Streaming potentials in gap osteotomy callus and adjacent cortex. A pilot study.

This study documented streaming potentials generated in vivo by maturing osteotomy calluses in 10 canine tibiae. Gap osteotomies were allowed to heal for 6 or 12 weeks and were stabilized by an external fixator. Then, with the dogs under anesthesia, electrical measurements were made from 3 silver-silver chloride electrodes placed surgically in direct contact with the callus, with adjacent cortical bone, and with the medullary canal (reference electrode). Streaming potentials were recorded during step loading and sinusoidal bending (0.1-30 Hertz) as the tibia was deformed by 2 threaded pins coupled to a servohydraulic device. Streaming potentials were generated at callus and adjacent cortical sites, but the magnitude was greater on the immature, flexible callus, where bending strain was concentrated; as the callus became increasingly rigid, strain and streaming potential magnitude were distributed more evenly over the callus and adjacent cortical fragments. When normalized to surface strain, mean streaming potential per strain was less dependent on the microscopic structure, although on individual specimens streaming potential per strain at callus and adjacent cortical bone sites tended to increase with decreasing porosity. Despite a wide variation in data in this pilot series, these observations are consistent with the natural history of callus maturation: the maximum magnitude of streaming potentials in callus appears to decrease as the strain gradient across the site decreases, whereas streaming potentials normalized to strain increase as bone matures and becomes more dense.

Inflatable brace-related streaming potentials in living canine tibias.

In a canine osteotomy model, application of a pressurized brace increased the density of periosteal bone and, at 12 weeks postfracture, yielded a stronger union compared with fractures treated by conventional cast, as determined by biomechanical testing. Pulsatile transcortical electric potentials were caused by the fluctuations in intramedullary pressure that result from active circulation. This report describes a collaborative effort designed to determine whether pressure fluctuations within an inflatable brace, placed over a canine calf, can affect endogenous transcortical electric potentials. Pressure within a brace placed over a canine hindlimb was observed to oscillate between 20 and 52 mm Hg during normal ambulation in 3 dogs. Manual pulsatile inflation of a similar brace, causing brace pressure fluctuations between 12 mm Hg and 130 mm Hg, produced fluctuating transcortical electric potentials ranging from 1.2 microvolts to 87 microvolts in anesthetized canines. These electric potentials were proportional to intramedullary pressures between 3.4 mm Hg and 59 mm Hg. Transcortical electric potentials resulting from the application of a pressurized brace, rather than conventional casting, may be part of the mechanism by which the changes in fracture healing are achieved.

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.

Aramid-epoxy composite internal fixation plates: a pilot study.

Mechanical tests were conducted on an aramid-epoxy composite laminate in vitro and in vivo to determine its suitability for internal fixation plates. This material, fashioned into blank test coupons the size of the standard 4-hole AO-ASIF plates, had a tensile modulus of elasticity significantly lower than bone. In three-point bending, blank test coupons exhibited a low yield strength that would limit utility in significant load-bearing situations, but changes in the layer configuration of the composite could be expected to improve this characteristic. Under destructive loads, these specimens appeared to be less subject to catastrophic failure than carbon fibre composites. Using 4-hole test coupons fastened to a plastic tube simulating bone, four-point bending tests showed that strain-shielding was significantly reduced by aramid composite relative to carbon fibre composite or metal plates. Finally, in-vivo tests on canine femora demonstrated that aramid composite plates were well tolerated and caused less strain shielding during weightbearing, but significant differences in cortical atrophy and porosity beneath steel versus aramid plates were not apparent. Although the plates were relatively flexible, they could not be preformed during surgery like a metal plate.

Intraarterial protamine sulfate reduces the magnitude of streaming potentials in living canine tibia.

Using previously described techniques, transcortical streaming potentials were measured at two middiaphyseal sites on one tibia of each of nine anesthetized canines during sinusoidal bending (approximately 0 to -200 mu epsilon periosteal surface strain) at 2 Hz. Measurements were made for 60 minutes prior to and up to 180 minutes following bolus injection of protamine sulfate (42-126 mg/kg) dissolved in Hanks Balanced Salt Solution, directly into the femoral artery without interrupting circulation. Shortly after injection, the protamine sulfate caused a clear reduction in the magnitude of streaming potentials. Subsequent injections of additional protamine sulfate resulted in further reductions, and in several instances, voltage sign reversals. This study represents the first observation that circulating proteins may alter electromechanical transduction in living bone, and suggests the possibility that specific agents, which are known to affect bone remodeling, may do so, in part, by altering these endogenous electrical potentials.

