Abstract:
A medical drilling device for drilling of human or animal tissue carries a drill ( 7 ) that is electrically contacted by a bore electrode ( 9 ) and a backing electrode ( 10 ). Both electrodes ( 9, 10 ) are charged by a resistance-measuring device ( 11 ) in a way, that electrical resistance of tissue ( 2, 3, 4 ), preferably impedance, as measured between the drill ( 7 ) and the backing electrode ( 10 ) becomes measurable. By means of a monitoring device ( 13 ), the surgeon is provides with information about the type of the respective tissue and the depth of drilling.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage application of International Application No. PCT/EP2004/000886, filed on Jan. 30, 2004, which claims priority of German application number 103 03 964.3, filed on Jan. 31, 2003. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention deals with a surgical drilling device and subsequent drilling procedure aiming at drilling holes in live human or animal bones. The device is equipped with a drilling drive rotating a drilling tool that conducts electrical currency. 
     2. Description of the Prior Art 
     Orthopedic surgery and trauma care require bone drilling when repair of fractures is indicated. The holes resulting from drilling accept screws to hold implants such as plates, prostheses or to exert pressure between broken and subsequently reduced bone fragments. The screws inserted have to fit allowing a length range of no more than one millimeter. Conventional measuring of length is using a mechanical slide gauge, which is unhandy, time consuming and erratic, sometimes requiring repeated x-ray-control and corrections, yet even tissue injury in rare cases. 
     The bones in question usually are of two major structures: cortical bone, which for example is stress carrying entity in tube-shaped long bones, and spongious bone, which e.g. is accumulated in the vicinity of joint surfaces. 
     Passing with a drill through a tube-shaped bone perpendicular to the long axis means penetrating at first the cortical layer, followed by spongious bone and reaching out through the opposite cortical layer to touch soft tissue surrounding the bone. The opposite cortex is normally covered by soft tissue, whereas the at the starting point, the small area, where the drill is attached before drilling on the nearside cortex is being either surgically exposed or reached transcutaneously by palpation with the aid of a socket. This socket protects soft tissue from immediate contact with the drill and equally helps to guide the drill, to give it the direction of choice. 
     Alternative matters to determine the depth of the drilled hole are provided by ultrasound techniques or use of lasers. These methods refer to variable reflections from soft tissue, cortical and spongious bone. These methods, however, are technically demanding and need separate measuring—as with the slide gauge—by interrupting the drilling act. 
     When drilling through bones, at the end the surgeon has to take care of the adjacent soft tissue after penetration the opposite cortex, thus to prevent this tissue from being injured. To make sure that this does not happen, some surgeons interrupt the drilling act in order to palpate with the non-rotating drill whether or not the cortex is penetrated. It is the object of this invention to simplify this manoeuvre by sparing the surgeon this interruption and simultaneously indicating the depth of the bore hole. This technical ability is provided by the demand number one of this invention. Furthermore, the invention in question refers to a medical drilling device which is equipped with a bore electrode (incorporated in the drill), a backing electrode as well as a measuring device for electrical resistance connecting both electrodes. Thus, the bore electrode is directly connected with the current conducting drilling device, while the backing electrode is mounted at an opposite distance to the drilling device, e.g. on the skin of the patient. 
     With the aid of the resistance measuring device, the two electrodes are exposed to continuous or alternating electrical current. Subsequently, the electrical resistance between the bore electrode and the backing electrode can be measured and indicated on a monitoring screen. 
     SUMMARY OF THE INVENTION 
     In an advantageous model of the invention, the resistance measuring device (RMD) produces an alternating current and registers resistance by the dimension of impedance. With this arrangement, it is possible to measure the electrical resistance of the tissue between the drilling device and the backing electrode particularly clean, i.e. without being disturbed by electrical fields or other static loads. 
     The basis of these mechanisms lies in the knowledge, that human and animal tissue particularly the contrast between soft and bone tissue, are characterised by different impedance levels. This allows to literally locate the tip of the drill on its way through the tissue. It was demonstrated that the tip of the drill by penetrating the opposite cortex and touching soft tissue leads to a significant change of impedance. Seeing that as an electrical or digital signal, the surgeon can conclude, that the tip of the drill has just penetrated the opposite cortex. By this, he can avoid injuring soft tissue and simultaneously know about the length of the needed screw. 
