Patent Description:
Although the tool system will be most often used for removing a cemented stem in hip revision surgery, the method can also be used to remove a cemented stem in shoulder revision surgery, or other cemented bone implant removals.

Removal of a well-fixed implant in cemented total hip arthroplasty can be very challenging. Indications for removing a well-fixed implant may include sepsis, recurrent dislocation due to femoral component malposition and/or inadequate offset. Attempting to remove a well-fixed implant from proximal exposure alone can result in extensive bone loss due to the inability to disrupt the bone/prosthesis interface distally. One of the most difficult aspects of Revision Total Hip Replacement (RTHR) surgery is that the surgeon may have very limited information about the location and amount of bone cement in the femur during the surgical procedure. The perforation and fracture of the femur during the removal of bone cement in revision total hip arthroplasty (THA) are serious complications that considerably affect the postoperative protocols and clinical results.

The extended trochanteric osteotomy is the most common osteotomy used in cemented femoral revision and allows improved access to the implant/bone or implant/cement interface.

Indications for the extended proximal femoral osteotomy include revision of well-fixed cemented femoral components or removal of a loose femoral stem with a well-bonded cement mantle. The use of an extended trochanteric osteotomy in these situations will minimize intraoperative complications and will result in predictable healing. However, comparison of surgeries with and without osteotomy clearly show that trochanteric osteotomy prolongs the surgery, is associated with more blood loss, and causes a slower postoperative rehabilitation course. Besides that, additional fractures (<NUM>%), nonunions (<NUM>%),<NUM> malunion (<NUM>%), and reoperations (<NUM>%) are reported.

Bone cement can be safely removed using the known ROBODOC system, which uses high speed milling. No serious complications have been reported using this system and full weight bearing is achieved early in the postoperative course because of circumferential preservation of the femoral cortex. However this system is expensive and time consuming because of the need to implant locater pins before the revision surgery and a mean robotic milling time of <NUM> (range17-<NUM>). Besides that there is a risk of heat injury during the ROBODOC milling process of cement removal.

The invention aims at an improved method, preferably resulting in a shortened surgical time, a reduced morbidity, a decreased blood loss, less fractures and/or less reoperations.

To that end the method in accordance with claim <NUM> is provided.

Preferably the stem comprises a chainsaw blade and the cutting element comprises a driven cutting chain, provided with the two laterally extending protrusions on the chainsaw blade. More preferably the cutting tool is a small electrical chainsaw, such as a Bosch™ NanoBlade™-type chainsaw, provided with the two laterally extending protrusions on the chainsaw blade. Preferably the protrusions are pins.

Preferably the position of at least one of the protrusions on the chainsaw blade is adaptable in the direction of the side edges of the chainsaw blade, so as to accommodate for both relatively deep and shallow cement cuts. Preferably the chainsaw blade has a wedge shape, wherein the outer end where the cutting chain is arranged to cut is the widest end of the wedge shaped chainsaw blade. Preferably the protrusions extend laterally on opposite sides of the blade, and the grooves are provided in corresponding opposite sides of the guide member.

Preferably the longitudinally running grooves are substantially U-, V- or Y-shaped, wherein the grooves are profiled such that the outer cutting end of cutting element, when inserted into the prosthetic cavity and being moved to at least substantially to the bottom end thereof, wherein the protrusions move through first ones of said legs of said U-, V- or Y- shaped grooves, and back to the upper end thereof, wherein the protrusions move through the other legs of said U-, V- or Y-shaped grooves, is forced to follow a substantially U- shaped path corresponding to the profile of the line of intersection of the cement/bone interface on two opposite sides of the intersection.

Preferably said guide member is produced by means of a 3D-printer. Preferably the profiles of said grooves are calculated from said profile of said line of intersection of the prosthetic cement/bone interface by means of a computer loaded with a computer program comprising instructions to carry out said calculation. Preferably said 3D-printer is controlled by said computer loaded with said computer program, said computer program further comprising instructions to control said 3D-printer.

The invention will now explained in more detail by means of exemplary embodiments, as shown in the figures, wherein:.

The femoral stem of a prosthesis <NUM> is held in place in the femur of a patient by a cement <NUM>, which is normally polymethylmethacrylate (PMMA). If the prosthesis <NUM> has to be removed for any reason it can normally be pulled relatively easily from the cement <NUM> leaving a closed end tubular cement plug <NUM> within the bone <NUM> which must then be removed to allow a fresh prosthesis <NUM> to be inserted and re-cemented.

In the process of removing the prosthetic cement <NUM>, the present invention allows it to be cut to the depth of the cement <NUM> in a longitudinal line without cutting into the bone <NUM>. Normally three or four of such cuts are made so as to be spaced apart around the perimeter of the cavity.

