Patent Publication Number: US-2015066032-A1

Title: Bone resector

Description:
The present invention relates to a surgical tool for cutting both cortical and cancellous bone. More particularly but not exclusively, it relates to a surgical tool for bone resection in minimal access surgical techniques. 
     It is known, for example from our British patent application No. GB2420979A, to cut both cortical and cancellous bone in the course of surgical procedures, using ultrasonically activated instruments having a cutting edge with a saw-tooth profile. 
     In many situations, conventional powered oscillating saws having sharp tooth profiles and lateral tooth offsets are also effective. However, joint replacement procedures (amongst others) are increasingly often being carried out through incisions of reduced dimensions, to reduce soft tissue trauma. While this has clear benefits in respect of post-operative healing, it places greater demands on the surgeon&#39;s skill and dexterity to achieve the correct bone facet geometry for implant location, working through such restricted incisions. The use of such minimally invasive techniques can thus paradoxically produce increased risks of significant collateral damage to sensitive tissue structures adjacent the desired operative site. A conventional sharp-toothed powered saw can readily cut ligaments, vascular and nerve tissue with only transient contact with its sharp cutting edges. 
     Ultrasonically-vibrated blades need not be as sharp, cutting only when activated. They are also tunable to transmit energy selectively into hard, bony matter in preference to soft tissue. They hence tend to cause less accidental trauma. Unfortunately, such tools currently perform their prime function of cutting bone significantly more slowly than conventional oscillating saws, and so have not been as widely adopted as had been expected, particularly when their greater complexity and cost is taken into account. 
     A further issue that has been encountered is that ultrasonically-vibrated osteotomes can lead to localised heating as ultrasonic energy is dissipated into the bone. This may lead to localised bone necrosis and consequent poor healing. 
     A further problem with conventional oscillating saws is that a portion of the oscillatory motion tends to be transmitted from the tool into the surgeon&#39;s hand. This low-frequency vibration can be uncomfortable, may lead to more rapid fatigue in the surgeon&#39;s hand and fingers, and with prolonged exposure might even result in problems such as “white finger”. 
     It is hence an object of the present invention to provide improved surgical bone-cutting tools that obviate at least some of the above problems, while allowing rapid and accurate bone resection with minimal damage to adjacent soft tissues or to the remaining bone. 
     According to a first aspect of the present invention, there is provided a surgical tool adapted to cut osseous material comprising cutting head means having elongate cutting edge means, said cutting head means being operatively connected both to means to generate ultrasonic vibrations and to means to displace the cutting head means reciprocally. 
     The reciprocal displacement means preferably acts generally parallely to the cutting edge means. 
     Preferably, the reciprocal displacement means is adapted to produce an oscillatory motion at a frequency of 250 Hz or lower. 
     Advantageously, such oscillatory frequency is at least 20 Hz. 
     Optionally, said oscillatory frequency is between 40 and 60 Hz, for example being at approximately 50 Hz. 
     Preferably, said means to generate ultrasonic vibrations is adapted to generate said vibrations at a frequency of at least 20 kHz. 
     Advantageously, said ultrasonic vibrations are generated at a frequency of 60 kHz or below. 
     Optionally, said ultrasonic vibrations are generated at a frequency of approximately 40 kHz. 
     Preferably, the relative amplitudes of the ultrasonic vibrations and the oscillatory motion of the cutting head are such that a peak velocity of the cutting head due to the ultrasonic vibrations is greater than a peak velocity resulting from the oscillatory motion. 
     Advantageously, the peak cutting head velocity due to the ultrasonic vibrations is at least twice that resulting from the oscillatory motion. 
     The peak cutting head velocity due to the ultrasonic vibrations may be at least three times that resulting from the oscillatory motion. 
     The peak cutting head velocity due to the ultrasonic vibrations is preferably no more than ten times that resulting from the oscillatory motion. 
     Advantageously, the peak cutting head velocity due to the ultrasonic vibrations is no more than seven times that resulting from the oscillatory motion. 