     It is of particular advantage, that continuous resistance measurement is possible during the drilling procedure, thus interrupting the drilling process is not necessary any more. A figure of electrical resistance is also given by applying continuous current. However, it needs to be verified whether this figure is sufficient to differentiate the various tissue types. It can be imagined, that continuous current is easily disturbed and thus lacking reliable data. 
     A particularly advantageous model of the invention offers a monitoring technique that indicates optical and acoustical signals based on measured resistance and the change of respective figures. This enables the surgeon to locate his bore tip. The data can be provided by absolute digits or by means of relative differences. The data processing can be arranged in a way that an automatic switch-off is integrated or a signal indicating the particular tissue quality, where the tip of the drill is presently located. 
     The drilling device, as characterised by this invention, does not only allow to determine time of penetration, but also to monitor the surgeon&#39;s conduct in particularly difficult, 3-dimensional regions such as in pelvic surgery. The surgeon is being navigated through narrow spongious bone without risk of accidentally penetrating adjacent cortex. 
     It is particularly useful, when the drilling device is equipped with an electrically conducting drill that is covered by insulating material. Due to this insulation, electrical current runs through the core of the drill only allowing electrical contacts at two points: At the tip of the drill and in a ring-like area on the bore shaft. In the last mentioned ring-like area, a voltage generating continuous or alternating current can be applied. The resulting current is conducted without disturbances to the drill&#39;s tip. At the end, electrical resistance between the tip of the drill and the backing electrode depending on the tissue&#39;s specific impedance can be precisely measured. 
     It is mandatory, that the rest of the bore shaft, which is mounted to a drilling chuck, is also electrically insulated to avoid errors in resistance measurements induced by the drilling chuck or the drive mounted behind. 
     Further development of the invention includes a device which can measure the drilling depth, i.e. by measuring the drilling advancement. Provided that measuring starts at a time, when the drill hits the first cortical surface, a precise measurement of the drilling hole&#39;s depth is possible. It is planned to indicate the measured advancement on a monitoring device, that can be observed by the surgeon and the assisting nurse, who usually selects the appropriate screw length. The time, when the drill hits the first cortical surface, is registered by a sensor operated by the surgeon. So is the time, when the drill leaves the opposite cortex. 
     It is obvious, that all materials and devices have to be stable enough to undergo sterilisation at 130° centigrade. 
     In summary, this invention refers to a drilling device that enables the surgeon to measure the length of the drilling hole and subsequently the length of the needed implant (screw). Simultaneously, i.e. without interrupting the procedure, it is possible to continuously locate the drill&#39;s tip and thus navigate through difficult, 3-dimensional tissue components as well. 
     Subsequently, this invention is going to prevent from repeated trials of implant fixation in case of wrong measurements and for that will safe operating time as well as repeated X-Ray-controls, which render unnecessary by this invention. 
     The above mentioned and further advantages and characteristics of the invention will be explained by the following drawings and practical examples: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  Schematic drawing to explain a first possible design of the invention related drilling device and the respective drilling procedure. 
         FIG. 2  Schematic drawing of a second possible design of the invention 
         FIG. 3  Schematic drawing of a third design of the invention. 
         FIG. 4  Schematic drawing of a fourth design of the invention 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically shows the principal construction of the invention-related drilling device during a drilling procedure through live bone. The bone ( 1 ) in shape of a tube like bone is drawn in cross section. The centre is filled with spongious bone  2  (spongiosa) being surrounded from a cylinder of cortical bone  3  (cortex). The more solid cortex is covered by soft tissue  4 , such as muscles, vessels etc. 
     The soft tissue  4  has been removed at the entry point of the drill  5 , exposing cortical bone. At this point a protective socket  6  is used, firstly to prevent the drill from contacting soft tissue, secondly to avoid skidding from the curved surface, thirdly to help guiding the drill  7  in the right direction through the bone. The socket is gently sheeting the drill, allowing enough freedom for unhindered rotation. 
     As shown in  FIG. 1  the tip  7   a  of the drill  7  has first penetrated the cortex and afterwards the softer spongious bone  2  to settle again in cortical bone  3 . For reasons of better definition this part of cortical bone is called opposite cortex  8 . At the end of the procedure, the drill will have penetrated the opposite cortex and have reached adjacent soft tissue. At this time it&#39;s the skill of the surgeon to stop drilling, before soft tissue is going to be injured. 
