Wednesday, March 6, 2013

How much does frequency matter for LSJL?

In the LSJL lengthening studies a frequency of 5 Hz is used.  Frequency is the hardest thing to mimic in our home made clamping method.  Even though this is an axial loading study it is still by the LSJL scientist Hiroki Yokota and it'll provide insights on how important frequency is for LSJL results.

Resonance in the mouse tibia as a predictor of frequencies and locations of loading-induced bone formation

"we conducted axial tibia loading using low, medium, or high frequency to the mouse tibia. The experimental data demonstrated dependence of the maximum bone formation on location and frequency of loading{But does frequency influence chondrogenic differentiation?}. Samples loaded with the low-frequency waveform exhibited peak enhancement of bone formation in the proximal tibia, while the high-frequency waveform offered the greatest enhancement in the midshaft and distal sections. Furthermore, the observed dependence on loading frequencies was correlated to the principal strains in the first five resonance modes at 8.0–42.9 Hz. Collectively, the results suggest that resonance is a contributor to the frequencies and locations of maximum bone formation."

"When loading is applied to such a material at or near its resonant frequencies, additional energy is absorbed and the material tends to vibrate at greater amplitude than when loading is applied at other frequencies. These vibrations propagate through the material in specific ways, or modes, based on the geometry and characteristics of the material."

"he tibia is composed of a shell of dense, stiff cortical bone that is thinnest on the outside of each epiphysis and thickest throughout the diaphysis. Inside the epiphysis a matrix of less dense, weaker trabecular bone is present. An epiphyseal plate is found at the border between the each epiphysis and diaphysis, which consists of hyaline cartilage. Each type of tissue likely contributes to the frequency response of the tibia."<-So when the epiphyseal plate is absent that affects the optimal frequency for chondroinduction.

"(C57BL/6 male, ∼13 weeks old) were used in this study."<-So these mice were definitely growing.

7N were used versus 0.5N in LSJL.

Low Frequency: 1-17Hz<-So LSJL is in this range
Medium Frequency: 18-34Hz
High Frequency: 35-51Hz

The animals experienced all 200 repetitions at each of the frequencies within the range.

So 8% region would be the region closest to the growth plate range and that was the range the responded most to low frequency.
It's hard to tell how chondrogenesis was affected in the 8% region.

"When a periodic load is applied at or near one of an object’s resonant frequencies, it tends to absorb more energy and oscillate at greater amplitudes than at other loading frequencies. In the case of the tibia, loading at frequencies near the resonant frequencies of the bone may be causing more energy to be dissipated and larger displacements in certain areas of the bone than loading at other frequencies with equal amounts of force. This may lead to increased strain rates, amplified intramedullary fluid flow, increased fluid shear stresses on bone cells, and enhanced cellular response in areas that absorb the most energy"<-So the correct frequency is a bonus but is not required.

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