically. In the other pattern, forces increased steadily throughout distraction, reaching maximum values at the end of distraction of approximately 16 N. Histologic observations indicated hyperplasia of the growth plate without fracture, however, only a small increase in limb length was detectable. Hence, if a significant increase in leg length is required close to skeletal maturity, then fracture of the growth plate must occur."

Note this is close to skeletal maturity.  Distraction if it increases height is likely to do so by increasing peak chondrocyte hypertrophy(which by the previous study is about 113%) so you'd likely see a 13% increase in height.  However, since it was so close to the end of development that 13% seemed small so we really don't know how much distraction(without fracture, at about 16N) can increase height overall.  It is something worth exploring.

Here's a study involving CH Turner that showed that even though bone loading reduced growth rate.  The growth plates greatly increased in thickness and size.

Modulation of appositional and longitudinal bone growth in the rat ulna by applied static and dynamic force.

"Using the noninvasive rat ulna loading model, we tested the hypothesis that brief-duration (10 min/day) static loads have an inhibitory effect on appositional bone formation in the middiaphysis of growing rat ulnae. Several reports have shown that ulnar loading, when applied to growing rats, results in suppressed longitudinal growth. We tested a second hypothesis that load-induced longitudinal growth suppression in the growing rat ulna is proportional to time-averaged load, and that growth plate dimensions and chondrocyte populations are reduced in the loaded limbs. Growing male rats were divided into one of three groups receiving daily 10 min bouts of static loading at 17 N, static loading at 8.5 N, or dynamic loading at 17 N. Periosteal bone formation rates, measured 3 mm distal to the ulnar midshaft, were suppressed significantly (by 28-41%) by the brief static loading sessions despite normal (dynamic) limb use between the daily loading bouts. Static loading neither suppressed nor enhanced endocortical bone formation. Dynamic loading increased osteogenesis significantly on both surfaces. At the end of the 2 week loading experiment, loaded ulnae were approximately 4% shorter than the contralateral controls in the 17 N static and dynamic groups, and approximately 2% shorter than the control side in the 8.5 N static group, suggesting that growth suppression was proportional to peak load magnitude, regardless of whether the load was static or dynamic. The suppressed growth in loaded limbs was associated with thicker distal growth plates, particularly in the hypertrophic zone, and a concurrent retention of hypertrophic cell lacunae[so the loading may increase final growth overall]. Negligible effects were observed in the proximal growth plate. The results demonstrate that, in growing animals, even short periods of static loading can significantly suppress appositional growth; that dynamic loads trigger the adaptive response in bone; and that longitudinal growth suppression resulting from compressive end-loads is proportional to load magnitude[so if you want to grow taller then heaviest load matters more than average load used] and not average load."

Note axial loading rather than lateral loading was used in this study.

"In the growing male Sprague-Dawley rat of this body mass, 17 N elicits a compressive strain of approximately 3500 με on the medial surface of the ulnar midshaft, which is greater than peak compressive strains normally encountered during unrestricted running (1200 με) or jumping down from a 30 cm platform (2500 με). Putting these strains into a human physiology context, peak compressive strains in the adult human tibial shaft during vigorous exercise reach approximately 1200 με"<-So this load is almost 3 times as much as that during normal running.

Look at how much thicker the 17N loaded growth plate is.  "This difference was also reflected in the normalized number of hypertrophic cell lacunae, which were nearly twice as numerous in the loaded side in both 17 N groups"  So the 17N loaded mice had about 4% less growth but about twice as many hypertrophic growth plate lacunae.

"The longitudinal growth suppression observed in the two 17 N groups was associated with a number of changes in the distal ulnar growth plate, including an increase in distal growth plate height—particularly at the hypertrophic zone, and a concomitant accumulation of hypertrophic cell lacunae. The inverse association between longitudinal growth (ulna length) and growth plate height found in this study was unexpected and is inconsistent with a number of other in vivo loading studies that have shown a positive, proportional association between growth plate height and longitudinal growth rate; i.e., shorter bones have been associated with thinner growth plates and vice versa[thicker growth plates are associated with longer bones]A few in vivo loading studies have reported thicker growth plates associated with shorter long bones in overloaded limbs, but these effects were associated with a diagnosis of dyschondroplasia (osteochondrosis).  We showed in a subsequent study, in which rats were sacrificed immediately after the last loading session, that the enlarged growth plates observed in this study were not the result of an accelerated growth reaction, or “growth recovery,” during the 4 day period between cessation of loading and sacrifice. The growth suppression observed in loaded limbs of the rats in our study could have been the effect of damage to the tissue; however, the lower load group (8.5 N) exhibited growth suppression as a result of loads normally encountered during natural ambulation in vivo. It remains unclear whether the changes in growth plate activity and longitudinal growth were the result of mechanical adaptation or a response to damage."<-So the increase in growth plate size is not merely a result of catch-up growth but could potentially be a permanent adaptation and thereby a permanent increase in height.