According to <FIG> the area of a prosthesis <NUM> and the femur <NUM> of a patient <NUM> with a cemented total hip arthroplasty, which should undergo revision surgery, is being scanned by a CT scanner <NUM>. As shown in <FIG> the scans of a multitude of planes of intersection of the prosthesis <NUM> and the bone cement <NUM> are then projected on a computer screen <NUM> and a virtual 3D model (shown on the right) of the prosthesis <NUM>, the bone <NUM> and the bone cement <NUM> is made from said scans. <FIG> shows the prosthesis <NUM> and its dimensions, which may be derived from the virtual 3D model, but which may also be determined by determining the manufacturer and the type of the prosthesis <NUM>.

The cement <NUM> to be removed is cut by an electrical chainsaw <NUM> as shown <FIG> and for instance in <FIG>. The chainsaw <NUM> comprises a blade <NUM>, a driven cutting chain <NUM>, a grip <NUM> and a power button <NUM>. Two pins <NUM>, <NUM>, to be engaged by the guiding grooves <NUM>, <NUM> of the guide members <NUM>, are mounted on the blade <NUM>, one pin <NUM> near the bottom end of the blade <NUM> and one pin <NUM> at a higher point on the blade <NUM>, more near the center of the length of the blade <NUM>. As shown in <FIG> the pin <NUM> at the bottom end and the pin <NUM> at the higher point preferably extend at opposite sides of the blade <NUM>. As shown in <FIG> the pins <NUM>, <NUM> can be fixed to the chainsaw blade in several different mutually adjacent holes <NUM> which are distributed along the width of the blade <NUM>, so as to accommodate a wide variety of distances A and B, corresponding to a wide variety of possible cement thicknesses. <FIG> shows an alternative embodiment, wherein the pins <NUM>, <NUM> are mounted in a rotatable disc <NUM>, to achieve the same.

<FIG> show screen shots of the process of designing a patient specific guide member <NUM> from a scan. First in <FIG> the shape and location of the stem of the prosthesis <NUM> is determined, which determines the shape and location that the guide member <NUM> will have during removal of the cement. Then in <FIG> the profile of the line of intersection of the prosthetic cement/bone interface on the left side of the guide member <NUM> (shown as a dashed line on the left side of the cement <NUM>) is determined from the scan. As shown in <FIG> the shape of the two grooves <NUM>, <NUM> are determined such that the chainsaw <NUM> will be guided in the correct manner along the line of intersection of the prosthetic cement/bone interface. Generally the distance A as shown in <FIG> between the groove <NUM> for guiding the bottom end of the chainsaw <NUM> and the line of intersection of the prosthetic cement/bone interface should be equal to the distance B between the pin <NUM> and the cutting edge of the chainsaw <NUM> as shown in <FIG>. The shape of the groove <NUM> for guiding the higher end of the chainsaw blade <NUM> prevents rotation of the chainsaw <NUM> and should be designed such that the cutting edge of the chainsaw's cutting chain <NUM> is forced to follow exactly the designed path (i.e. the line of intersection of the prosthetic cement/bone interface). As shown in <FIG>, B the guide member <NUM> is further designed to be provided with a flange <NUM> extending laterally from the top side for abutment against the top of the femur. A slot <NUM> for accommodating the blade <NUM> of the chainsaw <NUM> is provided in the guide member <NUM> from the top to the bottom thereof. The slot <NUM> may split the guide member in two halves, which are then held together by at least the flange <NUM>. In an alternative embodiment, if the guide member is made of a material that can be cut by the chainsaw, such as plastic, the slot <NUM> may not be (fully) present in the guide member <NUM> before the chainsaw <NUM> makes the cut through the cement, in which case the slot <NUM> is made by the chainsaw <NUM> during said cutting.

The so designed guide member <NUM> is then produced, preferably by means of a 3D printer. Materials to print the guide member <NUM> are for example metal or plastic.

<FIG> shows the femur <NUM>, with the prosthesis <NUM> fixed to the femur <NUM> by the cement <NUM>. Below the stem of the prosthesis <NUM> the cement <NUM> has formed a plug <NUM>. In <FIG> the prosthesis is removed from the cement <NUM>. <FIG> shows the insertion of the guide member <NUM> in the prosthetic cavity. Then, while the chainsaw <NUM> is turned on for cutting, the pin <NUM> is inserted in the groove <NUM>, then the pin <NUM> is inserted in the groove <NUM>, and then the chainsaw <NUM> is moved to the bottom of the cavity until it reaches the plug <NUM>. The chainsaw <NUM> may or may not be able to cut entirely through the plug <NUM>. <FIG> shows the situation where the chainsaw <NUM> is withdrawn from the cavity without entirely cutting through the plug <NUM>, thereby leaving the plug <NUM> in place for later removal.

The process as shown in <FIG> is then repeated three times, at <NUM> degrees angular intervals and with different patient specific guide members <NUM> designed for that line of intersection of the prosthetic cement/bone interface, such that the tubular body of cement <NUM> is cut in four sections.