     Preferably, the ultrasonic vibrations comprise longitudinal ultrasonic vibrations directed generally parallelly to the oscillatory motion and to the cutting edge means. 
     The cutting head means may comprise an elongate waveguide with the cutting edge means disposed adjacent a distal end thereof. 
     The cutting edge means may comprise an elongate array of tooth means. 
     Said tooth means may each comprise saw tooth means. 
     In a preferred embodiment, the means to displace the cutting head means reciprocally is provided with first counterweight means for the cutting head means, reciprocally displaceable out of phase with the cutting head means. 
     Advantageously, the first counterweight means is displaceable substantially in antiphase therewith. 
     A centre of mass of the cutting head means and the first counterweight means may remain substantially stationary. 
     Advantageously, the means to displace the cutting head means reciprocally displaces both the cutting head means and the means to generate ultrasonic vibrations. 
     The reciprocal displacement means may then be provided with second counterweight means for both the cutting head means and the means to generate ultrasonic vibrations, reciprocally displaceable out of phase therewith. 
     The second counterweight means may be displaceable substantially in antiphase therewith. 
     A centre of mass of the cutting head means, the means to generate ultrasonic vibrations and the second counterweight means may thus remain substantially stationary. 
     Preferably, the reciprocal displacement means comprises a rotatable generally cylindrical body having a first and a second track means each extending continuously around the body, with the cutting head means and optionally the means to generate ultrasonic vibrations being moveably engaged with the first track means, and the respective counterweight means being moveably engaged with the second track means. 
     Advantageously, each said track means comprises groove means. 
     The cutting head means and counterweight means may each then be provided with coupling pin means constrained to move within respective groove means. 
     Preferably, each track means extends around the cylindrical body at an angle to a rotational axis thereof, with the first track means being angled in an opposite sense to the second track means. 
     A longitudinal disposition of each track means thus varies around a circumference of the cylindrical body. 
     When the cylindrical body is rotated, the cutting head means and counterweight means, being coupled to respective track means, are driven to move reciprocally, and out of phase each with the other, optionally in antiphase each with the other. 
     Preferably, the reciprocal displacement means is provided with motor means, adapted to drive the cylindrical body rotatingly. 
     Advantageously, said motor means is provided with means to select a desired speed of rotation of the body. 
     Preferably, the tool comprises manually graspable and manipulable outer casing means, enclosing at least the reciprocal displacement means and the means to generate ultrasonic vibrations. 
     Advantageously, the tool comprises an elongate outer casing means having the cutting head means extending longitudinally therefrom. 
     In a preferred embodiment, said cutting edge means is provided with a plurality of teeth, arrayed therealong. 
     Each said tooth may have a hooked profile. 
     A tip of each said hooked tooth may extend generally towards a distal end of the tool. 
     Said profile may be suitable for use in any osteotome, particularly ultrasonically-vibratable osteotomes. 
     According to a second aspect of the present invention, there is provided a method of cutting osseous material comprising the steps of providing a tool as described in the first aspect above, applying a cutting edge means thereof to a zone of osseous material to be cut, activating both the reciprocal displacement means and the means to generate ultrasonic vibrations, and guiding the tool manually until a desired cut or facet has been produced. 
     Preferably, the method is adapted to cut cortical and/or cancellous bone as part of a surgical procedure. 
     Advantageously, the method comprises the steps of creating an incision leading from a body surface to the bone to be cut and introducing cutting head means of the tool therethrough. 
     The method may comprise the step of cutting bone to prepare for implantation of a prosthetic device, such as an orthopaedic joint replacement. 
     The method may comprise the step of cutting bone to remove an implanted prosthetic device, for example as part of a revision procedure for an orthopaedic joint replacement. 
