Abstract:
A device for determining the internal structure of a bone along a path directed into the bone (or other tissue) is disclosed. The device comprises a nozzle fluidically connected to a liquid reservoir for providing a liquid jet directed at the bone in the direction of the path; an ultrasonic transducer for generating ultrasonic waves through the liquid jet and for detecting echoes of the ultrasonic waves caused by changes in the acoustical impedance in the bone characterizing changes in the structure of the bone along the path; and an analyzer for interpreting the echoes into meaningful information relating to the location of the structural changes along the path.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and device for improved depth measurement during surgery. More specifically it provides a medical tool that either includes a liquid jet or to which a liquid jet can be added, and an ultrasound wave generator that transmits ultrasound waves and detects echoes through this jet of liquid, for determining the internal structure within the bone along a path in front of the tool, and measuring locations of structural changes along that path. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention is hereinafter explained with reference to dental surgery. However the applications of the present invention are not limited to dental surgery only, and in fact include any surgery involving the use of a medical tool, which is intended to penetrate into tissue and bone such as in orthopedics, general surgery and other such fields. 
         [0003]    For planning an implantation procedure, which is performed by an oral and maxillofacial surgeon, in the mandible or the maxilla, the patient undergoes an x-ray imaging, computer tomography (CT) and a panoramic imaging. From these images, the surgeon is able to plan the implantation procedure and also to take-out the data about the mandible, such as the depth of the canal that contains the nerve and the thickness of this bone. These imaging methods do not provide the surgeon with required information in real time, concerned with the distance between the bottom part of the drill and the top part of the canal that contains the nerve. 
         [0004]    The present invention completes the information obtained by the CT and the panoramic imaging, by guiding the surgeon during the drilling process thus enhancing prevention of injuries during the surgery. It provides a method and device for measuring the distance between the bottom part of a dental drill and vulnerable parts (such as the canal that contains the nerve in the lower mandible) in a patient&#39;s bone during an implantation surgery process. For example, the invention can be integrated into a dental drill to provide the dental surgeon with real time guiding location of the drill tip in the patient&#39;s bone during implantation surgery. 
         [0005]    With the present invention the surgeon can know where the drill tip is relative to sensitive areas of the patient&#39;s bone. This improves the efficiency, safety and reliability of the surgical procedure, particularly in cases where the depth of the drill must be carefully monitored and guided, in order to prevent injury to vulnerable areas in the bone, such as nerves or blood vessels that lie on the drill&#39;s path. 
         [0006]    The present invention measures the location of the bottom of the drill relative to the surrounding bone by means of ultrasound (US) radiation and makes that information immediately available to the surgeon. The ultrasound is transmitted and received via the jet of liquid running from the drill. 
         [0007]    US transmission through a liquid jet was described as a confirmation sensor of a drill in its position in a drilling machine (U.S. Pat. No. 5,850,184), as a dental tool for removing plaque from teeth and for cleaning teeth (U.S. Pat. No. 5,013,241), a high velocity pulsating jet stream for the removal of the dental plaque (U.S. Pat. No. 4,012,842), A thermal imaging system for detecting cracks in tooth, by applying an US dental cleaning tool that transmits US energy through a jet of water to the tooth. This causes cracks in the tooth to heat up and a thermal camera is used to detect the thermal radiation emitted by the heated cracks (U.S. Pat. No. 6,437,334B1). 
         [0008]    Another possibility is to sweep angularly with the jet of water that carries the US, and by this one is able to obtain an image of a section or of a volume. 
         [0009]    US radiation is a proven technology used in many medical diagnostic applications and was also applied for bone diagnostics—density, structure and velocity of propagation (U.S. Pat. No. 6,030,221, U.S. Pat. No. 5,006,984, U.S. Pat. No. 5,115,813, U.S. Pat. No. 5,402,781, U.S. Pat. No. 5,518,008, U.S. Pat. No. 5,564,423, U.S. Pat. No. 5,651,363). These parameters are important in order to investigate the correct value of the distance mentioned earlier. The radiation levels applied during the use of the present invention are in an accepted range for diagnosis and therefore pose no harm to the patient or the surgeon. Temperature gradients of the radiated area are negligible, especially when the US radiation is transmitted via a liquid jet as in the case of the present invention. 
