a system and method for healing bone fractures is described herein . the system is a dynamic system for applying high frequency , low displacement and low stress stimulation to a fracture site for accelerating the healing process and improving bone strength . the system includes an energy source which transfers energy to the bone . an actuator provides a force and a displacement to the bone . a feedback means collects information about predetermined parameters , for example resistance to displacement or a resistance force . the feedback means may be used to control the energy source to provide more or less energy to the bone . the actuator is thereby adjusted to provide a predetermined amount of work or energy to the broken or fractured bone joint . the actuator may be programmed to provide cyclical displacement or cyclical forces to the bone according to a formula or program . in addition , as the bone heals the actuator means is adjusted to account for smaller displacements and higher forces . referring now in specific detail to the drawings in which like reference numerals identify similar or identical elements throughout the several views , and initially to fig1 a system 10 is shown for therapeutically enhancing bone growth . system 10 includes an external fixator 12 which is coupled to bone portions 14 and 16 by screws or pins 18 inserted through bone portions 14 and 16 . alternately , fixator 12 may be coupled to bone portions 14 and 16 non - invasively by clamps ( see fig4 ). fixator 12 secures bone portions 14 and 16 and acts as a strength by - pass for transferring weight across a fractured or broken portion 20 . fixator 12 has a translatable intermediate portion 23 which provides relative translation between a first end portion 22 and a second end portion 24 of fixator 12 . an actuator 26 is provided to produce relative motion between first end portion 22 and second end portion 24 , thereby moving bone portions 14 and 16 closer or further apart . in one embodiment , actuator 26 includes &# 34 ; terfenol - d &# 34 ;, which is a magnetostrictive material , and actuates fixator 12 proportionally to a magnetic field applied thereto . &# 34 ; terfenol - d &# 34 ; actuators are commercially available from etrema products , inc . other actuator materials may include piezoelectric materials and the like . actuator 26 is controlled by a controller 28 or may be adjusted manually . controller 28 outputs signals to control actuator 26 based on feedback information . feedback information is based on monitored parameters which represent the present state of bone portions 14 and 16 . fixator 12 may include lvdt &# 39 ; s for measuring dynamic displacement between bone portions 14 and 16 or actuator 26 . in addition strain gauges or load cells may be used to measure displacements and forces respectively from the fixator 12 , actuator 26 or bone portions 14 and 16 , depending on the conditions of treatment . data gathered for forces , strains , displacements , etc . is collected by a data acquisition system 30 and transferred to controller 28 . controller 28 processes feedback data and sends output signals to actuator 26 to modify parameters as needed . during operation , bone portions 14 and 16 are mechanically stimulated by applying a waveform , preferably sinusoidal , at a predetermined frequency . the frequency may be selected from within a range of about 5 hz to about 20 khz , preferably between about 25 hz to about 100 hz . the power necessary to drive the actuator may be delivered by a portable power source , for example , batteries , or may be derived directly from household current . healing of a fracture occurs in stages , in the first stage bones are separated . mechanical stimulation using system 10 provides displacement amplitudes preferably up to 200 microns , but never to exceed a prescribed stress , by actuating actuator 26 with a proportional control signal . as the bone portions heal , callus forms therebetween . at this point , treatment must be modified in accordance with the feedback obtained from bone portions . displacement is decreased and force is increased by a predetermined amount . in one embodiment , the product of the force in the bone and the displacement amplitude ( work ) is maintained approximately constant . for example , as the bone becomes stronger more force may be applied but less deflection , thereby maintaining a relatively constant product . other relationships between force and displacement may be derived to further improve the healing rate and bone strength as described below . in a preferred embodiment , stress is determined within the bone by measuring the normal force directly and estimating the projected area of the bone fracture . by determining an optimal stress window , the healing rate may be further increased . an indication for the energy density imparted to the bone for a given time can be conveniently expressed as stress / frequency ( pascals / hz ) where stress is the measured or estimated stress between bone portions , and frequency is the frequency of mechanical stimulation . in this way , energy may be induced into the bone by various means to achieve a desired stress to frequency ratio . one aspect of system 10 , is a mechanical limiter 32 which prevents overloading bone portions 14 and 16 when angiogenesis and woven bone callus healing stages have begun . initially the force applied to bone portions should be significantly less than the dynamic loads applied during normal use , for example , walking forces for a tibia . as healing ensues forces may be increased , but must remain below a threshold so as to not reverse the healing process . mechanical limiter 32 must therefore be adjusted to account for this threshold . mechanical limiter 32 may be a strain gauge or load cell that disables controller 28 or actuator 26 if the threshold is exceeded . referring to fig2 a method for enhancing healing rate and increasing bone strength is shown in flow chart 50 . in block 52 , bone portions 14 and 16 ( fig1 ) are repositioned in proper alignment to begin the healing process . if a fixator , such as an intra - medullary nail or rod , or an external fixator , is used , it is applied in block 54 . also in block 54 , any feedback equipment is setup to measure feedback parameters such as force , strain or displacement amplitude between bone portions . in block 56 , initially , displacement and force are balanced to cyclically stimulate the bone fracture to provide a desired energy absorption rate . after callus begins to form , energy is applied to the bone fracture to create a stress field therein to enhance healing by absorbing energy at the fracture . energy sources for inducing this stress field may include electrical or magnetic energy , ultrasound energy , mechanical energy , etc . in block 58 , feedback data is obtained which may include a measured force and / or displacement between bone portion during stimulation . also calculations of formulas such as the calculation of work may be completed to determine if parameters should be adjusted . in block 60 , it is decided whether the monitored parameters need to be adjusted . if adjustment is needed , criteria , such as stress / frequency , are checked to determine the amount of adjustment needed for the monitored parameters . the parameters are adjusted accordingly in block 64 and the path is returned to blocks 56 , 58 and 60 until no further adjustment is necessary . in block 66 , the cyclical stimulation is applied for a predetermined amount of time . the time may range from between about 5 minutes to about 500 minutes daily , and preferably from about 10 minutes to about 30 minutes . treatment time may be monitored by controller 28 ( fig1 ) directly and system 10 may be turned off directly when the appropriate treatment time has elapsed in block 68 . referring to fig3 an alternate embodiment of system 10 is shown as system 100 . system 100 includes an intra - medullary nail or rod 102 inserted into an interior of a bone 104 . bone 104 is fractured into bone portions 106 and 108 . rod 102 provides a strength by - pass across a fracture portion 110 to transfer loads across fracture portion 110 . strain gauges and load cells may be directly mounted on rod 102 to provide feedback data to data acquisition system 30 and controller 28 . rod 102 has a first end portion 112 and a second end portion 114 which are anchored within bone 104 . rod 102 includes an actuator 116 which operates substantially as described above with reference to fig1 . actuator 116 applies loads and displacements to bone 104 to enhance the healing rate in accordance with this disclosure . both systems 10 and 100 provide a patient with mobility . fixator 12 and rod 102 may be disconnected from data acquisition system 30 and controller 28 to permit free movement of a recovering bone fracture patient in between treatment sessions . referring to fig4 a system 200 is shown . a patient stands on an impact or shaker table 202 . clamps 204 are externally attached to , for example a leg or legs of a patient . clamps 204 are provided with feedback devices which monitor relative deflections between clamp positions , estimate forces applied to a bone , for example a tibia 206 and measure strain and stress within the bone being treated . clamps 204 may include accelerometers to measure acceleration of the patient &# 39 ; s bone to aid in estimating forces applied thereto . shaker table 202 functions as the actuators described above providing mechanical stimulation to the bone being treated . a cyclical excitation of shaker table 202 is used to generate a deflection within the bone being treated . shaker table 202 parameters such as amplitude and frequency are adjusted according to feedback data collected from data acquisition system 30 by controller 28 . controller 28 supplies waveforms , preferably sinusoidal , of a given amplitude and frequency to shaker table to ensure appropriate energy absorption into the bone being treated . systems 10 , 100 and 200 are used to treat patients according to the method described in fig2 . system 200 does not require block 52 and portions of block 54 which involve fixation for a bone fracture . recent studies undertaken by the inventors conclusively show that smaller stresses and displacements will significantly accelerate fracture healing if the loading is undertaken at higher frequencies . in