The present invention relates to an improved apparatus for the removal of bone cement in the course of revision arthroplasty. More particularly, but not exclusively, it relates to removal of bone cement during a hip joint prothesis revision. Other joint revision operations may also benefit from use of the apparatus but for the sake of convenience, the following description will refer to hip joint revisions, which term must be taken to include all other revision operations.

It is known from our UK Patent No. 2229660B and our coopending European patent Application No. 92917959.6 and from Engelbrecht US 4248232 and Hood US Des 341202 that the local heating effect associated with the application of ultrasound to plastics materials can be employed to soften and effect the removal of bone cement during revision arthroplasty. Specifically, it has been shown in our previous patent and applications that a fundamental requirement of the cement removal technique is to influence the flow of softened material so that a small quantity of cement can be isolated and retained to be captured in the head of the activated instrument in order to extract it efficiently from the operating site, over a series of penetrating cycles. An important characteristic of such instruments is the presence of an axisymmetric disc close to the distal end of the instrument, and critically, of one or more connecting ports between the distal face and an annular cavity proximal to said disc.

It is also known from our application No. EP 92917959.6 that the flow of softened cemen is encouraged by the use of an annular cutting edge optionally incorporating a taper eithe towards or away from the distal end of the probe.

It is an object of the invention to improve the flow and control of softened plastics materia as it transfers to the proximal disc cavity region by increasing the flexural disc amplitude an in particular by creating a displacement gradient between an outer annular cutting edge an the longitudinal axis of the instrument.

According to a first aspect of the present invention there is provided an apparatus for removal of bone-cement comprising means to generate an ultrasonic signal, elongate stem means t transmit said signal to a distal end thereof, a plurality of spoke members extending radiall from a location adjacent said distal end and connecting with an annular member surroundin and spaced from said distal end, a plurality of openings, each defined by said stem means, said annular member and two of said spoke members, wherein said annular member i provided with a distally facing cutting edge, and wherein said spoke members are s configured as to transmit to and allow axial movement of said annular member with respec to said stem means.

Preferably said stem means terminates distally as a substantially hemispherical or part- hemispherical working surface.

The spoke members are advantageously disposed equiangularly around said stem means.

Four spoke members may be provided, thereby providing four openings. Preferably however eight spoke members are provided, to give eight openings.

Each spoke member may have a distally facing concave surface and may additionally or alternatively have a proximally facing concave surface.

Each spoke member is preferably so shaped as to have an intermediate narrowest portion, defined between one or two respective concavities on the distal and/or proximal faces thereof.

In this case, any distal concavity may have an axial extent of up to one half of the axial extent of the annular member.

Also, any proximal concavity may have n axial extent less than that of the distal cavity.

According to a second aspect of the present invention, there is provided tool means for use in the apparatus described above.

According to a third aspect of the present invention, there is provided a tool element for removal of bone-cementing plastics material, comprising an elongate shaft having a central axis and adapted to longitudinal transmission of ultrasonic energy from a proximal excitation end to a distal plastics-engaging end, said plastics-engaging end comprising a solid inner-body portion of substantially the diameter of said shaft and a solid annular outer-body portion radially offset from and in axial overlap with said inner-body portion, said outer-body portion extending between axially spaced circumferentially continuous distal and proximal limits, wherein the distal limit has a longitudinal section characterised by an acute angle which defines a circumferentially continuous distal cutting edge, and an angularly spaced plurality of radial members being of less axial extent than the axial extent of said outer-body portion and providing an axially compliant support of said outer body portion with respect to said inner-body portion.

Preferably said plurality of radial members is defined by and between a corresponding plurality of like equally spaced longitudinally extending passages substantially within the radial offset of said outer-body portion from said inner-body portion.

Advantageously said distal limit defines an end face which has an annular distally open concavity within a region of radial offset of said outer-body portion from said inner-body portion.

In this case, the axial depth of said distally open concavity may be substantially equal to or less than half the axial space between said limits.

Said proximal limit may define an annular proximal end face which has an annular proximally open concavity within the region of radial offset of said outer-body portion from said inner- body portion.

In this case the axial depth of said proximally open concavity may be less than the axial depth of said distally open concavity.

Said acute angle defining the distal cutting edge may be in the range of 20° to 40°.

Said outer-body portion may have a frusto-conical outer surface which converges in the proximal direction and wherein said outer surface terminates distally at said distal cutting edge.

In this case, the convergence of said outer annular surface is in the range from substantially zero to 15 degrees with respect to said axis.