         [0010]    The present invention is particularly advantageous for surgery where depth measurements in the mandible and the maxilla are required, as for example, in mandible intra-osseous implantations. 
         [0011]    Another advantage of the present invention for such surgery is that the US radiation is applied through an existing jet of liquid (usually distilled water) flowing from the dental tool. Therefore no additional medium is required, keeping the dimensions of the drill fitted with present invention small and well suited for work within restricted volumes, like the mouth. 
         [0012]    The present invention provides the surgeon with real-time information; is relatively inexpensive compared to other, less satisfactory imaging alternatives; and saves time for both the patient and the surgeon (since there is no need to move imaging equipment in and out or to interrupt the surgical procedure). 
         [0013]    In summary, it is a main object of the present invention to provide a probe for use in dental surgery for measuring distance from a bottom of a drill to surrounding bone, the probe provided with a mechanism for transmitting and receiving ultrasonic signals, a mechanism for processing the ultrasonic signals to determine the distance to the tissue, and a mechanism for communicating that information to the surgeon. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0014]    There is thus provided in accordance with a preferred embodiment of the present invention, a device for determining the internal structure of a bone along a path directed into the bone, the device comprising: 
         [0000]    a nozzle fluidically connected to a liquid reservoir for providing a liquid jet directed at the bone in the direction of the path;
 
an ultrasonic transducer for generating ultrasonic waves through the liquid jet and for detecting echoes of the ultrasonic waves caused by changes in the acoustical impedance in the bone characterizing changes in the structure of the bone along the path;
 
an analyzer for interpreting the echoes into meaningful information relating to the location of the structural changes along the path.
 
         [0015]    Furthermore, in accordance with some preferred embodiments of the present invention, the device is incorporated in a surgery tool. 
         [0016]    Furthermore, in accordance with some preferred embodiments of the present invention, the surgery tool is a drill. 
         [0017]    Furthermore, in accordance with some preferred embodiments of the present invention, the drill is hollow, allowing the liquid jet to pass through the drill. 
         [0018]    Furthermore, in accordance with some preferred embodiments of the present invention, the liquid jet is outside the drill, directed to a tip of the drill. 
         [0019]    Furthermore, in accordance with some preferred embodiments of the present invention, there is provided a method for determining the internal structure of a bone along a path directed into the bone, the method comprising: providing a liquid jet directed at the bone in the direction of the path; generating ultrasonic waves through the liquid jet and detecting echoes of the ultrasonic waves caused by changes in the acoustical impedance in the bone characterizing changes in the structure of the bone along the path; and interpreting the echoes into meaningful information relating to the location of the structural changes along the path. 
         [0020]    Furthermore, in accordance with some preferred embodiments of the present invention, the method is incorporated in surgery. 
         [0021]    Furthermore, in accordance with some preferred embodiments of the present invention, the surgery involves drilling into the bone. 
         [0022]    Furthermore, in accordance with some preferred embodiments of the present invention, the method is implemented simultaneously with the drilling. 
         [0023]    Furthermore, in accordance with some preferred embodiments of the present invention, the method is implemented when drilling is halted. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0024]    The invention is described herein, by way of example only, with reference to the accompanying Figures, in which like components are designated by like reference numerals. 
           [0025]      FIG. 1  illustrates how US waves are transmitted through, and reflected from, soft and hard layers of the body in accordance with a preferred embodiment of the present invention. 
           [0026]      FIG. 2A  illustrates a US transducer integrated into a liquid jet in an interactive ultrasound-based depth measurement device in accordance with a preferred embodiment of the present invention. 
           [0027]      FIG. 2B  illustrates front-end electronics located close to the US transducer in order to reduce signal loss in an interactive ultrasound-based depth measurement device in accordance with a preferred embodiment of the present invention. 
           [0028]      FIG. 3A  illustrates an interactive ultrasound-based depth measurement device integrated with a surgical dental drill in accordance with a preferred embodiment of the present invention. 