So heavy axial loading could be a way to grow taller.  Greater than jumping off a 30cm(66 inches) box loading for example.

Altered Cellular Kinetics in Growth Plate according to Alterations in Weight Bearing

"Unloading condition on the hind-limb of Sprague-Dawley rats was created by fixing a tail and lifting the hind-limb. Six rats aged 6 weeks old were assigned to each group of unloading, reloading, and control groups of unloading or reloading. Unloading was maintained for three weeks, and then reloading was applied for another one week thereafter. Histomorphometry for the assessment of vertical length of the growth plate, 5-bromo-2'-deoxyuridin immunohistochemistry for cellular kinetics, and biotin nick end labeling transferase-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL) assay for chondrocytes apoptosis in the growth plate were performed.
The vertical length of the growth plate and the proliferative potential of chondrocytes were decreased in the unloading group compared to those of control groups. Inter-group differences were more significant in the proliferative and hypertrophic zones. Reloading increased the length of growth plate and proliferative potential of chondrocytes. Apoptotic changes in the growth plate were not affected by the alterations of weight bearing."

"To create the unloading state by lifting the lower extremities of rats, tygon tubing was fixed to the dorsal skin, and subsequently, the tail was immobilized using a tape. The tubing connected to the rat was connected to the restraint equipment of the lower-limb suspension equipment, and the angle was always maintained constant. The animals were thus free to move in a 360° arc. The rats were suspended with 30° head-down tilt in order to initiate a fluid shift similar to that experienced during space flight. This method of suspension resulted in a total loss of weight-bearing function in the hind limbs. The fore limbs were in contact with the plastic grid floor of the model, but some loss of weightbearing might have occurred."

"The length of the growth plate of the suspended group was shorter than that control group because of the decrease in the length of the proliferating and hypertrophic zones, while the resting zone were not changed. After reloading, we observed that the length of the hypertrophic zone was recovered, while the proliferating zone didn't."

Growth plate explants respond differently to in vitro static and dynamic loadings.

"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."

Here's the growth plates under static and dynamic loading.  Control distal ulna growth plates were actually thicker than dynamicly loaded growth plates.

"Cyclic intermittent tension applied on the maxillae cranial base growth plate of growing rabbits resulted in significantly greater maximum growth plate height and in vitro cyclic loading applied on cranial base growth plate explants caused a significant increase in the growth plate height."

"Although the Hueter–Volkman principle stipulates that sustained static compressive forces retard bone growth, it does not stipulates on the effects of dynamic loading. It is conceivable that dynamic compressive forces could increase chondrocyte proliferation and stimulate bone growth, in the same way as a minimum compressive activity is required for normal bone development. Under unconfined compression, radial expansion was unconstrained, and free fluid flow was enabled across the cylindrical boundary surface of the biphasic tissue. As opposed to static loading, dynamic compression could have contributed to increase the fluid flow movement within the growth plate explants, and this in-and-out fluid flow could be related to chondrocytes columnar disorganization. Furthermore, it is conceivable that increased content of aggrecan and collagens in the reserve zone brings rigidity to the tissue and may contribute to preservation of the proliferative and hypertrophic zones ratio."

Expression of matrix metalloproteinases in human growth plate chondrocytes is enhanced at high levels of mechanical loading: A possible explanation for overuse injuries in children.

"Human growth plate chondrocytes were subjected to mechanical forces equal to either physiological loads, near detrimental or detrimental loads for two hours. In addition, these cells were exposed to physiological loads for up to 24 hours. Changes in the expression of MMPs -2, -3 and -13 were investigated. Expression of MMPs in cultured human growth plate chondrocytes increases in a linear manner with increased duration and intensity of loading. Physiological loads have the same effect on growth plate chondrocytes over a long period of time as detrimental loads applied for a short period."