According to a fourth aspect of the present invention there is provided an apparatus comprising a tool element as described in the third aspect above, in combination with piezo ceramic transducer means adapted to impart ultrasonic energy to said proximal end of said elongate stem.

Embodiments of the invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which:

Figure 1 shows schematically a tool head of an apparatus embodying the invention;

Figure 2 is a diagrammatic illustration showing flexural displacement amplitude across the tool head during use of the apparatus;

Figure 3 is a diagrammatic illustration of a waveform showing longitudinal displacement amplitude along the stem and tool head of the apparatus;

Figure 4 shows use of the apparatus in removing bone cement from a revision cavity;

Figure 5 shows in more detail the tool head of the invention;

Figure 6 shows a front elevation of the tool head;

Figures 7A and 7B show in side elevation and plan view a tool head including eight holes; and

Figures 8A and 8B show similarly a tool head including only four holes.

Referring now to the drawings, Figure 1 shows the tool head end of the apparatus, partially in cross section.

Figures 2 and 3 show respectively the waveforms, generated in use, across the working face of the tool head, and along the stem.

As is shown most clearly in Figure 2, the ultrasonic energy is transmitted along the stem 1 and therethrough to an annular cutting ring 2, which is able to flex axially with respect to the stem 1. This occurs by increasing the flexural tool head peripheral amplitude in an axial direction along a displacement gradient from the stem to the cutting ring.

This is achieved by minimising any conical distal extension of the probe, and by shaping the cross section of the connecting parts or spokes 3. A divergent distal taper of 45° included angle aids the flow of plastics material into ports 4, each defined between any two spokes 3. This is further encouraged by the flexural displacement gradient associated with the compression standing wave shown in Figure 3. Reduction of the distal extension of the probe stem beyond the disc increases the scope for developing flexural displacement in this region and so enhances the effectiveness of the peripheral cutting edge.

The relevance of this design to the enlargement and penetration of the distal cemented region of a femur is shown in Fig 2. Insertion of the probe tip into the cavity revealed by removal

of a prosthesis therefrom results in peripheral contact before the end of the stem or 'tip touches any part of the bone cement at the distal end of the exposed bore. The shape of th cement cavity could generally deny contact to a conical distal extension of the stem 1 , whic extension would serve only to increase surface drag during material transfer, and could als cause perforation of the bone, even when not activated.

Furthermore, when seeking the removal of cement in the wider proximal cavity formed b prosthesis removal, it is possible to angle the instrument so that the first contact betwee cement/bone and probe is made at the periphery of the tool, particularly if this includes a enhanced conical taper.

An exemplary embodiment of the invention will now be described particularly with referenc to Figures 5 and 6.

In these Figures, the following definitions and relationships apply.

R, = mean radial locus of centres of passages, i.e., where W ₂ (min. width) applie R ₂ = Lower effective limit of spread R ₂ , R ₃ . R ₃ = Outer effective limit of spread R ₂ , R ₃ .

W, = Overall axial width of outer annular body

W ₂ = Minimum axial width of radial arm (spoke) connection

W ₃ = Depth of distal recess

W ₄ = Depth of proximal recess

a. = 35° to 45° α ₂ = 0° to 15° α ₃ = 10° to 20°

If β _! = β ₂ , and β, + β ₂ = τ/2 the mean circumferentially arcuate width of each spoke is τR,/4

Also

W, > 2W ₂

W ₂ ~ W ₃

Use of apparatus according to these dimensions and relationships has shown a freer flow of plastics cement material through the openings and therefore an improved rate of removal of the plastics cement material.

In a further embodiment, such as is shown in Figure 7, there are eight holes 4 and eight spokes 3. Given a substantially similar radius of the tool head, this increase in the number of spokes causes a consequent reduction in the width of each and hence a greater axial flexural displacement of the annular cutting edge 2.

Indeed, it has been found that with a tool head of diameter 11.8mm with eight ports each of diameter 1.5mm, for a vibrational amplitude at the centre of 51 +. 2.5 μm, the vibrational amplitude at the periphery is 74 +_ 4 μm.

It has also been found that the flow of plastics cement material is so enhanced that the material will not only pass more easily through the ports 4, but will also be encouraged to pass up the stem 1 , possibly as far as the first nodal point.

It is preferred that the ratio between the diameters of the tool head and of the stem is between 1.75 - 2.25: 1.

Hence the apparatus enables improved removal of cement with, in some circumstances, improved patient safety.