Oscillatory and step response electromechanical phenomena in human and bovine bone.

Bone generates electrical potentials or strain-related potentials when mechanically deformed. These electrical potentials may be partially responsible for the remodeling process which bone experiences in response to mechanical loads. A large body of evidence suggests that streaming potentials are the dominant mechanism responsible for strain-related potential generation in fluid saturated bone. Recently, biphasic poroelastic theory has been coupled to electrokinetic theory in an attempt to further elucidate this mechanism. The work reported here expands the experimental evidence in support of this theory. Experimental results which characterize electrical and mechanical phenomena in both wide frequency oscillatory and step response testing of 47 human and bovine bone specimens are reported. These results also include data from experiments in which the viscosity and conductivity of the solutions permeating 10 human and bovine bone specimens were varied. A specially designed mechanical testing of bone specimens. Testing procedures, which were optimized for strain-related potential measurement, were developed. These included procedures for varying testing solution viscosity and conductivity. The experimental data was analyzed using descriptive and inferential statistical techniques and linear and nonlinear regression methods. The following objectives were established for the work reported here: (1) to confirm the existing experimental results; (2) to attempt to obtain a successful correlation between theoretically predicted and experimentally observed mechanical phenomena; (3) to extend the experimental results over the wider frequency range of 0.05-100 Hz; (4) to test the theory in the time domain; (5) to test the theory for changes in viscosity and conductivity of the permeating fluid; and (6) to extend the experimental testing to include human bone specimens. All of these objectives were met except that a highly variable correlation between the theoretically predicted and experimentally observed mechanical phenomena was obtained. Patterns in these data are explored and the possible causes of the highly variable correlation are discussed. The results strongly suggest an electrokinetic origin for the observed electrical potentials. The microporosity of bone appears to be the location in which this mechanism operates.

Biochemical mediators of the effects of mechanical forces and electric currents on mineralized tissues.

Cyclic nucleotides (cAMP and cGMP) and prostaglandin E (PGE) have been implicated as possible mediators of the effects of external stimuli on bone cells. The objective of this study was to determine changes in relative levels of these substances in mineralized tissue cells in response to mechanical and electrical stimuli, by the use of a combined immunohistochemical-microphotometric procedure. Canine teeth of eight 10-12 month-old female cats were tipped distally with 80 g force for either 1 h or 14 days. After 1 h, a slight elevation of staining intensity in alveolar bone osteoblasts and periodontal ligament (PDL) cells was observed at sites of tension and compression. After 14 days of treatment, this effect was markedly increased. Fifteen female cats, 10-12 months old, received electric stimulation (20 micron amperes d.c.) to the gingiva of 1 maxillary canine for 1, 5, 15, 30, or 60 min. At the cathode, significant increases of staining intensity in periosteal osteoblasts for cAMP, cGMP, and PGE were found at 15 and 60 min. At the anode, a significant rise in the staining intensity of these cells for PGE was seen at 15 min; at 60 min, cGMP and PGE, but not cAMP, were elevated. These results demonstrate the usefulness of the immunohistochemical technique in detecting relative changes in mineralized tissue cell content of cyclic nucleotides and prostaglandins in response to local application of physical stimuli of short and long duration.

The increase in vivo by electric currents of the 99mTc-methylene diphosphonate uptake and cyclic AMP content of the bone of the cat mandible.

Technetium-99-methylene diphosphonate (99mTc-MDP) bone images were obtained from live cats whose mandibles had been stimulated non-invasively and unilaterally with electric current (12 +/- 2 microA) for 1 week. The images were obtained using a gamma camera interfaced to a digital computer for quantitative comparison of the mandible. Radionuclide uptake by the stimulated sides increased by 59 +/- 26 per cent when compared with the contralateral control sites. Immunohistochemical staining of mandibular sections for adenosine 3',5'-monophosphate (cyclic AMP) showed that periosteal osteoblasts opposite the electrodes were intensely stained. Thus 99mTc-MDP scintigraphy is a sensitive non-invasive technique for determining in vivo enhanced bone remodelling activity. The immunohistochemistry indicates that the stimulation of bone cells was limited to surfaces adjacent to both cathode and anode.

Penetration of shearing and nonshearing pins into dentin.

An in vitro study was made of the penetration into human dentin of the shearing and nonshearing portions of TMS Minim two-in-one pins when placed manually and mechanically. It was found that the penetration of the first (shearing) portion of the pin was unaffected by the placement method. In contrast, the second (nonshearing) pin always penetrated further than the shearing pin. Mechanical placement resulted in better seating.

Electric currents, bone remodeling, and orthodontic tooth movement. I. The effect of electric currents on periodontal cyclic nucleotides.

Osteogenesis has been found to occur in response to the application of electric currents to bone. The objective of this experiment was to study the effects of D.C. electric currents on periodontal tissues in cats. Cyclic nucleotides, compounds known to be involved in cellular activation, were studied by immunohistochemistry in the involved tissues. Three groups of three young adult cats each were treated for 1, 3, and 7 days, respectively, by a device delivering 15 microamperes of direct current to bone osteoblasts and PDL cells stained intensely for cAMP and cGMP were observed adjacent to the cathode and anode, and bone apposition was found near the cathode. These results suggest that electric stimulation enhances cellular enzymatic phosphorylation activities in periodontal tissues and may be a potent tool in accelerating alveolar bone turnover.

