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Abstract
The environment and mechanical forces may modulate the rate of bone growth. This is a key concept in the progression of infantile and juvenile musculoskeletal deformities. The mechanical modulation of growth is an increasing interest in the development and improvement of minimally invasive approaches that aim at modulating local growth while preserving the natural growth and functions of bone and bone segments. Bone longitudinal growth occurs in the growth plate. Although several approaches advocated in pediatric orthopedics are based on growth modulation, optimal loading parameters, allowing better control over the growth modulation, are not clearly defined. Recent studies have investigated the effects of dynamic versus static loads applied to growth plates, but have used uncontrolled loading parameters that were nonequivalent in terms of average strain or not standardized. This study aims to determine the differential effects between static and cyclic loadings, equivalent and standardized, on the growth plate to determine which type of loading has the greatest potential for growth modulation.
Growth plate explants from 4-week-old swine were divided into four groups: baseline, control, static loading and equivalent cyclic loading. A 10% compressive strain is applied on the explants at a rate of 1.5E-03 s-1 and then kept constant. A sinusoidal compressive strain (0.1 Hz) ranging from 7% to 13% (mean of 10%) is used for the dynamic group. The loads are maintained over a 48 hours period using a micromechanical testing system installed in an incubator (37 ° C, 5% CO2). The control group is incubated under the same conditions but without loading. For the baseline group, the explants were treated immediately after dissection. Following the tests, histomorphometry of growth plates is characterized from explants fragment embedded in the methylmethacrylate. The total thickness of the growth plate and the combined height of the proliferative and hypertrophic zones are measured on histological sections stained with Toluidine blue, and the combined ratio of the two lower zones compared to the total thickness is evaluated. Statistical analyses are conducted to compare the response to static and dynamic loads. The expression of aggrecan, type II collagen, type X collagen and MMP13 in the extracellular matrix is characterized by immunohistochemistry on paraffin sections. Qualitative analyses are completed to assess the level of expression of each protein as well as its location in the growth plate. A comparative analysis shows trends and differences between groups.
The results indicate that static compression leads to a significant decrease in the combined ratio of the proliferative and hypertrophic zones relative to the total thickness of the growth plate. In the case of dynamically loaded samples, this ratio is preserved when compared with control samples. A major disruption of the columnar organization of chondrocytes is observed in the proliferative and hypertrophic zones of dynamically loaded samples, however the structural organization of chondrocytes is retained in the statistically loaded samples. This disruption of the columnar arrangement in the dynamically loaded samples could probably be related to increased cell proliferation, which would force the accommodation of additional cellular and/or extracellular material added to the proliferative zone. The results also show a reduction in the expression of aggrecan, type II collagen and type X collagen after static loading. In contrast, dynamic compression contributes to the synthesis of aggrecan and type II collagen in the extracellular matrix of growth plate, suggesting that dynamic loads preserve the composition of the growth plate. Considering that the extracellular matrix and the proliferation and hypertrophy of chondrocytes are intimately related to the longitudinal growth process, growth could possibly return to normal after orthopedic treatment involving growth modulation based on dynamic loads.
The main limitations of the project are the use of an experimental in vitro model and the limited size of the samples. In return, this study is the first to complete a standard benchmarking that provides relevant information on the differential effects of static and dynamic loading equivalent in terms of average strain.
The research hypothesis that static and equivalent cyclic loads result in distinct changes in histomorphometry of the growth plate and in the expression pattern of the extracellular matrix proteins is confirmed. The type of loading plays an important role in the mechanobiological response of the growth plates and the longitudinal growth process is affected differently by a static or dynamic compression. The realization of a future in vivo study will include measurement of cell proliferation, chondrocytes hypertrophy and growth rate. The growth modulation potential of dynamic versus static loads could be demonstrated in concrete terms.
Keywords : growth plate, static/dynamic compression, mechanobiology, histomorphometry, immunohistochemistry