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.

Relationships between bone structure in the iliac crest and bone structure and strength in the lumbar spine.

The purpose of this study was to examine the relationship between histomorphometric variables of cancellous bone structure and ultimate compressive strength (UCS) in the second lumbar vertebra (L2) and to determine whether structural variables in the iliac crest are predictive of the same variables and of UCS in L2. At autopsy, 7.5 mm diameter cores were removed from the iliac crest and from L2 of 29 subjects who had died suddenly without bone disease. Cancellous bone volume (BV/TV, %) was significantly lower in L2 than in iliac crest due to lower trabecular number (Tb.N, per mm) and thickness (Tb.Th, microns). There were significant correlations between iliac crest and L2 for BV/TV, Tb.N and trabecular separation (Tb.Sp, microns), but not for Tb.Th. BV/TV was negatively correlated, and Tb.Sp was positively correlated with age at both sites. Tb.Th was not significantly correlated with age in the iliac crest, but a significant negative correlation was observed in L2. The UCS of vertebral cores was negatively correlated with age. BV/TV and Tb.Th in L2 were positively correlated with UCS in L2.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

A new manual method for assessing two-dimensional cancellous bone structure: comparison between iliac crest and lumbar vertebra.

We have developed a simple method for the direct analysis of two-dimensional trabecular structure. Using this method, we have compared the structure of the left and right iliac crest and the second lumbar vertebra and studied the relationship between two-dimensional trabecular structure and the ultimate compressive stress of bone samples from the second lumbar vertebra. Transverse cores were taken from the left and right iliac crests and second lumbar vertebrae of 35 subjects (12 females) who died suddenly. The trabecular structure was subdivided into nodes (node count) and free ends (free end count), which were expressed per square millimeter of cancellous space. A node was defined as the point of junction between two or more trabeculae; a free end was defined as the end of a trabecula that was unconnected in the plane of the section to any other trabecular element. The following struts were defined by drawing a line between the nodes and the free ends: node to node, node to free end, free end to free end, and cortex-derived struts. Strut lengths were measured and expressed as a percentage of the total strut length and per square millimeter of cancellous space. Intraobserver variation was generally low, with coefficients of variation for repeat measurements in the range 3.9-17.8%. There was no statistically significant difference between left and right iliac crests for any of the variables measured. With the exception of cortex-derived struts, there was no statistically significant correlation between crests for any variable.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamic electromyography. I. Numerical representation using principal component analysis.

A complete description of human gait requires consideration of linear and temporal gait parameters such as velocity, cadence, and stride length, as well as graphic waveforms such as limb rotations, forces, and moments at the joints and phasic activity of muscles. This results in a large number of interactive parameters, making interpretation of gait data extremely difficult. Statistical pattern recognition techniques can simplify this problem. For this approach to be successful, first it is necessary to reduce the number of interactive parameters to a manageable set. In this study, we present an application of principal component analysis as a means for representing graphic waveforms in a parsimonious manner. In particular, we concentrate on representing the phasic muscle activity recorded using surface electrodes from ten major muscles of the lower extremity of 35 normal subjects during level walking. A 32 point vector is created in which each point of the vector represents the normalized area under the curve of a portion of rectified and smoothed electromyographic signal, expressed as a function of gait cycle. Principal components are computed and the first few weighting coefficients are retained as features to represent the original EMG data. We show that the corresponding basis vectors span parts of the gait cycle where the most variability between individual subjects exists. We also show that the basis vectors can be used to represent the EMG data of subjects not originally used to generate the basis vectors.

Dynamic electromyography. II. Normal patterns during gait.