an animal study , the effect of high frequency dynamic loading on promoting healing of tibia fractures in sheep was investigated . the protocol included the utilization of an external fixator which had been modified to provide dynamic loading of the fracture site ( fig1 ). initial displacements of about 25 micrometers were permitted across a fracture gap of approximately 3 millimeters . peak forces were limited to about 40 newtons , or an estimated peak stress of 350 kpa . dynamic loading was applied at about 30 hz for 1000 seconds per day . fractures exposed to this treatment regimen were found to increase in torsional strength at a rate significantly faster than untreated fractures ( fig5 ). the average effect of treatment was a threefold rise in fracture callus stiffness at the end of the 10 week treatment regimen ( fig6 ). moreover , bone mineral content was also found to be approximately threefold greater in the treatment group following 10 weeks of treatment ( fig7 ). referring to fig5 torsional stiffness and strength of bones used in experimentation were monitored for sheep having midshaft ovine tibial 3 mm osteotomies . after 10 weeks of applying the method of fig2 using system 10 , strength and stiffness were measured for 5 control groups and one group using &# 34 ; terfenol - d &# 34 ; actuators in accordance with the present invention . the control group had fractures stabilized by fixators that were locked in position . the &# 34 ; terfenol - d &# 34 ; group experienced mechanical stimulation by actuating fixators used to stabilize the fractures . as shown in fig5 and 6 , stimulated fractures had significantly more torsional stiffness ( p & lt ; 5 %) than the controls . as shown fig7 bone mineral content ( bmc ) data was collected . bmc in the stimulated group was significantly greater both in the fracture zone ( p & lt ; 1 %) and in four periosteal regions adjacent to the fracture gap ( p & lt ; 0 . 1 %). the combined studies by the inventors on inhibition of bone loss by dynamic mechanical loading and acceleration of bone fracture healing by dynamic loading have led to the identification of an algorithm for the efficient promotion of fracture healing by dynamic loading . the algorithm relates the product of the stress applied to the fracture site and the duration of the treatment regimen to the frequency of the applied dynamic loading . these three factors can be shown to be related by the expression : log ( stress * duration )≈ 4 . 5 - 0 . 3 * log ( frequency ) where frequency is in hertz ; stress is in kpa and duration is in minutes . this algorithm is valid through the acoustic range ( from 5 khz up to 20 khz ) and for durations extending from about 5 to about 500 minutes . during the early stages of fracture healing , stresses below this threshold level can be utilized under displacement control , until the callus is sufficiently rigid to support the above stress levels . however , displacements greater than 10 % of the fracture gap are not preferred . importantly , this algorithm demonstrates that the use of large , potentially damaging , stresses can be eliminated by either increasing the loading frequency or increasing the duration of loading or both . stresses more than twice the above prescribed levels should be avoided , as they may contribute to the incidence of delayed or non - union . because for a given modulus of the bone tissue , a stress can be converted to a strain within the tissue , the above formulation may alternatively be viewed as energy transfer needed to the fracture site . as the modulus of the fracture site changes dramatically as a function of time and healing ( e . g . phases of inflammation , repair and remodeling ), conversion to strain requires that the displacements output by the device will change as a complex interdependent function of time and stage of healing . having described preferred embodiments of a method and system for healing bone fractures and treating osteoporosis ( which are intended to be illustrative and not limiting ), it is noted that the modifications and variations could be made by those skilled in the art in light of the above teachings . it is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention defined by the appended claims .
a system for therapeutic treatment of bone includes a source of energy for stimulating a bone , a feedback loop for receiving response information from the bone generated by the stimulation and an adjustment device for adjusting the energy source according to predetermined criteria . a method of therapeutically treating bone includes the steps of providing a source of energy for stimulating a bone , feedback means for receiving response information from the bone generated by the stimulation and adjustment means for adjusting the energy source according to predetermined criteria , applying energy to cyclically stimulate the bone , collecting response information from the bone due to the stimulation , comparing the response information to the predetermined criteria and adjusting the energy source to obtain a response in accordance with the predetermined criteria . the system and method ensure , augment and accelerate the bone healing process .