Improved tool for removal of plastics material

Abstract: A tool for use in removing plastics material from a hole comprises a work surface adapted to contact the material. Piezo electric transducer means operatively connected through a work horn to the work surface cause it to vibrate ultrasonically and thereby to heat locally the plastics material. A cavity is adapted to received heated plastics material. The cavity is connected to a working zone adjacent the work surface. The work surface comprises a head flange and on a reverse side thereof a substantially annular cutting edge formed on one side of a conical face angled to the general direction of the tool.

Description: The present invention relates to an improved tool for use in removal of plastics material. A tool of this general type is disclosed in our GB 2229660 but further improvements have been discovered. The tool is particularly, but not exclusively, useful in removing plastics cement from such bores in bones as may be used in hip, or other joint, replacements (hereinafter referred to, for convenience, as hip joint replacements). In a hip joint replacement operation, a metal implant is provided with a long projection which is inserted into a hole drilled in the medulla of the femur and is held firmly in place by means of a plastics cement. On average, such replacements can be expected to last five to ten years. However, due to repetitive shearing forces during daily use, either the bone/cement interface or the cement/metal interface may weaken and the implant will become loose, requiring revision. Sometimes, the metal of the hip replacement may fracture or the plastics components of it may wear out. In these cases, revision is necessary although in most cases the bone/cement interface usually remains quite strong. In order to revise any loose or damaged implant, all or most of the plastics cement must be removed before inserting a new prosthesis and re-cementing. Removal of the old cement presents a number of difficulties. It is time-consuming and may cause fracturing of the bone. It involves the careful and tedious use of hand tools such as hammers and cement cutting chisels. High speed burrs have been used, but they frequently perforate the bone and make re-cementing more difficult and not so effective. GB 2229660 discloses a tool which is powered ultrasonically and is provided with a forward facing working surface connected by a plurality of holes to the rear of that surface. The tool may be inserted into the plastics cement filled hole, which cement then melts under the influence of the ultrasonic vibration and passes through the holes to the rear, from where it can be removed following a reverse movement of the tool. However, the bone may be thin. It is an object of the present invention to provide an improved tool for removal of plastics material such as cement from a bore, particularly one in a bone, which overcomes the above disadvantage. According to the present invention there is provided a tool for use in removing plastics material from a hole comprising a work surface having a substantially annular cutting edge adapted to contact said material, piezo electric transducer means operatively connected through a work horn to said work surface to cause it to vibrate ultrasonically and thereby to heat locally said plastics material, cavity means adapted to receive said heated plastics material, wherein the work surface comprises a head flange and on a reverse side thereof the substantially annular cutting edge is formed on one side of a conical face angled to the general direction of the tool. The angle is preferably between 20 and 25 degrees. Preferably, at least the piezo electric transducer means is sealingly encased in a first enclosure of waterproof plastics material and exterior thereof a second enclosure of stainless steel or the like material. Advantageously, the waterproof plastics material is an acetal plastics material. This arrangement allows the tool to be autoclaved or otherwise sterilised for use for another patient. Embodiments of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: - FIGURE 1 is a schematic side elevation, shown partially in cross-section, of a tool head embodying the invention; FIGURE 2 is a cross setional view of a handle incorporating ultrasonic generating means to power the tool head of Figure 1; and FIGURE 3 shows in cross section a further embodiment of handle for use with the invention. Referring now to the drawings, there is shown in the Figures a tool comprising a piezo-electric ceramic transducer 1, connected along a longitudinal axis to a coupling horn 2, which in turn is connected along the longitudinal axis to a work horn 3. At the far end of the work horn 3 is a cavity 4 surrounded by an annular cutting edge 5. As shown in the Figures, the length of the piezo electric ceramic transducer 1 is half a wavelength, the length of the coupling horn 2 is a full wavelength, while the length of the work horn 3 (which includes the annular cutting edge 5) is an integral number of half wavelengths ensuring that the total probe length can penetrate to the required depth. The term "wavelength" is used to represent the wavelength of the ultrasonic wave generated by the piezo electric ceramic transducer in the material concerned. The preferred material for the work horn and annular cutting edge is titanium or an alloy thereof. At an ultrasonic vibrational frequency in the region of 30-35 kHz, the wavelength of the ultrasonic wave in the titanium alloy is in the region of 70-90mm. It is well known that many common plastics materials will transmit high frequency vibrations without the significant internal losses which would cause bulk heating of the material. It is also known that when the ultrasound is transmitted through two closely contacted components, the interface can experience a considerable heating effect which under the correct circumstances will produce welding. Such heating can also occur at the interface between a vibrating tool or metal component and the plastics material, the heating being sufficient to melt the plastic. The present invention utilises this effect to drill an annular hole into the plastics material. The plastics cement material used for hip joint replacements is generally a powder of polymethylmethacrylate beads of diameter less than 100 µm held together in situ by a polymerised methyl methacrylate monomer. This material is prone to creep and is susceptible to localised heating on ultrasonic vibration. The property of creep may be utilised in that, during removal of a core of plastics cement material, the existing cement which remains may be forced into improved engagement with fissures or surface imperfections in the bone by virtue of the ultrasonic vibrations imparted to the cement, and thereby stabilise the interface. At the work surface, the annular cutting edge can be manipulated by the user of the tool to enable the bore diameter to be widened or, by applying pressure to one side of the tool, to create a hole of oval profile. The present invention is described with reference to removing a plastics cement from a hip joint replacement during revision of the prosthesis. In this case, the hip bone or femur has a blind hole filled with plastics cement which had originally surrounded the prosthesis, but which has a void where the prosthesis used to be. In order to operate the tool, there is shown in Figure 1 an instrument which has a sharp cutting edge on the reverse side of the head flange. The cutting edge is formed on one side of a conical face whole angle "a" critically determines the controlled cutting action of the scraper. There are no connecting holes between front and rear faces of the flange but as the probe is pulled in the reverse direction, it cuts into the cement on one side of the femoral cavity and the removed material collects in the recess behind the flange. This mechanism permits removal of discrete volumes of cement to ensure complete preparation of the endosteal surface ready for recementing. The optimum angle is between 20° and 25°, this has been found to permit cement removal safely and easily, with the application of only light force. The instrument is of particular use in cases of severe bone resorption which has left areas of extreme weakness in the femur. Great care is required to avoid perforation or fracture of the femur under these circumstances using conventional instruments. This embodiment of the invention achieves this difficult objective without risk. The reverse scraper is also of great use when preparing access to the distal plug and generally precedes the piercing operation effected with a multi-port instrument. As shown in Figures 2 and 3, the piezo electric transducer part of the tool may be encased, first in a layer of acetal plastics material 22 and then in a layer 23 of stainless steel. This arrangement will allow the tool to be autoclaved or otherwise sterilised in order to permit its use on further patients. As shown in Figure 3, the handset part of the tool may incorporate a switch 25 and still be autoclavable. The design philosophy takes account of the conditions prevailing in operating theatres and in particular the strict sterilisation requirements. Several sizes and shapes of oscillatory instrument should ideally be available to the surgeon and a particular case might demand very specialised probe designs. Whereas it is possible to interchange probes on a single handset this procedure is not only inconvenient but results in reduced efficiency of the system if the critical probe/horn interface becomes contaminated with foreign matter. It is desirable therefore to provide a number of independent handsets which can be selected by the surgeon without the need for reconnection or adjustment of switching functions. This dictates the use of a switch incorporated in each handset. Since the handset and cable assembly must be suitable for autoclave sterilisation, the switch assembly requires a special seal design to withstand the temperature and pressure conditions encountered during the sterilisation process. Figure 3 shows a handset which includes a coaxial switch button operating a sub-miniature micro-switch via a cylindrical moulded seal. The switch is contained in a metal cylindrical sleeve which supports the seal and ensures that it remains water tight even under pressure. This design permits the construction of an oscillatory system offering maximum operating flexibility with inherent reliability. Furthermore there is no need for foot switches which for up to four handsets would involve impractical complications.