Human gait is a complex phenomenon. Many descriptors are needed to completely describe gait in terms of the biomechanics involved. The descriptors, when expressed as a function of the gait cycle, are complex waveforms. For each of these variables, a single "normal" pattern with bands of deviation has generally been accepted as a reference in clinical/research use to explain the abnormalities in a patient's walking pattern. In fact, one observes many "normal" patterns, and a body of research has been devoted to explaining the differences between these patterns in terms of walking speed, age, cadence, sex, etc. It would be simpler in one sense to start with the fact that different people walk with different patterns, not one pattern with bands of deviation. Numerical representation of the waveforms simplifies the analysis and interpretation of waveform data and facilitates comparison between subjects or groups of subjects. When combined with pattern recognition techniques, it also is useful for identifying subpatterns within a group. In this article, the numerical representation of electromyographic data by Karhunen-Loeve expansion are combined with cluster analysis to obtain patterns of dynamic phasic activity of 10 muscles of the lower extremity. From the 35 normal subjects walking at self-selected speed, two to four patterns are developed for each of the muscles and the physiological significance of the patterns are discussed.

Transcortical streaming potentials are generated by circulatory pressure gradients in living canine tibia.

Electrical potentials associated with the pulse pressure have been observed in a canine tibia model in vivo. As the medullary pressure rises during pulsing, the periosteal bone surface becomes positive with respect to the endosteal surface. This pattern is consistent with streaming potentials generated by outward flow of fluid through bone with a negatively charged matrix (negative zeta potential). Both the medullary pressure and electric potential oscillations are halted by occlusion of the femoral artery. Furthermore, systemic administration of epinephrine decreases the amplitude of the medullary pressure and the electric potential by the same fraction. Streaming potentials generated by blood flow are distinct from those generated by mechanical deformation and may have additional significance in relation to fracture healing and/or etiology of osteoporosis.

Gait analysis of patients who have Paget disease.

Eighteen patients who had Paget disease were evaluated in our gait-analysis laboratory. The results were compared with those of ten healthy age-matched control subjects in order to quantitate the biomechanical changes and to describe the specific patterns of walking that occur secondary to bowing of a lower extremity. Kinetic and kinematic data were acquired using infrared video cameras and force platforms; electromyographic data were obtained using surface electrodes. Velocity and cadence were decreased and stride time and double-limb support time were increased in the patients who had Paget disease compared with the control subjects. Frequently, the knee of the limb that was affected by Paget disease was flexed during stance and flexed less during swing. When the involved knee was in varus angulation it also had an increased adduction moment, which may be related to the bowing deformity. Although the patterns of ground-reaction force were similar in the patients and the control subjects, the magnitudes of forces were reduced in the patients. Phasic muscle activity was similar in the two groups.

An improved design of electrodes for measurement of streaming potentials on wet bone in vitro and in vivo.

Streaming potentials are generated by mechanical stress in wet bone and may constitute a control mechanism for bone remodeling. Measurement of streaming potentials in bone has attracted considerable effort in past years but quantitative studies have been hampered by relatively poor repeatability when using Ag.AgCl electrodes which contact bone via a wick moistened with electrolyte. Improvement now has been achieved with an electrode design that limits the specific area of contact of an agar/salt bridge by means of a silastic seal, thus permitting the same equipotential surface to be contacted for each set of measurements. This reduces variations caused by bone structure and impedance, and facilitates quantitative comparisons of the response of bone samples to selected variables. The new design also permits considerable qualitative improvement in recordings made from bone during locomotor function in experimental animals in vivo.

Repeatability of kinematic, kinetic, and electromyographic data in normal adult gait.

The repeatability of gait variables is an important consideration in the clinical use of results of quantitative gait analysis. Statistical measures were used to evaluate repeatability of kinematic, kinetic, and electromyographic data waveforms and spatiotemporal parameters of 40 normal subjects. Subjects were evaluated three times on each test day and on three different test days while walking at their preferred or natural speed. Intrasubject repeatability was excellent for kinematic data in the sagittal plane both within a test day as well as between test days. For joint angle motion in the frontal and transverse planes, the repeatability was good within a test day and poor between test days. Poor between-day repeatability of joint angle motion in the frontal and transverse planes was noted to be partly due to variabilities in the alignment of markers. Vertical reaction and fore-aft shear forces were more repeatable than the mediolateral shear force. Sagittal plane joint moments were more repeatable than frontal or transverse plane moments. For electromyographic data, repeatability within a day was slightly better than between test days. In general, the results demonstrate that with the subjects walking at their natural or preferred speed, the gait variables are quite repeatable. These observations suggest that it may be reasonable to base significant clinical decisions on the results of a single gait evaluation.