"Samples of the infant growth plate were obtained from the supernumerary digit of six children (three boys and three girls) with polydactylism[extra fingers or toes] at the time of excision"

Before loading cells were of polygonal state:

"Mechanical loading induced changes in both the orientation and the morphology of the chondrocytes, which were linearly dependent on the duration and intensity of loading. The cells aligned in clusters and lined perpendicularly to the direction of the force{this may be one of the reasons that lateral loading can increase height as it causes the chondrocytes to align more columnar}. The morphology of the cells changed from a polygonal to a more spindle-like shape with enhanced networking. Non-loaded cells did not change morphologically throughout the experiments."<-If LSJL does not put any load directly on the growth plate chondrocytes(LSJL loads more the epiphysis and articular cartilage) this should not occur.

"MMP-2 and MMP-13 showed a similar pattern of expression, with a significantly increased expression after 24 hours of loading and when the loading frequency was 4 Hz"<-LSJL upregulated MMP2 and the loading was 5Hz.

"Expression of MMP-3 was initially reduced after one hour of physiological loading, but then increased with a significantly higher expression after 24 hours of loading compared with controls"<-MMP3 was upregulated 1 hour after the last loading and 49 hours after the first loading during LSJL.

"In vivo loading of 1 Hz corresponds to a normal walking frequency, 2 Hz to a high walking frequency, 3 Hz to normal running frequency and 4 Hz to high running frequency or cycling."

Chondrocytes in response to load in this experiment:

 Fig. 2
This seems to differ from the LSJL histology.  In LSJL the cells do not seem more aligned in clusters and perpendicularly to the force.  There also does not seem to be a difference in shape between the normal and contralateral growth plate.

Physeal cartilage exhibits rapid consolidation and recovery in intact knees that are physiologically loaded

"A 4.7T MRI scanner continuously scanned a knee joint in the sagittal plane through the central load-bearing region of the medial compartment every 2.5 min while a realistic cyclic loading was applied. A custom auto-segmentation program was developed to delineate complex physeal cartilage boundaries. Physeal volume changes at each time step were calculated. The new auto-segmentation was found to be reproducible with COV of the volume measurements being less than 0.5%. Time-constants of physeal cartilage consolidation (1.31±0.74 min) and recovery (1.63±0.70 min) were significantly smaller than the values (5.53±1.78/17.71±13.88 min for consolidation/recovery) in articular cartilage. The rapid consolidation and recovery of physeal cartilage may due to a relatively free metaphyseal fluid boundary which would allow rapid fluid exchange with the adjacent cancellous bone{this wouldn't be the case for LSJL-induced ectopic growth plates}. This may impair the generation of hydrostatic pressure in the cartilage matrix when the physis is under chronic compressive loading, and may be related to the premature ossification of the growth plate under such conditions."

Maybe lateral loading alters the generation of hydrostratic pressure?

" a tennis player's playing arm, which is exposed to repeated tensile loads through swinging activities, exhibits extended bone length compared with the contralateral side"

This was the device used:
Full-size image (44 K)
It's unclear how lateral this loading modality is.  It seems like it could in fact be considered lateral loading in comparison to some of the methods elucidated in the LSJL patent.

"Skeletally immature sheep hind limbs (approximately 2 years old) were held at a 45 degree flexion angle to simulate the normal standing position"

Full-size image (64 K)
"Example of cartilage segmentation. (a) Articular surface and subchondral bone interface for the articular cartilage, and (b) two boundaries of growth plate with surrounding bones."

"The articular and physeal cartilage continuously deformed over the entire experiment."

Volume decreased after compression and then returned to normal after time with some possible over compensation.

"The epiphyseal boundary of the physis has dense subchondral bone which is similar to the dense subchondral bone of the articular cartilage, but the zone of cartilage calcification and endochondral ossification at the metaphyseal boundary of the physis is full of open marrow spaces and vascular channels."

Cell Proliferation is promoted by Compressive Stress during early stage of chondrogenic differentiation of rat BMSCs.

"dynamic compression increased cell quantity and viability remarkably in the early stage of chondrogenesis, during which the expression of Ihh, Cyclin D1, CDK4 and Col2α1{up} were enhanced significantly. Possible signal pathways implicated in the process were explored in our study. MEK/ERK and p38 MAPK were not found to function in this process while BMP signaling seemed to play an important role in the mechanotransduction during chondrogenic proliferation."

"The ERK pathway may be a negative regulator of Sox9 mRNA expression in hydrostatic pressure-induced mechanotransduction"

Cyclin D1 controls progression through the cell cycle and complexes with CDK4.

The dynamic compression was at 0.5Hz at 10-40kPa.

Developmental biomechanics. Effect of forces on the growth, development, and maintenance of the human body.

Clubfoot(feet beat outward or inward) and tibial torsion are examples of how mechanical forces can shape growth.

"Generally, chondrogenesis occurs with intermittent loading, and osteogenesis occurs with continuous loading."

