Abstract
BackgroundInterstitial fluid flow induced by loading has demonstrated to be an important mediator for regulating bone mass and morphology. It is shown that the fluid movement generated by the intramedullary pressure (ImP) provides a source for pressure gradient in bone. Such dynamic ImP may alter the blood flow within nutrient vessel adjacent to bone and directly connected to the marrow cavity, further initiating nutrient vessel adaptation. It is hypothesized that oscillatory ImP can mediate the blood flow in the skeletal nutrient vessels and trigger vasculature remodeling. The objective of this study was then to evaluate the vasculature remodeling induced by dynamic ImP stimulation as a function of ImP frequency.MethodsUsing an avian model, dynamics physiological fluid ImP (70 mmHg, peak-peak) was applied in the marrow cavity of the left ulna at either 3 Hz or 30 Hz, 10 minutes/day, 5 days/week for 3 or 4 weeks. The histomorphometric measurements of the principal nutrient arteries were done to quantify the arterial wall area, lumen area, wall thickness, and smooth muscle cell layer numbers for comparison.ResultsThe preliminary results indicated that the acute cyclic ImP stimuli can significantly enlarge the nutrient arterial wall area up to 50%, wall thickness up to 20%, and smooth muscle cell layer numbers up to 37%. In addition, 3-week of acute stimulation was sufficient to alter the arterial structural properties, i.e., increase of arterial wall area, whereas 4-week of loading showed only minimal changes regardless of the loading frequency.ConclusionsThese data indicate a potential mechanism in the interrelationship between vasculature adaptation and applied ImP alteration. Acute ImP could possibly initiate the remodeling in the bone nutrient vasculature, which may ultimately alter blood supply to bone.
Highlights
Bone mass and morphology accommodates changes in mechanical demands by regulating the site-specific remodeling processes which consist of resorption of bone, typically followed by bone formation
Nutrient Arterial Wall Area The tunica media of the nutrient arteries demonstrated up to a 50% increase in area when subjected to intramedullary pressure (ImP) stimulation (Figure 2)
It is important to point out that nutrient arteries from the age-matched animals showed an average of 4% natural differences in arterial wall area between the left and the right ulnae (0.121 ± 0.01 mm2 vs. 0.126 ± 0.015 mm2), demonstrating minimal left and right handedness in turkeys
Summary
Bone mass and morphology accommodates changes in mechanical demands by regulating the site-specific remodeling processes which consist of resorption of bone, typically followed by bone formation. From mechanotransduction point of view, bone remodeling is regulated by various parameters within the mechanical milieu, i.e., strain magnitude, frequency, duration, rate, and cycle number [7,8] Stress injuries were initially thought to emerge from repetitive vigorous activity, inducing an accumulation of fatigue microfractures and resulting in material failure [9] This hypothesis of repetitive loading related fatigue microdamage as the sole causative factors for stress injuries has been shown to be inconsistent based on two key findings: a) the number of loading cycles associated with stress fracture in recruits and athletes are well below the fatigue fracture threshold, and that there is not enough duration for an accumulation of microdamage to contribute to material failure within the early onset of stress fractures [9,10,11], and b) the stress fracture site tends to occur close to the neutral axis of bending at the mid-diaphysis rather than the site with maximum strain magnitude [12,13]. The objective of this study was to evaluate the vasculature remodeling induced by dynamic ImP stimulation as a function of ImP frequency
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