Streaming potentials in healing, remodeling, and intact cortical bone.

Electrical fields have been implicated in accelerated bone healing and as a transduction mechanism for mechanically driven bone remodeling. Applied mechanical or electrical stimulation of bone remodeling suggests that this depends on the magnitude, frequency, and duration of the stimulus. The magnitude of endogenous electrical fields, manifest by streaming potentials (SPs) across canine cortical bone, were measured as a function of bending frequency in vivo and then in vitro at healing drill holes and at remodeling (ipsilateral) and normal, intact (contralateral) control sites in canine tibia. SP magnitudes normalized to periosteal strain were smaller for drill holes at 2 and 4 weeks postsurgery relative to either remodeling (P < 0.05 at 10 Hz) or normal intact (P < 0.001 at 10 Hz) controls both in vivo and in vitro. SPs of 12 week drill holes were similar to SPs of remodeling controls and tended to be smaller than SPs of normal intact controls. Mean SP normalized to bone impedance was approximately the same for all sites, suggesting that the smaller SPs during healing and remodeling relate to smaller bone impedance and/or larger porosity. SP as a function of bending frequency for normal sites was similar to that observed previously. SP versus frequency for drill holes and remodeling controls was more variable, probably because of variations in bone microstructure, and displayed a higher frequency content. The observed differences in SP magnitude and frequency response to loading associated with stages of healing indicate that endogenous electrical fields do indeed respond to the structural changes in healing and remodeling and are therefore capable of providing structural feedback information for the repair and remodeling process.

Electric field stimulation can increase protein synthesis in articular cartilage explants.

It has been hypothesized that the electric fields associated with the dynamic loading of cartilage may affect its growth, remodeling, and biosynthesis. While the application of exogenous fields has been shown to modulate cartilage biosynthesis, it is not known what range of field magnitudes and frequencies can alter biosynthesis and how they relate to the magnitudes and frequencies of endogenous fields. Such information is necessary to understand and identify mechanisms by which fields may act on cartilage metabolism. In this study, incorporation of 35S-methionine was used as a marker for electric field-induced changes in chondrocyte protein synthesis in disks of cartilage from the femoropatellar groove of 1 to 2-week-old calves. The cartilage was stimulated sinusoidally at 1, 10, 100, 10(3), and 10(4) Hz with current densities of 10-30 mA/cm2. Incorporation was assessed in control disks maintained in the absence of applied current at 37, 41, and 43 degrees C. The possibility that applied currents would induce synthesis of the same stress proteins that are caused by heating or other mechanisms was assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and examination of gel fluorographs. Total radiolabel incorporation in cartilage that had been stimulated relative to incorporation in the controls increased with current density magnitudes greater than 10 mA/cm2. The increase was greatest at 100 Hz and 1 kHz, and it depended on the position on the joint surface from which the cartilage samples were taken. Together, these results suggest that endogenous electric fields could affect cartilage biosynthesis. Stress proteins were not induced at any current density when the electrodes were electrically connected but chemically isolated from the media by agarose bridges. Stress proteins were observed for disks incubated at temperatures greater than 39 degrees C (no field) and when the stimulating platinum electrodes were in direct contact with the media bathing the cartilage disks. Therefore, the increase in incorporation of 35S-methionine due to applied fields with the use of chemically isolated electrodes did not appear to be associated with stress response.

A comparative analysis of streaming potentials in vivo and in vitro.

Streaming potentials (SPs) measured in vivo at a specific site on intact cortical bone (canine tibia) have been compared with measurements from the same site in vitro, tested as an excised bone strip soaked in Hank's balanced salt solution. The amplitude of SPs per periosteal strain in vitro was larger in 13 tibias than in vivo (by an average x6.5 at 1 Hz), but values per transcortical strain difference were similar. In vitro, SP magnitudes rose more sharply to an asymptotic value as a function of bending frequency than did in vivo signals, possibly because of a difference in the internal state of canaliculi and/or Haversian systems. Similarly, SP response to step-loading decreased to zero more slowly with time in vitro than in vivo. Difficulties encountered in preliminary measurements due to electrical shunting through electrolyte and soft tissues suggest the need for caution in using both in vivo and in vitro SP measurements to extrapolate to electric field strengths on the cellular level.

Streaming potential measurements at low ionic concentrations reflect bone microstructure.

Streaming potentials (SPs) have been proposed as one transduction pathway for mechanically driven bone remodeling. The fluid spaces in which SPs are generated will determine, in part, the structural information that they can provide to bone cells. Streaming potential measurements across cortical bone strips soaked in a range of saline concentrations were used to estimate the mean radii of fluid spaces that contribute to generation of electrokinetic fields. Using a cylindrical pore model, a pore radius of less than 200 A fit SP magnitude as a function of concentration. This pore size was shown to be consistent with estimates obtained from data reported earlier for SP as a function of concentration using a non-specific model, but was smaller than previously reported estimates for pore radius. A pore size in this range indicates that flow either in bone microporosity, or canaliculi that are substantially occluded by cellular material, must generate streaming potentials. Further, the fact that such small pores generate SPs in bone indicates that SPs could provide information regarding local matrix structure to bone cells.

Bone streaming potentials and currents depend on anatomical structure and loading orientation.

Bone streaming potentials (SPs) and streaming currents (SCs) may be a remodeling signal to cells, and might also be used to probe bone pore structure and fluid flows. For SPs or SCs to serve as either a remodeling signal or as a probe for pore structure, they must depend on bone structure. This study was undertaken to address two related questions. First, will differences in Haversian and laminar bone structure and fluid flow direction produce measurable differences in SP and SC? Second, do differences in SP or SC relate to differences in macroscopic bone impedance or large pore structure? SPs and SCs were measured across Haversian and laminar bone specimens with fluid flow driven in different directions by sinusoidal four-point bending. Data were grouped by bone type and flow direction (Haversian tissue, laminar tissue with radial flow, and laminar tissue with tangential flow) and flow direction alone (tangential and radial). SPs were larger for Haversian tissue and for laminar tissue with radial flow than for laminar tissue with tangential flow. SP and SC magnitude, and impedance were larger for radial than tangential flow. No difference in SC magnitude, SP or SC kinetics, or macroscopic bone impedance was observed between Haversian tissue, laminar tissue with radial flow, and laminar tissue with tangential flow. Thus, since laminar tissue with tangential flow had more vascular connections in the direction of fluid flow, SP was smallest for greatest vascular connectivity. The relation between SP or SC and impedance was inconclusive.