           [0029]      FIG. 3B  depicts a tip of a drill similar to the one shown in  FIG. 3A . However in this example, the drill itself is hollow, allowing a liquid jet to pass through. 
           [0030]      FIG. 4  illustrates how US waves propagate through the jet of liquid in a dental drill integrated with an interactive ultrasound-based depth measurement device in accordance with a preferred embodiment of the present invention. 
           [0031]      FIG. 5  illustrates a detailed block diagram of analog and digital circuitry for a dental drill with an interactive ultrasound-based depth measurement device in accordance with a preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    The present invention is based on transmitting ultrasonic (US) waves through a guided jet of liquid, such as distilled water, targeted to a bone or other tissue or a combination thereof (hereinafter—bone), in order to determine the internal structure along a path in the direction of the impinging jet. The echoes of the US waves are used to determine changes in the acoustical impedance of the bone characterizing changes in the internal structure, and determine their location along that path. 
         [0033]    In one preferred embodiment, according to the present invention, the device is incorporated in a drill or other penetrating medical device, so as to provide the surgeon with information on the path along which the tool is penetrating the bone, and specifically distances to structural changes in the bone, thereby preventing or greatly reducing the risk of inflicting damage to vulnerable tissue. 
         [0034]    However, the present invention is not limited to such incorporation, and a basic device, in accordance with some preferred embodiments of the present invention, would include only the provision of US transducer for provision of US waves carried by a liquid jet towards the surface of the bone, and for detecting echoes reflected from structural changes within the bone along a path in front of the impinging jet. 
         [0035]    The measurement provided by the present invention is based on US wave propagation through a material. When there is a change in the velocity of propagation (i.e. a change in the acoustic impedance), reflections are obtained. These echoes are detected and provide finally electronic signals, which are finally interpreted as distances (given the velocity of propagation through the examined medium is known from these US measurements) where the reflections appeared. 
         [0036]    It is known that the velocity of propagation of US waves through the bone varies. It is different for the upper and lower jaws. It is also different between males and females. It may vary with age. It also varies in different locations within the bone. In order to overcome this and take it into consideration in the measurement several methods may be incorporated. These include, for example, measuring the time intervals between the transmitted pulses and the received echoes, and at the same time measuring the velocity of propagation in the bone. 
         [0037]    Accordingly, in order to properly conduct a distance measurement, it is important to measure simultaneously both the time between the transmitted and received waves as well as the velocity of propagation at the desired depth. 
         [0038]    The US waves are transmitted through the jet of liquid, which is directed toward the bottom part of the bore produced by the drill, and into the bone under the bottom of the bore. Abrupt changes in the acoustic properties of the matter traversed by the US waves produce US echoes (reflections). 
         [0039]      FIG. 1  illustrates in principle how this property of US waves is put to use in the present invention. Power signals  32  are sent from power  33  circuits to US transducer  68 . The control electronics  31  controls the power circuits  33 . In the following case, US transducer  68  transmits US waves  34  through soft layers  26  and through a bone  22 . US waves  34  encounter nerve canal  20  (containing nerve  24 ), which generates a distinct (mirror like) reflection  36 . When reflection  36  returns from canal  20  via bone  22  and soft layers  26 , a similar (attenuated) reflection is obtained, but with an opposite polarity. Reflection  36  is received by US transducer  68 , which passes data signals  38  back to signal processing unit  37 , which calculates (by means of a software) the distance to the cause of the reflection and presents the results to the surgeon, typically as text or graphic output  39 . 
         [0040]    In a preferred embodiment of the present invention it is possible to preprogram the US examination of the patient with a typical US wave duration of 1 μsec or less, comprising sinusoidal waveforms that are typically in a frequency range of 10 to 20 MHz and with every wave lasting a fraction of a period to several periods. These are sample specifications and they can vary according to the application. They are presented here for illustrative purposes and do not limit the scope of the present invention. 