<FIG>, <FIG> show an alternative embodiment of the patient specific guide member <NUM>, wherein the grooves <NUM>, <NUM> are designed such that the saw can cut the cement <NUM> along the line of intersection of the prosthetic cement/bone interface on both the left side and the right side of the guide member <NUM>. This results in the grooves <NUM>, <NUM> being substantially V-, Y- or U-shaped. In this example the groove <NUM> has a Y-shape, and the groove <NUM> has a V-shape. In this embodiment the tubular body of cement <NUM> can be cut into four sections by means of only two different guide members <NUM>.

<FIG> shows the guide member <NUM> being pulled out of the femur, leaving the four sections of cement <NUM> as a result of the cuts <NUM> made with the chainsaw <NUM>. The sections of cement <NUM> are then removed in a conventional way by means of an osteotome (a surgical chisel). <FIG> shows the femur <NUM> after remaining cement is removed from the bone, but leaving the bottom plug <NUM>.

For cutting the plug <NUM> a generic plug cutting guide member <NUM>' may be used, which has a shape which is generally the same as the stem of the prosthesis <NUM>, and which has two sets of grooves <NUM>', <NUM>" shaped such that the chainsaw <NUM> is guided straight down to the plug <NUM> in two mutually perpendicular orientations of the chainsaw, as shown in <FIG> and <FIG>. <FIG> show the insertion of the chainsaw <NUM> into the generic plug cutting guide member <NUM>' in a first direction, with the pins <NUM>, <NUM> being guided in the first grooves <NUM>' to make a first cut through the plug <NUM>, and <FIG> shows the insertion of the chainsaw <NUM> into the generic plug cutting guide member <NUM>' in a second perpendicular direction, with the pins <NUM>, <NUM> being guided in the second grooves <NUM>" for making a second perpendicular cut through the plug <NUM>. Then, after the guide member <NUM>' is removed, the remaining cement parts can be easily removed.

According to <FIG> the remaining prosthetic cement pieces of the plug <NUM> are removed by means of a tool <NUM> comprising an elongated stem <NUM>, a hook <NUM>, and a grip <NUM>. As shown in <FIG>, the hook has an X-shaped cross section, seen from below, wherein the four legs of said X-shape extend laterally from the outer end of the stem <NUM> at <NUM> degrees intervals. The hook <NUM> is inserted through the X-shaped opening made by the first and second cuts as described above with reference to <FIG>, until the hook <NUM> extends below the pieces of said plug <NUM>. Then the stem <NUM> is rotated around its axis by approximately <NUM> degrees such that the hook <NUM> engages the bottom surface of the remaining pieces. Then the tool <NUM> is forcibly moved upwards, by pulling the grip <NUM> or if necessary by means of hammering against the lower end of the grip <NUM>, thereby removing all four remaining pieces of the plug <NUM>.

Claim 1:
A method for providing a tool system, the tool system being suitable for removing prosthetic cement (<NUM>) from a bone (<NUM>) of a patient undergoing a joint prosthesis (<NUM>) replacement operation, the method comprising the steps of:
scanning the patient in the area of the joint prosthesis (<NUM>) so as to obtain at least one profile of a line of intersection of a prosthetic cement/bone interface at its intersection with a generally longitudinally pre-selected plane; and
providing a cutting tool (<NUM>), the cutting tool being suitable for forming a running cut substantially completely through the prosthetic cement (<NUM>) along the line of intersection;
wherein the cutting tool (<NUM>) comprises:
an elongated stem (<NUM>) having dimensions to be able to be inserted into a prosthetic cavity located in said prosthetic cement (<NUM>), at least substantially to the bottom end thereof;
a cutting element (<NUM>) arranged on said stem (<NUM>) and arranged to cut the prosthetic cement (<NUM>) at one outer end of said stem (<NUM>);
a grip (<NUM>) arranged on the other outer end of said stem (<NUM>) for engagement of the cutting tool (<NUM>) by an operator; and
wherein the stem (<NUM>) of the cutting tool (<NUM>) comprises two laterally extending protrusions (<NUM>, <NUM>), which protrusions (<NUM>, <NUM>) are located along the length of the stem (<NUM>) at a mutual distance apart;
characterized in that the method further comprises providing a guide member (<NUM>);
wherein the guide member (<NUM>) is substantially shaped as a stem of the prosthesis (<NUM>) in the prosthetic cavity located in said prosthetic cement (<NUM>); and
wherein the guide member (<NUM>) is provided with two longitudinally running grooves (<NUM>, <NUM>) which each are arranged to engage one of said protrusions (<NUM>, <NUM>) of the cutting tool (<NUM>), wherein the grooves (<NUM>, <NUM>) are profiled such that the outer end of the stem (<NUM>) at which the cutting element (<NUM>) is arranged to cut the prosthetic cement (<NUM>), when inserted into the prosthetic cavity and being moved to at least substantially to the bottom end thereof, will be forced to follow a path corresponding to the profile of the line of intersection of the cement/bone interface.