Introduction

The interstitial fluid of bone fluid flow is supplied by flowing blood. Blood flow is determined by three kinds of muscles: cardiac, smooth, and skeletal. Cardiac muscle establishes baseline blood pressure. Smooth muscle controls vessel diameter and skeletal muscle creates intermittent intravascular pressure pulses. For the tibia the relevant skeletal muscle is the gastrocnemius which functions as a muscle pump. This study tested the hypotheses: 1) skeletal muscle-caused pressure pulses increase cortical blood flow, 2) extravasation of vascular fluid and, consequently, interstitial bone fluid flow are enhanced by resultant increased microvascular pressure and 3) bone healing is enhanced by increased bone fluid flow.

Scientific truth is an oxymoron. The goal of modern science is an understanding of the natural world. Truth is the goal of empiricism. In orthopaedic research conflict develops between these goals because empiricists seek to discover ways to improve musculoskeletal health and scientists seek to understand how the musculoskeletal system functions. When resources are limited, a hard choice must be made concerning which path to pursue. The conflict actually has a long history in Western culture that can be traced to differences between Greek and Roman approaches to discovering truths about the natural world. For ancient Greeks, no truth was complete unless a cause-and-effect connection could be established following analysis of observations. For Romans, truths were empirical. They were solutions to solved problems; an aqueduct that did not leak or a healed fracture. Empirical approaches to problems have been a characteristic of Homo sapiens since the Stone Age. They defined the original methodology of medicine and so established it as a “truth” profession. The Romans added engineering to the list of truth professions, although they did not classify it as a profession. Engineering and medicine functioned as truth professions until the 20th century.

Science was much slower to mature. The term “scientist” was not coined until 1834. It was not established as a distinct profession until it was freed from scholasticism by the natural philosopher, Francis Bacon, who introduced inductive logic, and Galileo Galilei in the 17th century. They and Isaac Newton launched the Enlightenment which stimulated scientific research for over a century. By the mid-19^th century, progress in science was sufficient to convince many scientists that they were members of a truth profession that would eventually be able to explain all life functions in terms of physics and chemistry. This reductionist view prevailed until 1927, when it was shown to be invalid by Werner Heisenberg. As a consequence of his ‘Principle of Uncertainty’, science is no longer a “truth” profession. Instead, scientific analysis has become a statistical methodology devoid of final proofs. In place of proofs, the scientist must formulate falsifiable hypotheses that are the reverse of those being proposed. In this approach, developed mainly by Karl Popper, observations are analyzed statistically and if they significantly disagree with the falsifiable hypothesis it may be said to have been disproven and one can say that the data support the proposed hypothesis.

Members of truth professions began to perform experiments and employ science as an approach to truth in the 20th century thereby developing an “empirical science”. Since these investigators are constrained by their profession to favor truths, they tend to minimize the Greek and maximize the Roman components of their research. The result has been a dichotomy in science between those whose research success is measured by its contribution to “cures” and those whose research success is measured by its contribution to understanding. In orthopaedics, the dichotomy separates analytical scientists from engineers and physicians. In addition, caught in between are the bioengineers. There is a need for better communication between all.

The concept that fluid percolating through bone matrix is the basis for mechanotransduction of stress stimuli to modeling and remodeling has proved challenging⁴. Traditional solid mechanical models in which piezoelectricity is the mechanotransduction process are more intuitive to orthopaedic practitioners nurtured on the fundamentalism of Wolff’s “Law”. However, the scientific evidence from Anderson and Ericksson¹, Piekarski and Munro⁵, and Hillsley and Frangos³ strongly support a bone fluid flow (BFF) paradigm, which, when coupled with the Utah paradigm of Frost² forms the most scientifically validated explanation to date of osteon response to mechanical stress in vivo.

In order to make this explanation more intuitive we have developed a metaphorical rendering of the model. The rendering adds an extension to include our current model for the effects of skeletal muscle contraction on the basic multicellular unit (BMU). In the metaphor, cortical bone is the continent nation HaroldFrostland. Its cities are fed by waterways which serve as metaphors for each of the fluid transporting pathways from arteries to inter-collagen spaces of bone matrix.

Bounding the continent are oceans wherein underwater earthquakes—metaphors for skeletal muscle contractions—generate solitary pressure waves (tsunamis) and a ripple effect in continental waterways. Critical for penetration by and effectiveness of such solutions is that flow resistance in the waterways must allow pressure buildup. Locks/valves serve to provide this resistance. When pressure is sufficient filtration at transfer points—metaphors for where nutrient exchange occurs--is enhanced. By this means, muscle contraction enhances delivery of fluid not only to bone cells; but around them as well, stimulating mechanotransduction.