         [0041]      FIG. 2   a  illustrates a US transducer integrated into a liquid jet in a interactive ultrasound-based depth measurement device  81  in accordance with a preferred embodiment of the present invention. US transducer  68  is located in vessel  67  through which liquid  52  (usually distilled water) flows. Liquid  52  starts in main reservoir  50  and is pumped by pump  54 , when tap  56  is open, through pipe  58  to front reservoir  62 . Liquid output from front reservoir  62  passes out of funnel and tube  72  (typically with a diameter in the range of 1 mm to 2.5 mm) and flows against the adjacent tissue or directly on the bone. The pump is preferably a pump that delivers constant mass flow rate, due to medical considerations, but this is not a requirement of the ultrasonic measuring system, hence the present invention is not limited to such pump. An example of such pump is OsseoCare™ Drilling Equipment from Nobel BioCare AB, Sweden and which also includes a torque limiter in order to avoid the mechanical overloading of the bone tissue. US transducer  68  is connected electrically  64  to electronic circuitry, such as a control unit, a power unit, a signal processing unit, an output unit, and sub-units of the transducer. US transmit and receive signals  70  from/to US transducer  68  follow the path  74  of liquid  52 . 
         [0042]    It should be noted that what is germane to the present invention is that the US transducer be located such that the wave that it transmits and reflection that it receives pass through the liquid jet, such that the jet provides the medium between the transducer and adjacent tissue or bone. Several mechanisms for providing a liquid jet are known. Which specific mechanism is used to generate the liquid jet for the present invention can vary and is not essential to the invention as long as the transducer of the present invention is situated such that it transmits and receives via the jet to the adjacent tissue or bone. In a preferred embodiment of the present invention, the liquid jet is adapted to serve both for the regular operation of a dental drill (cooling the bone and the drill and cleaning the bore from the drilled materials) and for operation of the interactive ultrasound-based depth measurement of the present invention. 
         [0043]      FIG. 2   b  illustrates an interactive ultrasound-based depth measurement device  81  in accordance with another preferred embodiment of the present invention, wherein front-end electronics  76  are located close to US transducer  68  in order to reduce signal loss and therefore increase the signal-to-noise ratio (S/N). Front-end electronics  76  contains parts of the electronic circuitry, such as a pre-amplifier and parts of the signal processing unit.  FIG. 3   a  illustrates an interactive ultrasound-based depth measurement device  81  integrated with a surgical dental drill  84  in accordance with a preferred embodiment of the present invention. Drill  84  is a standard dental drill, typically comprising power cable  80 , which supplies voltage to the electrical motor  82 , which turns shaft  84 , which is geared to turn drill bit  88  (which is locked in place with drill lock  90 ). The jet is directed along side the drill  88 , aimed at the tip of the drill, or a target just in front of the tip, having a certain angle defined between the jet and the drill. 
         [0044]      FIG. 3   b  depicts a tip of a dental drill similar to the one shown in  FIG. 3   a , but the liquid jet in this tip passes through the drill, via a duct provided along the drill. 
         [0045]    The actual distance measurement using the device of the present invention may be carried out while the drill is in use (such as in the case of the embodiment shown in  FIG. 3   b , where the jet passes through the drill), or when the drill is inactivated, or even retracted from the bore that was drilled, while at the same time directing the jet into that bore. When the jet is along side the drill, and defining a small angle with the drill, it may be practical to withdraw the drill from the drilled bore and direct the jet substantially perpendicular to the bottom of the bore for best results. 
         [0046]    Interactive ultrasound-based depth measurement device comprises the components described earlier in this specification with several components adapted for integration with drill  84 . These include: additional section of flexible pipe  53  running from funnel and tube  72  to metal tube  55 , which exits drill  84  at a point next to drill bit  88 , thereby creating egress next to drill bit  88  for liquid carrying transmitted US waves. The liquid serves both its normal function of cleaning and cooling the bore of the drill and its new function (as provided by the present invention) of acting as a medium for transmission of the US waves. 
         [0047]    (In the embodiment shown in  FIG. 3   a  or the one shown in  FIG. 3   b , front-end electronics  76  are installed to reduce signal loss (and therefore increases the S/N), as in  FIG. 2B . Alternatively, all depth measurement device  81  electronics can be implemented at the back end, connected to US transducer via electrical connection  64 .) 