     The invention-related drilling device holds a drill  7 , that is being contacted by a bore electrode  9 . Vis-à-vis to the drill  7  or to its tip  7   a  a backing electrode  10  is mounted preferably at an easily accessible site (i.e. patient&#39;s skin). 
     The bore electrode  9  and the backing electrode  10  are connected to a resistance measuring device  11 , that is exposing a continuous or alternating current/voltage to the electrodes  9 ,  10 , and thus measuring electrical resistance 
     The drill  7  is a conventional drill made of electrically conducting steel. Except for the tip  7   a , however, and an annular surface around the proximal drill shaft in 12 cm distance from the tip, the drill&#39;s surface is electrically insulated, i.e. by galvanisation or eloxation. 
     The bore electrode  9  is getting into contact with the electrically conducting annular surface of the proximal drill  12 . Thereby the continuous or alternating voltage of the bore electrode  9  is transmitted to the annular surface  12  and therefore conducted directly to the tip of the drill  7   a.    
     In principal, with this method measuring of the resistance against continuous current of the tissue between the tip of the drill  7   a  and the backing electrode  10  will be possible. Preliminary experiments however led to the conclusion, that applying alternating voltage and measuring impedance will yield particularly precise data. 
     Measuring of impedance happens with as low as possible voltages, i.e. 1 volt. The alternating voltage is generated by the resistance-measuring device, subsequently impedance is calculated from the measured current. For this purpose the so-called lock-in-technique seems very suitable. This technique is widely known, which is why further description seems unnecessary. The frequency of the alternating voltage typically ranges between 1 kHz and 100 kHz. Thereby, erratic currents caused by the 50 Hz buzzing or by high frequent couplings can be avoided. Subsequently, the current is only registered within the given range of frequencies (lock-in). Currents of other frequencies are being disregarded. In addition to the amplitude of impedance, its face is determined, too. With the aid of suitable band pass filters, the measured signal is being processed. 
     As it is of major importance for the surgeon—as explained above—to know about the time of penetrating the opposite cortex  8 , the change of impedance between the various tissue types should lead to suitable measures to detect it. This change of impedance, for example, can be monitored by using suitable filters to differentiate the signals according to their maximum. 
     The RMD  11  is connected to a monitoring device  13  for evaluating the data measured by the RMD. For this purpose, a computer system is hooked up to the monitoring device. This makes it possible to display the tissue type (cortex  3 , spongious bone  2 , soft tissue  4 ) and thus to locate the positions of the tip of the drill at a time. By penetrating the opposite cortex  8  and touching the adjacent soft tissue  4 , an optical as well as an acoustic signal can be generated. Furthermore, the monitoring device  13  can be used to indicate the depth of the drilled hole. 
       FIG. 2  shows a second design of the invention related drilling device, while the above-mentioned RMD stays the same. 
     In detail, according to  FIG. 1 ,  FIG. 2  shows, that the drill  7  is hooked up to a drilling chuck  14 , which is brought into rotation by a bore drive, which is not particularly illustrated. 
     The bore electrode  9  contacts a widely known carbon sliding-bow at the annular surface  12  of the drill  7 , which is electrically conducting. 
     For better guiding of the bore electrode  9 , it is integrated in a special case. This, in turn, is hooked up, for example by means of a bayonet-clutch, to the main case  16  carrying the drilling device. 
     In addition to the above described resistance measurement, the second design of the invention also includes the measuring of depth  17  of the drilled hole. The drilling-depth measuring device (DDMD)  17  in  FIG. 2  is constructed as follows: 
     Between the centre piece  16  of the drilling device and the tissue protecting socket  6 , a mechanically stretchable and compressible element is mounted, for example a spring  18 . This spring can be fixed on one side only, e.g. at the head piece  15  while being loosely attached to the protecting socket  6 . By advancing the drill  7 , the spring subsequently shortens, while it will extend when the drill is withdrawn. The power necessary to deform the spring can be measured by means of electrical devices for load indication (not drawn in the Fig). Thereby one can measure the degree of advancement of the drill in indicate the distance on the monitor  13  ( FIG. 1 ). 