    
    
     
       An embodiment of the present invention will now be more particularly described by way of example and with reference to the figures of the accompanying drawings, in which: 
         FIG. 1A  is a schematic longitudinal cross-section of an internal operative structure of a first bone resector tool embodying the present invention; 
         FIG. 1B  is a cross-section of a driving stud separated from the tool shown in  FIG. 1A ; 
         FIG. 1C  is a scrap radial cross-section of the driving stud shown in  FIG. 1B , in operation within the tool shown in  FIG. 1A ; 
         FIG. 1D  is a schematic longitudinal cross-section of an internal operative structure of a second bone resector tool embodying the present invention; 
         FIG. 1E  is a scrap elevation of a cutting head of the second tool shown in  FIG. 1D ; 
         FIG. 2  is a side elevation of a drive converter element separated from the tool shown in  FIG. 1A  or the tool shown in  FIG. 1D ; 
         FIG. 3  is a side elevation of a driveshaft separated from the tool shown in  FIG. 1A  or the tool shown in  FIG. 1D ; 
         FIG. 4  is a side elevation of the drive converter element shown in  FIG. 2 , together with its drive arrangements and a counterweight cylinder coupled thereto; 
         FIG. 5  is a side elevation of the drive converter element shown in  FIG. 2 , together with a blade driving cylinder coupled thereto; and 
         FIG. 6  is a side elevation of either of the tools shown in  FIGS. 1A and 1D , including its outer casing in sectioned and partially disassembled form. 
     
    
    
     Referring now to the Figures and to  FIG. 1A  in particular, an acoustic system  1  of a first bone resector tool  100  comprises a longitudinal mode ultrasonic transducer  8  (typically comprising a stack of piezo electric-elements) connected by a horn arrangement  4  to an elongate exchangeable blade portion  2 . The blade portion  2  has a cutting head  6  at its distal end, provided with one or more lateral cutting edges. (The cutting edge(s) are not shown in detail in  FIG. 1A , but may typically comprise an array of saw teeth, set in a desired geometry. The present invention is believed to be of use with most or all known forms of osteotome blade geometries). 
     The particular tool  100  shown produces ultrasonic vibrations in its blade portion  2  which have a maximum longitudinal displacement amplitude, at a distal tip  6 A of the cutting head  6 , of between 80 and 140 μm. The ultrasonic transducer  8 , horn  4  and blade portion  2  are tuned such that the distal tip  6 A is at an antinode of the ultrasonic vibrations. The displacement amplitude at a proximal end  6 B of the cutting head  6  will be about 60% of that at the distal tip  6 A. 
     It is found that ultrasonic vibrations in the near ultrasonic region are suitable, for example in the range 20-60 kHz. A frequency of close to 40 kHz is currently preferred. This produces a peak blade velocity at the distal tip  6 A of 10-50 m.s −1    
     The acoustic system  1  is held within elongate cylindrical housing  10 , with the blade portion  2  projecting distally therefrom. At its proximal end, the housing  10  is fastened by a screw coupling  21  to a blade driving cylinder  5 A, the function of which is described below. 
     An electric motor  17 , located adjacent a proximal end of the tool  100  and acting through a gearbox  9  and a driveshaft  24  (see  FIG. 3 ), drives a shaft  7  of a drive converter element  3  located generally centrally of the tool  100 . The electric motor  17  drives the converter element  3  to rotate continually in a single direction (as shown by arrow  11 ) at a controllable speed. 
     The converter element  3  comprises a cylindrical body having a first  19 A and a second  19 B groove extending around its circumference. Each groove  19 A,  19 B comprises a single continuous loop, extending within a plane at an angle to a radial plane through the body of the converter element  3 . Each groove  19 A,  19 B is inclined at the same angle, but in opposite directions/senses. Thus, at a first point on the circumference of the converter element  3 , the grooves  19 A,  19 B are relatively close together, but they diverge around the circumference from the first point, until at a second point diametrically opposite to the first they are relatively remote, each from the other. Continuing around the circumference from the second point, the grooves  19 A,  19 B converge back again towards the first point. The grooves  19 A, 19 B thus each undergo a lateral displacement x, as measured along the longitudinal axis of the converter element  3  and the tool  100  as a whole. (See  FIG. 2  for a view of the converter element  3  in isolation).The blade driving cylinder  5 A extends around a distal portion of the converter element  3 , and is coupled to the converter element  3  by means of a driving stud  12  travelling within the first groove  19 A. 