Pressure sores: a biochemical test for early detection of tissue damage.

This study was undertaken to seek biochemical indicators in blood associated with the onset of early tissue damage which may progress to the formation of a pressure sore. Changes in serum creatinine phosphokinase (CPK), inorganic phosphate, and lactate dehydrogenase in systemic blood of pigs were investigated before, during, and after local indentation. Pressures of 540mmHg and 700mmHg were applied for six hours on both sides of the scapula and backs of five anesthetized animals. Two hours after release of indentation, serum CPK levels showed marked elevation and remained elevated even after one week. The elevated levels of CPK were shown to correspond to defined pathology of the tissue as determined histologically. On the other hand, inorganic phosphate began to decrease after release of indentation and recovered to the preindentation level after one day. Lactate dehydrogenase did not change significantly throughout the experiment. These results offer important evidence of the potential of CPK as a systemic indicator of muscle damage at an early stage of pressure sore formation.

External ultrasound can generate microampere direct currents in vivo from implanted piezoelectric materials.

Under development is an internal fixation plate that incorporates a piezoelectric element to generate current when excited mechanically by either weight bearing or external application of ultrasound. The intent is to deliver this current to electrodes at a fracture or osteotomy site to aid in prevention or treatment of nonunion. The present study examines quantitatively the ability of external ultrasound to generate current from small piezoelectric ceramic elements implanted in tissue. An ultrasonic transducer (2.25 MHz, 10-20 V input, less than 10 mW/cm2 output) was employed to excite small test coupons of a piezoelectric ceramic in vitro and in vivo with various materials, including water, PVC gel, cortical bone, and living soft tissues, interposed. In all instances, it was possible to generate currents of up to 20 microA after rectification; currents up to 1 mA were achieved in some cases. The work indicates that external ultrasonic energy could effectively power small internal devices designed to stimulate bone healing, without the need for implanted batteries or percutaneous leads.

Method for histological preparation of bone sections containing titanium implants.

A thin sectioning technique involving hand grinding has been developed to produce 20-40-microns-thick sections of bone-titanium implant sites. Components include: 1) surface staining of sections prior to mounting on slides so bone labels (oxytetracycline-HCl and 2,4-bis(N,N-dicarbomethyl)aminomethylfluorescein (DCAF] can be seen in sections viewed with transmitted light, 2) a pneumatic sample press for bonding sections to slides with a thin, uniform glue line and without trapped air bubbles, and 3) bonding methyl methacrylate embedded sections to clear acrylic slides with methyl methacrylate monomer to provide enhanced bond strength and grinding properties compared to those obtainable with glass slides. Sample cracking and distortion is minimized and the tissue-implant interface can be kept intact. The expense of start-up equipment for this technique is minimal.

Design considerations in development of a prototype, piezoelectric internal fixation plate: a preliminary report.

The piezoelectric internal fixation plate represents a new concept in orthopaedic implants. The purpose of this device is to provide stable bone fixation while delivering internally generated, microampere direct currents to prevent or treat nonunion of a fracture or osteotomy. Clinically, currents of this type have been effective in treatment of nonunion, but application has required separate, implanted, or external battery or radiofrequency powered circuits. The "piezoplate" being developed contains an integral piezoelectric element that generates current in response to either physiological loading such as weightbearing or to externally applied ultrasound. Currents are processed by a rectifying circuit for delivery to bone by electrodes. Specially designed series/parallel piezoelectric elements and dual processing circuits are required to generate optimum rectified currents from the low-frequency, high-voltage signals generated by weightbearing, as well as the high-frequency, low-voltage signals produced by ultrasound. This paper reports on the current status of development and describes design parameters of this device which combines the modalities of mechanical fixation and electrical stimulation in a single implant.