Claims: A tool for use in removing plastics material from a hole comprising a work surface having a substantially annular cutting edge (5) adapted to contact said material, piezo electric transducer means (1) operatively connected through a work horn (3) to said work surface to cause it to vibrate ultrasonically and thereby to heat locally said plastics material, cavity means (4) adapted to receive said heated plastics material, characterised in that the work surface comprises a head flange and on a reverse side thereof the substantially annular cutting edge is formed on one side of a conical face angled to the longitudinal direction of the tool . A tool as claimed in Claim 1, characterised in that said angle of the conical face is an angle of between 20 and 25 degrees. A tool as claimed in either Claim 1 or Claim 2, characterised in that at least the piezo electric transducer means is sealingly encased in a first enclosure of waterproof plastics material and exterior thereof a second enclosure of stainless steel or the like material. A tool as claimed in Claim 3, characterised in that the waterproof plastics material is an acetal plastics material. A tool as claimed in either Claim 3 or Claim 4, characterised in that switch means are provided on a handset of said tool, said switch means being sealed to allow the tool to be autoclaved or otherwise sterilised for use for another patient.

Inventors: BRADNOCK BRIAN ROBERT DENIS PE, YOUNG MICHAEL JOHN RADLEY, BRADNOCK, BRIAN ROBERT DENIS PETER, YOUNG, MICHAEL JOHN RADLEY
Applicants: BRADNOCK BRIAN R D P, YOUNG MICHAEL JOHN RADLEY, BRADNOCK, BRIAN ROBERT DENIS PETER, YOUNG, MICHAEL JOHN RADLEY
Application year: 1992
Main classification: A61N7
IPC codes: A61N7, A61F2