     A counterweight cylinder  5 B extends coaxially around the gearbox  9  and a proximal portion of the converter element  3  and is coupled to the converter element by means of a driving stud  12  travelling within the second groove  19 B. 
     As shown in  FIG. 1B , each driving stud  12  comprises a locating screw  16  extending into a metal bush  18  within a high-density polyethylene (HDPE) block  14 . As shown in  FIG. 1C , the locating screw  16  fastens the driving stud  12  to the blade driving cylinder  5 A or the counterweight cylinder  5 B, respectively, with the low-friction HDPE block  14  located within the respective first  19 A or second groove  19 B. 
     Thus, as the converter element  3  is rotated, the respective driving studs  12  must follow their respective grooves  19 A,  19 B (NB: there are spline arrangements, omitted for clarity, to prevent the cylinders  5 A,  5 B merely rotating along with the converter element  3 ). The driving studs  12  and their respective cylinders  5 A,  5 B are thus compelled to travel axially of the tool  100 , first outwardly towards the remote ends of the tool  100  and then back towards each other. Because of the opposite inclination of the grooves  19 A,  19 B, the cylinders  5 A,  5 B thus move 180° out-of-phase (i.e in antiphase). 
     The blade driving cylinder  5 A is mounted securely to the housing  10 , the enclosed ultrasonic transducer  8  and the blade portion  2  of the tool  100 . Thus, the entire acoustic system  1  is displaced reciprocally along the longitudinal axis of the tool  100 , in particular producing a reciprocal longitudinal motion of the cutting head  6 . 
     The particular tool  100  shown is set up for this reciprocal/oscillatory motion to be at a frequency of about 50 Hz, with the lateral displacement x of the groove  19 A, the blade driving cylinder  5 A and the cutting head  6  being of the order of three to ten millimetres. 
     The counterweight cylinder  5 B is constructed to have a mass as close as possible to the total mass of the blade driving cylinder  5 A and the acoustic system  1 , including the housing  10  and the blade portion  2 . Thus, as the converter element  3  rotates and the counterweight cylinder  5 B is also displaced with the same lateral displacement x at the same reciprocal/oscillatory frequency, a centre of mass of the counterweight cylinder  5 B, blade driving cylinder  5 A and acoustic system  1  should remain substantially stationary. Whereas a convertional vibrating saw at a frequency of around 50 Hz would tend to give rise to vibrations transmitted into a user&#39;s hand (possibly causing discomfort, fatigue and even tissue damage after prolonged exposure), the tool  100  shown should produce minimal or zero tangible vibrations in the user&#39;s hand. This should allow longer periods of use and greater accuracy in use, since the user&#39;s hand should avoid fatigue for longer. 
     A second bone resector tool  101 , shown in  FIG. 1D , is very similar to the first bone resector tool  100 . Its longitudinal mode ultrasonic transducer  8 , horn  4  and blade portion  2  are shown in more detail, as are the arrangements used to fasten the ultrasonic transducer  8 , horn  4  and blade portion  2  together. The second tool  101  operates in an identical manner to the first tool  100 . 
     The cutting head  6  of the second tool  101  is also shown in more detail in  FIG. 1D , and in particular in  FIG. 1E . The cutting head  6  of the second tool  101  has two lateral cutting edges, which converge slightly towards its distal tip  6 A. Each cutting edge is provided with an array of cutting teeth  6 C. Each cutting tooth  6 C has a hooked or “shark-tooth” profile, with a pointed tip of each hooked tooth aligned towards the distal tip  6 A of the cutting head  6 . The cutting teeth  6 C are defined by an array of slanting notches  6 D, each notch having an inner end with a profile comprising a portion of a circle. 
     While this form of cutting head  6  is of particular benefit when incorporated into bone resector tools  100 ,  101  as described above, it is believed that it would also be of benefit in other bone resector tools (osteotomes), particularly those in which the cutting head  6  is ultrasonically vibratable. 