"With continuous compression in a constant direction, bones become connected by bars of cartilage that act as shock absorbers and as a flexing system. A synchondrosis is an example."

"With intermittent compression and a range of movement between bones, articular cartilage is formed. A pathological example is cartilage forming in a pseudarthrosis.
With a decreasing magnitude of intermittent compression and an increasing amount of tension, symphyses are formed.
With intermittent loads of tension and compression of equal magnitudes along with sliding, condylar cartilage develops.
With increasing intermittent tension, sutures are formed such as joints in the skull.
With continuous tension in one direction, thick collagenous tissue develops into tendons and fascia."

"An increase in compression or tension outside the normal range of loading inhibits chondrocyte mitosis and results in retarded or halted growth in length."<-however an increase in compression or tension below this range seems to stimulate growth.

"An increased compression on one side of an epiphysis may slow growth on that side, while the normal proliferation of cells is occurring on the other side."<-Is this what's occurring in the LSJL growth plates?  With the growth plates extending out in the central direction?

"A load applied to an epiphyseal plate that is not parallel to the direction of growth (perpendicular
to the epiphyseal plate) will deflect the growth along the line of the deforming force."

"As long as the load is maintained, new growth will be deflected in the direction of the load resulting in a lateral displacement of the epiphysis."

"a torsional load will lead to a rotational deflection of the growth columns around the circumference of the epiphyseal plate. Newly formed bone will grow away from the epiphysis in a spiral pattern, which gives a torsional change."

Intermittent compression on cartilage thickens it whereas sustained, static compression thins it.

In vivo cyclic compression causes cartilage degeneration and subchondral bone changes in mouse tibiae.

"Cyclic compression at peak loads of 4.5N and 9.0N was applied to the left tibial knee joint of adult (26-week-old) C57BL/6 male mice for 1, 2, and 6 weeks. Only 9.0N loading was utilized in young (10-week-old) mice.
Mechanical loading promoted cartilage damage in both age groups of mice, and the severity of joint damage increased with longer duration of loading. Metaphyseal bone mass increased with loading in young mice, but not in adult mice, whereas epiphyseal cancellous bone mass decreased with loading in both young and adult mice. In both age groups, articular cartilage thickness decreased, and subchondral cortical bone thickness increased in the posterior tibial plateau. Mice in both age groups developed periarticular osteophytes at the tibial plateau in response to the 9.0N load, but no osteophyte formation occurred in adult mice subjected to 4.5N peak loading.{seems like cyclic compressive loading enhanced endochondral ossification}
This noninvasive loading model permits dissection of temporal and topographic changes in cartilage and bone and will enable investigation of the efficacy of treatment interventions targeting joint biomechanics or biologic events that promote OA onset and progression."

"a load level of 9.0N generates a strain of 1,200 με on the tibial mid-shaft of 10-week-old mice. In vivo tibial loading was applied for 1,200 cycles at 4 Hz for 5 days per week at each peak load."

Note though that in A posterior the cartilage seems to penetrate the bone.  However that is young mice.

The effect of mechanical stretch stress on the differentiation and apoptosis of human growth plate chondrocytes.

"The study is aimed to investigate the effect of stretch stress with different intensities on the differentiation and apoptosis of human plate chondrocytes. In the present study, the human epiphyseal plate chondrocytes were isolated and cultured in vitro. Toluidine blue staining and type II collagen immunohistochemical staining were used to identify the chondrocytes. Mechanical stretch stresses with different intensities were applied to intervene cells at 0-, 2000-, and 4000-μ strain for 6 h via a four-point bending system. The expression levels of COL2, COL10, Bax, Bcl-2, and PTHrp were detected by quantitative RT-PCR. Under the intervention of 2000-μ strain, the expression levels of COL2, COL10, and PTHrp increased significantly compared with the control group (P < 0.05), and the expression level of PCNA was also increased, but the difference was not statistically significant (P > 0.05). Under 4000-μ strain, however, the expression levels of PCNA, COL2, and PTHrp decreased significantly compared with the control group (P < 0.05), and the expression level of COL10 decreased slightly (P > 0.05). The ratio of Bcl-2/Bax gradually increased with the increase of stimulus intensity; both of the differences were detected to be statistically significant (P < 0.05). In conclusion, the apoptosis of growth plate chondrocytes is regulated by mechanical stretch stress. Appropriate stretch stress can effectively promote the cells' proliferation and differentiation, while excessive stretch stress inhibits the cells' proliferation and differentiation, even promotes their apoptosis. PTHrp may play an important role in this process."

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