Introduction: Insufficiency of poroelastic bone bending as the sole mechanism driving bone interstitial fluid flow (BIFF) to account for the shear stress required to activate mechanoreceptors on osteocytes and osteoblasts, has stimulated a search for alternative or complementary mechanisms in the quest for a comprehensive bone remodeling model. Some investigators, noting that a substantial amount of interstitial fluid is exchanged with blood and lymphatic capillaries, have suggested that this exchange may play a substantial role in both microtransport through the collagen matrix and lacunar-canalicular transport. In order to accept the vascular system as a significant source of transport driving BIFF, it must first be demonstrated that capillary filtration, the proc ess by which fluid is transported from blood vasculature, is sufficiently convective to drive interstitial percolation. We have proposed that while, as shown by Otter et al., resting transmural vascular pressures are sufficient to generate streaming potentials across cortical bone, it is likely that these forces must be complemented by muscle pump contractions during exercise to generate convective percolation flows which will develop the required mechanotransducer shear stress activation threshold. To determine a minimal baseline for a muscle pump driven BIFF (MPD-BIFF) model, we have investigated the role of repetitive skeletal muscle contractions, uncoupled from gravitational loading, on blood flow and capillary filtration in cortical bone of the rabbit tibia. We tested the hypothesis that these effects increased when the muscle pump was activated.

Materials & Methods: The experimental model combined non-invasive, low magnitude transcutaneous neuromuscular stimulation (TENS), with real-time measurements from intravital microscopy (IVM) of optical bone chamber implants. Sling suspension of rabbits was utilized to eliminate gravitational reaction forces throughout TENS and data collection. TENS-induced muscle contraction forces were measured in situ, resultant bone strains were calculated, and systemic circulatory parameters were monitored, in order to eliminate these factors as contributors to blood flow changes. Blood flow rates and capillary filtration were measured by video-image analysis of 1 μm fluorescent microspheres and dextran-conjugated fluorescein isothiocyanate (FITC) and rhodamine (ITC) injected intravascularly during IVM. Bone formation, angiogenesis, and mineral apposition rates (tetracycline labelling) were analyzed from weekly microscopy pictures. Changes in bone mineral content and density were determined with CT scans obtained at implantation and termination.

Results: Mechanical loading and baseline systemic circulation did not significantly contribute to the findings. Rhythmic muscle contractions were shown to increase cortical blood flow, rate of capillary filtration, rate of bone apposition, and angiogenesis.

Discussion: The hypothesis was supported by the data. However, since no measurements were made on single capillaries, we could not confirm previous reports by this laboratory of convective extravasation.

Introduction: Tissue engineered scaffolds require vascularization to 1) enhance nutrient exchange and 2) provide cells needed to build new tissue. Cell-seeded scaffolds; bioreactors-- require rapid penetration of vessels or enhanced fluid percolation to keep their contents alive until normal nutrient exchange can be established. Bone fluid flow depends on a pumping system which drives percolation through its own matrix. Recent interest in the pumping mechanism has resulted in bone fluid flow models, which link the pumps to bending of bone by muscle contraction and compression-tension cycles from weight-bearing during locomotion. The present authors have proposed that capillary filtration, the source of the percolating fluid, is sufficiently enhanced by soliton pressure waves in blood driven by the muscle pump during exercise to provide a significant hydraulic pressure component to bone fluid percolating through bone and any bone-implanted scaffold. A proposal and some preliminary results from a pilot project suggesting enhancement of capillary filtration by the muscle pump is presented.

Materials and Methods: Optical bone chambers were implanted in adult New Zealand White female rabbits. Chamber construction and implantation were as usual1. At the third week post-op, chamber ends were exposed and weekly intravital microscopy commenced. Transcutaneous electrical stimulation was administered with a ToneATronic® TENS at 85V, 80mA and 2Hz. The stimulator was applied externally over the gastrocnemius muscle. A fluorescence digital image was obtained before 30 minutes of application of transcutaneous electrical nerve stimulation (TENS) after injection of FITC-D70. Blood samples were obtained from an aural vein in the ear opposite that being injected with the fluorescent dye after each injection. Blood concentration of dye was determined with a SPEX Fluoromax-3 spectrofluorometer for both serum (absolute concentration) and whole blood (to detect differences which would make fluorescence in vessels an inaccurate indicator of red blood cell color contamination). For analysis, four vessels were chosen and the average dye concentration profiles before and after 30 minutes of stimulation were obtained.

Results: Results are shown in Figure 1. Extravasated dye levels in TENS rabbits were markedly higher than those in controls. Analysis of profiles using an erfc-based diffusion-convection discrimination model2 showed that extravasation was convective.

Discussion: These data are consistent with significant contribution to convective percolation of bone fluid through implanted scaffolds by muscle pump-driven extravasating fluid. They do not, however, answer two critical questions: 1) Is the magnitude of this convection a major component of flow through the scaffold? 2) What are the relative contributions of skeletal muscle-generated intravascular pressure solitons and incompressible fluid transmission of bone bending pressure to the convective flow observed? Additional studies with released gastrocnemius muscles are in progress.