Electric currents, bone remodeling, and orthodontic tooth movement. II. Increase in rate of tooth movement and periodontal cyclic nucleotide levels by combined force and electric current.

Piezoelectric currents in mechanically stressed bone were implicated in the activation of bone cells. The objectives of this experiment were to determine the usefulness of exogenous electric currents in accelerating orthodontic tooth movement and to study the effect of electric-orthodontic treatment on periodontal cyclic nucleotides. Maxillary canines were tipped in five cats by 80 g force. Two groups of five cats each were treated by an electric-orthodontic procedure to one maxillary canine for 7 and 14 days, respectively. Teeth treated by force and electricity moved significantly faster than those treated by force alone. Enhanced bone resorption was observed near the anode (PDL compression site), while bone formation was pronounced near the cathode (PDL tension site). Staining for cyclic nucleotides was increased when electric stimulation was added to the mechanical force. These results suggest that orthodontic tooth movement may be accelerated by the use of locally applied electric currents.

An in vitro study of electrical osteogenesis using direct and pulsating currents.

Although many investigators have electrically stimulated osteogenesis in vivo, this phenomenon has not been reported in vitro. In this work direct and pulsating currents were applied to in vitro fetal rat tibiae. The results indicated that electric current did cause osteogenesis in vitro, precipitates of calcium compounds formed in the cathode region, and direct current was more effective in stimulating bone growth than pulsed current delivering the same total charge (coulombs).

The influence of functional use of endosseous dental implants on the tissue-implant interface. I. Histological aspects.

The tissue-implant interfaces of functional and non-functional endosseous dental implants were compared histologically for up to one year post-operatively. Nonmineralized connective tissue zones (a "fibrous capsule") existed in all functional interfaces. Direct, or nearly direct, bone apposition to implants occurred in non-functional interfaces. The origin of this result and its significance in dental implantology is discussed.

The influence of functional use of endosseous dental implants on the tissue-implant interface. II. Clinical aspects.

Functional and non-functional endosseous dental implants were clinically compared in beagle mandibles for up to one year post-operatively. Differing biomechanical conditions led to clinical differences between functional and non-functional implants. Typical clinical tests, however, did not always reveal detailed histological differences between implant-tissue interfaces of functional and non-funcional implants.

Microelectrode studies of stress-generated potentials in four-point bending of bone.

A microelectrode technique has been developed to enable the study of stress-generated potentials (SGP) in bone to a spatial resolution of 5 micrometers. The technique has been used to measure the electrical potentials as a function of bone micromorphology in four-point bending. Electric fields ranging from 30 to 10(3) times greater than is measured by conventional macroscopic methods have been discovered at the Haversian canals for human and bovine cortical bone. The amplitude and direction of the electric field in the osteons depend specifically upon the amplitude and the sign (i.e., compression or tension) of the stress. The implications of this finding with regard to the origin of SGP and their possible physiological significance are considered.

Microelectrode study of stress-generated potentials obtained from uniform and nonuniform compression of human bone.

By use of a previously developed microelectrode technique, the effect of nonuniform stresses on the stress-generated potentials (SGP) in bone were studied to a resolution of 5 micrometers. Comparison was made between uniformly and nonuniformly applied compression of human cortical bone. It was found that the radial electric fields for osteons in a specimen under uniform compression were equivalent, and such specimens possessed no macroscopic SGP; for nonuniform compression, the electric fields of osteons differed, and a macroscopic SGP was measured. The magnitude of the macroscopic SGP thus appears to be dependent upon local stress differences and, hence, on the SGP of local regions.

Stress-generated potentials in bone: effects of collagen modifications.

The effect of collagen modification on the magnitude of stress-generated potentials has been studied. A whole bone model has been developed for determining the effects of collagen crosslinking on the electrical signals. Increased crosslinking results in larger electrical signals, and such increases result whether in vivo or in vitro changes in the collagen are made.

An in vitro investigation of the anodic polarization and capacitance behavior of 316-L stainless steel.

Determinations were made of how the corrosion-resistant properties of the passive film on 316-L stainless steel are influenced by the material's mechanical and surface states, and the variable pH and PO2 conditions of the interstitial fluid. Cold-rolled and annealed specimens were surface-prepared, commercially and in the laboratory, respectively, as if for orthopedic implantation. Passive film behavior was studied by the anodic polarization and pulse-potentiostatic capacitance methods. The pH and PO2 of the Ringer's test solution were varied to include interstitial fluid values occurring postoperatively and onto recovery. The anodic polarization behavior of all specimens was found to be pH- and PO2-independent. Breakdown potentials of annealed specimens were 800-950 mV (SCE), in contrast to previously reported values of approximately 350 mV. This substantial increase is related to the influence of surface preparation and, in particular, to the optimization of electropolishing time which acts to produce a microscopically smooth surface, free of debris and disarrayed material. Capacitance behavior of annealed material for potentials greater than 400 mV was consistent with a model involving the entry of chloride and metal ions (mostly Fe) into the passive film. This entry is related to the onset of pitting.