METAL CABLE

Abstract: A titanium alloy cable comprises a plurality of individual wire filaments (20). Groups of wire filaments are twisted together in a first helical direction around a central wire filament to make a strand (10). A plurality of strands are twisted together in a second helical direction around a core strand to make a strong, flexible cable. The first helical direction has the opposite sense to that of the second helical direction. In one embodiment, the cable has one hundred and thirty three wire filaments, each of diameter 0.0025 inches (64(my)m), arranged in nineteen equivalent strands and the titamium alloy is Ti-6 wt % A1-4 wt % V.

Description: METAL CABLE The present invention relates to a metal cable and, in particular, to a cable for use in surgical procedures. A number of surgical techniques have been developed over recent years which involve the fastening of loops of wire around bony structures. Some of these procedures are, for example, concerned with the stabilising of certain types of fracture but others may involve the lashing of various types of prosthesis or like device to the skeleton. Examples of such procedures and of devices to be used in them are to be found in US Patents 4,604,995 (Stephens et al.), 2,501,978 (Wichman) and 4,790,303 (Steffee). Traditionally, a stiff stainless steel wire filament is moulded into a shape and pulled or pushed around the bone. The stiff wire is fastened by tightening together the free ends of the wire as described for example in U.S. patents 2,455,609 (Scheib), 1,304,620 (Steinkoenig) and 914,182 (Pfeffer). Basically, a mechanical operation simultaneously tightens and fastens the loop. The use of this stiff wire is not ideal since it may cause serious and irreparable damage to the underlying soft tissues, and for this reason a more flexible cable would be preferable. During the surgical procedure a great deal of strain is placed on the wire which may be sufficient to cause the wire to deform plastically. If this happens the structures, which should have been held firmly by the wire loop, may still be able to move relative to one another. In addition, the high strain may weaken the wire to such an extent that it subsequently breaks after the operation has been completed and the wound closed. This could easily produce sharp wire ends which may protrude into, and cause serious damage to, the surrounding tissues. In either event, if the high strains damage the wire irrevocably, the entire procedure will have to be repeated. The problems associated with a metal wire filament may, at first sight, be obviated by the selection of alternative non-metallic materials for the filament. For example, polymers or fibre reinforced composites might seem particularly suitable because of the way in which they have been successfully applied to a large number of other situations. However, polymers are prone to stretching, and fibres have a tendency to splinter. Neither of these characteristics are acceptable in this particular application. Instead, a wire cable comprising filaments of surgical grade stainless steel, a material which is compatible with human tissue, has been preferred, see US patent 4,790,303 (Steffee). The wire cable has to be strong enough to withstand the high strains which may be encountered during the surgical procedure, a factor which tends to necessitate an increase in the diameter of the cable. However, by increasing the diameter to provide basic tensile strength, there is an unavoidable loss in the flexibility of the known braided wire cables. Hence there is a need for an improved cable which has the strength and flexibility required by the surgical procedures. A further disadvantage of stainless steel is that it is not compatible with magnetic resonance imaging. Magnetic resonance imaging (MRI) is an application of the well documented technique of nuclear magnetic resonance (NMR) and is used to provide a non-invasive clinical image of the human body. Multiple projections are combined to create virtual sections through a body, providing a very powerful diagnostic aid. In MRI, a strong magnetic field is applied to a specimen, and the absorption of radio frequency - radiation is monitored. The magnetic field must be exceptionally well controlled and homogenous over the bulk of the sample. The technique is highly sensitive and resolution can be severely reduced by the presence of even small amounts of ferrous material within the magnetic field. Hence, the progress of a fracture stabilized with a steel cable, for example, could not be monitored by MRI because of the presence of the iron in the material. Aluminium alloys usually provide an alternative to ferrous based materials. However, whilst aluminium is compatible with MRI, it is not suited for use in the human body, since traces of dissolved aluminium are toxic. In accordance with the present invention there is provided a metal cable comprising a plurality of individual wire filaments; the wire filaments comprising titanium and having a diameter in the range 0.0015 inches to 0.005 inches (approximately J38μm to 127μm). Pure titanium has a hexagonal close packed (h.c.p.) structure (α) below 882"C, and a body centred cubic (b.c.c.) structure (β) above this temperature. The existence of this phase transformation opens up a wide range of possibilities for alloying. Some alloying elements stabilize one crystal form, some the other, and alloys having either structure, or a duplex mixture of both, are well known. One particular alloy having 6wt% aluminium and 4wt% vanadium, Ti-6wt%Al-4wt%V, has a duplex α/β structure. It is heat treatable to obtain a yield stress in the range 800-1200MNm- 2 . There are no problems of compatibility of this alloy within the human body - the aluminium component is all in solid solution and is therefore protected from dissolving by the titanium matrix. Furthermore, many