Knee loading dynamically alters intramedullary pressure in mouse femora

Abstract

Dynamic mechanical loads have been known to stimulate bone formation. Many biophysical factors such as number of daily loading cycles, bone strain, strain-induced interstitial fluid flow, molecular transport, and modulation of intramedullary pressure have been considered as potential mediators in mechanotransduction of bone. Using a knee loading modality that enhances anabolic responses in mouse hindlimb, we addressed a question: Do oscillatory loads applied to the knee induce dynamic alteration of intramedullary pressure in the femoral medullary cavity? To answer this question, mechanical loads were applied to the knee with a custom-made piezoelectric loader and intramedullary pressure in the femoral medullary cavity was measured with a fiber optic pressure sensor. We observed that in response to sinusoidal forces of 0.5 Hz and 10 Hz, pressure amplitude increased up to 4-N loads and reached a plateau at 130 Pa. This amplitude significantly decreased with a loading frequency above 20 Hz. To confirm alteration of intramedullary pressure, real-time motion of microparticles in a glass tube inserted to the femoral medullary cavity ex vivo was visualized. Taken together, these data reveal that knee loading dynamically alters intramedullary pressure as a function of loading intensities and frequencies.

Figure 1

Pressure calibration. (A) Schematic illustration of an experimental setup for pressure calibration using a femur in vivo. Two surgical holes were generated for connecting a pressure sensor and a water column to a femoral cavity. Pressure calibration was conducted using the femur in vivo and a glass tube filled with a saline solution. (B) Relationship between the applied pressure in Pa and the sensor signal in mV. The best fit regression line is y = 0.057x + 2.236 (r² = 0.99; water column), and y = 0.049x −0.488 (r² = 99; in vivo femur).

Figure 2

Experimental setup for pressure measurements with the femur in vivo and baseline intramedullary pressure. (A) Syringe consisting of a pressure sensor and an optic fiber. (B) Schematic illustration of an experimental setup. (C) Baseline intramedullary pressure. At t = 30 s, the pressure in the syringe was released to an ambient atmospheric pressure.

Figure 3

Pressure measurements in response to 0.5 Hz dynamic loading. (A) Input voltage to the piezoelectric loader at 0.5 Hz. (B) Pressure signal for sham loading (control). (C) Pressure signal with knee loading. (D) Fourier spectrum corresponding to the data in C.

Figure 4

Effects of loading intensities and frequencies on intramedullary pressure. (A) Alteration in pressure amplitudes as a function of actuator voltages at 10 to 100 V (0.5 to 4 N). (B) Alteration in pressure amplitudes as a function of loading frequencies at 0.5 to 50 Hz.

Figure 5

Real-time monitoring of the microparticles with knee loading. (A) Piezoelectric loader with a femur ex vivo mounted on a NIKON microscope. (B) Schematic illustration of the microparticle experiment. (C) Time elapse images at 250-msec intervals in response to the loading at 2 Hz. The left panel shows the raw images, and the right panel presents a trace of 4 particles. (D) Temporal alterations in particle positions. Based on 4 particles the sinusoidal motion model, depicted in the dotted line, was built as αsin(2πft + θ₀) − βt