     The converter element  3  is shown in more detail in  FIG. 2 . The grooves  19 A,  19 B are as described above. Not shown above was an axial bore or passage  23 , which receives a driveshaft  24  as shown in  FIG. 3 . The cylindrical shaft  26  of the driveshaft  24  is provided with a flat  27 . A radial aperture  13 A extending through the converter element  3  into its axial bore  23  ( FIG. 2 ) allows a radial screw  13  ( FIG. 1 ) to engage with the flat  27  to secure the driveshaft  24  to the converter element  3 . A proximal fitting  28  of the driveshaft  24  allows it to be connected to the gearbox  9 . 
       FIG. 4  shows the counterweight cylinder  5 B coupled to the converter element  3  by its driving stud  12  following the second groove  19 B. In the disposition shown, the counterweight cylinder  5 B is at its maximum displacement towards the centre of the tool  100 . 
     In contrast,  FIG. 5  shows the blade driving cylinder  5 A coupled to the converter element  3 , but in a disposition in which the blade driving cylinder  5 A is at its maximum displacement towards a distal end of the tool  100 ,  101 . (Note the gap  7 C between a distal end of the converter element  3  and the blade driving cylinder  3 . 
       FIG. 6  shows additional features of the tool  100 ,  101  as a whole. The internal operative structures shown in  FIG. 1  are enclosed in a three-piece casing  30 ,  31 ,  32 . A proximal cap  31  and a distal cap  32  are both detachably mounted to a main casing  30 , with seals  33  provided at the respective joints to protect the internal workings of the tool  100 , eg. from fluid ingress. 
     The main casing  30  encloses respective spaces  17 C,  9 C to hold the motor  17  and gearbox  9  (not shown), the converter element  3 , both cylinders  5 A,  5 B and a proximal portion of the ultrasonic generator  8 . 
     The proximal cap  31  has an opening  34  for power cables and control cables (it is common for such tools to be activated by means of a foot pedal, rather than by a finger-operated switch on the tool itself). 
     The detachable distal cap  32  allows access to the ultrasonic generator  8 . 
     A further feature of this tool  100 ,  101  is that the blade portion  2  is detachable, using a threaded fitting  35 . Blades having alternative cutting head  6  geometries may thus be fitted, and worn or damaged cutting heads  6  may be exchanged. 
     The tool  100 ,  101  shown thus have a cutting edge that is both vibrated ultrasonically and displaced reciprocally on a macroscopic scale at a much lower frequency. Combining ultrasonic activation and macroscopic blade reciprocation in this way creates a significant advantage in cutting efficiency. With sufficient ultrasonic amplitude, the physical force required to cut the bone is reduced to close to zero, while the reciprocating action displaces embrittled bone tissue with very little reactive force. This creates a vibration-free sensation as a surgeon cuts into the bone, with clear benefits for accuracy, comfort and reduced fatigue. The counterbalanced macroscopic reciprocating drive mechanism described above further enhances this substantially vibration-free action. 
     High amplitude ultrasound on its own heats the tissue on which it acts. Rapid and efficient removal of each layer of heated tissue by the macroscopic blade displacement avoids the bone necrosis that would otherwise be produced as this heat is dissipated into surrounding tissues. 
     This mechanism has been shown in animal model studies to produce an effective and safe method of bone resection. The studies indicated very low levels of bone necrosis, even without the saline irrigation that is conventionally employed for cleaning and cooling the cut site. Soft tissue disruption was negligible. 
     To gain maximum benefit in comfort and efficiency for the system shown, it has been found that the ultrasonic velocity amplitude should exceed the low frequency macroscopic velocity amplitude, preferably be a factor of between three and seven times. This ensures that the relative oscillatory movement of the cutting edge against bone tissue benefits substantially from friction vector reversal continuously throughout almost the entire cutting cycle of the reciprocating blade. 
     It should be appreciated that (regardless of frequency) holding a vibrating blade against tissue will produce a net heating effect. Only by moving the blade progressively through the target tissue can cutting be effected and heated tissue removed from the immediate surgical site. Manually-impelled bodily movement of the blade is impractical within the parameters described, so the combined action of the present invention has major practical benefits.