titanium alloys, and Ti-6wt%Al-4wt%V in particular, are compatible with the MRI technique. The important mechanical properties of the metal cable are strength and flexibility. The strength depends upon the overall size of cable, the number of individual wires in the cable, and the type of material from which the wires are made. A cable that undergoes repeated and severe bending must have a high degree of flexibility to prevent premature breakage and failure due to fatigue. Greater flexibility in the cable is obtained by using small wires in larger numbers. In a preferred embodiment, groups of wire filaments are twisted together in a first helical direction around a central wire filament to make a strand. A plurality of strands are twisted together in a second helical direction around a core strand to make a strong, flexible cable. Desirably, the first helical direction has the opposite sense to that of the second helical direction. If the wires in the strands lay in the opposite direction to that of the strands in the cable, then any filament failure is more likely to occur on the outer surface of the cable where it may be detected before catastrophic failure ensues. " In an especially preferred embodiment, the cable has a 19x7 configuration with each of the nineteen strands consisting of seven wire filaments. The diameter of each wire filament is 0.0025 inches (64um), and the diameter of the cable after swaging is 0.033 inches (0.84-nra). The combination of a 19x7 configuration with wire filaments of diameter 0.0025 inches (64μm) results in a cable with a highly desirable strength versus flexibiltiy ratio. A metal cable comprising titanium, in accordance with the invention, will now be described in detail, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a cross-section through a cable according to the invention in the unswaged condition; Figure 2 shows an enlarged individual strand in cross section according to Figure 1; Figure 3 shows a configuration for a 19 x 7 cable; Figure 4 shows a configuration for a 7 x 7 cable; Figure 5 shows schematically a beam deflection test; and Figure 6 shows graphically the results of beam deflection tests. The cable comprises a plurality of strands 10. Each strand 10 comprises a plurality of wire filaments 20. Eighteen strands 10 are arranged substantially symmetrically about a central core strand in two concentric rings. Therefore the cable consists of nineteen strands altogether. Each strand consists of seven individual wire filaments 20. Therefore there are one hundred and thirty three (133) wire filaments in the cable cross-section. The cable, designated a 19x7 cable, may be manufactured in the following way:- The individual wire filaments are produced by wire drawing. Drawing is the process by which metallic wire is pulled through a die in the presence of lubricants to create wire of a diameter equal to that of the die. There are several parameters in the drawing process which ultimately affect the strength, . ductility, and flexibility of the wire. For example, differences in the drawing speed may effect the degree of strain hardening imparted to the wire. For this reason, the drawing process is generally very slow to prevent the wire from breaking. The wire is drawn from a diameter of about 0.020 inches (0.5mm) in a series of drawing steps until it is approximately 0.003 inches (75μm) in diameter. The drawn wire is chemically etched or 'milled' in hot hydrofluoric acid to further reduce the diameter of the wire. The exposure to the acid is strictly measured so that the final diameter of the wire may be controlled to a tolerance of about ±0.0001 inches (±2.5μm) . The very thin wire of typical diameter 0.0025 inches (64μm) is received on spools which are placed in a stranding machine. The wire is first "stranded" into 1x7 helical strands (i.e. producing a single strand from seven individual wires). The strands are laid in the right hand direction, where the 'lay' of a strand refers to the direction of the helical path in which the wires are arranged. The resulting strands are taken up as 1x7 strands on individual spools for eventual, closing. Closing is the term which describes the stranding of strands to form the final cable in a process which is similar to the wire stranding process itself. The strands are layed in the left hand direction. The result is a cable, as shown in the drawing, comprising nineteen strands with seven wires in each strand, i.e. 19 x 7 cable. After the stranding process, the cable is subjected to a stress relieving heat treatment. The stress relief helps the material maintain its helical shape and decreases its natural tendency to unwind. This process is repeated after the closing phase for the same reason. The cable undergoes a swaging operation which creates flats or longitudinal facets on the wire. Care must be taken not to crush or distort the wires unintentionally. Swaging smoothes out the wire and helps ensure a uniform cross sectional area. It is of paramount importance to ensure that the cable is free of sharp edges and burrs. For this reason, any free end of cable must be specially prepared. A small bead of titanium is welded onto the end to capture all the fine wires. The end is then subjected to another swaging operation to create a short length of cylindrical material equal in diameter to the cable. The lay of the wires in the strands is opposite to that of the strands in the cable. Cross-laying has an advantage in that any failure of the cable usually begins on the outer surface of the cable and may therefore be detected. In contrast, if the wires and strands are laid in the same direction, failure typically initiates on the inner wires and is not noticed until catastrophic failure occurs. However, the latter form of laying does result in a marginal increase in flexibility of the wire. Therefore, particular situations dictate which type