         [0048]    With reference to  FIG. 1  and to  FIGS. 3   a  and  3   b , during the drilling process, control unit  31  triggers (automatically or manually) transmission of US waves  34  by transducer  68 . US wave reflections received back from different density tissues by transducer  68  are passed via link  64  to processing unit  37 , which calculates the distance from the bottom of the drilled bore in the mandible to the upper part of the canal  20  (that has a mirror-like reflection) and which contains the nerve and outputs the results to output unit  39  where the surgeon can view them. Typically this output is provided to the surgeon displayed on an alphanumeric or graphical display. 
         [0049]      FIG. 4  illustrates how US waves propagate through the jet of liquid in a dental drill integrated with an interactive ultrasound-based depth measurement device in accordance with a preferred embodiment of the present invention. Jet of liquid  52  flowing into bore  94  in bone  22  serves as medium for transmission  34  of US waves into bone  22  and reception of US reflections  36  from nerve canal  20 . 
         [0050]    In the case of the mandible, the depth is measured in real time between the bottom of bore  94  and the upper part of nerve canal  20 . The depth measurement results are preferably displayed as alphanumeric information, providing the surgeon with an accurate indication of the distance (depth) between the lower part of drilled bore  94  and the upper edge of the sensitive tissue, such as nerve canal  20 , that must be avoided with the drill. Alternative displays are also possible (for example graphic representation of the distance). 
         [0051]      FIG. 5  illustrates a detailed block diagram of a circuitry for a dental drill with an interactive ultrasound-based depth measurement device in accordance with a preferred embodiment of the present invention. This block diagram operates in an analog  200  mode, and others—which operate in a digital  300  mode. These circuits provide improved noise filtering and signal amplification of transmission of US waves and reception of US wave reflections. The timing of the amplified signals is measured, and assuming known propagation velocity of the ultrasound pulses through bone, the distance from the bottom of the drill bore to the nerve canal is measured and displayed, as described above. 
         [0052]    Power is supplied from power supply  100  to transmitter  104 , which receives signal from signal generator  102 . Transmitter  104  transmits US wave and activates the electronic switch  106 , which activates receiver preamplifier and amplifier  110 , which receives reflection of wave from different density tissue and time-gain compensation  108 . Analog signal is converted  112  to digital and after time-gain compensation  114  is filtered  116 , compressed  118 , and stitched and offset  120 . Calculated time of reflection return is converted to depth measurement  122  and the result is output to display  124 . 
         [0053]    In a preferred embodiment of the present invention, depth measurement using the present invention can be performed in real time while the drilling is in process or in steps: drilling for a short distance (depth), typically 1 mm to 2 mm and then measuring the distance between the bottom of the drilled bore and the upper edge of the nerve canal before proceeding. 
         [0054]    In another preferred embodiment of the present invention, the measurement method can be applied for side measurements (for example, through the gums). One use of this application is identifying the three-dimensional location of the nerve canal (rather than just the linear direction to it). 
         [0055]    In another preferred embodiment of the present invention, depth measurement can be applied for other medical applications involving drilling, such as bone implantation, to avoid drilling injury to nerves or blood vessels beneath the drill. 
         [0056]    In another preferred embodiment of the present invention, depth measurement can be applied for veterinary medical applications. 
         [0057]    In another preferred embodiment of the present invention, it is possible to store the above-mentioned depth measurement on magnetic or other media—for further analysis, comparison, and documentation. 
         [0058]    In another preferred embodiment of the present invention, it is possible to incorporate software to control the data transmission to the signal-processing unit and to enable configuring parameters of the invention, for example, to customize the device for pediatric patients. It should be clear that the description of the embodiments and attached Figures set forth in this specification serves only for a better understanding of the invention, without limiting its scope as covered by the following Claims. 
         [0059]    It should also be clear that a person skilled in the art, after reading the present specification could make adjustments or amendments to the attached Figures and above described embodiments that would still be covered by the following Claims.