     With a signal indicator (not shown here), the surgeon can determine the start-off for measuring the drilling depth. The surgeon can start the signal indicator at the moment he begins drilling. After this onset, the advancement of the drill is continuously measured. Occasional interruption of the drilling procedure does not harm the measurement, because the intercepted computer will calculate only the absolute advancement against the starting point. 
     In combination with the above-described RMD, the surgeon is continuously being informed about the actual drilling depth, the eventual type of tissue and tissue borders. 
       FIG. 3  schematically shows a third design of the invention, which essentially refers to the second design. It only differs in that the DDMD here relies on the principle of magnetic or electromagnetic tools. 
     For this purpose, the tissue protecting socket  6  is equipped with adding, attachment or support  19 , which on the one hand allows the drill to pass on its way through an opening  20 , on the other hand carries a magnetic spool  21 . This spool  21  is completely encapsulated to ease cleaning and sterilisation. This special adding/attachment support  19  to the socket can also be mounted separately to the case of the drilling device. 
     In the centre case  16  or the head piece  15  of the drilling device, a ferrite-rod is mounted being also encapsulated for the known reasons. The ferrite-rod  22  intrudes the spool  21  as spool core. Then it is inductivity of the spool from which the advancing of the drill can be calculated and indicated on the monitoring device. 
       FIG. 4  finally shows a fourth design of the invention, which differs from the fore-going designs only concerning the DDMD  17 . In contrast to the fore-mentioned mechanical and magnetic methods, this design deals with optical measuring. 
     For this purpose, the drill  7  carries coloured rings, e.g. in a distance of 0.5 mm. The colours can change in the following way: White, Red, Green, White, Red, Green, White. 
     The tissue protecting socket  6  carries a second measuring socket  23 . In this measuring socket  23  two (not illustrated) light sources are focused at the drill ( 7 ), where-by the light from the first source, e.g., can be reflected from the green as well as from the white colour ring, whereas the light of the second source can only be reflected from the red and the white colour ring. The reflected light from each source is going to be detected by a photo diode (not illustrated). In a special processing unit (not illustrated) the change in colours detected from the reflections is registered, the sequence is counted and thus direction and degree of bore advancement can be indicated via EDV. 
     Example: The system can conclude for advancement if the colour changes from white into green, while drawing back the drill is indicated when white changes into red. The resolution of the system amounts to 0.5 mm. A special cleansing device is mounted at the entry of the tissue-protecting socket in order to protect the colour rings from being polluted with blood and bone debris. 
     The designs as presented in  FIGS. 2 and 4  are shown only to explain principle alternatives in the construction of the DDMD. The finally chosen design of this invention has to take into account, that the drilling device can also be used by hand (alternatively by robots), because the surgeon wants to feel the tissue quality. Therefore, the DDMD needs to provide possibilities to determine by hand the drilling speed and the drilling direction and to allow intermediate interruption of the drilling process. This, of course, does not prohibit fixing the DDMD in a special tool, such as a robot. 
     To improve the precision of measurement, in addition to the change of impedance, the torque of the drilling drive can be measured and used for integrated calculations. 
     Depending on the tissue borders and the specific tissue resistance, the drill will de- or accelerated. It is known, that deceleration is also due to friction between the cylindrical sidewalls of the drilling hole and the drill itself. It is known, however, that deceleration is predominantly caused by shear load at the tip of the drill. The measurement of torque can be done in different ways: It is possible, to measure the uptake of current by the drive and therefrom draw conclusions on the torque. 
     The integrated information out of torque of the drill and change of impedance can be evaluated by the processing unit in order to get precise information on the tissue qualities and the location of the tip of the drill at a time. Once the tip of the drill leaves the spongious bone to enter the opposite cortex, not only a change of impedance, but also an increased torque can be measured. 
     This torque depends on the shear load, which itself depends on the exerted pressure the surgeon is applying. Therefore, it can be useful to additionally measure this pressure by a separate tool. 
     The information about the exerted drilling pressure helps to calculate a standardised relative torque from the measured absolute torque. This might even more improve the precision of measurement. 
     What has been described above are preferred aspects of the present invention. It is of course not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, combinations, modifications, and variations that fall within the spirit and scope of the appended claims.