of lay is preferred depending upon whether maximum reliability or maximum flexibility is required. The wire filaments may be made from unalloyed titanium e.g. ASTM F67. Although this material is not as strong as some titanium alloys, such as Ti-6wt%Al-4wt%V, it is at least as flexible and easier to draw. The cross-section of the cable in Figure 3 shows two concentric rings of strands. The outermost shell of strands may not be required. The loss of twelve strands in the cable would greatly reduce its ultimate tensile strength. However, this would be compensated by a significant increase in flexibility. The resulting cable i.e. 7x7 configuration may have sufficient flexibility and strength for use as a suture in certain surgical procedures. The diameter of the cable is controlled by the diameter of the individual wires in each strand, and also by the number of strands in the cable. The 19x7 configuration can comprise wires of diameter in the range 0.0015 to 0.005 inches (38μm to 127μm) with a tolerance of ±0.0001 inches (±2.5μm). The lower limit of this range is controlled by practical limitations. The corresponding cable diameter, in the unswaged condition, would be in the range 0.022 to 0.075 inches (0.56mm-1.9mm). The 7x7 configuration could employ wires of similar dimensions, although only wires in the range 0.0015 to 0.0025 inches (38um-64pm) would be suitable for the intended application. The corresponding cable thickness, in the unswaged condition, would be in the range 0.014 to 0.022 inches (0.36mm-0.56ram). The use of wire filaments of diameter 0.0025 inches (64μm) in a 19x7 configuration results in a cable possessing good flexibility and strength. The diameter of such a cable after swaging is 0.033 inches (0.84mm). Other construction patterns, besides a 19x7 configuration, using wire filaments of appropriate diameters could be used to give a cable of diameter 0.033" (0.84mm) as is illustrated in table 1. (For the sake of clarity, the 7x7x3 notation represents a cable with seven groups of seven strands where each strand has three wire filaments). However, some of these alternatives may not have adequate strength and others may not have sufficient flexibility. Typically, as the diameter of the wire filament decreases so the flexibility of the resultant cable increases. Also, as the total cross-sectional area of all the wire filaments in the cable approaches the cross-sectional area of a monofilament having the same diameter as the cable, so the strength of the cable approaches a maximum. The 19x7 configuration with wire filaments of diameter 0.0025" (64μm) has a particularly advantageous strength v flexibility ratio. Beam deflection tests were used to compare the flexibility of 19x7 cable, having 133 wire filaments of diameter 0.0025 inches (64μm), with a 20 gauge monofilament wire, approximately 0.032" (0.81mm) in diameter, and also with a 19x19 cable. The 19x19 cable has 361 wire filaments of diameter 0.0025 inches (64μm) and an overall diameter of 0.050 inches (1.3mm). The test is illustrated in figure 5, and the results shown graphically in figure 6. A sample 50 is clamped such that a length "1", where 1 is 1.00 inches (25.4mm) projects horizontally from the clamp 51. A range of loads 52 are applied to the unsupported protuberant end of the sample to produce a variety of deflection readings. The maximum deflection "d" at the tip of the sample is measured and recorded provided that, when the load is removed, the sample returns to the horizontal starting position. The graph of figure 6 plots deflection against load applied for the different samples. The gradient of each slope obtained gives a relative indication of the flexibility of the sample. The 19x7 cable shows a gradient of 4.25; the 19x19 cable shows a gradient of 0.77; and the monofilament wire shows a gradient of 0.02. Clearly, both the 19x7 and 19x19 cables are considerably more flexible than monofilament wire. TABLE 1: CONFIGURATION WIRE FILAMENT DIAMETER inches (μm) 7 7 3 19 x 7 12 x 7 19 x 3 7 x 7 7 3 1 x 19 1 x 12

Claims: CLAIMS : 1. A metal cable comprising a plurality of individual wire filaments; the wire filaments comprising titanium and having a diameter in the range 0.0015 inches to 0.005 inches (approximately 38um to 127μm) . 2. A metal cable according to claim 1 wherein a group of wire filaments are twisted together in a first helical direction around a central wire filament to make a strand. 3. A metal cable according to claim 2 wherein the said strand consists of seven wire filaments. 4. A metal cable according to claim 2 or claim 3 wherein the cable comprises a plurality of strands, including a core strand with peripheral strands twisted in a second helical direction around the core strand. 5. A metal cable according to claim 4 wherein the peripheral strands are arranged substantially symmetrically around the said core strand in at least one peripheral ring. 6. A metal cable according to claim 5 wherein six peripheral strands are arranged substantially symmetrically around the said wire strand in a peripheral ring. 7. A metal cable according to claim 6 wherein a further twelve peripheral strands are arranged substantially symmetrically around the said peripheral ring in a second peripheral ring. 8. A metal cable according to claim 4 when appendant to claim 2 wherein the first helical direction has the opposite sense to that of the second helical direction. 9. A metal cable according to any of the preceding claims wherein the wire filaments further comprise aluminium and vanadium substantially to provide the alloy Ti-6wt%Al-4wt%V as hereinbefore referred to in the description.

Inventors: CROCKARD HUGH ALAN, MELOUL RAPHAEL, CROCKARD, HUGH, ALAN, MELOUL, RAPHAEL
Applicants: CROCKARD HUGH ALAN, HILLWAY SURGICAL LTD, MELOUL RAPHAEL, HILLWAY SURGICAL LIMITED
Application year: 1993
Main classification: A61B17
IPC codes: A61B17, A61C7, D07B1